Everything has a price. And the price of powerful rockets with nuclear propulsion is of course the dread horror of deadly atomic radiation. But the danger can be brought under control with appropriate counter-measures, and by treating the power plant with the respect it deserves. And the same measures will come in handy if your ship is an interplanetary warship that may be facing hostile nuclear warheads.

But it is important not to over-react. There is a lot of silly media hype about plutonium being "the most toxic substance known to man", there is a general agreement among experts in the field that this is false.

The characteristic blue glow you've seen in photographs of "swimming pool" reactors is called Cherenkov Radiation. If you the blue Cherenkov glow around an object IN THE AIR (not at the bottom of a swimming pool reactor), you'd better be viewing it through several inches of lead glass or you have already taken a lethal dose, it is far too late to do anything about it, you are already dead. This comes under the heading of "not treating radiation with the respect it deserves."

As a side note, Cherenkov Radiation is caused by radioactive particles exceeding the speed of light in the medium. The term "c" is not "the speed of light", it is the speed of light in a vacuum. The maximum speed of light is much slower in air and even slower in water. The practical upshot of this is that there is no Cherenkov Radiation in the vacuum of space, and to get the same level of glow seen in a swimming pool a radiation source in air will have to be much more radioactive.

Types of Radiation

RocketCat sez

OK, listen up people! Rockets in space will need to protect the crew from space radiation. Atomic rockets will have to protect the crew from the atomic engine radiation. And combat rockets should (but may not be able to) protect the crew from the radiation of hostile nuclear explosions and particle beam weapons fire.

The radiation you will worry about is Ionizing Radiation. You don't give a rat's behind about non-ionizing radiation because the worst it will do is give you a sunburn. Ionizing radiation will kill you.

There are two kinds of radiation: [1] rays and [2] particles.

Science geeks call rays "Electromagnetic Radiation" (a fancy word for exotic light), but you just have to know that the deadly ones are X-Rays and Gamma Rays. Combat them with a radiation shield made out of dense stuff, such as lead or tungsten. Otherwise the far wall will have a splendid x-ray picture of your skeleton writhing in its death agonies. This is why your dentist puts a lead vest on you for teeth x-rays.

Particle radiation is protons ("proton storm"), neutrons ("neutron radiation"), electrons ("beta particles"), alpha particles ("alpha particles") and heavy primary nuclei ("HZE ions"). Combat them with a radiation shield made out of low density stuff: water, liquid hydrogen, lithium hydride, paraffin, hydrogenated polyethylene composite, or something else stuffed with hydrogen. Otherwise read the radiation chart for the hideous details of your fate, which will convince you that euthanasia might not be such a bad idea after all.

  • Protons come from planetary radiation belts ("Van Allen belt"), solar flares ("proton storms"), and cosmic rays. And particle beam weapons.
  • Neutrons come from fission & fusion reactors, fission & fusion rockets engines, nuclear & thermonuclear bombs, some radioactive elements and by cosmic rays slamming into the hull. That is, mostly man-made.
  • Electrons come from planetary radiation belts and inside auroral borealis and australis. Also some radioactive elements and particle beam weapons.
  • Alpha particles come from cosmic rays and solar flares. And some radioative elements.
  • HZE particles come from cosmic rays
  • X-rays come from high speed electrons crashing into metal. You'll find x-rays screaming out of fission explosions and where electron particle beam weapons rake the metal spacecraft hull. That is, mostly man-made.
  • Gamma-rays come from tortured atomic nuclei. You'll also find gamma-rays screaming out of fission explosions and from nuclear fission reactors. That is, mostly man-made.

Put yer radiations shields so it hits the anti-particle shields first then the anti-ray shield second! Otherwise the particles hitting the anti-ray shield will act like a huge dentist x-ray machine and generate a lethal storm of x-rays past the anti-ray shield. The shield will kill the crew!

The general term for dangerous unhealthy everybody-panic-now kind of radiation is "ionizing radiation." This is because the radiation is capable of ionizing atoms which compose the material being irradiated. Materials such as the poor crew's tender vulnerable pink bodies and internal organs. Non-ionizing radiation such as visible light and radio waves can be safely ignored (by which I mean a laser beam can chop you into bits but it won't give you cancer).

Ionizing the atoms composing the proteins of a living thing is much like using a machine gun to fill a running automobile engine full of bullets. Proteins are the tiny molecular machines that make cells work. If you ionize an atom of a given protein it either splits or crumples up into a tangled wad, rendering it useless. Destroy enough of the proteins and the cell dies. Destroy enough cells and you die.

Radiation also smashes DNA molecules like a jackhammer. This can kill the cell or turn it cancerous. No, it won't turn you into a mutant, but any future childen you have will be another matter.

The sun's ultraviolet light is a form of radiation that can give your skin a sunburn. Ionizing radiation is more penetrating, so it is capable of giving you a lethal "sunburn" on your internal organs.

Ionizing radiation comes in two types:

In the context of this website, you will be dealing with radiation in several areas.

  1. Radiation from the space environment
  2. Radiation from spacecraft nuclear propulsion and nuclear power plants
  3. Radiation from space combat weapons

Finally it is important to understand the subtle distinction between radiation and radioactivity. Radiation are the deadly rays and particles that kill you. An element is radioactive if that element emits the deadly rays and particles that kill you.

What's the practical difference?

If you say that the nuclear rocket engine is emitting radiation, this means it is emitting deady rays, which all nuclear rockets tend to do.

But if you say that the nuclear rocket engine is emitting radioactivity, this means that the reactor core has been breached, and it is spewing powdered nuclear reactor rods in the form of a lethal cloud of atomic fallout.

Radiation from Space

Astronauts traveling from planet to planet are exposed to the natural radiation of space. This is generally always particle radiation (but little or no neutrons), with zero gamma rays or x-rays. One usually only encounters gamma rays and x-rays from artificial sources, such as nuclear reactors and nuclear weapons (unless you are near a black hole or other extreme environment). Exposure time to natural space radiation is "chronic" (see below).

There are three sources of natural space radiation:

Here is a good overivew of naturally occuring sources of space radiation.

The ionizing radiation in space is comprised of charged particles, uncharged particles, and high-energy electromagnetic radiation. The particles vary in size from electrons (beta rays) through protons (hydrogen nuclei) and helium atoms (alpha particles) to the heavier nuclei encountered in cosmic rays, e.g., HZE particles (High Z and Energy, where Z is the charge). They may have single charges, either positive (protons, p) or negative (electrons, e), multiple charges (alpha or HZE particles); or no charge, such as neutrons. The atomic nuclei of cosmic rays, HZE particles, are usually completely stripped of electrons and thus have a positive charge equal to their atomic number.

The ionizing electromagnetic radiation consists of x-rays and gamma-rays which differ from each other in their energy. By convention X-rays have a lower energy than the gamma-rays with the dividing line being at about 1Merv. In general, x-rays are produced either by the interaction of energetic electrons with inner shell electrons of heavier elements or through the bremsstrahlung or braking radiation mechanism when deflected by the Coulomb field of the atomic nuclei of the target material. Gamma-rays are usually products of the de-excitation of excited heavier elements.

Ionizing radiations vary greatly in energy. Electromagnetic radiations have energy quanta determined by their wavelength or frequency. The energy of particulate radiation depends on the mass and velocity of the particles. Figure summarizes the main types of ionizing radiation including their charge, mass, and source.

Figure Sources and Characteristics of Electromagnetic and Particulate Ionizing Radiations in Space.
NameNature of radiationChargeMassSources

Primary: Solar corona, stars, galaxies, terrestrial atmosphere in auroral zone.

Secondary: Spacecraft structure in some parts of the radiation belts, in the auroral zone, and in interplanetary space following some solar flares

Gamma rayElectromagnetic00Stars, galaxies, unknown sporadic sources, and spacecraft atmosphere.
ElectronParticle-e1meRadiation belts and auroral regions.
ProtonParticle+e1840 me or 1 amuGalaxy cosmic rays, radiation belts, and solar flares.
NeutronParticle01841 me

Primary: Galactic cosmic ray atmosphere albedo neutrons.

Secondary: Galactic cosmic ray interaction with spacecraft structure.

Alpha particle (helium nucleus)Particle+2e4 amuGalactic and solar.
HZE particle (heavy primary nuclei)Particle=> +3e=> 6 amuGalactic and solar.

Galactic Cosmic Radiation

Galactic Cosmic Radiation is particle radiation coming from outside the solar system (some is from outside of the galaxy), from more or less the entire sky. Most of them come from supernovae, it is unclear what is the origin of the rest.

The intensity of the cosmic ray bombardment varies with the strength of the solar wind. The wind is not causing the cosmic ray flux, it is apparently providing some shielding from cosmic rays. At a distance of about 94 astronomical units from Sol the solar wind undergoes termination shock which forms the inner edge of the heliosphere. This provides the cosmic ray shielding, in an amount proportional to the current solar wind strength.

But even with maximum protection from solar wind, the cosmic ray flux never drops down to zero.

In a spacecraft or space station the crew is protected from galactic cosmic rays by hull armor. The armor has two layers: hydrogen-rich armor (e.g., paraffin) to stop particles and dense armor (e.g., tungsten) to stop electromagnetic radiation. It is vitally important that the hydrogen-rich armor is on the outside with the dense armor as an inner layer. This ensures that the cosmic rays are stopped before they hit the dense armor.

If the dense armor was on the outside so the cosmic rays hit it first, the charged particles in the GCR would create deadly "Bremsstrahlung" ("secondary") radiation and kill the crew. Basically you would have designed the spacecraft to be a huge x-ray machine with the crew at the emitter. That would be bad.

But in any event keep in mind that it is incredibly difficult to shield against GCRs.

NASA engineers fret about this because the transit time for a Mars mission with the currently available pathetically weak propulsion systems will expose the crew to more space radiation than is allowed. NASA's Curiosity space probe measured the radiation dosage inflicted by traveling through space to Mars, specifically from galactic cosmic rays and solar proton storms. The dosage was 1.84 millisieverts per day (0.00184 sieverts). However, keep in mind this was measured during the peak of Sol's 11-year activity cycle, when GCR flux is relatively low due to shielding from the heliosphere. Also keep in mind this is with zero radiation shielding.


A Forbush decrease is a rapid decrease in the observed galactic cosmic ray intensity following a coronal mass ejection (CME). It occurs due to the magnetic field of the plasma solar wind sweeping some of the galactic cosmic rays away from Earth. The term Forbush decrease was named after the American physicist Scott E. Forbush, who studied cosmic rays in the 1930s and 1940s.


The Forbush decrease is usually observable by particle detectors on Earth within a few days after the CME, and the decrease takes place over the course of a few hours. Over the following several days, the galactic cosmic ray intensity returns to normal. Forbush decreases have also been observed by humans on Mir and the International Space Station (ISS), and by instruments onboard Pioneer 10 and 11 and Voyager 1 and 2, even past the orbit of Neptune.

The magnitude of a Forbush decrease depends on three factors:

  • the size of the CME
  • the strength of the magnetic fields in the CME
  • the proximity of the CME to the Earth

A Forbush decrease is sometimes defined as being a decrease of at least 10% of galactic cosmic rays on Earth, but ranges from about 3% to 20%. Reductions of 30% or more have been recorded aboard the ISS.

The overall rate of Forbush decreases tends to follow the 11-year sunspot cycle. It is more difficult to shield astronauts from galactic cosmic rays than from solar wind, so future astronauts might benefit most from radiation shielding during solar minima, when the suppressive effect of CMEs is less frequent.

From the Wikipedia entry for FORBUSH DECREASE

Radiation on the way to Mars, and why it isn't such a huge risk as we think it is

News coverage of a mission to Mars will often result in claims about radiation on the way to Mars, that it's either a huge problem or will even cause everyone to die on board. However, there isn't a large amount of truth to this, in my view. Radiation is an issue but not a major one and not one that can't be resolved.

Radiation levels on the way to Mars

The readings performed by the Curiosity rover on the way to Mars show that the astronauts would be exposed to a total of 1.8 milllisieverts per day, with surface levels being about 0.64 mSv per day. Assuming a 500 day surface stay and 360 days in space, the total radiation dose the crew would be exposed to is roughly 1.01 Sievert over the total duration of the trip. This is associated with a total death risk by cancer of... five percentage points. It would go up from 21% to 26%. The radiation limit for ESA astronauts is 1 Sievert, which means that ESA astronauts would be only barely out of the limit, even if provided only with the thin metal shielding on Curiosity. Only a relatively small amount of radiation protection would be required to get the mission dose under the acceptable limit. According to an ESA study from 2004, only 9 grams per square centimeter of radiation protection is required to get within the acceptable limit, which actually is no additional shielding at all for their habitat design. The NASA limit of 2/3rds of a Sv are more problematic, however.

Source 1, source 2

Also, this is for Galactic Cosmic Rays, or GCRs. These particles are highly energetic and require a load of shielding to get it down to terrestrial levels. Curiosity flew during a solar minimum, which means that the sun's own radiation was at a minimum, however the sun's magnetic field is also weaker, which actually increases the amount of GCRs a ship would be exposed to. During a solar maximum, GCRs are reduced significantly and only solar particles provide a significant danger. Solar particles are less energetic and can be shielded against far more effectively.

Solar storms

These are the kind of radiation events that actually form a real danger during the trip. However, solar particles are more easily stopped than GCRs, and the risk they provide can be made almost completely negligible by the addition of a storm shelter for the crew.

The shielding required can be as "low" as 25 g/cm2 to prevent the astronauts from being under serious risks. By putting this shelter in the middle of your spacecraft, like in Mars Direct, you can use your supplies (food and water) to keep the crew safe. Other sources note 300 kg/m2 (30 g/cm2) of water also sufficient to keep the dose reasonable.

Source, source

So how do we solve this problem?

Using ESA astronauts instead of NASA ones, obviously.

In more seriousness, additional shielding (hydrogen-based shielding like water and plastic are optimal), careful mission planning and crew selection (an old male has lower risk than a young female) and good placement of equipment and supplies on board can significantly reduce the radiation risk posed to the crew. In any case, as long as there is shielding against solar storms, the risk of radiation is fairly small and not life threatening during the mission.

Is it a problem? Yes. Is it unsolvable? No. Is it going to cause the astronauts to fry on their way there? Not at all.

Trapped Radiation

Back in the 1950s the pulp scifi stories tried to scare the readers with references to the deadly belts of radiation found in space. In reality they are not instant death, but they are unhealthy to linger inside. They damage the electronics of satellites as well.

What happens is that planets with strong magnetic fields will created zones of radiation by trapping energetic particles from the solar wind. Sort of like a cosmic roach motel: "Radiation checks in, but they don't check out!" From the standpoint of life on the planet's surface, the magnetic field is a good thing. The radiation from the solar wind is either diverted or trapped in the radiation belts, in either case it is much better than hosing the planet's surface with deadly radiation. If the planetary life becomes intelligent enough to develop space flight, then it should be intelligent enough to invent radiation shielding.

Please note that the radiation in the belts never reach the planet itself.

There are known radiation belts around Terra, Jupiter, Saturn, Uranus and Neptune.

In a spacecraft or space station the crew is protected from trapped radiation by storm cellars. The crew has to occupy the storm cellar as long as the ship is within the radiation belt. This is why ion drives are not favored for breaking out of low Terra orbit, because such low-thrust propulsion can take almost a year to climb out of Terra's Van Allen Radiation Belt.

The zones around Terra are called the Van Allen radiation belts, after James Van Allen discovered them in 1958.

There are two belts.

The Inner Belt starts at an altitude from 400 km to 1,200 km, depending on latitude, and ends at an altitude of about 6,000 km, with its most lethal area 3,500 km out (1.55 Terran radii). The South Atlantic Anomaly can potentially disrupt satellites in polar orbits, but usually does not pose a problem for manned spaceflights. Except for the ISS. The radiation is high-energy protons (400 MeV).

The Outer Belt ranges from 13,000 km to 60,000 km, with its most lethal area 27,000 km out (5.22 Terran radii). The Outer Belt is affected by solar winds, and is thus flattened to 59,500 km in the area directly between the Earth and the Sun, and extends to its maximum distance in the shadow of the Earth. The radiation is high-energy electrons (7 MeV).

A safe channel exists between the belts from 9,000 km to 11,000 km.

if you were in a ship with 2 g/cm2 of hull shielding, the maximum radiation intensity zone of the inner Van Allen belt would give you a radiation dose of 0.2 Grays per hour. Hull shielding of 25 g/cm2 would reduce that to 0.05 Grays/hr. The report cites a permissible limit of 2 Grays total for a mission. The Apollo lunar missions dealt with the belts using a trajectory that missed the inner belt and zipped through the outer belt as fast as possible.

Since Terra's rotational and magnetic axes do not intersect at Terra's Center (see diagram), there is a deadly spot in the inner belt called the South Atlantic Anomaly. The inner edge of the belt proper is usually 1,000 kilometers from Terra's surface, but the anomaly gets as close as 200 kilometers. Satellites and space stations need extra radiation shielding for when they periodically pass through the anomaly. The ISS has extra shielding for that reason. Astronauts have seen phosphene shooting lights in their eyeballs, laptops have crashed, control computers experience transient problems as they pass through the anomaly.

Also of note, the Starfish Prime nuclear test temporarily (for five years) made the radiation levels in the Van Allen belts much worse (crippled a third of all satellites in low Earth orbit). Yes, this means that somebody can easily intensify the radiation just by popping off a few fission bombs. Science fiction writers can easily imagine scenarios where this could be used to attack a planet or to defend a planet.

Since the Van Allen Belts will destroy expensive satellites as well, there have been proposals to drain the radiation out of the belt.

The planet Jupiter has radiation belts similar to Terra, except the radiation is thousands of times stronger. Io, Europa, and Ganymede are inside the radiation belt, Callisto is outside. Volcanic gas from Io makes things more complicated. In 1973 Pioneer 11 was surprised by radiation levels around Jupiter ten times greater than NASA had predicted. This is why Pioneer did not send back photos of the moon Io since the radiation belt had fried its imaging photo polarimeter. Work on the Voyager space probe came to a screeching halt as they frantically redesigned it to cope with the radiation, but still be assembled in time for the launch window.

According to Cairan:

Satellite Daily
rate (Sv)
rate (Sv)
Io 36 13149 2 min 4 min 20 min 40 min 1.33h 2 h 2.67h 4 h 6.67h 33.33h
Europa 5 1972 13.33 min 26.66 min 2.22h 4.44h 8.89h 13.33h 17.78h 26.67h 44.44h -
Ganymede 0.08 29.22 15h 30h 6.25d 12.5d 25d 37.5d 50d 75d - -
Callisto 0.0001 0.037 1.37y 2.74y - - - - - - - -

The single year limit is 50 mSv, while the maximum 5-year cumulative exposure is 100 mSv (or 20 mSv per year). LD stands for Lethal Dose, LD x/y means "x" percent of individuals die within "y" days. LD 50/30 thus means half of people exposed at this level of radiation would die within 30 days.

For shielding purposes to limit exposure to below regulatory levels for 5-year periods, the number of halving thickness of shielding material stands as follows:

Satellite Number of
Io 20 3.6 m
Europa 17 3.06 m
Ganymede 11 1.98 m
Callisto 1 18 cm

Solar Energetic Particles

The sun (or any other star) occasionally belches out fast-moving clouds of deadly particle radiation (the technical term is Coronal Mass Ejection or CME). These are called a solar proton storm, solar storm, solar proton event (SPE), or proton storm. Planets lucky enough to have magnetic fields (like Terra) are protected from such storms. Planets without magnetic fields and spacecraft, are bombarded by the radiation. The International Space Station has an orbit low enough that it is protected by Terra's magnetic field, though it does periodically plough through the South Atlantic Anomaly.

The storm can be sped up or slowed down by interations with the solar wind and interplanetary magnetic field. They reach velocities from 20 to 3,200 km/s with an average speed of 489 km/s. This means the transit time from Sol to the mean radius of Terra's orbit will be from 13 hours to 86 days, with 3.5 days as the average

Solar storms typically have a duration from one to two days.

Please note that solar storms are blobs of radiation that are quite a bit larger than a planet, but smaller than the solar system. So, for instance, a storm that hits Terra might totally miss Mars or Ceres. Or force the crew of spacecraft heading to Luna to take shelter in the storm cellar, while astronauts near Mars will be safe.

In a spacecraft or space station outside of a planetary magnetic field the crew is protected from solar storms by storm cellars. The crew will have to occupy the storm cellar for several hours during the peak of the storm, maybe up to a couple of days.

Researchers have been looking into equipping spacecraft with artificial magnetic fields for protection, but so far this has proven difficult.

From NASA-STD-3000 Man-Systems Integration Standards. (ed note: Naturally any NASA document will be silent on nuclear weapons, so there is no mention of gamma-ray or x-ray shielding.)

Radiation from Nuclear Power

If the spacecraft uses a nuclear propulsion system, or has a nuclear power reactor, these are also sources of both electromagnetic and particle radiation. The exposure time is "chronic." Typically the crew is protected by use of shadow shields.

In some propulsion systems, such as open-cycle gas core nuclear thermal rockets and Orion nuclear pulse rockets, the exhaust is radioactive. This means the shadow shield has to cover a broader arc.

The type of radiation emitted depends upon whether the reactor is fission or fusion, and which fuel is used.

There is a first order approximation here to calculate the radiation flux from a fission reactor or fission nuclear thermal propulsion system.

Fission fuel is radioactive. If the reactor core is breached the fuel can spread radioactive contamination. Also the neutron radiation emitted by the reactor in normal operation can cause neutron activation.

Radiation from Weapons

If the spacecraft is in a combat situation, it will be targeted by nuclear warheads and particle beam weapons. In these cases the exposure time is "acute."

Typically the crew is protected by use of storm cellars and hull armor.

Nuclear warheads emit both electromagnetic and particle radiation

Obviously particle beam weapons only emit particles. Having said that, if the particle beam hits a metal hull bremsstrahlung will create a flood of x-rays. This only happens if the particle in the beam are charged, it doesn't work with neutron beams.

Effects of Radiation


The effect of radiation upon the crew is rather complicated (translation: I don't understand it) so take the following explanation under advisement.

Radiation is meaured with lots of different confusing units. To make it worse, each measurement has both a traditional obsolete deprecated unit and a new modern scientific metric unit. There are units for the amount of radiation emitted, units for the amount of radiation absorbed by an inert object, and units for the amount of radiation absorbed by a living being. And on top of that, metric units often have prefixes for various powers of 10: milli-, micro-, etc. There is a table of prefixes here.

Naturally, the United States Nuclear Regulatory Commission requires the use of the non-metric obsolete deprecated units curie, rad and rem as part of the Code of Federal Regulations 10CFR20. The United States hangs on like grim death to its stupid ramshackle non-decimal system of units, instead of adopting the metric system like the rest of the scientific and civilized world.

ACTIVITY: The amount of radiation emitted by a chunk of radioactive material is measured by the traditional obsolete unit the Curie or the new metric unit the Becquerel (Bq). One Curie is equal to the amount of radiation emitted by one gram of radium. One Becquerel is equal to one decay per second. 1 Curie equals 3.7 × 1010 Becquerels.

ABSORBED DOSE: The amount of radiation absorbed by an inert object is measured by the traditional obsolete unit the Rad or the new metric unit the Gray (Gy). One Rad is a dose of radiation causing 100 ergs of energy to be absorbed by one gram of matter. One Gray is 1 joule of radiation absorbed by one kilogram of matter. 1 Rad equals 0.01 Gray. An even more obsolete term is the Roentgen, currently it is defined as 1 Roentgen equals 0.0096 Gray.

MGy is the symbol for megagray, or 106 gray. mGy is the symbol for milligray, or 10−3 gray. cGy is the symbol for centigray, or 10-2 gray.

DOSE EQUIVALENT: The amount of radiation absorbed by living being is measured by the traditional obsolete unit the Rem or the new metric unit the Sievert (Sv). 1 Rem equals 0.01 Sievert.

Sieverts are determined from Grays. The effects of Acute radiation exposure are figured by the exposure in Grays. The effects of Chronic radiation exposure are figured by the exposure in Sieverts. Radiation quality factors seem to mostly matter for chronic doses.

You see, as far as an inert lump of matter is concerned, all forms of radiation are pretty much the same. But when you get to living beings, different kinds of radiation do different levels of long-term chronic internal organ damage per joule. Some types of radiation are better at killing people than other. For example, if 1 Gray of gamma radiation does 1 unit of damage to a stricken crewperson, 1 Gray of alpha particles will do 20 units of damage to the crewperson.

What this boils down to is that each type of radiation has a quality factor Q. You look up the Q factor for the radiation in question, take the radiation dose in Rads, multiply by Q, and you will have the "dose equivalent" in Rems. Or take the dose in Grays, multiply by Q and you will have the dose equivalent in Sieverts. There is a list of Q factors here.

You will sometimes see radiation exposure expressed in units of mGy/a or mGy/yr. This stands for milliGrays per annum, where 1 milliGray = 0.001 Gray and 1 annum is 8760 hours = 365 days = 1 year. This appears when talking about the radiation exposure suffered by inanimate objects on an extraterrestrial planet. For instance a rover space probe on Mars will suffer 73 mGy/a from cosmic rays and proton storms, while a space probe on Terra is typically only exposed to 0.4 mGy/a from cosmic rays.

The corresponding measure for organic creature exposure is mSv/a or mSv/yr. This is of course milliSieverts per annum.

Calculating Dosage

The size of the dose depends on two things: the intensity of the radiation, and the duration of exposure. Crewpersons who do not want to die a hideous radioactive death will do well to reduce the size of the dose. You reduce the intensity by getting as far away from the source of radiation that you can (allowing the inverse-square law to reduce the intensity) and trying to get some radiation shielding between you and the source of the radiation. You reduce the duration by performing the first two techniques as quickly as possible (i.e., don't just stand there with a stupid expression on your face, run for your life!). There is also a difference between "acute" and "chronic" exposure. An example of an acute exposure is being in the general neighborhood of a nuclear weapon when it goes boom: the exposure duration is measured in fractions of a second. An example of chronic exposure is the day-to-day job of a nuclear rocket engineer: exposure duration is measured in months. Obviously the only things you can do to reduce an acute dose is to always be inside plenty of shielding and only fight enemies who use tiny nuclear weapons.

To figure the dose in Grays, take the radiation from the "event." Calculate how many radiation joules managed to penetrate the shielding and intersect the cross section of a person and divide by the body mass of the unlucky crewperson.

Since I know you are impatient, I'm first going to give you the quick-and-dirty equations. I'll then give you how I derived them, to allow you to skip over it if you are not interested. The equations have build-in assumptions. Assume this is radiation from an exploding nuclear warhead. 80% of the energy is in the form of x-rays with an average energy of 10 keV. Each fissioning nuclei will produce 2 or 3 neutrons and about half will escape to become radiation. The neutrons will have an average Sievert quality factor of 10. The crewmember will have an average cross-section of 0.445 m2 and a mass of 68 kilograms. Finally assume no radiation shielding.

Gx = 1.78e9 * (Y / R2)

Gn = 7.2e8 * (Y / R2)


  • Gx = person's acute radiation dosage from x-rays (Grays)
  • Gn = person's acute radiation dosage from neutrons (Grays)
  • Y = weapon yield (kiloton TNT)
  • R = person's distance from weapon's detonation center (meters)

Ignoring all the warning broadcasts, Floyd the dufus meteor-miner is poaching on an asteroid in the Trojan asteroid cluster claimed by the Jupiter-Equilateral corporation. 30 kilometers away (30,000 meters), the unaware local JE mining supervisor detonates a one kiloton nuclear mining charge on the surface of 4805 Asteropaios. Unfortunately for Floyd, there is a clear line of sight between him and the detonation.

  • Gx = 1.78e9 * (Y / R2)
  • Gx = 1.78e9 * (1 / 30,0002)
  • Gx = 1.78e9 * (1 / 900,000,000)
  • Gx = 1.78e9 * 0.00000000111
  • Gx = 1.98 Grays of x-rays
  • Gn = 7.2e8 * (Y / R2)
  • Gn = 7.2e8 * (1 / 30,0002)
  • Gn = 7.2e8 * (1 / 900,000,000)
  • Gn = 7.2e8 * 0.00000000111
  • Gn = 0.8 Grays of neutrons

Floyd will live, assuming that he can quickly get into a pressurized environment before his space helmet fills up with vomit.

How was this derived? In a round about fashion. First you figure out the radiation flux in joules of radiation per square meter. You take the amount of joules in the detonation and divide them by the surface area of a sphere with radius R.

ssphere = 4 * π * R2

There are about 4.19e12 joules per kiloton of nuclear detonation, and 80% of that is x-rays. Putting it all together:

  • Fx = joules / surfaceArea
  • Fx = (Y * 4.19e12 * 0.8) / (4 * π * R2)
  • Fx = 2.67e11 * (Y / R2)

As a general rule, figure an average person has a cross section of about 0.445 m2 and a mass of 68 kilos. How was the cross section calculated? Given a mass of 68 kilos (150 pounds), and a height of 168 centimeters (5 feet, 6 inches) the Body Surface Area Calculator says the surface area is 1.78 square meters. The average cross section will be approximately one quarter of the surface area, or 0.445 m2.

To obtain Grays, we take the radiation flux, multiply it by the cross section of the person (0.445), and divide by their mass (68).

  • Gx = ((2.67e11 * 0.445) / 68) * (Y / R2)
  • Gx = 1.78e9 * (Y / R2)

Figuring neutron radiation is a bit more involved. Each kiloton requires the fissioning of approximately 1.45e23 nuclei. Each fission produces 2 or 3 neutrons, with an average production of 2.5. About half (0.5) will escape the nuclear reaction to become radiation. The neutron flux is therefore:

  • Fn = (Y * 1.45e23 * 2.5 * 0.5) / (4 * π * R2)
  • Fn = 1.8e23 * (Y / R2)

Neutrons have a Sievert quality factor ranging from 2 to 20 for neutrons of energies 0.01 MeV to 2.0 MeV, with an average quality factor of 10. So according to the references I've found, in the absence of specific data, you can assume that a neutron flux of 2.5e11 neutrons per square meter is about 0.01 Sievert or 0.001 Grays. This means 1 Gray equals a neutron flux of 2.5e14 neutrons per square meter. So simply divide Fn by 2.5e14 to get Grays:

  • Fn = (1.8e23 * (Y / R2)) / 2.5e14
  • Fn = 7.2e8 * (Y / R2)


Quality Factors
Type of radiationQuality
Gamma rays and bremsstrahlung1
Beta particles, electrons, 1.0 MeV1
Beta particles, 1.0 MeV1
Neutrons, thermal energy2.8
Neutrons, 0.0001 MeV2.2
Neutrons, 0.005 MeV2.4
Neutrons, 0.02 MeV5
Neutrons, 0.5 MeV10.2
Neutrons, 1.0 MeV10.5
Neutrons, 10.0 MeV6.4
Protons, greater than 100 MeV1-2
Protons, 1.0 MeV8.5
Protons, 0.1 MeV10
Alpha particles (helium nuclei), 5 MeV15
Alpha particles, 1 MeV20
Galactic Cosmic Rays1 to 30

There are two kinds of radiation exposure: acute and chronic.

The important point is that acute radiation damage will heal. Chronic damage does not. Acute damage goes away with time, chronic damage gradually accumulates over a lifetime. On the other hand, acute exposure can cause death by radiation burns a few weeks after the exposure. Chronic never causes death from radiation burns, instead it might kill you with cancer decades after the exposure.

Acute is a sudden dose that occurs over a few seconds to minutes. Chronic is a dose that occurs over a few days to years. Acute radiation syndrome is damage due to raw energy which burns internal organs (the technical term for such direct tissue damage is "nonstochastic effects" or "deterministic effects"). Chronic radiation syndrome is damage to the cellular DNA, leading to cancer and genetic defects in the victims future offspring (the technical is, surprise-surprise, "stochastic effects"). Chronic doses also cause skin ulceration and blindness due to cataracts scarring.

For our purposes, acute doses happen due to reactor accidents, solar storms, nearby nuclear explosions, and hits by particle beam weapons. Chronic doses are due to the unavoidable radiation filtering through the reactor shielding, the normal background radiation in space, and prolonged stays in regions like the Van Allen radiation belts.

There is no nonstochastic effects from chronic radiation dosage because the body has time to repair the damage while the dose is absorbed. There is no time to repair with an acute dose. For example, if a person suffers a (chronic) dose of 4.5 Sieverts over one year, they will have a much higher chance of cancer for the rest of their life. If they suffer an acute dose of 4.5 Grays in a second, they have an LD50 dose and have a 50-50 chance of being dead within 6 weeks due to radiation burns.

Deterministic effects mean that X amount of acute radiation exposure will cause Y amount of tissue damage. Stochastic effects mean that X amount of chronic radiation exposure will raise your chance of getting cancer by Y percent.

Remember acute exposure is measured in Grays, chronic exposure is measured in Sieverts.

For acute doses, simply figure the exposure in Grays suffered by the crewperson, and refer to the Acute radiation syndrome chart below. When a person receives an acute dose, they suffer what is listed under "Immediate symptoms." Then they appear to get better, but this is only temporary. After the Latent phase time passes, the person will start to suffer what is listed under "Post-latent symptoms".

As a side note, when NASA is sterilizing containers of meat for astronaut supplies, the recommended dosage is 44,000 Grays to kill off all the bacteria. You can find dosages for other food sterilization here.

To figure chronic doses, one has to calculate the Dose Equivalent. Split the radiation into the x-ray/gamma-ray Grays and the neutron Grays. Multiply the Grays by the radiation type's Quality Factor to get the Equivalent Dose (in units called "Sieverts"). Gamma-rays have a quality factor of 1.0, neutrons have a quality factor ranging from 2 to 20 for neutrons of energies 0.01 MeV to 2.0 MeV (just use the average of 10). Add the two Sievert doses together and look it up in the Chronic radiation syndrome chart.

Doctors go further, calculating the Effective dose, which is a weighted average of the equivalent dose to each organ depending upon its radiosensitivity. But that's probably a bit too much detail for our purposes. Rocketeers will always keep close watch on their radiation dosimeters that measures their current chronic dose. There will also be a crewperson or officer who is assigned the job of logging and monitoring the chronic dose of every person on board. If a crewperson gets close to their maximum allowable dose, they may be restricted to the more shielded sections of the ship, or even grounded from shipboard duty until they recover.

Atomic rocketeers will also without fail have a package of potassium iodide tablets on their persons at all times. Why? If the reactor core is breached, the mildly radioactive fuel and the intensely radioactive fission fragments will be released into the atmosphere. While none of the fission fragment elements are particularly healthy, Iodine-131 is particularly nasty. This is because one's thyroid gland does its level best to soak up iodine, radioactive or not. Thyroid cancer or a hoarse voice from thyroid surgery might be common among atomic rocket old-timers. The tablets prevent this by filling up the thyroid first, before the Iodine-131 arrives. The instant the reactor breach alarm sounds, whip out your potassium iodide tablets and swallow one.

Radioactive contamination of an area is measured by a "swipe" or "smear" survey. A small piece of absorbent paper is rubbed over a 100 square centimeter area in a S-shaped pattern. A radiation detector then is used on the paper to measure the disintegrations per minute (dpm). Depending on the standard used, an area is considered "contaminated" if the dpm is above 100 - 500.

Note, in your research you may run across the terms "rem" and "rad." These are sort of obsolete terms. One Sievert equals 100 rems. One Gray equals 100 rads. LD50 is the radiation dose that is expected to kill 50% of an exposed population.

Acute Radiation Syndrome Chart

Dose (Grays)Immediate symptomsLatent phasePost-latent symptomsPrognosis
0 - 0.5No obvious effectNoneNo obvious effect, except, possibly, minor blood changes and anorexia.Certain survival
0.5 - 1.0Vomiting and nausea for about 1 day in 10 to 20% of exposed personnel. Fatigue, but no serious disability.days to weeksIn this dose range no obvious sickness occurs. Detectable changes in blood cells begin to occur at 0.25 Gy, but occur consistently only above 0.50 Gy. These changes involve fluctuations in the overall white blood cell count (with drops in lymphocytes), drops in platelet counts, and less severe drops in red blood cell counts. These changes set in over a period of days and may require months to disappear. They are detectable only by lab tests. At 0.50 Gy atrophy of lymph glands becomes noticeable. Impairment to the immune system could increase the susceptibility to disease. Depression of sperm production becomes noticeable at 0.20 Gy, an exposure of 0.80 Gy has a 50% chance of causing temporary sterility in males. At 0.75 Gy there is a 10% chance of nausea.Almost certain survival
1.0 - 2.0Mild acute symptoms occur in this range. Symptoms begin to appear at 1 Gy, and become common at 2 Gy. Typical effects are mild to moderate nausea (50% probability at 2 Gy) , with occasional vomiting, setting in within 3-6 hours after exposure, and lasting several hours to a day. This will be followed by other symptoms of radiation sickness in up to 50% of personnel.10 - 14 daysTissues primarily affected are the hematopoietic (blood forming) tissues, sperm forming tissues are also vulnerable. Blood changes set in and increase steadily during the latency period as blood cells die naturally and are not replaced. A reduction of approximately 50% in lymphocytes and neutrophils will occur. There is a 10% chance of temporary hair loss. Mild clinical symptoms return in 10-14 days. These symptoms include loss of appetite (50% probability at 1.5 Gy), malaise, and fatigue (50% probability at 2 Gy), and last up to 4 weeks. Recovery from other injuries is impaired and there is enhanced risk of infection. Temporary male sterility is universal. The higher the dosage in this range, the more likely the effects, the faster symptoms appear, the shorter the latency period, and the longer the duration of illness.Fatality rate is about 10%
2.0 - 3.5Nausea becomes universal, the incidence of vomiting reaches 50% at 2.8 Gy and 100% at 3 Gy. Nausea and possible vomiting starting 1 to 6 hours after irradiation and lasting up to 2 days. This will be followed by other symptoms of radiation sickness, e.g., loss of appetite, diarrhea, minor hemorrhage7 - 14 daysIllness becomes increasingly severe, and significant mortality sets in. Hematopoietic tissues are still the major affected organ system. When symptoms recur, the may include epilation (hair loss, 50% probability at 3 Gy), malaise, fatigue, diarrhea (50% prob. at 3.5 Gy), and hemorrhage (uncontrolled bleeding) of the mouth, subcutaneous tissue and kidney (50% prob. at 4 Gy). Suppression of white blood cells is severe, susceptibility to infection becomes serious. At 3 Gy the mortality rate without medical treatment becomes substantial (about 10%). The possibility of permanent sterility in females begins to appear. Recovery takes 1 to 3 months.Fatality rate 35% to 40%
3.5 - 5.5Nausea and vomiting within half an hour, lasting up to 2 days. This will be followed by other symptoms of radiation sickness, e.g., fever, hemorrhage, diarrhea, emaciation.7 - 14 daysHair loss, internal bleeding, severe bone marrow damage with high risk of bleeding and infection. Hemopoietic Syndrome. Mortality rises steeply in this dose range, from around 50% at 4.5 Gy (LD50) to 90% at 6 Gy (unless heroic medical intervention takes place). Hematopoietic tissues remain the major affected organ system. The symptoms listed for 2.0-3.5 Gy increase in prevalence and severity, reaching 100% occurrence at 6 Gy. When death occurs, it is usually 2-12 weeks after exposure and results from infection and hemorrhage. Recovery takes several months to a year, blood cell counts may take even longer to return to normal. Female sterility becomes probable. Survivors convalescent for about 6 months.Fatality rate 50% within 6 weeks
5.5 - 7.5Severe nausea and vomiting within 15 - 30 minutes, lasting up to 2 days, followed by severe symptoms of radiation sickness, as above.5 - 10 daysHair loss, internal bleeding, severe bone marrow damage leading to complete failure of blood system, high risk of infection, moderate gastrointestinal damage. Gastrointestinal Syndrome. Survival depends on stringent medical intervention. Bone marrow is nearly or completely destroyed, requiring marrow transfusions. Gastrointestinal tissues are increasingly affected. The final phase lasts 1 to 4 weeks, ending in death from infection and internal bleeding. Recovery, if it occurs, takes years and may never be complete. Survivors convalescent for about 6 months.Death probable within 3 weeks
7.5 - 10Excruciating nausea and vomiting within 5 - 15 minutes, lasting for several days5 - 7 daysHair loss, internal bleeding, severe bone marrow damage leading to complete failure of blood system, high risk of infection, severe gastrointestinal damage.Death almost certain within 3 weeks. Complete recovery impossible.
10 - 20Immediate nausea occurs due to direct activation of the chemoreceptive nausea center in the brain. The onset time 5 minutes.5 - 7 daysVery high exposures can sufficient metabolic disruption to cause immediate symptoms. Above 10 Gy rapid cell death in the gastrointestinal system causes severe diarrhea, intestinal bleeding, and loss of fluids, and disturbance of electrolyte balance. These effects can cause death within hours of onset from circulatory collapse. Following an initial bout of severe nausea and weakness, a period of apparent well-being lasting a few hours to a few days may follow (called the "walking ghost" phase). This is followed by the terminal phase which lasts 5 - 12 days. In rapid succession prostration, diarrhea, anorexia, and fever follow. Death is certain, often preceded by delirium and coma. Therapy is only to relieve suffering.Certain death in one week or less.
20 - 80Immediate disorientation and coma will result, onset is within seconds to minutes.NoneCNS Syndrome. Metabolic disruption is severe enough to interfere with the nervous system. Convulsions occur which may be controlled with sedation. Victim may linger for up to 48 hours before dying.Certain death
> 80ComaNoneThe U.S. military assumes that 80 Gy of fast neutron radiation (from a neutron bomb) will immediately and permanently incapacitate a soldier. Lethal within 24 hours due to damage to central nervous system.Certain death

Chronic Radiation Syndrome Chart

CriteriaGeneral PublicOccupational WorkersAstronauts
30-day limit0.0004 Sieverts (0.4 milli-Sieverts)0.004 Sieverts1.5 Sieverts
annual limitAdult: 0.05 Sieverts
minor: 0.005 Sieverts
Colonist: 0.02 Sieverts
Pregnant colonist: 0.0066 Gray
0.05 Sieverts3 Sieverts
Male career limitN/A2 + 0.075 x (age - 30) Sieverts4 Sieverts
Female career limitN/A2 + 0.075 x (age - 38) Sieverts4 Sieverts
Pregnancy limit
(9 months)
0.005 SievertsN/AN/A
accident limit0.25 Sieverts1 SievertN/A
acute limitN/AN/A0.1 Sieverts

One study suggested for a pregnant woman it is 0.005 Sievert total for the duration of the pregnancy. Another later study suggested an annual limit of 0.0066 Grays, using Grays instead of Sieverts because nobody knew the Q factor for embryonic tissue and obviously nobody was about to try experimentation to find the answer.

Colorful Chronic Terms

Writer Allen Steele uses the following terms:

  • Singed: receiving a radiation dose that put one close to a chronic limit
  • Cooked: receiving a radiation dose that put one over a chronic limit, could be career-ending
  • Fried: receiving a radiation dose that put over the lethal LD50 limit, could be life-ending

However, when the doctor checked her suit dosimeter, he discovered that the amount of radiation to which the young woman had been exposed while marooned on Amalthea had reached seventy-two rems (0.72 sv). The maximum number of rems allowed per annum under union health regulations was seventy-five (0.75 sv). Although she was still safe from contracting leukemia, she couldn’t expect to continue working in the Jovian system; a single EVA, even in the outer fringes of the system, would certainly push her over the limit.

This discovery prompted Yoshio to check both Old Bill’s and his own dosimeters. More bad news. Since Yoshio himself had never left the Marius, he had received barely five rems (0.05 sv) during the mission, well within the safety limits—but Old Bill, even though he had been protected by his exoskeleton, had received almost twenty rems (0.20 sv). Under the same union codes, the maximum radiation exposure allowed within a thirty-day period is twenty-five rems (0.25 sv), with a career limit of four hundred rems (4.0 sv).

In spacer parlance, William Smith-Tate had been singed. Had he been cooked, his career would have been automatically over and he would have spent the rest of his life grounded on Earth; if he had been fried, he would have already been suffering a slow, nasty death in the infirmary. Considering the circumstances, he was lucky to have been only singed—yet for the next month he could not leave the vessel under any circumstances save for the most dire emergency.

At risk was not only his own health, but also his EVA certification. The rules were necessarily tough, because otherwise the major insurance companies—chief among them Lloyd’s of London, ConSpace’s principal guarantor—would refuse to underwrite industrial space efforts. Indeed, after a spacer receives more than four hundred rems (4.0 sv), Pax Astra automatically rescinds that person’s EVA certification, and a spacer who can’t step outside of an airlock might as well ship back to Earth. His career is over.

From THE WEIGHT by Allen Steele (1995)

I got my data on risks and passive shielding from NASA’s own site and documents. See, titled Spaceflight Radiation Health Program at JSC (no cited reference newer than 1992).

Two Risks:

There is galactic cosmic radiation (GCR) and there are solar flare events (SFE). GCR is a very sparse trickle of extremely high-energy particles (mostly protons) that are very hard to passively shield, and which can induce secondary showers of other dangerous particles in some materials. The risk is modulated by the solar wind which varies with the solar activity level.

SFE radiation is mostly proton and heavier particles emitted from the sun sporadically, from eruptions on its surface. These come in very concentrated but brief events, comprising much lower-energy particles that are far easier to passively shield.

GCR: Roentgen equivalent man (REM) per year = 42 + 18 sin(360° (t, year) / (11 year)), max 60 at solar min, min 24 at solar max. Solar max is max sunspot activity, with more frequent eruptions. Although, eruptions can occur throughout the cycle. (1 REM = 0.01 Sievert or Sv)

SFE: from 1968 to 1970, events every month or so ranging from 2 REM to 50 REM (0.02 Sv to 0.5 Sv) accumulated during each event; from 1970 to 1972, events about every 6 months ranging from about 50 REM to about 100 REM (0.5 Sv to 1 Sv) accumulated during each event; and in 1972 right between Apollo 16 and Apollo 17, one event right at 5000 accumulated REM (50 Sv). There was one event of about 5 REM (0.05 Sv) during Apollo 16. Figure 1 is a plot of SFE events during the Apollo program, direct from the NASA document’s Figure 10. The quoted numbers are for somebody outside a spacecraft wearing only a spacesuit, per NASA’s figure.

By way of comparison, the Earthly natural background in the US is near 300 milli-REM (0.30 REM)(0.003 Sv) per year. Worldwide is not significantly different.

A rough general rule: 500 REM (5 Sv) accumulated in a short time (hours or days) is considered lethal to 100% of those so exposed.

NASA’s Astronaut Exposure Rules:

These vary with the affected organ (eyes, skin, and 5 cm inside the body as representative of blood-forming organs or “BFO”), but the lowest values are for 5 cm inside. NASA’s exposure rules limit exposure to 50 REM (0.5 Sv) accumulated in any one year, 25 REM (0.25 Sv) in any single month, and a career limit that varies with age and gender, but peaks at 400 REM (4 Sv) accumulated over an entire career.

These are illustrated with Tables 1 and 2 lifted from the NASA document, and presented here as Figures 2 and 3, respectively. Use the equations in Figure 3 to calculate career limits. These exposure limitations represent approximately twice the exposures nuclear workers are allowed to face, with a single-digit percentage increase in cancer risk expected.

Effectiveness of Shielding Materials:

These divide into the effects of aluminum, water, and hydrogen, which fairly well bounds a lot of possible materials. The true risk here is the SFE event, of a very large magnitude, such as the 5000 REM (50 Sv) 1972 event. That’s outside in nothing but a space suit. Inside the command module, the effect of the spacecraft structure reduces the exposure to 500 REM (5 Sv). That’s a 10:1 reduction for the spacecraft hull (remember, SFE particles are lower-energy and far easier to shield passively).

Based on the NASA document’s Figure 9, given here as Figure 4, the plot for the 1972 event reduces 500 REM (5 Sv) inside the command module to 20 REM (0.2 Sv) at 20 g/cm2 of aluminum shielding added to the effects of the spacecraft hull. Use the density of aluminum (2.7 g/cm3) to find the actual physical thickness of this aluminum to be 7.4 cm. You would want an aluminum shield that thick or thicker to survive a 1972-magnitude SFE event, and stay barely within the month exposure limit.

The effect of the various materials as passive shielding for GCR is given in Figure 5, which is lifted from the NASA document’s Figure 6. In terms of shield mass per unit area of hull, hydrogen is the most effective, and aluminum the least, with water in between. Note how the curves flatten at larger masses per unit area, leading to stronger differences in the amount of shield material required.

Here we ignore the shielding effect of the spacecraft hull structure as negligible against the more energetic radiation. For 20 g/cm2 (7.4 cm thick) aluminum, 60 REM/year (0.6 Sv/year) reduces to 40 REM/year (0.4 Sv/year), which is well within the annual limit. You get the same exposure at only 10 g/cm2 water at 1 g/cm3, which is 10 cm thick, and 3 g/cm2 hydrogen, which at 0.07 g/cm3 is some 43 cm thick.

The problem with “over-killing” the shielding is the secondary shower of dangerous particles created by the high-energy GCR particles. This gets to be a problem if you make the shield too thick, and it simply doesn’t happen with the lower-energy SFE particles. That makes passive shielding a real trade-off for design purposes. This is less a problem with spacecraft design, and more of a problem with surface habitation design and construction. The temptation would be to pile too much regolith atop the roof, causing the secondary scatter problem.

My own recommendation would be to use 15-20 g/cm2 water, some 15-20 cm thick, which would reduce 60 REM/year (0.6 Sv/year) GCR to about 30 REM/year (0.3 Sv/year). Against the GCR, the same protection obtains at some 50 g/cm2 aluminum, which is about 18.5 cm thick. These are similar thicknesses (15-20 cm water vs 18-19 cm aluminum), but very different masses per unit area: 15-20 g/cm2 water vs 50 g/cm2 aluminum. Water is simply the lighter shield for the same effect, by about a factor of 2.5 to 3.

We don’t have anything but aluminum to look at in Figure 4 the for the SFE event. At the same 50 g/cm2 aluminum that looks good against GCR, the 500 REM (5 Sv) SFE event, as measured inside the command module, gets reduced to a 2 REM/event (0.02 Sv/event) exposure. Assuming the same attenuation ratio and mass per unit area ratio between aluminum and water that we saw with GCR in Figure 5, then we should see the same 500 REM to 2 REM (5 Sv to 0.02 Sv) reduction of SFE radiation that 50 g/cm2 of aluminum provides, with only 15-20 g/cm2 water, which is 15-20 cm thick. That’s an assumption requiring verification.

Lessons for Spacecraft Design:

Water is the best shielding material, because it is the lightest, while providing practical thicknesses, unlike hydrogen. My best guess is that storable propellants should resemble water in their shielding properties. They are light molecules made of light atoms, like water, and have densities far more comparable to water than to liquid hydrogen.

The recommended water shield is 15-20 g/cm2 (15-20 cm thickness), which could be water, wastewater, or even frozen food. It could also be storable propellants like the hydrazines and NTO oxidizer. Increase the thicknesses for wastewater, ice, or frozen food: a good guess is a factor of 1.5 to 2 increase in thickness over straight water.

Against GCR and ignoring any effects of the spacecraft hull, 15-20 cm of water should reduce 60 REM/year (0.6 Sv/year) of GCR to something near 30 REM/year (0.3 Sv/year). This might, or might not, be practical for the entire spacecraft, but putting water in one form or another around the sleeping quarters might be, in addition to a designated radiation shelter space.

There is some benefit of the spacecraft hull reducing SFE from a 5000 REM/event (50 Sv/event) outside the hull to a 500 REM/event (5 Sv/event) inside the hull. Since that is still a lethal dose, further shielding is absolutely required! That same 15-20 cm of water should reduce an inside-the-hull 500 REM/event (5 Sv/event) to something nearer 2 REM/event (0.02 Sv/event), which is well within the monthly limit, even if multiple such events occur spaced rapidly together. This gives us a lot of margin in the case of an event far larger than the 1972 SFE event.

Any spacecraft design should incorporate its flight control station within the designated-shelter radiation shielding so that critical maneuvers may be flown regardless of the solar weather. Shielding about the sleeping quarters is also recommended for purposes of reducing GCR exposure.

By using the shadow-shield effect, this kind of shielding might be obtained with a combination of water/wastewater/frozen food items located about the sleeping quarters and flight control station, combined with propellant tanks for the next burn, that are docked about the periphery of these regions, outside the spacecraft hull. These are all items you already must have, anyway, so that extra shielding mass is not added to the design. See Figure 6 for a concept sketch.

Exposures Calculated for a Mars Mission at 60 REM/Year (0.6 Sv/year) GCR with Three 1972-Class SFE Events:

The mission is 9-month transit/13-month near (or on) Mars/9-month transit. One SFE event occurs during each transit, and the other occurs while the crew is on or near Mars. Calculations are made with and without the sleeping quarters shielded, for 1/3 of clock time during each day. Shielding about the sleeping quarters and the designated shelter is spacecraft hull plus 15-20 cm water-equivalent for SFE, just 15-20 cm water-equivalent for GCR. How this might actually be done was shown conceptually in Figure 6 above.

The first radiation exposure year is 3 months pre-mission on Earth, then 9 months in transit to Mars. The second radiation-exposure year is 12 months on Mars. The third radiation-exposure year is one month on Mars, 9 months in transit to Earth, and 2 months post-mission on Earth. Earthly exposure is at the 0.3 REM/year (0.003 Sv/year) rate.

The 9 month transit is 0.75 year. Without any shielding effects at all, 45 REM (0.45 Sv) are accumulated during transit for the year in which transit occurs.

If there is sleeping quarters shielding, its presence cuts the GCR to a rate of 30 REM/year (0.3 Sv/year) while sleeping. Then based on clock times, a 2/3-1/3 split occurs between the 60 and 30 REM (0.6 and 0.3 Sv) rates: that is a rate of 50 REM/year (0.5 Sv/year) applied to a 9 month transit. Thus the crew accumulates 37.5 REM (0.375 Sv) during the transit, which goes toward the total accumulated exposure during the year in which the transit takes place.

While on or near Mars, the planet blocks half the spherical “sky”, for a net in-space unshielded GCR rate, assumed unattenuated by the planet’s atmosphere, of 30 REM/year (0.3 Sv/year), accumulated during each year spent on Mars. The stay is 13 months, so without sleeping quarters shielding, 30 REM (0.3 Sv) counts toward the first full year on Mars, and one month’s worth (2.5 REM (0.025 Sv)) counts toward the second year on Mars and in-transit home.

If there is shielding about the sleeping quarters on Mars, the same 2/3-1/3 split applies to rates of 30 and 15 REM/year (0.3 and 0.15 Sv/year), reducing the effective exposure rate to 25 REM/year (0.25 Sv/year). In that case, the year on Mars accumulates 25 REM (0.25 Sv), and the 13th month accumulates 2.1 REM (0.021 Sv).

Add 2 REM (0.02 Sv) to the accumulation in any one month for SFE events during the transits and during the stay on Mars. There must be a designated radiation shelter for SFE events, even while on Mars, or the exposures could easily be lethal. That assumes no attenuation of the radiation by Mars’s thin atmosphere.

The result is depicted graphically in Figure 7. Again, shielding is 15-20 cm of water-equivalent.

These results show a marginal yearly exposure during year 3, at just barely under the 50 REM (0.5 Sv) annual limit, for the case of no sleeping quarters shielding. With sleeping quarters shielding, this reduces well under the limit. Worst case monthly exposures are well under the limit for both cases. Two such missions will approach career limits, in either case.

Note that if there is no shielded place for SFE events, then during any event in the same class as the 1972 event, the exposure will be fatal at 500 REM (5 Sv) received over a matter of hours. There simply must be a solar flare shelter somewhere. This is true during the transits and on Mars.

Note also that during times when the GCR is under 60 REM/year (0.6 Sv/year) out in space, exposures inside the ship (either case) will be much lower. It is only the worst-case 60 REM/year (0.6 Sv/year) space environment that is analyzed here.

Note also that it is the shorter-than-a-year transit time that reduces in-transit unshielded exposure to 45 accumulated REM! (0.45 accumulated Sv) Extending the transit time by using repeated aerobraking passes to capture at Mars, instead of a one-time rocket burn, will quickly violate the annual exposure limit! The same thing applies to electric propulsion using spiral-out/spiral-in flight plans. (Not to mention exposure times passing through the Van Allen Belts at Earth.)

Once the unshielded one-way flight time exceeds 10 months, the annual exposure limit gets exceeded in a max GCR year. Once that long-transit situation obtains, you must shield the entire habitable volume of the ship, not just a designated shelter and perhaps the sleeping quarters.

Finally, if the entire habitable volume of the spacecraft could be shielded at the 15-20 cm water-equivalent level, exposures would be cut essentially in half, to only around 30-something REM (0.3-something Sv) per radiation-exposure year in the transits, and 15 REM/year (0.15 Sv/year) on Mars, even in a 60 REM/year (0.6 Sv/year) part of the solar cycle. Year 1 would be 32.08 REM (0.3208 Sv), year 2 would be 17 REM (0.17 Sv), and year 3 would be 33.3 REM (0.333 Sv), for a mission total of 83.4 REM (0.834 Sv). Three, or possibly even 4, such missions in 60 REM (0.6 Sv) GCR years might be feasible, before hitting career exposure limits.

Final Comments:

First, this kind of radiation shielding will inevitably prove to be absolutely necessary, but it will look nothing at all like what we have ever before done with our spacecraft designs. When “they” show you spacecraft design concepts that look like what we have done before, you already know that ”they” have not thought this problem through!

Second, the shield design concept shown in Figure 6 above is entirely compatible with a “long” ship design that is spun end-over-end like a rigid baton for artificial gravity. A design like that is also entirely unlike anything we have ever before done, but it is rather well-understood from an engineering viewpoint, and would require far less technology development and demonstration than any sort of cable-connected spin gravity design.

Third, it is quite evident that worst-case GCR risks are slight over-exposure for late-in-life cancer, while the SFE risks really are lethal doses leading to an ugly death within hours. Thus, when “they” point to GCR as the radiation risk that precludes humans going into deep space, you already know that (1) they are lying for nothing but fear-mongering purposes, and (2) “they” are truly ignorant of the real radiation risk. Such claims are simply not credible.

From SPACE RADIATION RISKS: GCR VS SFE by Gary Johnson (2018)

Banana Equivalent Dose

An amusing unit of radiation exposure is the Banana equivalent dose. It provides some perspective, and can be used to calm down scientifically illiterate people who go hysterical when they hear the "R" word.

As it turns out, ordinary bananas are very slightly radioactive due to their potassium-40 content. Under this scale eating one banana exposes you to 0.1 μSv or 0.0000001 Sievert.

Living within 50 miles of a nuclear power plant for one year will give you a dose of half a banana. Living within 50 miles of a coal power plant for one year will give you a dose of three bananas (a pound of coal contains only small traces of radioactive elements, but such plants typically burn 4 million tons of coal every year).

Living in a stone, brick, or concrete building for a year will expose you to a dose of 700 bananas. The average dose from the Three Mile Island accident to someone living within 10 miles is 800 bananas. One mammogram is 30,000 bananas. A chest CT scan is 58,000 bananas.

If you spend one hour at the site of Chernobyl nuclear disaster in the year 2010 you will receive a dose of 60,000 bananas.


Lets say I'm telling a story about some hiker heading up into the mountains. I mention that when passing under a cliff a pebble came loose and landed on him. That might sting but it wouldn't occur to you to ask if he died. Lets say that instead I mention that a boulder had landed on him. Then you'd expect him to be quite dead.

That's the nice thing about these old English words that've been around a while and encode distinctions that make sense in our everyday lives. The difference between the pebble and the boulder is just a matter of degree but it's one where the quantitative difference is big enough to become qualitative. If we just had one word whoever I was telling the story to would have to ask questions and because we run into rocks so often I'd know when I'd have to use an adjective.

When you're talking about scientific things, though, you don't often have this choice of words. Energy is energy and you're expected to use a number if you want to say whether it's a lot or a little. Same with radiation. The radiation emitted by your cell phone is so subtle you'd never be able to perceive it but a large enough amount of that same sort could cause incredible pain or even cook you alive.

So "radiation" as a word can be dangerously ambiguous even before considering how it's spectrum effects things. "Ionizing radiation" is a completely different beast in terms of hazards even though we often use the shortcut "radiation" to refer to it too.

And the word "radiation" quickly brings us to the word "fallout." In the first hydrogen bomb test at Bikini Atoll the explosion was even large than scientists anticipated. Huge amounts of calcium from the coral the atoll was made up of were sucked into the fireball. They were bombarded with neutrons there and transmuted to elements that were ferociously radioactive. The bunker built for the observers was barely enough to save them from the radiation as particles of radioactive material rained out of the sky. Far downwind, the crew of the Lucky Dragon fishing boat were also exposed. They were burned and sickened and one of them died.

The public soon learned that radioactive fallout was a deadly serious matter. Bombs that explode high in the air aren't so bad, the nitogen, and oxygen that make up most of the air can get whacked by neutrons and not turn into anything too unpleasant. But the ground is another matter and nukes detonated close to the ground to destroy command bunkers, missile silos, hardened aircraft hangers, or sub pens would send plumes of deadly fallout downwind.

There's a website, Nukemap, you can use to play around with if you feel like frightening yourself about nuclear wars. According to it a 5 megaton ground explosion in New York could, if the wind is blowing the wrong way, deposit about 1 Gray/hour's worth of fallout here in Boston. The single Gray from the first hour would be enough to give me radiation sickness but not kill me but it would keep building up unless I could get to shelter. I'm not quite sure how to figure the decay curve but without a shelter from the radiation I'd either die in a day if I was lucky or die in a couple of weeks if I wasn't.

So fallout, in this context, is an immediately lethal threat. A deadly danger which people believed, entirely accurately, would probably kill you if you were exposed to it.

In this context it's very understandable that when people heard that the Three Mile Island nuclear plant had released radioactive fallout, they were just a tad concerned.

The problem is that we were using the same word, fallout, to refer to both the 10 gray doses that'll kill you dead quickly in a nuclear war and the 0.0008 gray doses you might get around Three Mile Island which would increase your lifetime odds of getting cancer by about 5 in a million assuming the most pessimistic model of radiation induced cancer. There's a unit people have put together called a micromort for thinking about very small chances of death. Assuming that all cancer is deadly the radiation was 5 micromorts which is about as dangerous as going scuba diving or driving 1000 miles by car. Not so small a risk that it doesn't deserve respect but small enough that it doesn't deserve fear.

Fukushima had similar levels of radiation released. Expose millions of people to a several micromort risk and you will get deaths at a population level but again, not at a level worth treating with fear rather than caution. Chernobyl, well, Chernobyl occupied a "stone" level between the "boulder" level of World War III's fallout and the "pebble" fallout of Three Mile Island.

I've talked like the effect of using one word for two very different situation has been all about making people too afraid of small amounts of fallout but that's not the only direction this confusion goes in. Growing up after the Cold War and MAD and only worrying about radiation from power plants I had this idea that fallout was something that would maybe give you cancer after a few years and that mostly people had fallout shelters during the Cold War as a matter of long term health rather than short term survival. I was very badly misled by my understanding of the term. I think that this is a problem for people of my generation that while we might be more afraid of radiation from reactor meltdowns than we should be we also treat the idea of fallout from a nuclear war with inappropriately low levels of terror. Terror is the appropriate response to the continued existence of the huge US and Russian nuclear arsenals and when we think about the dangers that may face humanity it's important to remember that the shadow of the mushroom cloud hasn't gone away.


Number of

Half-life applies to many things, but in our area of interest, it determines how long it takes hideously radioactive elements to decay into safe non-radioactive elements. Scientists use half-life instead of full-life because [1] we want to know the rate and [2] full-life is typically freaking huge.

Half-life is also useful for figuring out how long you'll get useful power out of an RTG.

So, say there is a slug of Strontium-90 that is emitting about 10 sieverts of radiation. We will say that "safe" means radiation at a level about equal to background radiation emitted by the ground, about 0.21 millisieverts (0.00021 sieverts). How long will it take to for the strontium-90 to decay to a safe level?

Assume that strontium-90 decays directly into a non-radioactive isotope (it doesn't, but let's not complicate things). Assume that if the amount of strontium-90 is reduced by half due to decay, the amount of radiation will also be reduced by half. The fraction of an element undecayed after n half-lives is 1/2n.

After playing around with numbers, I found that 16 half lives will have an undecayed fraction of 1/216 = 1/65,536 = 0.000015. This means 10 sieverts will become 0.00015 sieverts (below 0.00021 sv background) after 16 half lives.

Strontium-90 has a half life of 28.8 years. Sixteen half-lives is 16 * 28.8 = 460.8 years.

Now, in reality you'd have to figure the radioactive decay products, figure their radiation level, and figure their decay time.

An atom of a radioactive element decays by emitting radiation. So as a general rule, the shorter an isotope's half-life, the more intensely radioactive it is. Specifically, the activity of a lump of isotope (in Becquerels) is:

Abq = N * (ln[2] / t½)


Abq = (radio)activity (Becquerels)
N = number of atoms in the lump of isotope
ln[2] = Natural logarithm of 2, about 0.69315...
t½ = half-life of the isotope (seconds)
Uranium 2357.13×108 years
Uranium 233160,000 years
Plutonium 23924,100 years
Curium 2458,500 years
Plutonium 23887.7 years
Hydrogen 3
12.32 years
Polonium 210138.376 days

The question arises "how many atoms are in a gram?" The answer was told to you in chemistry class, when your eyes glazed over as the professor talked about "molar mass" and the "Avogadro constant". Avogadro constant is about 6.02214179×1023 mol-1. This means if you made a pile of 6.02214179×1023 Uranium-235 atoms it would weigh exactly 235 grams. A pile of that number (one "mole") of Plutonium-239 would weigh exactly 239 grams.

The point is, you can use this to convert between atomic mass units and grams. Basically you divide Avogadro constant by the atomic mass of the element to find the number of atoms of that element in one gram. So Strontium-90 contains 6.02214179×1023 / 90 = about 6.69126865×1021 atoms per gram.

The radiation flux depends upon the energy per atomic decay. These are generally listed in terms of MeV or megaelectron volts. 1 MeV = 1.6021773×10-13 joules. For instance, strontium-90 undergoes beta-decay, at 0.546 MeV per decay. Multiply this by the Becquerels to get the total radiation flux in joules.

The total flux can then be used to calculate the dosage inflicted on anybody unfortunate enough to be exposed to the lump.


Say there is a 500 gram lump of pure Strontium-90 lying in a field. What is the radiation flux?

First the number of Strontium-90 atoms per gram:

Napg = 6.02214179×1023 / 90
Napg = 6.69126865×1021

Then the number of atoms in a 500 gram lump:

N = Napg * Ngrams
N = 6.69126865×1021 * 500
N = 3.34563432×1024

Now the Becquerels. Strontium-90 has a half-life of 28.79 years or 9.07921×108 seconds.

Abq = N * (ln[2] / t½)
Abq = 3.34563432×1024 * (0.69315... / 9.07921×108)
Abq = 2.55420570×1015

Strontium-90 does beta decay at 0.546 MeV per decay. In joules this is:

Jdecay = MeVdecay * 1.6021773×10-13
Jdecay = 0.546 * 1.6021773×10-13
Jdecay = 8.7478880×10-14

So the total radiation flux is:

Fluxtotal = Abq * Jdecay
Fluxtotal = 2.55420570×1015 * 8.7478880×10-14
Fluxtotal = 223 joules

Moron Floyd gets within 2 meters of the strontium-90 lump before he notices the WARNING! HIDEOUS RADIOACTIVE DEATH sign. Over the next second, what is his radiation exposure?

A sphere with a radius of 2 meters has a surface area of:

ssphere = 4 * π * R2
ssphere = 50 m2

Radiation flux at 2 meters is:

Flux = Fluxtotal / ssphere
Flux = 223 / 50
Flux = 4.5 joules/m2

Floyd is an average person with a cross section of about 0.445 m2 and a mass of 68 kilos. His exposure in Grays is:

Grays = (Flux * CrossSection) / Mass
Grays = (4.5 * 0.445) / 68
Grays = 0.03

Beta particles have a quality value of 1.0. This means the exposure in Sieverts is also 0.03. This isn't serious but Floyd is still an idiot. He will receive further exposure as he runs away.



(ed note: A nuclear powered spacecraft, the Ymir, comes back to Terra orbit, with a huge chunk of cometary ice as payload. Unfortunately the crew is dead. A team is sent in the ship New Caid to discover what happened)

      In the meantime, Markus was able to make contact with the computers on the other side of the port (inside the Ymir), and verify that there was breathable air and other amenities.
     It was damned cold, though: about twenty degrees below freezing.
     “That was Sean doing us a favor,” Markus said. “He turned the thermostat down before he died. His body will be frozen solid.” (instead of at room temperature and rotting away) For Ymir had no lack of power from its nuclear generators, and its electrical systems were still working.
     Markus entered a command that would turn the command module’s environmental systems back on and bring the temperature back up. He pressurized the tiny space between Ymir’s hatch and New Caird’s. Then he opened the latter.
     They were all looking now at the slightly domed exterior surface of the hatch that would lead into Ymir’s command module.
     Someone had written on it with a felt-tipped marker. He had drawn the trefoil symbol used to warn of radiation hazards and beneath it had written the Greek letters alpha, beta, and gamma (symbols of deadly ionizing radiation). Then, as a darkly humorous doodle, he had added a crude skull and crossbones.

     Markus was the first to recover. He spiraled out of the pilot’s chair and propelled himself aft to the inner hatch of the airlock. There he punched a virtual button on a screen, which had the effect of locking the inner hatch. He was not letting Vyacheslav come in. He reached up with one hand and adjusted his headset. “Slava,” he said, “can you hear me? Good. Listen. We have contamination. You may have picked some of it up on your space suit. Before you come inside, I would like you to go over to Jiro’s external external radiation detector and see if we pick anything up.
     Jiro was already scanning the hatch with his Eenspektor (Eenspektor is a gamma spectrometer, modern equivalent of a Geiger counter), fortunately without results.
     Outside they could hear Vyacheslav cycling the airlock again and clambering back out. Using external handholds on the hull he made his way to the place where the external gamma spec was mounted, and devoted a couple of minutes to turning this way and that, directly in front of them, paying particular attention to his gloves, his knees, his boots—anything that had come into contact with the ice. No bursts of radiation were noticed, and so he was given clearance to go back to the airlock and enter New Caird.
     They had brought warm clothes, which seemed advisable when going on a journey to a huge piece of ice. Jiro put his on. Dinah reached for the stuff sack in which she had stored hers, but Markus held up a restraining hand. She noticed he was making no effort to dress for the occasion. Jiro was going down there alone.
     “I am going to overpressurize us a little bit,” Markus said, working with an interface on his pad. Dinah felt pressure building against her eardrums. Markus didn’t explain himself, and didn’t have to: they wanted clean air from New Caird to waft into Ymir, as opposed to potentially contaminated air coming in here.
     Jiro then pulled a disposable one-piece bunny suit over his cold-weather gear. For they had come prepared to find the ship contaminated. He slung his Eenspektor over the outside of the bunny suit. Dinah handed him a respirator mask, so that he wouldn’t breathe radioactive dust into his lungs, and he pulled it on over the bunny suit’s hood and checked it for a good seal against his face. He pivoted into the space between the ships, operated the external latch on Ymir’s hatch, and jerked forward slightly as the overpressure in New Caird pushed it open. He let himself drift into the command module, then got himself turned around so that his feet were oriented toward the “floor.” Meanwhile Markus pulled the hatch closed behind him.
     Vyacheslav by now had emerged from the airlock. He, Dinah, and Markus were listening to Jiro’s breathing on their headsets.
     “Sean bled to death,” Jiro announced.

     After a few minutes Jiro was able to set up a video link from a camera mounted to his head. They watched it on their tablets.
     The frozen body of Sean Probst was floating in a sleep sack that had been zip-tied to the ceiling. The porous fabric was stained dark brown. Very little of it had not been soaked with blood.
     Bumping lightly against him was an old-school Geiger counter, tethered by another zip tie. The word BUSTED had been written on it with the same felt-tip pen used to make the sign.
     After sweeping Sean’s body and the rest of the level with his Eenspektor, Jiro floated down the gangway to the next level “down.” The noise of the Eenspektor built steadily.
     “Oh, turn the f*cking sound off,” Markus said, and it went quiet. It would now display the counts per minute on its little screen, which only Jiro could see, but they wouldn’t hear the clicks.
     The next story was a sort of general meeting, dining, and muster room, mostly open space lined with storage lockers. The third, or middle, story was divided into sleeping compartments, toilets, and showers. The fourth was a laboratory and workshop space. Those functions continued down into the fifth and bottom-most story.
     “Cold here,” Jiro said, as he reached the bottom level. “Suddenly a lot of beta.”
     “Okay,” Markus muttered, “so the contamination is there. On the fifth level down.”
     It was cold, as they soon saw, because someone had left the door open: a manhole in the middle of the floor, big enough for a person in a space suit to climb through it and into a round shaft leading straight down into the ice. The entire length of the shaft was illuminated by white LEDs.
     “That is remarkable,” Markus said.
     Jiro descended into the tunnel headfirst and began to propel himself along it by the simple expedient of pulling on a knotted rope that had been fixed into its wall by ice anchors. He moved tentatively at first, then more rapidly. “There is a hatch at the far end—a hundred meters away, maybe,” Jiro said.
     “Radiation?” Markus asked.
     “Not so much,” Jiro said. “I do not think this was the route of the contamination.”
     The hatch at the end was adorned with a more formal rendering of the radiation hazard symbol. They all knew what was on the other side of it: a small pressurized module that was physically connected to the guts of the reactor. Jiro elected not to go through, instead turning around for a return to the command module.
     Then he turned back suddenly, and swept the beam of his headlamp across the ice wall of the tunnel. Some long slender object was embedded in the ice.
     Two long slender objects.
     Two human bodies. Dinah gasped as she recognized Larz’s strawberry-blond hair. Without making any comment, Jiro made his way back “up” the tunnel to the lower level of the command module. He turned his attention to a locker near the hatch. Its door was open. Mining tools and space suit parts were floating around in it. Others had spilled out into the room and were drifting around aimlessly, pushed by currents of air.
     “Jiro,” Markus said, “talk.”
     “Strong beta from here,” Jiro said. “This is where the contamination came from.”
     He drifted back up to the common room and found a garbage bag in a cabinet, then returned to the bottom level and went to work sorting through the tools and the clothing, holding each of them in turn up to the Eenspektor as he focused on its screen. From time to time he would grimace at the results and push the item into the garbage bag.
     Dinah, Markus, and Vyacheslav waited in New Caird for an hour, pretending to pass the time with tasks on the screens of their tablets.
     Then they heard Jiro’s voice again: “Prepare to put something out the airlock!” he was shouting.
     It took them all a few moments to understand Jiro’s thinking. New Caird and the command module of Ymir now formed a closed system. Since the latter was completely embedded in ice, the only way to remove something from that system—to take out the radioactive garbage—was to put it out New Caird’s airlock.
     There were some distant thuds. Dinah floated forward and opened the hatch to be greeted by a garbage bag, filled to the dimensions of a beach ball, and all wrapped up in duct tape. Propelled by a shove from Jiro, this entered New Caird. Dinah pushed it up to Markus, who intercepted it and tapped it sideways into the airlock. Vyacheslav slammed the hatch behind it. Then they heard a hiss, indicating that the lock had cycled. The bundle was now adrift in space.
     Jiro’s head, then the rest of him came through the port. He had stripped off the bunny suit and the respirator and presumably stuffed them into the garbage bag. He was sweaty and exhausted.
     “Just like old times, my friend?” Markus said, referring to Jiro’s earlier career running cleanup at Fukushima.
     “I don’t miss it,” Jiro said.
     It was warm in the command module now, so they didn’t need the parkas. But they all used bunny suits when they went into Ymir, and stripped them off before going back into New Caird. Contamination was “sneaky,” as Jiro put it. The beta emitted by a microscopic speck of fallout could be hidden from the Eenspektor’s view by just about any random obstacle—and the command module was cluttered with those. So Jiro’s initial sweep was no guarantee that tiny beta-emitting particles weren’t still hidden in there. If such particles found their way into a lung, or the digestive tract, fatal radiation damage was likely to result. He had, though, identified a space suit glove on the lower level as being heavily contaminated, and found lower levels of contamination on some other odds and ends that had gone into that garbage bag and out the airlock. With luck all serious sources of contamination had now been removed.

     Somewhat ruining their appetite, he reported on the findings of this impromptu autopsy as they got ready to eat a meal in the common room.
     “Sean bled to death out of his (anus),” he reported. “He had an internal rupture of the bowel.”
     “I picked up some beta through his belly,” Jiro added. “He was very emaciated at the end.”
     “Meaning?” Markus asked.
     “He swallowed a particle of fuel. Probably a fuel flea that got loose and somehow was tracked in here.”
     “Fuel flea?” Jiro had used the term before. No one else knew what it meant. It had gone in one ear and out the other, just another bit of the tech jargon that was so ubiquitous on Izzy (International Space Station). Now that fuel fleas were killing people, it was time to learn about them.
     “A tiny piece of uranium or plutonium that has gotten loose from a ruptured rod. As it throws off alpha particles, it zigs and zags around the room—conservation of momentum. So it hops around like a flea. The point is, it is small and it makes a lot of alpha. It lodged in a diverticulum in his bowel. It burned through his bowel wall and started a bleed that could not stop.
     Everyone pushed back their food.
     “Okay,” Markus said. “We eat in New Caird.”

     When they were all awake, they ate food from the stores they had brought with them and listened to a briefing from Jiro.
     “Let me tell you what happened to this expedition,” he said. And then he told them the story as he had pieced it together from the logs left behind by the dead.

     The failure of the radio, shortly after the beginning of the mission, had been caused by a defective part for which there was no replacement: a simple, stupid oversight. The longest leg of the trip—the year and a half spent coasting from the L1 gate to (Comet) Grigg-Skjellerup—had consisted of lengthy stretches of boredom interrupted by occasional panics, most of which had to do with the life support system. This was based on using sunlight to grow algae, a process that worked well in the lab but had turned out to be difficult to sustain on Ymir. The newest arklets in the Cloud Ark had benefited, in this respect, from lessons learned operating such systems in the time since Zero, but Ymir had been built and launched very early, using systems that now seemed painfully out of date.
     Once they had reached “Greg’s Skeleton” and thereby gotten access to vast amounts of water, they’d been able to make oxygen by splitting H2O, and life had improved. Until then, however, they’d been oxygen hungry and tense, trying to keep their consumption of air and food to a minimum by floating listlessly in their sacks watching the same DVDs over and over again. Health, and mental status, had suffered.
     They broke the shard (of ice for payload) from Grigg-Skjellerup using small mining charges planted by hand, or by robots programmed by Larz. Into its nose they embedded the command module, making themselves comparatively safe from cosmic radiation and bolides for the first time since the beginning of the mission. Life began to improve. They started excavating the access tunnel into the core. Into the aft end of the shard they inserted the reactor system, letting it melt its way into the ice. Around it, in the heart of the shard, they began to excavate a cavity and sculpt out hoppers: containers designed to hold broken-up ice produced by the mining robots. Twelve augers—long, spiraling ice movers, like the ones used to transport grain into elevators—were set up to convey that loose ice from the hoppers into the space surrounding the warm reactor vessel, where it would melt and be pumped into the core itself. Meanwhile, a separate corps of robots worked on the outside of the shard, melting the ice a little bit at a time, mixing it with the fibrous material they’d brought with them, and letting it refreeze into the much tougher material known as pykrete. The “steampunk” propulsion system had basically worked as planned—though not without a lot of tinkering and head scratching—on the first “burn” that had put it on the course back to L1. There had, however, been some problems with the augers that were used to feed ice into the reactor chamber. The augers received their inputs of ice from hoppers that had to be filled up by “mining” solid ice from the inside of the shard, a process for which robots were well suited, and so nothing worked at all without the assistance of a small army of robots conveying flakes of ice from mine head to hopper like ants dismantling a loaf of sugar. This part of it had actually worked. But some of the pieces of ice being mined by the robots had little rocks in them. These jammed the augers. Jams could often be repaired by operating the auger in reverse for a short time, but sometimes a robot, or even a person in a space suit, had to be sent to pry a rock out of the mechanism. An auger accident had led to the death of one member of the crew.
     During the months between that first burn and their arrival at L1, Larz did some programming work on the robots, trying to teach them not to collect rocky ice. They conducted a number of system tests intended to make sure that the problems they’d experienced the first time around wouldn’t be repeated during the critical second burn. These ranged from small-scale tests on individual robots all the way up to full dress rehearsals where the entire system would be energized and the reactor turned on to generate thrust for a few minutes.

     It had been during the first of those dress rehearsals when something had gone wrong in the core, resulting in damage to the jacket of a fuel rod.

     Jiro had an idea as to what had gone wrong. Ymir’s reactor used water—the melted ice of the comet core—as its moderator. In nuclear engineering, that meant a medium that slowed down the neutrons hurled out by fission reactions, making them more likely to stick around long enough to trigger more such reactions. In the absence of an effective moderator, the neutrons would mostly escape from the system without doing anything useful.
     Between being as dead as a doornail and running out of control was a narrow band of normal and healthy power output in which basically all commercial reactor operations happened. The essential problem with Ymir’s reactor was that its moderator—being a naturally occurring substance—was impure and unpredictable. The water that flooded into the chamber for the first dress rehearsal had been melted from ice a few months earlier, around the time of the initial “burn,” and had been sitting in the plumbing system ever since then. There, it had been in contact with rocks and grit that had made it through the augers. It had leached various minerals out of that rock, and become something other than pure water. When the reactor was started and the pumps turned on, that impure water was drawn through screens and filters intended to exclude all the debris. But it was nonetheless impure water, and when introduced to the core, it failed to perform its function as a moderator. The reactor was sluggish to get going. With the advantage of hindsight, it could be seen that its neutron economy was suppressed, poisoned by the impurities in the water. Overreacting to the slow start, the operators had pulled the control blades out farther than they would have otherwise. But once the first rush of impure water had been flushed through the system and blown out the nozzle, it had been replaced by relatively pure water, only just now melted from the ice. The reactor’s power had surged, producing a sudden buildup of fission products inside the fuel rods. Some of those would have been gases such as krypton and argon. The gases would have created pressure. Fuel rods were engineered to withstand it, but one of them had failed and ruptured. Possibly it had left the factory in excellent condition but been damaged en route by a nanometeoroid that had left a microscopic flaw. In any case, for whatever reason, the rod burst open and began to spill out the highly radioactive “daughters” of nuclear fission, which had become mixed with the steam being blasted out the rocket’s nozzle.
     Most of the fallout had, therefore, dissipated into space. But the whole point of a rocket nozzle was to convert the thermal energy in the gas—its heat—into velocity. The faster the steam went, the colder it got, until the steam near the nozzle exit was so cold that it actually began to condense into snow. Tiny particles of fallout made excellent nuclei around which a snowflake might begin to form. Some of that snow had stuck to the ice walls of the nozzle bell.

     The most likely explanation for what had happened next was that one of the robots crawling around in that area maintaining the shape of the nozzle had become contaminated with a mixture of alpha-emitting fuel fleas and beta-emitting daughters, and tracked the material to a location where it had been transferred to the glove of a space suit—possibly by a mechanism as simple as a spacewalker reaching down to brush some ice from the claw of a Grabb, or planting a foot in a location where a contaminated Grabb had stepped. The contamination had then been brought into the command module when the spacewalker came indoors. They might not even have known about the burst fuel rod, so they might not have been checking for contamination. Or, as suggested by Sean’s note, their Geiger counters might have broken down, one by one, rendering them blind to the presence of radioactivity in their environment. In any case, the particles had spread around the command module. Some men had inhaled them, some had swallowed them. They hadn’t been healthy to begin with.

From SEVENEVES by Neal Stephenson (2015)

"Wrecked. Null-gee and high radiation. I'll have to put you in the safe for a while." Deston shoved the oldster into a small room, gave him a line, and turned to Barbara. "My tell-tale reads twenty-pink - so we've got a few minutes...

...He glanced at his telltale. Thirty two. High red. Time to go...

...In the lifecraft he closed the port, cut in the launcher, and slammed on a one-gravity drive away from the ship. Then he shucked Barbara out of her suit and shed his own. He unclamped a fire-extinguisher-like affair; opened the door of a tiny room. "In here!" He shut the door behind them. "Strip, quick!" He cradled the device and opened four valves.

Fast as he was, she was naked and ready for the gush of thick, creamy foam from the multiplex nozzle. "Oh, Dekon?" she asked. "I've read about it. I rub it in good, all over me?"

"That's right. Short for 'Decontaminant, Complete; Compound, Absorbent, and Chelating; Type DCQ.' It takes care of radiation, but speed is of the essence. All over you is right." He placed the foam-gun on the floor and went vigorously to work. "Eyes, too, yes. Everywhere. Just that. And swallow six gulps of it . . . that's it. I slap a gob of it over your nose and mouth and you inhale once-hard and deep. One good one's enough, but if it isn't a good one you die of lung cancer, so I'll have to knock you out and give it to you while you're unconscious, and that isn't good - complications. So make it good and deep?"

"Will do. Good and deep." She emptied her lungs.

He put a headlock on her and slapped the Dekon on.

She inhaled, hard and deep, and went into paroxysms of coughing. He held her in his arms until the worst of it was over; but she was still coughing hard when she pulled herself away from him.

From SUBSPACE EXPLORERS by E.E."Doc" Smith (1965)

Note that Doc Smith to the contrary, chelating decontamination doesn't quite work this way.

Neutron Activation

While human beings and other living things suffer harm from nuclear radiation, inanimate objects are not fond of it either. Charged particle radiation can be deflected from your ship by magnetic fields, but vexing neutron radiation is uncharged. It hits whatever is in the way, you cannot divert it.

This is why the magnetic nozzles of fusion engines are an open lattice full of holes: to reduce the surface area of nozzle that can be hit by neutrons. Meaning if from the viewpoint at the center of the reaction the sky is 10% lattice and 90% holes, only 10% of the neutron radiation is damaging the engine.

Wonder Neutron helps kill strong bodies 3 ways: Neutron Activation, Neutron Embrittlement, and Neutron Heating.

Neutron Activation happens when an ordinary harmless atomic nucleus swallows a low-energy neutron from the radiation flux of, say, a nuclear thermal propulsion system. This changes the nucleus from a stable isotope into an unstable isotope, and all unstable isotopes are radioactive and emit gamma-rays. Or the neutron can actually split the atom, with much the same results but now including bonus fission fragments and neutrons in the induced radioactivity.

Translation: the steel girders and everything else too close to the reactor will eventually become radioactive in and of themselves, glowing with dangerous gamma-rays. Your nuclear engine structure will gradually be transformed into low-level radioactive waste. This makes it treacherous for the crew to leave the spacecraft, and drastically lowers the re-sale value. This is a good reason to make your spacecraft modular, so you can detach the nuclear engine and swap it for one that doesn't glow in the dark.

Low-activation elements are chemical elements that are resistant to neutron activation. These include tungsten, tantalum, vanadium, and chromium. Use these to construct nuclear reactors and nuclear rocket engines.

High-activation elements can easily be transmuted into glowing radioactive chunks of death by trival amounts of neutrons. These include nickel, copper, aluminium, molybdenum, cobalt and niobium. As a matter of fact, cobalt has been suggested as a component of a salted bomb. Avoid these when making your nuclear engines.

There is a nice list of common elements that can be transmuted into radioactive isotopes here, along with the half-life of said radionuclides.

Neutron activation is a good thing in a breeder reactor or a medical isotope generator, but very bad anywhere else. Heavy neutron radiation is usually never found naturally, it is found unnaturally in nuclear reactors, nuclear explosions and other artificial things made by intelligent creatures.

Neutron activation analysis can be used to determine how much neutron radiation a hapless victim was exposed to. The doctor can examine the victim to determine how much of the body's sodium was neutron activated into sodium-24 and how much phosphorus was activated into phosphorus-31. This will provide an estimate of the acute radiation dosage.



The gamma dose rates arising from the neutron activation of a test article (modular concept) (the Lockheed nuclear shuttle concept) has been calculated at axial detector locations at Station 46 and at a second point 20 feet below Station 46 for times of 50, 100, 200, and 500 hours following an irradiation history consisting of nine 400 second irradiation periods followed by four 1800 second irradiation periods with a 30 day interval between each irradiation period. These gamma dose rates in units of milli-tissue rads/hr (1 milliRad equals 0.00024 Gray) are as follows:

Times after Final Irradiation50 hr100 hr200 hr500 hr
At Station 461600.0 mTRad/hr151.0 mTRad/hr25.5 mTRad/hr17.0 mTRad/hr
At 20 feet below Station 4660.6 mTRad/hr5.53 mTRad/hr0.86 mTRad/hr0.605 mTRad/hr

From these data it appears that it is safe with the exercise of normal precautions for an individual to venture into the vicinity of the second detector point (20 feet below Station 46) after only a few days (100 hr) following the final irradiation, whereas a waiting period of a week or so (200 hr) will be required before the dose rate at the first detector point (Station 46) dimenishes to a comparable level.

The effects of fewer irradiation periods has been accessed by comparing the dose rates at 50 and 500 hours following the 1st, 9th, and 10th irradiation periods to those following the 13th irradiation period. The ratios of these dose rates are as follows:

Times after Irradiation50 hr500 hr

At 50 hours following an irradiation: Cu-64 with a half life of 12.8 hours is the predominant radiation source; Na-24 with a half life of 15 hours is also significant; all other source terms combined account for less than 5 percent of the dose rate. With 30 days between irradiation periods there is no buildup of these significant sources. This is examplified by comparing the dose rate after the 1st irradiation period with that following the 9th irradiation period. The jump in the dose rate following the 10th irradiation period shows that the dose rate is nearly proportional to the length of the irradiation period which increased from 400 to 1800 seconds.

At 500 hours following an irradiation the dose rate is dominated by isotopes with half lives greater than a few hundred hours. There is a considerable buildup of these sources from previous irradiations. This is shown by comparing the dose rates following the 9th Irradiation period to that following the 1st irradiation period or by a comparison of the 13th to the 10th. The effect of buildup is of greater significance with the shorter irradiation periods.

From an operational standpoint fewer irradiation periods will significantly reduce the waiting time for entry into the area following an irradiation. This is especially true when the longer lined isotopes are withholding entry.

by Lockheed Missiles & Space Co. (1971)


The radiation environment was defined during engine operation and after shutdown. These results were applied for: (1) radiation effects on the materials, components, and subsystems in the RNS, (2) nuclear heating of the propellant, and (3) orbital maintenance operations.

The PATCH point kernel code described in Section 4.5. 11 of Book 1 was used in the definition of the rad iation levels exterior to the engine. Data on engine radiation sources during reactor operation were based on the May 1969 Common Radiation Analysis Model (Reference 1) as modified by Reference 2.

Figure 3.7-1 shows the time to accumulate a dose of 1 rem at 2 different times after final shutdown. This is used to assess the constraints imposed on orbital operations by the residual radiation levels.

by McDonnell Douglas Astronautics Co. (1971)

(ed note: The charts use the obsolete radiation absorbed dose unit the Rad instead of the more modern Gray. Multiply Rads by 0.01 to get Grays. Since the quality factor of gamma radiation is 1, the dose equivalent of Rem will be equal to the Rad dose (and likewise the Sievert dose will be equal to the Gray dose).

A dose of 3.5 to 3.9 Sieverts means the astronaut is "Singed". A dose over 4.0 Sieverts means the astronaut is "Cooked", which means their NASA career is over (forbidden to work at any NASA job where they might be exposed to radiation). A dose over 4.5 Sieverts is "Fried", which is LD50 (50% chance of death).)


The orbital environment encompasses both natural and induced radiation effects, both of which impact orbital operations. An assessment was made of the residual activation of the components at the aft end of the propulsion module in order to determine accessibility. The natural environment of nuclear radiation was also evaluated and its impact on man was determined.

4.5.1 Residual Activation Analysis

Operational plans for the RNS include recycling of the modules to earth after deployment with the RNS. The feasibility and safety of this plan depends on the constraints which must be placed on personnel involved in handling the modules both in space and on the ground, since radiation levels of module components and materials could be biologically high due to induced activity related to the NERVA powered operation. Activation analyses of the propulsion module were conducted to determine the activity levels which could be expected after completing the full ten mission life cycle for the module at a rate of 8 missions per year. Other modules similar in construction, but farther from the reactor source, are assumed to be less of a problem.

Gamma dose rates arising from the neutron activation of materials in the propulsion module/engine adapter region and at the tank top were evaluated for detector locations as indicated in Fig. 4-68. The dose rates expected at these locations for various decay times following final shutdown are also included in Fig. 4-68. The materials and locations of the various components were assumed as shown in Figs. 4-69 and 4-70.

Total gamma does rates following the tenth mission are presented in Table 4-24. The dose rates due to each of the activated parts are listed in Table 4-25 for the case of the detector at 100 feet. This table in effect tabulates the relative source strength of each of the activated parts. The fractions of the total dose rate due to each type material at the various times after shutdown are tabulated in Table 4-26.

At 10 hours after shutdown, Manganese-56 with a half-life of 2.58 hours and Copper-64, with a half-life of 12. 8 hours account for approximately 98 percent of the total dose rate. The stainless steel alloys have a maximum weight fraction of 2 percent Manganese and the aluminum alloys have a weight fraction of 0.15 to 0.9 percent Manganese and 0.4 to 6.8 percent Copper.

At 100-hr decay time, many radioactive isotopes are significant: Copper-64, 12.8 hr; Chromium-51, 27.8 days; Iron-59, 45 days; Molybdenum-99, 66 hr; Cobalt-58, 71 days; and Sodium-24, 15 hr. The latter two isotopes are products of fast-neutron reaction. The Sodium-24 isotope results from an (n, α) reaction with aluminum, while Cobalt-58 and Manganese-54 isotopes result from (n,p) reactions with Nickel-58 and Iron-54, respectively. The dose rates from the stainless steel alloys are dominated by Chromium-51 and Iron-59 isotopes, while Cu-Copper and Sodium-24 dominate the dose rates from aluminum alloys.

At 1,000 hours, the predominant isotope is Cobalt-58, followed by Manganese-54, Chromium-51 and Iron-59. At 10,000 hours, the Manganese-54 isotope becomes predominant, followed by Cobalt-58. The Zinc-65 isotope emerges as relatively significant, while Iron-59 is relegated to a role of minor importance.

The analyses support the feasibility and safety of the tank exchange approach, but suggest that at least 100 hours of decay be allowed before initiation of the operation. This should be no real constraint however, since normal cooldown and mission operations will probably dictate delays greater than 100 hours.

(ed note: Figure 4-68 shows the neutron activation radiation dose rates at various locations, starting at the final engine shut down at the end of the 10th mission. The worst is at location 3, at 0.4 Sieverts per hour. This means a Fried dose in 12 hours. Now, 417 days after final engine shut down the radiation has decayed to the point where location 3's radiation is so weak that an astronaut would have to stay there more than years to get a Fried dose.

The report is of the opinion that any astronaut refurbishment, engine swap, or other operations on the spacecraft should wait until 100 hours (102 hours) after engine shut down. This will allow the neutron activation induced radiation to die down to a less than utter suicidal level.

Table 4-24 is similar to Figure 4-68. Except the locations are different and the doses are in milliRads per hour instead of Rads per hour. 1 milliRad equals 0.001 Rad and equals 0.000 01 Sievert. "Decay Time Hours" and "Hours After Shutdown" are the same, as are the column headers (10,000 = 104)

Table 4-25 is the dosage contributed by each activated part. The values are for the case of the detector at 100 feet. And because the study authors figured things were not complicated enough, the table is in microRads per hour. 1 microRad equals 0.000 001 Rad and equals 0.000 000 01 Sievert. So for instance Table 4-24 lists the 101 shutdown dose as 13.6 milliRad and Table 4-25 lists it as 13,600 microRad.

Figure 4-69 shows the model used to calculate the doses in Figure 4-68. The model shows the location of the various components and their composition. Table 4-26 shows the percentage of the neutron activation radiation contributed by each material, figuring their in the amount of each material and its suseptibility to neutron activation. )

by Lockheed Missiles & Space Co. (1971)

Low-energy neutrons can also produce adverse effects. Neutrons at low energies are more easily absorbed by atomic nuclei, usually resulting in the creation of a radioactive isotope. This too can change material properties and can cause problems with electronic circuitry.

Salted Bombs

Neutron activation is used on purpose in nasty salted bombs. The bomb is intentionally cased in cobalt or other element that is particularly good at being neutron activated into a hideously radioactive isotope. This provides enhanced quantities of radioactive fallout. "Salted", as in "sowing the earth with salt so nothing ever grows there again." The salted bomb is conveniently almost non-radioactive while in storage, the radioactive isotopes are instantly manufactured at ground zero.

And let's not forget the "enhanced radiation bomb" aka "Neutron bomb". This is a nuclear warhead specially constructed so that less of the bomb energy goes into x-rays and more goes into neutrons. It was designed to do less blast damage to buildings and vehicles but do more radiation damage to people. But of course the extra neutron flux will naturally do more neutron activation to any material object near ground zero. I found mention that a main battle tank close to the detonation point would suffer enough neutron activation to render it lethally radioactive for about 48 hours.

Neutron Embrittlement

Neutron radiation can make some materials become brittle by neutron-induced swelling and buildup of Wigner energy. High-energy neutrons striking metal gradually damage the metal's crystal lattice. This makes the metal more brittle and can eventually lead to failure. Steel has a so-called "ductile-to-brittle transition temperature", a temperature below which it becomes brittle. Neutron bombardment raises this temperature.

The brittleness can be healed by heating the material, this is called annealing. It might be possible to construct a reactor capable of annealing its structural members in place instead of removing it first. But of course you have to be very careful. You will be in for some real excitement if you accidentally catch the nuclear engine on fire.


(ed note: alloy is resistant t neutron embrittlement. The alloy is 38% tungsten, 36% tantalum, 15% chromium, and 11% vanadium. "High-entrophy alloy" is a fancy term for "alloy with four or more ingredients")

      LOS ALAMOS, N.M., March 5, 2019—A new tungsten-based alloy developed at Los Alamos National Laboratory can withstand unprecedented amounts of radiation without damage. Essential for extreme irradiation environments such as the interiors of magnetic fusion reactors, previously explored materials have thus far been hobbled by weakness against fracture, but this new alloy seems to defeat that problem.
     “This material showed outstanding radiation resistance when compared to pure nanocrystalline tungsten materials and other conventional alloys,” said Osman El Atwani, the lead author of the paper and the principal investigator of the “Radiation Effects and Plasma Material Interactions in Tungsten Based Materials” project at Los Alamos. “Our investigations of the material mechanical properties under different stress states and response of the material under plasma exposure are ongoing.”
     “It seems that we have developed a material with unprecedented radiation resistance,” said principal investigator Enrique Martinez Saez, a coauthor of the paper at Los Alamos. “We have never seen before a material that can withstand the level of radiation damage that we have observed for this high-entropy [four or more principal elements] alloy. It seems to retain outstanding mechanical properties after irradiation, as opposed to traditional counterparts, in which the mechanical properties degrade easily under irradiation.”
     Arun Devaraj, a materials scientist and project collaborator at Pacific Northwest National Laboratory, noted, “Atom probe tomography revealed an interesting atomic level layering of different elements in these alloys, which then changed to nanoclusters when subjected to radiation, helping us to better understand why this unique alloy is highly radiation tolerant.”
     The material, created as a thin film, is a quaternary nanocrystalline tungsten-tantalum-vanadium-chromium alloy that has been characterized under extreme thermal conditions and after irradiation.
     “We haven’t yet tested it in high-corrosion environments,” Martinez Saez said, “but I anticipate it should perform well there also. And if it is ductile, as expected, it could also be used as turbine material since it is a refractory, high-melting-point material.”
     Described this week in a paper in Science Advances, the project was a multi-institutional effort, involving researchers and facilities of Los Alamos National Laboratory, Argonne National Laboratory, Pacific Northwest National Laboratory, Warsaw University of Technology, Poland, and the United Kingdom Atomic Energy Authority.


Neutrons can cause damage to materials in a variety of ways. Their total integrated flux—defined as the fluence—must be limited to prevent damage. High-energy neutrons normally pass through most materials without any atomic collisions, simply because of the relatively small size of the nucleus. However, where collisions do occur, they can result in atomic displacements, where the atom is effectively driven out of its location in the crystal matrix. This phenomenon is referred to as neutron embrittlement and can be thought of as a microscopic strain hardening mechanism. If a large enough number of these events occur, as is the case in high-radiation environments, the material will lose many of its structural, electrical, and heat conduction properties. Structural elements can fail, heat exchangers can lose performance, and electrical conductors can increase in resistivity. Without appropriate protective steps, all of these reactions would result in a decrease in the performance of the MTF (Magnetized Target Fusion) propulsion system.


One of the main reasons for limiting the operating lifetimes of nuclear reactors is that metals exposed to the strong radiation environment near the reactor core become porous and brittle, which can lead to cracking and failure. Now, a team of researchers at MIT and elsewhere has found that, at least in some reactors, adding a tiny quantity of carbon nanotubes to the metal can dramatically slow this breakdown process.

For now, the method has only proved effective for aluminum, which limits its applications to the lower-temperature environments found in research reactors. But the team says the method may also be usable in the higher-temperature alloys used in commercial reactors.

The findings are described in the journal Nano Energy, in a paper by MIT Professor Ju Li, postdocs Kang Pyo So and Mingda Li, research scientist Akihiro Kushima, and 10 others at MIT, Texas A&M University, and universities in South Korea, Chile, and Argentina.

Aluminum is currently used in not only research reactor components but also nuclear batteries and spacecraft, and it has been proposed as material for storage containers for nuclear waste. So, improving its operating lifetime could have significant benefits, says Ju Li, who is the Battelle Energy Alliance Professor of Nuclear Science and Engineering and a professor of materials science and engineering.

Long-term stability

The metal with carbon nanotubes uniformly dispersed inside “is designed to mitigate radiation damage” for long periods without degrading, says Kang Pyo So.

Helium from radiation transmutation takes up residence inside metals and causes the material to become riddled with tiny bubbles along grain boundaries and progressively more brittle, the researchers explain. The nanotubes, despite only making up a small fraction of the volume — less than 2 percent — can form a percolating, one-dimensional transport network, to provide pathways for the helium to leak back out instead of being trapped within the metal, where it could continue to do damage.

Testing showed that after exposure to radiation, the carbon nanotubes within the metal can be chemically altered to carbides, but they still retain their slender shape, “almost like insects trapped in amber,” Ju Li says. “It’s quite amazing — you don’t see a blob; they retain their morphology. It’s still one-dimensional.” The huge total interfacial area of these 1-D nanostructures provides a way for radiation-induced point defects to recombine in the metal, alleviating a process that also leads to embrittlement. The researchers showed that the 1-D structure was able to survive up to 70 DPA of radiation damage. (DPA is a unit that refers to how many times, on average, every atom in the crystal lattice is knocked out of its site by radiation, so 70 DPA means a lot of radiation damage.)

After radiation exposure, Ju Li says, “we see pores in the control sample, but no pores” in the new material, “and mechanical data shows it has much less embrittlement.” For a given amount of exposure to radiation, the tests have shown the amount of embrittlement is reduced about five to tenfold.

The new material needs only tiny quantities of carbon nanotubes (CNTs) — about 1 percent by weight added to the metal — and these are inexpensive to produce and process, the team says. The composite can be manufactured at low cost by common industrial methods and is already being produced by the ton by manufacturers in Korea, for the automotive industry.

Strength and resilience

Even before exposure to radiation, the addition of this small amount of nanotubes improves the strength of the material by 50 percent and also improves its tensile ductility — its ability to deform without breaking — the team says.

“This is a proof of principle,” says Kang Pyo So. While the material used for testing was aluminum, the team plans to run similar tests with zirconium, a metal widely used for high-temperature reactor applications such as the cladding of nuclear fuel pellets. “We think this is a generic property of metal-CNT systems,” he says.

“This is a development of considerable significance for nuclear materials science, where composites — particularly oxide dispersion-strengthened steels — have long been considered promising candidate materials for applications involving high temperature and high irradiation dose,” says Sergei Dudarev, a professor of materials science at Oxford University in the U.K., who was not involved in this work.

Dudarev adds that this new composite material “proves remarkably stable under prolonged irradiation, indicating that the material is able to self-recover and partially retain its original properties after exposure to high irradiation dose at room temperature. The fact that the new material can be produced at relatively low cost is also an advantage.”

Radiation Heating

Very-high energy neutrons not only cause neutron embrittlement, they also impart thermal energy to the nucleus they hit. Meaning it heats things up. Gamma rays heat things up as well. For the most part, fast neutrons tend to heat up things like liquid hydrogen, while gamma rays heat up engine components.

So here's the score:

  • Very-high-energy neutrons: thermal heating of engine
  • High-energy neutrons: embrittlement of engine
  • Medium-energy neutrons: SAFE ZONE
  • Low-energy neutrons: activation of engine (becomes radioactive)

Neutrons with energies in the medium-energy zone have no harmful effect on the engine, the metals composing the engine are transparent to the neutrons. Those neutrons have too much energy to be absorbed by nuclei and cause neutron activation, but not enough energy to interact with nuclei to cause neutron embrittlement or thermal heating.

It would be wonderful if researchers could invent a shield that would somehow cause the neutrons to enter the safe zone (the technical term is "neutron moderation"). But that would be tricky. And I am unaware of any way to speed up low-energy neutrons, moderators in nuclear reactors can only slow neutrons down.


Fortunately, for most materials, there exists a range of energies at which the neutrons are too energetic to be absorbed yet low enough to pass through without interacting with the nuclei. Within this energy range, the material is effectively transparent to the neutrons.

Designing a neutron radiation shield capable of moderating neutrons (into the safe zone) with a range of initial energies would be a challenging task. It would probably necessitate the use of a composite shield, consisting of many different layers of materials, with the outer layers slowing down fast neutrons but allowing slower ones to pass to the next layer. Each successive layer would have to repeat this process until all the neutrons were moderated to the appropriate energy. Such a shield would act as a neutron filter and would require much less mass than conventional shielding. Unfortunately, the design of such a tailored shield was not possible within the scope of the HOPE study. With available techniques, at best, a large population of the high-energy neutrons can be reduced to a level of transparency. The fusion community is currently experimenting with the molten salt FLiBe (a mixture of lithium fluoride and beryllium fluoride, pronounced “FLiBe”). FLiBe has a good atomic cross section for slowing neutrons, a high vaporization temperature, and an acceptable viscosity.

In addition to its shielding properties, liquid FLiBe can also serve as a primary cooling fluid. This yields a particularly efficient design solution, since the material into which the neutron thermal energy is first deposited (the FLiBe) is also the medium that carries it away from the nozzle. The neutron energy must be pumped out of the system so that a nozzle operating temperature no greater than 1,500 K is maintained. This is in keeping with the maximum operating temperature that the hottest portions of the nozzle can withstand. It also ensures that the FLiBe remains below its vaporization point.

For these reasons, FLiBe was selected as the neutron shield material and active cooling fluid for the MTF nozzle.


Semiconductor electronics are also vulnerable to radiation (including particle beam weapons) due to mechanical disruption, as you can see from Anthony Jackson and Christopher Thrash (below).

However while the vacuum tubes (thermionic valves) in the Russian MiG fighters would not provide much protection from nuclear EMP, vacuum tubes are inherently more radiation resistant compared to semiconductors.

Modern rad-hardened electronics can survive a few hundred to a few thousand grays, and will generally continue functioning until destroyed; non-hardened electronics won't handle even one gray very well, and will crash more or less instantly. In general, more advanced chips, because they have smaller circuits, are more vulnerable to radiation than more primitive designs.

(ed note: here are some samples of rad-hard systems. The S950 is good for over 350 grays and uses a processor released in 2002.)

Anthony Jackson
Subject: Re: radiation and computers
From: t*** (Christopher Thrash)
Date: 19 Nov 2000 14:06:00 GMT
Message-ID: 6507
Newsgroups: sjgames.gurps.traveller

On 16 Nov 2000 08:15:36 GMT, a*** (Anthony Jackson) wrote:
> Realistically, what level of ionizing radiation will cause significant
> software errors (and possible a soft reboot) in hardened electronics?
> For that matter, what level of radiation will cause permanent damage,
> I know that some of the jupiter probes were fried by multiple passes
> through the jovian radiation belts.

From _The Effects of Nuclear Weapons_, Glasstone and Dolan, 1977, Sec. 8.73-8.88:

"The name commonly applied to the class of effects under consideration 
is "transient-radiation effects on electronics," commonly appreviated to the 
acronym TREE. In general, TREE means those effects occurring in an 
electronics system as a result of exposure to the initial radiation from an 
nuclear weapon explosion. The adjective "transient" applies to the radiation 
since it persists for a short time, i.e. less than 1 minute. The response, 
however, is not necessarily transient... 
"Radiation effects on electronics may be temporary or more-or-less 
permanent... The component responses of short duration are usually the result 
of ionization caused by gamma radiation and are dependent on the dose rate, 
e.g., in rads per second, rather than the dose. The more permanent effects 
are generally -- but not always -- due to the displacement of atoms in a 
crystal lattice by high-energy (fast) neutrons. In such cases the extent of 
the damage is determined by the neutron fluence, expressed in neutrons/cm2. 
When a permanent effect is produced in an electronic component by gamma 
radiation, the important quantity is usually the dose in rads."

Neutron fluence (Fn) at a distance R from a nuclear detonation is 
approximately given by:

Fn = 1.4 × 1012 Y/R2

where Y is in kilotons, R is in km, and Fn is in neutron/cm^2.

Dose (Dg) from prompt radiation of an explosion is approximately:

Dg = 4 x 105 Y(2/3)/R2

where Y is in kilotons, R is in km, and Dg is in rads (Si).

Damage Thresholds (gleaned from the text):
Transistors1011—1015 neutron/cm2
MOS Transisitors104 rads (silicon)
Capacitors1015 neutrons/cm2
Precision Resistors107 rads (carbon)/s
1014 neutron/cm2
NiCd Batteries107 rads (air)/s
1013 neutron/cm2
Hg Batteries10^16 neutron/cm2
Wiring Insulation:
Silicon Rubber2x1015 neutron/cm2
Polyethylene107 rads (carbon)
Teflon TFE104 rads (carbon)
Teflon FEB2×106 rads (carbon)
Polyolefins5×109 rads (carbon)
From _Space Mission Analysis and Design_, 3d Ed. (SMAD III), Wetz and Larson, 
1999, pp. 214-240:

Commercial Off the Shelf (COTS) and Rad Hard Parts Comparison:
CharacteristicsCOTSRad Hard
Total Dose103—104 rads105—106
Dose-Rate Upset106—108 rads(Si)/s>109 rads(Si)/s
Dose-Rate Induced Latchup107—109 rads(Si)/s>1012 rads(Si)/s
Neutrons1011—1013 n/cm21014—1015 n/cm2
Single-Event Upset10-3—10-7 error/bit-day10-8—10-10 error/bit-day
Single-Event Latchup
Single-Event Burnout<20 MeV-cm2/mg (LET)37-80 MeV-cm2/mg (LET)
LET is "linear energy transfer".
Christopher Thrash


The potential for damage to these components is a serious consideration for communications planners if (l) a nuclear detonation could generate strong electromagnetic fields that could couple into electronics and potentially cause damage. and (2) the electromagnetic fields produced could be radiated at such distances away from a detonation that other physical damage does not occur. These conditions are met by a secondary nuclear phenomena generated by detonations near the surface and at high altitudes known as Electromagnetic Pulse (EMP).

The gamma rays emitted from a detonation are reponsible for production of the EMP. Two distinct regions of EMP are produced by the low altitude detonation: a source region EMP which is generally confined to the region in which gammas are deposited and a radiated field which falls off in amplitude with 1/range relationship.

For near-surface bursts (less than 2 Km HOB) the source region extends some 3 to 6 Km from the burst. Within this source region, EMP field strengths on the order of hundreds of thousands of volts/meter and millions of amperes of current are generated. If a “hard wire" connection between two facilities each far enough away from a detonation to preclude any damage from blast, (thermal or radiation} intersects the source region, it may be severed. However. the remaining electrical connections provide a conducting path into each facility through which electrical current could flow.

Source region EMP can also affect facilities hardened against air blast which can be designed to withstand "near direct" hits. Conducting paths into these facilities provide entry paths for the source region EMP currents and the electronics housed within the facility are susceptible to EMP induced damage.

A radiated EMP is also generated by the near surface detonation and the radiated fields propagate as a plane wave obeying free space relationships. For small yields (less than 40 KT), it is possible to expose communications-electronic assets to field strengths on the order of 20,000 v/m at distances from the detonation that preclude any blast, thermal or radiation damage.

When a nuclear detonation occurs above the sensible atmosphere (HOB greater than 30 Km), ground based communications elements are exposed to a different nuclear environment. These high altitude detonations do not produce air blast, thermal, radiation or fallout effects that could significantly affect ground based communications facilities. However. these detonations do generate a strong radiated EMP.

The gamma rays emitted from a high altitude nuclear detonation deposit in the lower D region. producing a large deposition region limited only by line of sight from the burst. In the gamma deposition region. the gamma ray energy is converted to a downward moving electromagnetic plane wave. EMF fields on the order of 50,000 v/m are radiated over the large geographical area covered by this deposition region.

Generally, when the term EMP is mentioned in the context of communications, this high altitude electromagnetic pulse (EMP) is meant. While the pulse only lasts for microseconds, it has a rise time to peak amplitude (50,000 v/m) of 10-20 nanoseconds. This extremely fast rise time is one reason that protection against EMP is not provided by conventional lightning protection.

The deposition region for a single high altitude burst can be thousands of square kilometers, as shown in Figure 4, where areas of coverage for two heights of burst are shown.

High altitude EMP is of major significance due to its potential for widespread damage. As we stated earlier, EMP can couple into a facility by free field diffusion through unshielded walls or by any conducting path which may act as an antenna.

EMP coupling into equipments can result in functional and operational upsets or permanent damage due to burnout of electronic components. Many variables such as systems configuration, state-of-the-art of electronic technology and the amount of inherent or designed protection determine the susceptibility or vulnerability of equipments to EMP damage. Generally, the more advanced the state-of-the-art of the electronics, the more susceptible the equipment is to EMP damage. Vacuum tubes are generally l0 million times inherently harder to EMP than integrated solid state circuitry.

The same high altitude detonation that generates EMP also generates magnetohydrodynamic EMP (MHD-EMP). MHD-EMP is a late time, long duration (minutes), low frequency pulse, produced as the nuclear fireball rises and slightly distort the earth's geomagnetic field lines. This geomagnetic field movement induces electric potentials in the earth on the order of 7-30 volts/kilometer which can cause the protective systems of long haul cable power supplies to shut down. The effect of MHD-EMP is not unique to nuclear weapon detonations—there are recorded cases of long haul cables shutting down due to solar magnetic activity.

Technology for protection against EMP effects is advancing and available for application to electronics systems for which EMP protection is specified. Providing EMP protection for new facilities and equipments during the design phase is much less expensive than protection provided on a retrofit basis. EMP protection may create greater demands for configuration management and standard maintenance procedures if its continued effectiveness, once implemented, is to he insured.

The previous discussion introduced the effects of nuclear detonations on ground based facilities. However, aircraft are also being utilized as critical communications platforms. When airborne, aircraft are more survivable to blast and thermal damage in a strategic attack than ground based facilities due to their location uncertainty. Obviously the aircraft itself, as well as the personnel and equipment on board have potential vulnerabilities to the same effects that were discussed earlier. Yet the ranges at which a particular aircraft and equipment are vulnerable are very dependent on many factors which space does not permit addressing in this overview format.

Ground facilities are subject to a wide range of environments whose influence is felt at different ranges from a detonation and other effects (such as high altitude EMP) which are relatively independent of range. In the development of protection plans for a critical facility to enhance its survival to nearby detonations, care should be taken to insure that the system achieves a “balanced survivability" specifically designed for the command and control function the facility supports.

A balancd survival plan could, for example, only consider EMP protection if teh critical function that must survive needs to occur before blast and shock physical damage could be expected in a direct attack, or it could include protection against all effects expected from attacks against a nearby high priority target. The function supported by the facility is the key for determination of the strategy to be used.


Peer inside an antique radio and you'll find what look like small light bulbs. They're actually vacuum tubes—the predecessors of the silicon transistor. Vacuum tubes went the way of the dinosaurs in the 1960s, but researchers have now brought them back to life, creating a nano-sized version that's faster and hardier than the transistor. It's even able to survive the harsh radiation of outer space.

Developed early last century, vacuum tubes offered the first easy way to amplify electric signals. Like light bulbs, they are glass bulbs containing a heated filament. But above the filament are two additional electrodes: a metal grid and, at the top of the bulb, a positively charged plate. The heated filament emits a steady flow of electrons, which are attracted to the plate's positive charge. The rate of electron flow can be controlled by the charge on the intervening grid, which means a small electric signal applied to the grid—say, the tiny output of a gramophone—is reproduced in the much stronger electron flow from filament to plate. As a result, the signal is amplified and can be sent to a loudspeaker.

Vacuum tubes suffered a slow death during the 1950s and '60s thanks to the invention of the transistor—specifically, the ability to mass-produce transistors by chemically engraving, or etching, pieces of silicon. Transistors were smaller, cheaper, and longer lasting. They could also be packed into microchips to switch on and off according to different, complex inputs, paving the way for smaller, more powerful computers.

But transistors weren't better in all respects. Electrons move more slowly in a solid than in a vacuum, which means transistors are generally slower than vacuum tubes; as a result, computing isn't as quick as it could be. What's more, semiconductors are susceptible to strong radiation, which can disrupt the atomic structure of the silicon such that the charges no longer move properly. That's a big problem for the military and NASA, which need their technology to work in radiation-harsh environments such as outer space.

"The computer you and I buy is what NASA buys, but they won't want it exactly the same way," says Meyya Meyyappan, an engineer at NASA Ames Research Center at Moffett Field in California. "It takes them a few years to radiation-proof it. Otherwise the computer you put in the space shuttle or the space station basically will get zapped and stop working."

The new device is a cross between today's transistors and the vacuum tubes of yesteryear. It's small and easily manufactured, but also fast and radiation-proof. Meyyappan, who co-developed the "nano vacuum tube," says it is created by etching a tiny cavity in phosphorous-doped silicon. The cavity is bordered by three electrodes: a source, a gate, and a drain. The source and drain are separated by just 150 nanometers, while the gate sits on top. Electrons are emitted from the source thanks to a voltage applied across it and the drain, while the gate controls the electron flow across the cavity. In their paper published online today in Applied Physics Letters, Meyyappan and colleagues estimate that their nano vacuum tube operates at frequencies up to 0.46 terahertz—some 10 times faster than the best silicon transistors.

The team's device isn't the first attempt at miniaturizing the vacuum tube. Contrary to previous work, however, the researchers do not need to create a "proper" vacuum: The separation of the source and drain is so small that the electrons stand very little chance of colliding with atoms in the air. This is a huge benefit, says Meyyappan, because it opens the door to mass production.

Electronics engineer Kristel Fobelets at Imperial College London agrees. "Vacuum technology within a semiconductor fabrication line would make fabrication costs very high," she says. Still, she cautions, the nano vacuum tube is more of a "proof of concept" than a working device, since its operational requirements do not yet match modern transistors. As one example, about 10 volts is needed to switch the device on, whereas modern transistors operate at about 1 volt; in this respect, the nano vacuum tube isn't compatible with modern circuits.

Even so, the potential is great, says Meyyappan. The new vacuum tube's inherent immunity to radiation could save the military and NASA a lot of time and money, while its faster operation makes it a rare candidate for so-called terahertz technology. Sitting between the microwave and infrared regions of the electromagnetic spectrum, the terahertz region can pick out the "fingerprints" of certain molecules. The technology could therefore be used at airports to safely scan for illicit drugs, for instance.

So are vacuum tubes poised to make a comeback? Meyyappan thinks so. "We are combining the best of the vacuum," he says, "and the best of what we have learned in the past 50 years about integrated-circuit manufacture."


Vacuum tubes have been retro for decades. They almost completely disappeared from the electronics scene when consumers exchanged their old cathode ray tube monitors for flat screen TVs. Their replacement – the semiconductor – is generally the cheaper, lighter, more efficient, and easier to manufacture of the two technologies. But vacuum tubes are more robust in high-radiation environments such as outer space. And since electrons travel faster in a vacuum than through a semiconductor, vacuum tubes are an intrinsically better medium for electricity.

An international team of researchers from NASA's Ames Research Center in Moffett Field, Calif., and the National Nanofab Center in Korea have combined the best traits of both technologies by making a tiny version of vacuum tubes that could be incorporated into circuits. Their prototype, a vacuum channel transistor, is just 150 nanometers long and was made using conventional semiconductor fabrication methods. Its small size allows it to operate at fewer than 10 volts, much less than a retro vacuum tube requires; with further work, the device could be made to use about 1 volt, which would make it competitive with modern semiconductor technology.

In a paper accepted to the American Institute of Physics' (AIP) journal Applied Physics Letters, the authors write that such a transistor could be useful for applications in hazardous chemical sensing, noninvasive medical diagnostics, and high-speed telecommunications, as well as in so-called "extreme environment" applications for military and space.


Although vacuum tubes were the basic components of early electronic devices, by the 1970s they were almost entirely replaced by semiconductor transistors. But in the past few years, researchers have been developing "nanoscale vacuum channel transistors" (NVCTs) that combine the best of vacuum tubes and modern semiconductors into a single device.

Compared to conventional transistors, NVCTs are faster and more resistant to and radiation. These advantages make NVCTs ideal candidates for applications such as radiation-tolerant deep space communications, high-frequency devices, and THz electronics. They are also candidates for extending Moore's law—which states that the number of transistors on a computer chip doubles approximately every two years—which is expected to soon hit a roadblock due to the physical limitations of shrinking semiconductor transistors.

On the other hand, traditional tubes have certain disadvantages compared to semiconductor transistors, which caused them to become obsolete. Notably, vacuum tubes are very large and consume a lot of energy.

With the new NVCTs, size is no longer an issue because the new devices are produced using modern semiconductor fabrication techniques, and so can be made as small as a few nanometers across. Whereas traditional vacuum tubes look like light bulbs, NVCTs look more like typical and can only be seen under a scanning electron microscope.

To address the more pressing issue of energy consumption, in a new study researchers Jin-Woo Han, Dong-Il Moon, and M. Meyyappan at the NASA Ames Research Center in Moffett Field, California, have designed a silicon-based NVCT with an improved gate structure that reduces the drive voltage from tens of volts to less than five volts, resulting in a lower energy consumption. Their work is published in a recent issue of Nano Letters.

In an NVCT, the gate is the component that receives the drive voltage and, based on this voltage, it controls the flow of electrons between two electrodes. In contrast, in the old , electrons were released by heating the emitter of the device. Because the electrons traveled through a vacuum (the vacuum gap), they moved at very high speeds, which led to the fast operation.

In NVCTs, there is not actually a vacuum, but instead the electrons travel across a space filled with an inert gas such as helium at atmospheric pressure. Since the distance between electrodes is so small (as little as 50 nm), the probability of an electron colliding with a gas molecule is very low, and so the electrons move just as quickly through this "quasi-vacuum" as they do in an actual vacuum. Even with some collisions occurring, the gas molecules are not ionized due to the lower operating voltage.

Perhaps the greatest advantage of the new vacuum transistors is their ability to tolerate high temperatures and ionizing radiation, which makes them promising candidates for the harsh environments often experienced by military and space applications. In the new study, the researchers experimentally demonstrated that the NVCTs continue to operate at the same level of performance at temperatures of up to 200 °C, whereas conventional would cease to function at this temperature. Tests also showed that the new NVCTs are robust against gamma and proton radiation.

In the future, the researchers plan to further improve the performance of this "new old" technology.

"Future research plans include device modeling work at the nanoscale, including structure and material properties," Han told "Also we plan to study aging mechanisms to improve reliability and lifetime."


A nanoscale vacuum-channel transistor (NVCT) is a transistor in which the electron transport medium is a vacuum. In a traditional solid-state transistor, a semiconductor channel exists between the source and the drain, and the current flows through the semiconductor. However, in a nanoscale vacuum-channel transistor, no material exists between the source and the drain, and therefore, the current flows through the vacuum.

Theoretically, a vacuum-channel transistor is expected to operate faster than a traditional solid-state transistor, and have higher power output. Moreover, vacuum-channel transistors are expected to operate at higher temperature and radiation level than a traditional transistor making them suitable for space application.

Although vacuum-channel transistors are not currently commercially available, several implementations and prototypes of vacuum-channel transistors are reported in the literature such as vertical field-emitter vacuum-channel transistor, gate-insulated planar electrodes vacuum-channel transistor, vertical vacuum-channel transistor, and all-around gate vacuum-channel transistor.


The concept of using field-emitted electron beam in a diode was first mentioned in a 1961 article by Kenneth Shoulders. However, due to the technological difficulty of fabricating a field-emitter electron source, such a diode was not implemented.

As the field of microfabrication advanced, it became possible to fabricate field-emitted electron sources, thereby paving the way for vacuum-channel transistors. The first successful implementation was reported by Gary et. al. in 1986. However, early vacuum-channel transistors suffered from high gate threshold voltage and couldn't compete with solid-state transistors.

More recent advances in microfabrication have allowed the vacuum-channel length between the source and the drain to be shrunk, thereby significantly reducing the gate threshold voltage below 0.5V, which is comparable to the gate threshold voltage of current solid-state transistors.

As the shrinking of solid-state transistors is reaching its theoretical limit, vacuum-channel transistors may offer an alternative.

Simplified operation

A nanoscale vacuum-channel transistor is essentially a miniaturized version of a vacuum tube. It consists of a field-emitter electron source, a collector electrode, and a gate electrode. The electron source and the collector electrodes are separated by a small distance, usually of the order of several nanometers. When a voltage is applied across the source and the collector electrode, due to field-emission, electrons are emitted from the source electrode, travel through the gap and are collected by the collector electrode. A gate electrode is used to control the current flow through the vacuum-channel.

Despite the name, vacuum-channel transistors do not need to be evacuated. The gap traversed by the electrons is so small that collisions with molecules of gas at atmospheric pressure are infrequent enough not to matter.


The nanoscale vacuum-channel transistors have several benefits over traditional solid-state transistors such as high speed, high output power, and operation at high temperature and immunity to strong radiations. The advantages of a vacuum-channel transistor over a solid-state transistor are discussed in detail below:

High speed

In a solid-state transistor, the electrons collide with the semiconductor lattice and suffer from scattering which slows down the speed of the electrons. In fact, in silicon, the velocity of electrons is limited to 1.4×107 cm/s. However, in vacuum electrons do not suffer from scattering and can reach a velocity of up to 3×1010 cm/s. Therefore, a vacuum-channel transistor can operate at a faster speed than a silicon solid-state transistor.

Operation at high temperature

The band-gap of silicon is 1.11eV, and the thermal energy of electrons should remain lower than this value for silicon to retain its semiconductor properties. This places a limit on the operating temperature of silicon transistors. However, no such limitation exists in vacuum. Therefore, a vacuum-channel transistor can operate at a much higher temperature, only limited by the melting temperature of the materials used for its fabrication. The vacuum-transistor can be used in applications where a tolerance to high temperature is required.

Immunity to radiation

The radiation can ionize the atoms in a solid-state transistors. These ionized atoms and corresponding electrons can interfere with the electron transport between the source and collector. However, no ionization occur in the vacuum-channel transistors. Therefore, a vacuum-channel transistor can be used in a high radiation environment such as outer space or inside a nuclear reactor.


The performance of a vacuum-channel transistor depends upon the field emission of electrons from the source electrode. However, due to the high electric field, the source electrodes degrades over time, thereby decreasing the emission current. Due to the degradation of electrons source electrode, vacuum-channel transistors suffer from poor reliability.


Radiation Shielding

The three anti-radiation protection methods are Time, Distance, and Shielding. Time means minimize the duration of exposure, Distance means get as far away from the radiation source as you can, and Shielding means get some radiation armor between you and the radiation source.

Remember, outside the engine room hatch will be a decontamination booth. And I'm sure over the hatch will be mounted an alarm with a red rotating light, so you don't have to put your ear on the bulkhead to hear Astro say "Oh SH*****T!!!". Past the hatch will be a radiation-shielded corridor, with a dog-leg bend in it, so you can get in but radiation cannot get out (radiation has to travel in straight lines, but crewmen can zig-zag). Be sure you are wearing your dosimeter.


Shortening the duration of exposure is difficult to do. A nuclear engine burn for a specific amount of delta V takes as long as it does, it cannot be shortened. You cannot stop a solar proton storm in progress to shorten exposure. About the only thing you can realistically do is make sure you perform any vital repairs in a radioactive zone as quickly as possible, and if you unexpected discover something radioactive nearby you should run away.

Difficult as it is, NASA scientists were looking into duration shortening strategies for a proposed Mars mission. Using a minimum delta V / maximum duration Hohmann trajectory the trip from Terra to Mars will take the better part of eight months. The Mars Science Laboratory actually traveled the route and measured the cosmic ray radiation exposure with the RAD. The scientists were aghast when they found that the round trip dose was much higher than they estimated, about 0.66 sieverts round trip (about 1.8 milliSieverts per day). Which is bad news if the career radiation limit for astronauts is 1.0 sieverts.

They tried to design radiation shielding that would reduce the cosmic ray exposure to something reasonable. Unfortunately this cut so deeply into the payload mass that there basically wasn't a mission any more.

But now the are focusing on reducing the time of radiation exposure. How? By developing more powerful propulsion systems like VASIMR. If such propulsion can increase the spacecraft's delta V capacity enough, it can afford a trajectory with trip duration reduced. Which would shorten the duration of cosmic ray radiation exposure.


There was some 1950's era spacecraft designs that attempted to substitute distance for lead shielding (since distance weighs nothing) thus utilizing the inverse-square law. They were practical designs for exploration craft but pretty silly for warships. Crew cabins on the end of hundred meter booms, or dangling at the end of kilometer long cables, that sort of thing. While you wouldn't want to use the designs, you understand the motivation. When every gram is limited, you don't want to waste it on a shield made out of one of the heaviest elements in existence.

The break-even point is where the mass of the boom or cable is equal to the mass of the shadow shield. Meaning that distance doesn't exactly weigh nothing, but you get one level of radiation reduction with far fewer kilograms than you'd need if using a heavy radiation shield. One source suggested that occurred at a cable length of one kilometer with a one megawatt reactor (reduction factor of about 10-6).


Mile-Long Starship: Some bigger classes easily fall into this category or above: dreadnoughts and superdreadnoughts, grapeship megafreighters, the top end of highliners, colony seedships, mobile factories, that sort of thing, and – of course – city-ships.

Special note here to most lighthuggers, which have to accommodate vast quantities of deuterium and antideuterium and whose antimatter-pion-torch engines are so ridiculously lethal to be near that you want them on the end of a very long spine indeed.

Flyaway Engine

This crazy/brilliant concept was described in Lockheed's study of the Class 3 Resuable Nuclear Shuttle. Sources are Nuclear Shuttle Systems Definition Study Phase III Volume 2 part A, and Volume 2 part B.

The Class 1 reusable nuclear shuttle was designed to be fully assembled on Terra, then launched into orbit on a huge booster rocket. The Class 3, however, was to be boosted piecemeal and assembled in orbit. So every component was prefereably sized to fit in the Space Shuttle cargo bay.

The Problem

Due to neutron activation, each time you run the engine reactor it transmutes more and more of the engine structure into radioactive isotopes. This will make the engine emit harmful gamma rays even when it is totally turned off. Which makes the nuclear shuttle difficult to refurbish. At least without killing the maintenance crew.

For the grim details, go here.

The Solution

Some engineer started thinking outside of the box, and came up with a solution so crazy it just might work.

The class 3 nuclear shuttle has to be assembled in orbit, right? So they designed a special little drone thingy that latches onto the nose of the nuclear engine. The drone acts like a micro-tug, using little attitude jets to drag the engine into its place on the aft end of the shuttle.

However, it is impractical for the drone to get out of the way, being in between the aft of the shuttle and the fore of the engine. It is providing all the careful course changes to ensure the engine attaches firmly. It is easier if the drone stays attached to the fore of the engine, and docks to the aft of the shuttle. All the propellant pipes pass through the body of the drone. It will become an integral part of the nuclear shuttle. A pity that at this point it is pretty much worthless penalty mass.

The engineer got a pondering look in their eyes, and figured out how to make a liability into an asset.

Nuclear shuttle performs a mission. After the mission it returns to its parking orbit, with the engine dangerously radioactive.

Here's the clever part. The drone then detaches itself and the engine from the aft of the shuttle. It then moves the engine a few hundred kilometers away from the shuttle. Now the maintenance crew can safely refurbish the shuttle, with the radioactive engine far enough away to poise no danger.

When it is time for the shuttle to perform a new mission, the drone reattaches the engine. Brilliant, eh?

RocketCat sez

This. Is. AMAZING.

The sheer unmitigated audacity takes my breath away. The thinking-outside-the-box. The elegance of solution. The triumph of modular construction. The implications for ship repair. The possibilities for rocket engine robbery scifi plots. The gall of the researchers to actually present this concept to their bosses as a serious solution to the problem.

And the basic solution ain't all that far-fetched. If you can assemble once a freshly launched RNS with this flyaway gizmo, you certainly can take it apart and put it back together as many times as you like (as long as you top off the attitude jet fuel tanks). And the drawback of using distance as a substitute for radiation shielding is the mass cost of the long ship's spine. But this doesn't need a spine, there isn't any mass cost. Just park the blasted engine a few hecto-klicks retrograde in parking orbit and you are golden.

I can already see the wheels spinning in the minds of devious science fiction authors, with all sorts of amusing possibilities.

Honest Sherrie's Used Nuclear Engines will have a flock of these in stockpile orbit, ready to swap out a used or damaged engine when the tow-truck space tug hauls in a disabled spaceship for servicing. But Sherrie had better have top-of-the-line security on the flyaway computers. Otherwise a thief can remotely hack one and command the flyaway to steal itself.

Or a sinister villain can hack a flyaway and deorbit the radioactive engine to crash onto a major metropolitan area, for ransom or terrorist purposes. This may happen with the first prototype of the RNS, if NASA does not do a deep enough threat analysis.

4.6.2 Autonomous Engine Handling System Summary

Considerable effort was expended in refining and expanding the autonomous handling system for the NERVA engine during earth and lunar orbital operations. The premise is that the autonomous system will accomplish the following:

  • Separate from the RNS
  • Propel the engine on command to a specified distance away from the RNS
  • Maintain a specified station
  • Return to the RNS on command
  • Rendezvous, align, and dock to the RNS

This analysis has indicated that the concept is feasible; however, it should be noted that there was no attempt to optimize the system or to conduct trade analyses of variations in the basic concept.

The self-contained approach requires a separable engine interface module (fly-away module) as an integral part of the engine thrust cone structure. All power, propulsion, control, and communications subsystems required for engine handling would be included in this module and separated with the engine when removal is required. Operations

Initial On-orbit Vehicle Assembly. It is desirable to have the initial assembly point coincide with the remote separation joint. If the RNS structure is powered up and has minimum maneuvering capability, the entire assembly could be released from the Space Shuttle at a stand-off location and the automatic docking system used to fly the assembly in. No EVA should be required.

Engine Removal — Docking. The engine should be remotely separable and attitude stabilized (internally) for flyaway operations. Using the candidate control system as a reference, the flyaway maneuvers could be done in the following manner:

After main engine propellant lines are vented, the flyaway system is switched to internal power and a fault/status check made by the RNS computer. If all systems check, the engine module is armed and the remote separation joint is uncoupled. The RNS laser engine docking radar is slaved to track the receiver and the RNS computer used to compute maneuvering commands for the engine. During the separation, the computer will generate error signals (relative velocity, pitch, and yaw in terms of line-of-sight) to be coded PCM on the laser signal and demodulated by the module receiver. If two 25 lbF (111 N) thrusters are used to generate the 2 ft/sec separation rate, the acceleration time will be reduced to under 30 sec, simplifying the control problem. During the transit time (~ 1.5 hr.) the engine will be attitude stabilized by the line-of-sight (LOS) angle errors (XplXy) measured by the module laser receiver. After the engine has reached its storage station and has been stopped, all residual translation (relative to the RNS) will be removed to within the accuracy of the laser radar (3 cm/sec). Precise attitude and station will be determined by recursive filtering the position O/P of the laser radar over a time period (10-30 min), at the end of which all remaining residual rates will be removed. It should be possible to execute this maneuver with sufficient accuracy to reduce the residual relative drift rates between the engine and RNS to less than 1000 ft/2 weeks, eliminating the requirement for station-keeping maneuvers during that time span. At this point the engine system can be turned off (with the exception of the command receiver and rate limiting stabilization circuits) to conserve power.

During storage the engine would not be attitude stabilized but rate limited to prevent excessive rotation from building up. If analysis indicates these rates for a two-week period would not be excessive (or a rate damper is added, 5-10 lb), the rate sensor could be turned off and power reduced to less than 1 watt.

Retrieval. When it is desired to return the engine, a two-stage acquisition is required. The RNS laser must first acquire the 360-deg coverage corner reflectors on the engine. This should only take a few seconds. When the engine is acquired, a turn-on-acquisition command is transmitted via TLM to the engine. At that point the acquisition sensors (small solid-state IR detectors) coupled in an analog bridge will point the laser receiver toward the transmitted beam (pulsed mode). When the LOS angle is less than 15-deg the receiver will acquire the beam and the return maneuver will essentially be a reversal of the flyaway. During terminal docking, the measured roll misalignment (relative roll index) angle will be transmitted to the engine along with rate commands. The control sources (error signals) will be LOS angle of both vehicles, that is, the RNS will be maneuvered in pitch and yaw so that the LOS angle is zeroed, and the engine will be maneuvered in pitch, yaw, roll and longitudinal axis AV such that LOS angle and relative roll index are zeroed and the LOS commanded closing rate is maintained. After docking is complete, the flyaway module RCS propellant is resupplied from the RNS main RCS storage tanks, and the Ni-Cd batteries are recharged. Equipment in the engine module will be located so that maximum radiation shielding will be derived from the battery and propellant tanks.

by Lockheed Missiles & Space Co.(1971)


The engine flyaway module structure is shown in Fig. 2-8, It houses all the engine flyaway equipment, and is bolted to the interface of the NERVA engine. The structure of the flyaway module is a conical shell with skin and internal rings of 7075-T6 aluminum alloy. It has a major diameter of 93 in. and minor diameter of 58 in. by 46 in. long. A forward closure ring is used for orbital interface assembly with the RNS propulsion tank thrust structure. The aft closure ring is bolted in 12 places at 56 in. diameter to the NERVA engine upper thrust structure interface. The weight of the structure (2.11) is 375 lb.


The primary engine system for the RNS consists of the NERVA engine and an engine flyaway control module shown in Fig. 3-2. The engine is completely defined in Reference 3-1. The basic characteristics of the engine are:

Thrust = 75,000 lb
Specific Impulse = 825 sec
Propellant Flow Rate = 90.91 lb/sec
Weight (5.01) =27,728 lb

The engine flyaway control module provides the capability to fly the engine away in an autonomous mode. This module is a completely self-contained propulsive system that is attached to the engine on the ground. This module and the engine are launched in the Space Shuttle as a unit and are then connected to the propulsion module tank to complete the propulsion module.


3.3.1 Module Requirements

This self-contained approach for the module requires a separable engine interface adapter as an integral part of the engine thrust cone structure. All power, propulsion, control, and communications subsystems required for engine haadling would be included in this module and separated with the engine when removal is required. The following design goals were utilized:

  • Totally autonomous operation (i.e., no EVA or manual control)
  • Simple maintenance/resupply
  • RNS systems commonality
  • Power/weight efficiency
  • Technical feasibility

3.3.2 Module Description

Initial In-orbit Vehicle Assembly. It is desirable to have the initial assembly point coincide with the remote separation joint. If the RNS structure is powered up and has minimum maneuvering capability, the engine assembly could be released from the Space Shuttle at a stand-off location and the automatic docking system used to fly the assembly in. No EVA should be required.

Engine Removal — Docking. The engine should be remotely separable and attitude stabilized (internally) for flyaway operations. Using the candidate control system. Fig. 3-3, as a reference, the flyaway maneuvers could be accomplished in the following manner:

After main engine propellant lines are vented, the flyaway module is switched to internal power and a fault/status check made by the RNS computer. If all systems check, the engine module is armed and the remote separation joint is uncoupled. The RNS laser engine docking radar is slaved to track the receiver and the RNS NG&C computer used to compute maneuvering commands for the engine. During the separation, the computer will generate error signals (relative velocity, pitch and yaw in terms of LOS) to be coded PCM on the laser signal and demodulated by the module receiver. If two 25-lbF thrusters are used to generate the 2 ft/sec separation rate, the acceleration time will be reduced to under 30 sec, simplifying the control problem.

During the transit time (1.5 hr.) the engine will be attitude stabilized by the LOS angle errors XplXy) measured by the module laster receiver. After the engine has reached its storage station and has been stopped, all residual translation (relative to the RNS) will be removed to within the accuracy of the laser radar (3 cm/sec). Precise attitude and station will be determined by recursive filtering the position O/P of the laser radar over a time period (10-30 min) at the end of which all remaining residual rates will be removed. It should be possible to execute this maneuver with sufficient accuracy to reduce the residual relative drift rates between the engine and RNS to less than 1000 ft/2 weeks, eliminating the requirement for station-keeping maneuvers during that time span. At this point the engine system can be turned off (with the exception of the command receiver and rate limiting stabilization circuits) to conserve power. During storage the engine would not be attitude stabilized but rate limited to prevent excessive rotation from building up. If analysis indicates these rates for a two-week period would not be excessive (or a rate damper is added, 5-10 lb), the rate sensor could be turned off and power reduced to less than 1 watt, cutting battery weight by two-thirds.

Retrieval. When it is desired to return the engine, a two-stage acquisition is required. The RNS laser must first acquire the 360-deg coverage corner reflectors on the engine. This should only take a few seconds. When the engine is acquired, a turn on-acquisition command is transmitted via telemetry to the engine. At that point the acquisition sensors (small solid-state IR detectors) coupled in an analog bridge will point the laser receiver toward the transmitted beam (pulsed mode). When the LOS angle is less than 15 deg, the receiver will acquire the beam and the return maneuver will essentially be a reversal of the flyaway. During terminal docking, the measured roll misalignment (relative roll index) angle will be transmitted to the engine along with rate commands. The control sources (error signals) will be LOS angle of both vehicles; that is, the RNS will be maneuvered in pitch and yaw such that the LOS angle is zeroed, and the engine will be maneuvered in pitch, yaw, roll and longitudinal axis ΔV such that LOS angle and relative roll index are zeroed and the LOS commanded closing rate is maintained. After docking i s complete, the propellant is resupplied from the RNS main RCS storage tanks through normally vented plumbing, and the Ni-Cd batteries are recharged. Equipment in the engine module will be located such that maximum radiation shielding will be derived from the battery and propellant tanks.

3.3.3 Module Characteristics

The functional systems required to provide the capabilities of the engine flyaway module are as follows:

Battery Sizing. Peak power requirements during separation will be about 25-30 watts for 2 to 4 hours. During storage, the power required will be about 3 to 4 watts for 2 weeks. Thus, total power required for the mission (including retrieval maneuvers) will be about 56 amp-hr (28V system), or about 1.6 Kw hours. If the rechargeable battery is sized for a 2. 5 margin (to prolong useful life) the Ni-Cd battery will be sized for 4. 0 Kw-hr.

Table 3-1 is a weight statement of the flyaway module functional systems, radiation shield and structure.

The arrangement of the components in the module is shown in Figure 3-4.

by Lockheed Missiles & Space Co.(1971)


Remember the basic strategy. Use dense elements like lead, tungsten, and beryllium for x-ray and gamma-ray shielding. Use low-density elements like liquid hydrogen, dehydrated astronaut poo, lithium hydride, paraffin, hydrogenated polyethylene composite, or other hydrogen-rich compounds for particle radiation shielding.

Why? X-rays and gamma-rays are stopped by electrons, and high density elements have more electrons per cubic centimeter. Particle radiation is stopped by atomic nuclei and low density elements have more atomic nuclei per cubic centimeter than metals.

When shielding against neutron particle radiation, instead of using hydrogen compounds it is better to use neutron reflectors such as graphite, beryllium, steel, and tungsten carbide.

Beware of bremsstrahlung. If you place your shielding improperly you'll convert the inside of the spacecraft into a giant x-ray machine and fry your astronauts to death. To avoid this make sure the outermost shield layer (next to deep space or other radiation source) is particle shielding and the innermost shield layer (next to the astronauts) is the x-ray/gamma-ray shield layer. As a side note, only charged particles like protons and electrons cause bremsstrahlung, neutrons do not.

Why? Gamma-ray shielding is worse than useless against particle radiation. Charged particles hitting dense elements is the same operating mechanism used inside a dentist's x-ray machine. So if you have the gamma-ray shielding outermost, this is what happens:

  1. A charged particle from deep space slams into the x-ray/gamma-ray shielding
  2. This generates deadly x-rays which are emitted by the x-ray shielding
  3. The new x-rays find there is no x-ray shielding in front of them, only pathetic particle shielding
  4. The x-rays sail unharmed through the particle shielding, and kill the astronauts

When you place your shielding the right way, the particle shielding soaks up the particle radiation before it can hit the gamma-ray shielding.

In an attempt to avoid the penalty mass of particle shields, researchers are looking into diverting particle radiation using zero-mass bubbles of plasma or powerful magnetic or electrostatic fields.

Philip Eklund points out that in spacecraft combat an Orion drive rocket has built-in radiation armor. But it only works if you can keep the pusher plate aimed at the nuclear warhead. If you can manage that, you can laugh at most nuclear detonations.

On the other hand, there are certain propulsion systems that undergo catastrophic failure (i.e., they blow up) if minor damage happens to the fuel tanks. These include antimatter rockets, Zubrin's NSWR, and any form of metastable fuel.

Shield Rating

Radiation shielding is rated in "Tenth Value Thickness" or TVT. One TVT is the depth of shielding required to reduce the radiation to one tenth of its initial value (i.e., it stops 90% of the radiation). Twice the TVT will reduce the radiation to one one-hundredth of its initial value (stops 99%), and so on.

Sometimes one will encounter "Half Value Thickness" and "1/e". HVT is the depth required to reduce the radiation by one-half, and 1/e is the depth required to reduce the radiation to approximately 37% (specifically to 1/e where e is approximately 2.718).

Water has a TVT of 25.4 centimeters vs particle radiation (including neutrons), but only 61 centimeters vs gamma rays. Lead has a TVT of 5 cm vs gamma, and basically doesn't do diddly-squat vs particle radiation. Steel has a TVT of 11 cm vs gamma and also does poorly vs particle radiation. By my calculations carbon should have a TVT of 22.5 cm vs gamma rays, but I have no idea what its TVT is vs neutrons.

X-ray and gamma-ray shielding boils down to how much mass is between the radiation source and the crew. 45 g/cm2 is a TVT (meaning that behind each square centimeter of shield surface area is 45 grams worth of shield material of a thickness determined by the material's density). As a wild guess, the interior of the spacecraft has a density of about 0.25 grams per cubic centimeter. This means a crew member would get a "free" 1 TVT from X and gamma-rays if they were 1.8 meters from the hull, from the shielding provided by the bulkheads, machinery, pipes, and structural materials (45 g/cm2 / 0.25 g/cm3 = 180 cm). Keep in mind that 1 TVT isn't all that much, and the free shielding obviously goes up the further from the hull the crew is. This is an argument for putting the control room of a combat spacecraft near the center.

Cosmic rays will need a TVT of about 450 g/cm2. You will need 450 g/cm2 to get OSHA-legal exposure limits on a timescale of years, say for a space colony or long duration space mission.

Extremely high energy particle beam weapons act like cosmic rays, with a TVT peaking at a whopping 100 to 300 g/cm2.

If you have a thickness which stops a known amount of radiation of a known and constant type, then if you have a new thickness, you can calculate how much it stops by:

Stoppage = 1 - ((1 - AmountStoppedByKnownThickness)^(NewThickness / KnownThickness))

Example: if six centimeters of polka-dotted Kryptonite will stop 90% of x-rays, then eighteen centimeters of polka-dotted Kryptonite will stop:

Stoppage = 1 - ((1 - AmountStoppedByKnownThickness)^(NewThickness / KnownThickness))
Stoppage = 1 - ((1 - 0.9)^(18 / 6))
Stoppage = 1 - (0.1^3)
Stoppage = 1 - 0.001
Stoppage = 0.999 = 99.9%

Just to make our lives more difficult, mixed radiation such as is found in space has verying penetration. So if shield material X stops 90% of the quote "radiation" unqote, this will mean something like stopping 99% of the low-penetrating radiation and 50% of the high-penetrating radiation. And doubling the thickness of the shielding might only bring the radiation stoppage up to aroung 96%.

For comparison purposes, a typical NASA space suit has 0.25 g/cm3, the hull of an Apollo command module is rated at 7 to 8 g/cm3, the Space Shuttle is rated at 10 to 11 g/cm3, the storm cellar of the International Space Station is rated at 15 g/cm3, and future lunar bases are planned to exceed 20 g/cm3.

Now to calculate the radiation that penetrates a shield:

Rd = Rh * Vf(Ad / Vd)


  • xy = raise x to the power of y
  • Rd = Radiation dose that penetrates the armor (grays or whatever)
  • Rh = Radiation strength hitting the armor (grays or whatever)
  • Vf = Value Factor (0.1 for TVT, 0.5 for HVT, 0.37 for 1/e)
  • Ad = Armor depth (centimeters or whatever)
  • Vd = Value depth (e.g., 61 cm if armor is water and radiation is gamma rays)

PFC Floyd, on a nameless jungle planet, is surprised by a Blortch storm-trooper. Floyd jumps into a pool, dives to the bottom, and holds his nose. The Blortch unlimbers his deadly gamma-ray zap gun and caroms a bolt into the pool.

Poor Floyd has only three feet of water (91 centimeters) between him and the 100 Gray gamma-ray bolt. Water you will recall has a TVT of 61 centimeters vs gamma rays.

  • Rd = 100 * 0.1^(91 / 61)
  • Rd = 100 * 0.1^1.49
  • Rd = 100 * 0.032
  • Rd = 3.2 Grays

As Blortch storm-trooper oozes away, Floyd floats to the surface. Floyd is very ill with radiation sickness, but alive.


An absurdly powerful one-GEV particle beam weapon would have a 1/e of about 100 g/cm2. In Attack Vector: Tactical one armor layer is 5 g/cm2. If a warship had 14 layers of armor, a particle beam strike would be reduced by:

  • Rd = 1 * 0.37^((5 * 14) / 100)
  • Rd = 1 * 0.37^(70 / 100)
  • Rd = 1 * 0.37^0.7
  • Rd = 1 * 0.5
  • Rd = 0.5

in other words, by one-half.

Example (no doubt full of mistakes)

A one kiloton nuclear warhead from the Asteroid Revolutionary Navy goes off fifteen kilometer from the scoutship Peek-a-Boo. At fifteen klicks, the radiation flux will be about 1500 joules per square meter. 90% is x-rays and the rest is neutrons, or 1350 j/m2 x-rays and 150 j/m2 neutrons. The Peek-a-Boo has 5 centimeters of lead around the crew compartment, which is one TVT vs x-rays and zero TVT vs neutrons. Only 135 j/m2 of the x-rays penetrate, but the entire 150 j/m2 neutrons comes sailing on through.

Floyd is the only crewmember. He's about 68 kilos, and 168 centimeters tall, which gives him a surface area of about 1.78 square meters. Figure that Floyd's cross section is one quarter of his surface area, or 0.445 m2. He will intercept 60 joules of x-rays and 67 joules of neutrons. Dividing by his mass we discover that Floyd has been exposed to 0.88 Grays of x-rays and 0.98 Grays of neutrons. This makes a grand total of 1.86 Grays. He will start upchucking his lunch in a few hours but he'll live.



Likewise with radiation, there is only one known way to prevent damage: mass. Place material between you and the radiation source to reduce your exposure. (On Earth you can reduce exposure by adding distance, but that's not feasible in space where the radiation comes from all directions.) The exact amounts needed vary by the type of radiation, type of shielding material, method of exposure and allowable limits. For reference, the background dose on Earth is around 3.6 milliSieverts (mSv) per year. People working in radiation industries (nuclear power, medical imaging, pilots / air staff) are limited to 50 mSv per year. Astronauts face two limits: 500 mSv in a year and a lifetime limit that depends on age and gender but is typically 2,000 to 4,000 mSv. These values are expected to cause no more than a 3% increased risk of developing cancer, which NASA and the astronaut corps considers acceptable.

Colonists are not likely to make more than one round trip (if they even return), but dedicated crew might need to make several. The actual transit takes around six months (varies, could be 5, could be 7 depending on where we are in the cycle), so a single round trip should take on average one year. The deep space radiation environment averages about 740 mSv per year, way over the limit; this needs to be reduced to a manageable level using the least mass possible.

Radiation shielding can be specified in a couple of ways; most spacecraft design studies state it in terms of grams per square meter and assume that all the mass is aluminum. That's useless for an expandable module since there is little to no aluminum in the outer hull. Another way to define it is by attenuation depth; old fallout shelter manuals from the 60's will list halving thicknesses or tenthing depths of soil or concrete, meaning the thickness of a material necessary to cut radiation by half (or by 90%). Modern sources use the number e as the base, so an attenuation of 1 means reducing radiation by 1/e, or about 36.8%. The benefit of using e is that you can easily calculate how much shielding you need by taking the natural log of expected divided by allowable radiation. ln(740/500) is 0.392 units, so we need at least this much shielding for colonists. If we wanted a permanent workforce to operate habitats then the allowable radiation limit would be 50 mSv and the shielding required would be ln(740/50) = 2.69 units.

Different materials provide different levels of shielding. 1 unit of shielding requires 8cm of titanium, 17 cm of aluminum, 60 cm of water or 52 m of dry air at 1 atmosphere. The units are additive, so once you know the value of each layer of hull you can simply add them up and see if it is enough. The minimum value is 0.392 units and more would be better. The expandable hull provides about 0.02 by itself, but if we build in a 24 cm thick layer of water then it will provide about 0.4 units, leaving a tiny bit of margin. (My previous analysis suggested 20cm of water and assumed the hull, hardware and other mass would make up the deficit; this turned out to be wrong.)

These unitless numbers can be difficult to visualize, so let's look at what that means as a percentage. The fraction of radiation that penetrates a shield is 1 / e^(shield value). For our water shield of 0.4 units that's 0.67, or 67%. Another way to think of it is the percentage of radiation blocked, which is 1 - ( 1 / e^(shield value)). For our water shield of 0.4 units that's 0.33 or 33% as one might expect. A 2.7 unit shield suitable for career crew would allow 1 / e^2.7 or 6.7% of radiation through, blocking 93.3%; this would require a 1.6 meter layer of water (as deep as a residential swimming pool), a 46 cm aluminum shield or a 10.5 cm nickel-iron shield. I also have a design for a permanent colony that uses a meter of packed regolith and half a meter of water plus a thin metal shell to provide adequate protection (4 units).

Lastly, the radiation levels in space are not constant. Sometimes there are solar storms that push the levels much higher than normal for a short time. Surviving these storms requires a storm shelter, a secure area on the craft with much higher shielding than the rest of the habitat. For my modules this will be inside the rigid core section, taking advantage of the mass of all the levels and their equipment as well as a second layer of water shielding. This doubles as the water processing storage, so it is mass that was already needed. The ship points the engine at the sun and points all solar panels and radiators parallel, operating on battery power. If high radiation conditions go on for longer than the batteries can sustain then some of the solar panels will be put back into service; this will reduce their lifespan.


Shadow Shield

Shadow shields are specifically to protect the crew from radiation emitted by the spacecraft's nuclear engines and nuclear power plants.

The safe design would be to totally encase the engine and reactors in radiation shielding. But this sharply reduces the ship's payload since radiation shields literally weigh tons, and Every Gram Counts. Shadow shields are the bare minimum of shielding: it only stops the radiation heading for the habitat module and other vital parts of the spacecraft. The radiation freely sprays in all other directions, which makes it dangerous to approach an atomic rocket outside of the safe shadow cast by the shield.

Another implication is that the ship's docking port is probably best placed on the ship's nose. This will allow two ships to dock nose-to-nose, while keeping each other in the shadow of their shadow shields. Alistair Young calls this "Booping", a re-use of the word for "tapping your pet affectionately on the nose."

Note that the larger the distance between the crew and the atomic engine, the narrower the angle the shadow has to be, thus the smaller the shadow shield. Since the shadow shield is several tons of rocket mass that is not payload, the smaller it is, the better. Also note that radiation weakens with distance due to the inverse-square law, which is another argument in favor of plenty of distance. As previously mentioned, if the rocket has multiple atomic engines one wants them clustered closely or they will require a larger shadow shield, or even one shield for each engine. (for "cluster closely" read: "have the radioactive components as close to the axis of the spacecraft as possible")

I have seen a few Mars mission studies that try to use the shadow shield to protect against solar proton storms in lieu of a storm cellar. The idea is to point the engine at the Sun when a storm comes and hope it provides the same protection as the penalty-mass laden storm cellar which was omitted from the design. I do not have precise figures on how effective this strategy is, but the fact it is not a standard design feature in all Mars missions tells me the concept has problems.


The weight of a space shield is extremely dependent upon geometry. It is obvious that the shield weight will be a direct function of the solid angle that the shield shadow must subtend. In addition the shield geometry must be such that there are no paths or mechanisms by which radiation can bypass or be shunted around the shield. The presence of any structural material positioned such that it is illuminated by the source and can scatter radiation directly into the payload region can drastically reduce the shield net effectiveness. As a result of this consideration minimum weight shields dictate that the source have a minimum projected area and that all structural members be contained withm the conical region defined by the dose plane and the extremities of the source. The resulting ideal geometry is shown m Figure II-65. It is obvious that this consideration has a major influence on the configuration of a nuclear powerplant.

The approximate weight of LiH shields to limit the neutron dose to 1011 - 1012 over a 5-ft diameter payload section at a distance of 30 ft is shown m Figure II-66 as a function of reactor size and thermal power. It is apparent that reactor source size is a more important consideration than thermal power. Thus there is a prime incentive to minimize reactor size in order to reduce shield weight. However, a minimum weight reactor does not necessarily lead to a minimum weight reactor-shield combination.

From Figure II-65 it is apparent that separation distance has a major influence on shield weight. The shield weight of a typical case is shown in Figure II-67 as a function of separation distance. If the separation incurs an added structure, boom, weight and optimum separation distance for minimum shield-boom weight will result . It should be noted that large separation can also be advantageous for gravity gradient stabilization of a satellite.

From NUCLEAR SPACE POWER SYSTEMS by H. M. Dieckamp (1967)


When the reactor is idling, the shadow shield does not have to be as thick. In order to widen the area of shadow (for adding side tanks or whatever), the secondary shadow shield could extrude segments as extendable side shields.

Assessment of the Advantages and Feasibility of a Nuclear Rocket for a Manned Mars Mission by Dr. Steven D. Howe had a clever idea. It is a better way to widen the area of the shadow.

During a propulsive burn the neutron-stopping part of the shadow shield (Lithium Hydroxide LiH or something) and the gamma-ray-stopping part (Tungsten W or Mercury Hg) have to be on top of the engine, to shadow the rest of the spacecraft. After the burn, the reactor stops emitting neutrons and gamma rays. However, neutron activation has transmuted girders and other engine parts into radioactive isotopes. These will emit dangerous gamma-rays at a gradually decreasing rate. After a couple of days the gamma-ray flux will be reduced by about three orders of magnitude (1/1,000th). Which means a thinner shield can provide the same protection. A pity that the shield is welded in place.

In the report they say Not So Fast, there may be a way. What if you used liquid mercury instead of solid tungsten as the gamma shield? In the diagram above the mercury is in the upper tank during the burn. The thick layer can cope with burn-levels of gamma rays, at the price of only protecting a narrow shadow.

But after the burn, the neutron radiation stops and the gamma-rays are at a reduced level. The mercury is pumped into the lower tank. The mercury shield is a thinner layer which can cope with the reduced gamma-rays. The advantage is a thinner layer means there is more mercury to go around. The lower tank creates a mercury layer that covers more of the engine, expanding the size of the shadow. This could come in handy if the spacecraft is a tail-sitter.

     Shielding the crew from the reactor during the "propulsive burn" can be accomplished by the combination of a tungsten and LiH shield. Further, reduction in the neutron dose to the crew can be accomplished by incorporating a few meters of LH2 in a tank between the crew and engine. This tank, for example, might contain the 15% contingency LH2 and would be the last tank to be used.

     After the full power burn of the engine, the radiation from the reactor will be only gamma-rays and withln a few days the intensity will have dropped by over three orders of magnitude. The thickness of tungsten required to shield the reactor in transit will be substantailly less than for propulsive maneuvers. Thus, the tungsten shield may be designed to "unfold" around the reactor for post burn shielding which will provide a 2π (i.e., 360° or 2π radians) or greater shield around the reactor and allow docking or EVA activity. Another possibility is to use mercury as the gamma-ray shield. Change of configuration Is then accomplished by pumping the Hg Into preformed reservoirs as shown in Figure 2.


So the theory is you calculate effective radiation flux from the atomic engine, multiply it by the appropriate attentuation factors of the shadow shield, and figure the effective dose given the total burn time for the mission. The idea is to keep the effective dose is within acceptable limts.

The effective radiation flux is the atomic engine's total radiation flux reduced by the free shielding of the remoteness of the habitat module

Divide the radiation flux by the square of the distance between the engine and the habitat module to get the effective radiation flux. That is:

Fluxeff = Flux / d2

Flux = engine radiation flux (Severts/sec)
Fluxeff = effective radiation flux (Severts/sec)
d = distance between engine and habitat module (meters)
x2 = square of x (x × x)

Which brings us to the problem of calculating the radiation flux from the atomic engine.

Anthony Jacks says as a quick-and-dirty first order approximation, for a fission reactor, the radiation flux is one-half the power production (in kilowatts) of the reactor core (which is greater than the power output due to inefficiency). This assumes 1 kilowatt of reactor core power produces 1.26 kilowatts of radiaion, mostly neutrons.

Flux = 0.5 * Pcore

Flux = engine radiation flux (Severts/sec)
Pcore = power production of reactor core (kW)

As a rough guess, for atomic engines with a thermal power level of one megawatt to one gigawatt, the shadow shield will be from 1.0 to 0.1 kilograms per kilowatt. This assumes that the spacecraft is long and skinny, which reduces the angular size of the shield. The shield will be a composite of gamma ray shielding materials and neutron shielding materials.

In Space Propulsion Analysis and Design they give the specs on a typical shadow shield. Starting at the atomic engine, the gamma rays and neutrons first encounter 18 centimeters of beryllium (which acts as a neutron reflector), followed by 2 centimeters of tungsten (mainly a gamma-ray shield but also does a good job on neutrons), and finally 5 centimeters of lithium hydroxide (To stop the remaining neutrons. Hydrogen slows down the neutrons and lithium absorbs them.). This attenuates the gamma flux to a value of 0.00105, and neutron flux to 4.0e-9. This has a mass of 3,500 kilograms per square meter of shadow shield (ouch!).

For a rough estimate of the surface area of the shadow shield it should be a disk with a radius equal to the radius of the reactor core.

To estimate the size of the core is over my head but it is covered in SPAD. In the example from the book a 1000 megawatt reactor had an radius of 0.45 meters so the shield surface area is 0.64 m2. The mass would be 3,500 * 0.64 = 2,240 kg.

A 2000 MW had a radius of 0.75 meters, surface area 1.77 m2, mass 6,195 kg.

If an attenuation factor of 0.00105 for gamma and 4.0e-9 for neutrons is not enough, the factors can of course be increased by adding more thickness to the layers in the shadow shield. The SPAD has a handy table which I adapted:

Additional cm of
Lithium Hydroxide
for Neutron Attenuation
Additional cm of
Tungsten for
Gamma Attenuation
x0.5+0.205 cm
(+2.99 kg/m2)
+0.564 cm
(+109 kg/m2)
x0.2+0.477 cm
(+6.96 kg/m2)
+1.308 cm
(+252 kg/m2)
x0.1+0.683 cm
(+9.97 kg/m2)
+1.872 cm
(+361 kg/m2)
x0.01+1.365 cm
(+19.92 kg/m2)
+3.744 cm
(+722 kg/m2)
x0.001+2.048 cm
(+29.90 kg/m2)
+5.616 cm
(+1084 kg/m2)

Say a gamma attenuation factor of 0.00105 is not enough, you need 0.000525. This is a reduction factor of 0.000525 / 0.00105 = x0.5. Looking this up in the table reveals that the shield will need an additional 0.564 centimeters of tungsten, for a grand total of 2.0 + 0.564 = 2.564 centimeters. This will increase the mass of the shadow shield to 3,500 + 109 = 3,609 kg/m2. So a 1000 MW reactor would have a shadow shield mass of 0.64 * 3,609 = 2,309 kg


Many factors influence the geometry, composition, and mass of the radiation shield, including:

  • Size and nature of the power source
  • Type of radiation
  • Configuration of the spacecraft or platform and its payload (radiation flux level decreases by a factor of 1/distance2 from the radiation source)
  • Generic operational procedures and requirements for the mission
  • Length of the mission
  • Total permitted levels of radiation dosage

To design the shield correctly, we need to compare the radiation flux from the engine with the allowable dose. For example, if the engine is releasing 107 rem per year and the payload is allowed a dose of only 10 rem per year, we need to attenuate the radiation by a factor of 106. For shield design in a nuclear rocket, we usually attenuate the radiation flux to avoid excess propellant heating while the propellant is still in the tank. The structure of the tank, the propellant, and the additional distance of the payload from the radiation source then reduce the radiation further (see Fig. 8.22).

By combining different shielding materials, we can tailor the shield to a particular form of radiation. For example, usually a shadow shield (shields the reactor from the tank) is directly under the tank. Figure 8.23 shows a typical cross section of a shadow shield. In this example, the radiation first sees a neutron reflector material such as beryllium (Be). Next, it encounters a thin layer of heavy material used to shield gamma rays. Tungsten (W) is a good candidate for this part of the eld because it has a high neutron-absorption cross section plus high gamma attenuation. Finally, a lighter-weight material finishes attenuating the neutron flux. We often suggest using lithium hydride (LiH2) because it has good neutron-slowing properties from the hydrogen component and a high neutron-absorption cross section from the lithium component.

Table 8.11. Physical Properties of Shielding Material and Effectiveness for Sample Shield Layout
Parameter/Shield MaterialBeWLi-H2
Density (ρ — kg/m3)185019,300500
Molecular weight (gm/mol)9.01183.86Li—6.94;
σa - absorption cross section (barn)0.00919.2Li—71;
Attenuation factor for 1MeV gamma rays
(μ — cm-1)
Shield thickness (cm)1825
Gamma attenuation of incident beam0.15380.08540.8011
Number density (atom/(barn·cm))0.1240.0630.047
Σa - absorption cross section (cm-1)0.0011.2143.373
Attenuation of incident neutron beam0.98020.08824.7×10-8
Integrated gamma-flux reduction0.15380.001310.00105
Integrated neutron-flux reduction0.98020.08644.0×10-9

(ed note: Fig. 8.23 says tungsten (W) is 5 cm thick, table 8.11 says 2 cm thick. I cannot quite calculate which one is the correct figure.)

Table 8.11 summarizes the properties and resultant radiation attenuation across each thickness and for the overall shield. Because the size and mass of shields are significant, we keep the layers as thin as possible for an effective shield. But the shield shown in Fig. 8.23 can reduce the gamma ray flux by a multiplication factor of 0.001 05 and the neutron flux by 4.0 (10)-9, which is adequate for most applications. We can use the data in Table 8.12 to increase or decrease the attenuation. For example, adding 1.872 cm of tungsten further decreases the gamma-ray flux by a factor of 10.

The information in the last two lines is most important. As gamma rays pass through the complete shield (Fig. 8.23), they are attenuated by a factor of 1000 (0.00105). Similarly, neutrons are attenuated by 9 orders of magnitude (4×10-9).

For preliminary design, we use the shield cross section shown in Fig. 8.23 as our baseline. This shield configuration has a mass of 3,500 kg /m2.

(ed note: I tried calculating the mass but came up with 969 kg/m2. I am unsure what I am doing wrong.)

As a first cut in sizing our shield, we assume it has a radius equal to that of the reactor core (Rcore).

(ed note: I'm still trying to figure out how to calculate Rcore. The math in the book is rather dense. A supplied graph showed a 1000 megawatt reactor had an Rcore of 0.45 meters for a surface area 0.64 m2 and a mass of 3,500 * 0.64 = 2,240 kg. A 2000 MW had a radius of 0.75 meters, surface area 1.77 m2, mass 6,195 kg.)

We can change this baseline shield configuration a bit for lower or higher reactor power, burn duration, and reactor type. However, scaling this shield is highly non-linear and requires complicated computer analysis. The simplified approach we discuss here gives us an adequate estimate.

Table 8.12. Radiation Attenuation for Shielding Example
cm Thickness Required for
Neutron Attenuation
cm Thickness Required for
Gamma Attenuation
( )
( )
( )
( )
( )
( )
( )
( )
( )
( )
( )
( )
( )
( )
( )
( )
( )
( )

To reduce gamma rays by 50%, we would need a tungsten shield 0.564 cm thick.

As an example, NASA's Reusable Nuclear Shuttle concept used a NERVA NTR engine with 334 kiloNewtons of thrust with a shadow shield massing 1360 kilograms which protected a 10 degree half-angle area. The distance between the habitat module and engine (a bit less than 49 meters) provided extra protection, as did the mass of the propellant.

The crew protected by the shadow shield, distance, and propellant would still suffer a radiation dose of 0.1 sieverts every time the shuttle did a standard burn. Anybody outside of the shadow cast by the shield and closer to the engine than 16 kilometers would suffer a whopping 0.25 to 0.3 sieverts per hour. The safe distance outside of the shadow is no close than 160 kilometers.

A standard burn was a delta V between 1 and 2 kilometers per second.

NASA has a career limit of 4 sieverts for astronauts, so an astronaut exposed to 40 standard burns would be permanently grounded.

I have found minimal references to low mass shields for space nuclear reactors that were layered tungsten-lithium hydride, layered boron carbide-beryllium, and layered lithium hydride-beryllium. The lowest mass one is the tungsten-lithium hydride shield.

In Heinlein's "The Green Hills of Earth", atomic spacecraft designers are guilty of scrimping on shadow shields in order to save mass. The designers were under pressure to maximize payload mass without worrying about trivial incidentals like the health of the engine crew. This is why the jetmen working next to the atomic engines find it so hard to get insurance, and seldom have children. At least ones that are not mutants.


      Ten minutes later he was back. "Captain," he stated darkly, "that number two jet ain't fit. The cadmium dampers are warped."
     "Why tell me? Tell the Chief."
     "I did, but he says they will do. He's wrong."
     The captain gestured at the book. "Scratch out your name and scram. We raise ship in thirty minutes."
     Rhysling looked at him, shrugged, and went below again.
     It is a long climb to the Jovian planetoids; a Hawk-class clunker had to blast for three watches before going into free flight. Rhysling had the second watch. Damping was done by hand then, with a multiplying vernier and a danger gauge. When the gauge showed red, he tried to correct it — no luck.
     Jetmen don't wait; that's why they are jetmen. He slapped the emergency discover and fished at the hot stuff with the tongs. The lights went out, he went right ahead. A jetman has to know his power room the way your tongue knows the inside of your mouth.
     He sneaked a quick look over the top of the lead baffle when the lights went out. The blue radioactive glow did not help him any; he jerked his head back and went on fishing by touch.
     When he was done he called over the tube, "Number two jet out. And for crissake get me some light down here!"
     There was light — the emergency circuit — but not for him. The blue radioactive glow was the last thing his optic nerve ever responded to.

     Rhysling obliged, then said, "You youngsters have got it soft. Everything automatic. When I was twisting her tail you had to stay awake."
     "You still have to stay awake." They fell to talking shop and Macdougal showed him the direct response damping rig which had replaced the manual vernier control which Rhysling had used. Rhysling felt out the controls and asked questions until he was familiar with the new installation. It was his conceit that he was still a jetman and that his present occupation as a troubadour was simply an expedient during one of the fusses with the company that any man could get into.
     "I see you still have the old hand damping plates installed," he remarked, his agile fingers flitting over the equipment.
     "All except the links. I unshipped them because they obscure the dials."
     "You ought to have them shipped. You might need them."
     "Oh, I don't know. I think—" Rhysling never did find out what Macdougal thought for it was at that moment the trouble tore loose. Macdougal caught it square, a blast of radioactivity that burned him down where he stood.
     Rhysling sensed what had happened. Automatic reflexes of old habit came out. He slapped the discover and rang the alarm to the control room simultaneously. Then he remembered the unshipped links. He had to grope until he found them, while trying to keep as low as he could to get maximum benefit from the baffles. Nothing but the links bothered him as to location. The place was as light to him as any place could be; he knew every spot, every control, the way he knew the keys of his accordion.
     "Power room! Power room! What's the alarm?"
     "Stay out!" Rhysling shouted. "The place is 'hot.'" He could feel it on his face and in his bones, like desert sunshine.
     The links he got into place, after cursing someone, anyone, for having failed to rack the wrench he needed. Then he commenced trying to reduce the trouble by hand. It was a long job and ticklish. Presently he decided that the jet would have to be spilled, pile and all.
     First he reported. "Control!"
     "Control aye aye!"
     "Spilling jet three — emergency."
     "Is this Macdougal?"
     "Macdougal is dead. This is Rhysling, on watch. Stand by to record."
     The ship was safe now and ready to limp home shy one jet. As for himself, Rhysling was not so sure. That "sunburn" seemed sharp, he thought. He was unable to see the bright, rosy fog in which he worked but he knew it was there. He went on with the business of flushing the air out through the outer valve, repeating it several times to permit the level of radiation to drop to something a man might stand under suitable armor.

From THE GREEN HILLS OF EARTH by Robert Heinlein (1947)

(ed note: twins "Castor" and "Pollux" are on the hull of their father's solid-core nuclear thermal rocket)

      Padeyes were spaced about twenty feet apart all over the skin of the ship. They had been intended for convenience in rigging during overhauls and to facilitate outside inspections while underway; the twins now used them to park bicycles. They removed the bicycles from the hold half a dozen at a time, strung on a wire loop like a catch of fish. They fastened each clutch of bikes to a padeye; the machines floated loosely out from the side like boats tied up to an ocean ship.
     Stringing the clusters of bicycles shortly took them over the 'horizon' to the day side of the ship. Pollux was in front carrying six bicycles in his left hand. He stopped suddenly. 'Hey, Grandpa! Get a load of this!'
     'Don't look at the Sun,' Castor said sharply.
     'Don't be silly. But come see this.'
     Earth and Moon swam in the middle distance in slender crescent phase. The Stone was slowly dropping behind Earth in her orbit, even more slowly drifting outward away from the Sun. For many weeks yet Earth would appear as a ball, a disc, before distance cut her down to a brilliant star. Now she appeared about as large as she had from Luna but she was attended by Luna herself. Her day side was green and dun and lavished with cottony clouds; her night side showed the jewels of cities.
     But the boys were paying no attention to the Earth; they were looking at the Moon. Pollux sighed. 'Isn't she beautiful?'
     'What's the matter, Junior? Homesick?'
     'No. But she's beautiful, just the same. Look, Cas, whatever ships we ever own, let's always register them out of Luna City. Home base.'
     'Suits. Can you make out the burg?'
     'I think so.'
     'Probably just a spot on your helmet. I can't. Let's get back to work.'
     They had used all the padeyes conveniently close to the hatch and were working aft when Pollux said, 'Wups! Take it easy. Dad said not to go aft of frame 65.'
     'Shucks, it must be "cool" back to 90, at least. We've used the jet less than five minutes.'
     'Don't be too sure; neutrons are slippery customers. And you know what a stickler Dad is, anyway.'
     'He certainly is,' said a third voice.
     They did not jump out of their boots because they were zipped tight. Instead they turned around and saw their father standing, hands on hips, near the passenger airlock.

From THE ROLLING STONES by Robert Heinlein (1952)

Keep in mind that these are called "shadow" shields because it is too expensive to put radiation shielding all around the hot stuff ("expensive" in terms of reduction of payload mass). This means that if one ventures outside of the spacecraft, you run the danger of moving out of the shadow and into the deadly glow of the unshielded engine. When the spacecraft is designed, it is also important to ensure that no part of the ship scatters the lethal radiation around the shadow shield and into the crew. The heat radiators, for instance. If lifting off from a planet with an atmosphere, said atmosphere can also create pesky neutron backscatter.

This does make exiting a landed ship somewhat challenging, and makes an argument for a ground crew wearing lead suits. In the Tom Corbett books, any ship that was to be on the ground for more than three days would have its liquid fissionable fuel removed by the "hot soup" wagon. Keep in mind that the neutron flux from the engine would transmute the elements composing the rocket's structure, making the aft end of the spacecraft radioactive even if all the fissionables are removed. Spaceship designers should also construct the aft end of the spacecraft out of materials that are not only strong, but that will transmute into materials of still acceptable strength.

You will find more discussion on embarking/debarking from a radioactive rocket here.

In the interest of radiation safety, the corridor to the atomic engine room is going to have dogleg bends in it. Radiation travels in straight lines but people don't have to. This allows the crew to quickly move out of direct line of sight with the reactor. The corridor exit will have an adjacent decontamination booth.


(ed note: Interstellar explorers from Terra land on a newly discovered planet. There are remarkably humanoid primitive aliens living there. The ship's linguist Kung Su learns the alien language, which is suspiciously similar to Chinese. Captain Griffin has a talk with some natives, and learns some interesting bits of the alien's history.)

      "Kung," Griffin asked over coffee next afternoon, "how well up are you on Chinese mythology?"
     "Oh, fair, I guess. It isn't my field but I remember some of the stories my grandfather used to tell me."
     "What is your legend of creation?" Griffin persisted.
     "It's pretty well garbled but I remember something about the Son of Heaven bringing the early settlers from a land of two moons on the back of his fire-breathing dragon. The dragon got sick and died so they couldn't ever get back to heaven again. There's a lot of stuff about devils, too."
     "What about devils?"
     "I don't remember too well, but they were supposed to do terrible things to you and even to your unborn children if they ever caught you. They must have been pretty stupid though; they couldn't turn corners. My grandfather's store had devil screens at all the doors so you had to turn a corner to get in. The first time I saw the lead baffles at the pile chamber doors on this ship it reminded me of home sweet home. By the way, some young men from the village were around today. They want to work passage to the next planet. What do you think?"

(ed note: The punchline is that Terran Chinese are descendants from the same primitive aliens, who incidentally can be found on thousands of other planets. They moved from planet to planet by hitchhiking in atomic space ships of an extinct high-tech race of "Barbarians".)

From BLESSED ARE THE MEEK by G. C. Edmondson (1955)
read online at Project Gutenberg

     "Does that give anyone a notion of why the Mayflower was assembled out in an orbit and will never ever land anywhere?"
     "Too hot," somebody said.
     "'Too hot' is an understatement. If the Mayftower had blasted off from Mojave space port the whole Los Angeles Borough of the City of Southern California would have been reduced to a puddle of lava and people would have been killed by radiation and heat from Bay City to Baja California. And that will give you an idea of why the shielding runs right through the ship between here and the power plant, with no way at all to get at the torch."
     We had the misfortune to have Noisy Edwards along, simply because he was from the same bunk room. Now he spoke up and said, "Suppose you have to make a repair?"
     "There is nothing to go wrong," explained Mr. Ortega. "The power plant has no moving parts of any sort."
     Noisy wasn't satisfied. "But suppose something did go wrong, how would you fix it if you can't get at it?"
     Noisy has an irritating manner at best; Mr. Ortega sounded a little impatient when he answered. "Believe me, son, even if you could get at it, you wouldn't want to. No indeed!"
     "Humph!" said Noisy. "All I've got to say is, if there isn't any way to make a repair when a repair is needed, what's the use in sending engineer officers along?"
     You could have heard a pin drop. Mr. Ortega turned red, but all he said was, "Why, to answer foolish questions from youngsters like yourself, I suppose." He turned to the rest of us. "Any more questions?"
     Naturally nobody wanted to ask any then. He added, "I think that's enough for one session. School's out."

     I told Dad about it later. He looked grim and said, "I'm afraid Chief Engineer Ortega didn't tell you the whole truth."
     "In the first place there is plenty for him to do in taking care of the auxiliary machinery on this side of the shield. But it is possible to get at the torch, if necessary."
     "Huh? How?"
     "There are certain adjustments which could conceivably have to be made in extreme emergency. In which case it would be Mr. Ortega's proud privilege to climb into a space suit, go outside and back aft, and make them."
     "You mean—"
     "I mean that the assistant chief engineer would succeed to the position of chief a few minutes later. Chief engineers are very carefully chosen, Bill, and not just for their technical knowledge."
     It made me feel chilly inside; I didn't like to think about it.

From FARMER IN THE SKY by Robert Heinlein. 1950.
Stay Shadowed

"But," I hear you say "surely you only need to have the crew habitat module inside the shadow cast by the radiation shield? It's not like the ship's girders can get cancer from radiation, right?"

Nope, nice try, but you'd do best to keep every single part of the ship inside the radiation shadow. Especially the heat radiators, which are huge extended structures that want lots of room. There are three reasons why:

  1. Neutron radiation can cause Neutron Embrittlement. Becoming brittle is not healthy for struture in general and load-bearing members in particular. Unless you have a perverse reason to want your spacecraft to snap like a twig when you goose the rockets.
  2. Neutron radiation can cause Neutron Activation. Having components of the spacecraft transmuted into radioactive isotopes is a health hazard. Especially since the isotopes will not be behind a radiation shield. They will be free to spray the habitat module with deadly radiation.
  3. Spacecraft strutures protruding outside of the radiation shadow can cause "backscatter", bouncing deadly radiation around the shadow shield and irradiating the habitat module. This is one reason why nuclear powered aircraft never caught on, the very atmosphere itself would cause backscatter.

In the first diagram below, note how the lower part of the external propellant tanks are cut at an angle so they do not stick outside of the shadow (the "half-cone sections"). In the other diagrams, note how the square heat radiators are trimmed to a triangular profile when they are near the shadow shield. This also gives the viewer an indication of the outline of the (otherwise invisible) radiation shadow.

Hull Armor

Hull armor is specifically to protect the ship and crew from the natural radiation from space, and from hostile weapons fire.

Different kinds of armor are required for different kinds of ionizing radiation: particle radiation or electromagnetic radiation. Neutron, cosmic rays, solar protons and the like are particle radiation (because they are subatomic particles). X-rays and gamma-rays are electromagnetic radiation. Particle shielding is generally something with lots of hydrogen in it, like water, liquid hydrogen propellant tanks, lithium hydride, paraffin or a hydrogenated polyethylene composite. X-ray/gamma-ray shielding is generally something very very dense, like lead, tungsten, or an alloy with a lot of heavy elements in it.

The hull armor will be arranged differently than than shadow shield.

First off, the armor is probably only going to be on the habitat module, and any radiation-sensitive equipment. It is not going to be over the entire spacecraft.

Secondly, unlike a reactor, cosmic rays and solar storms contain charged particles, mostly protons. Charged particles can create "Bremsstrahlung" or braking radiation. (Keep in mind that the hull of the spacecraft will probably never encounter natural gamma rays in the space environment. Gamma rays will probably only come from artificial sources, such as nuclear weapons.)

You see, gamma shielding is worse than useless against charged particle radiation. Such particles striking lead actually creates deadly x-rays, making the radiation problem much worse (the same principle is used in a doctor's x-ray machine). Please note that this only applies to charged particles, neutrons from the reactor do not generate Bremsstrahlung.

And please do not confuse "neutral particle beams" with "neutron particle beams." The former will produced Bremsstrahlung, the latter will not. Neutral particle beams are beams of protons and electrons (which are charged) in a neutral electrical balance. Neutron particle beams are beams of neutrons (which are uncharged).

So for the hull shielding it is best to arrange things so that the incoming radiation first encounters the paraffin to soak up all the particle radiation, then have a layer of tungsten to stop the gamma rays.

Anthony Jackson on the topic of Carbon as radiation shielding says:

Carbon's decent (better than aluminum or steel, worse than hydrogen or hydrocarbon plastics) against neutrons and cosmic rays (including particle beams), and has the useful secondary property of not becoming radioactive when bombarded with such particles. It's inferior against gamma rays and electrons (electrons are not hard to shield against even with a bad material, however). Within the context of a space radiation environment, it's probably overall a good material.

Anthony Jackson

This graph is from Proceeding of the Symposium on Manned Planetary Missions 1963/1964 page 92.

For solar proton storms occuring during missions lasting from 300 to 700 days, the graph shows the radiation dosage the crew will receive to their skin given aluminum shield weight. The dose to the crew's blood forming organs will be roughly half the skin dose.

The curves are for the probability of exceeding the listed radiation dosage, probabilities of 0.001, 0.01, and 0.1 (i.e., one in a thousand, one in a hundred, and one in ten).

For example, say you were concerned with the crew having a skin dose over 103 rads (10 grays) and the mission was 700 days. Find 103 on the vertical scale on the left. Look at the three curves: 700 days at P=0.001, 700 days at P=0.01, and 700 days at P=0.1

You draw a horizonal line starting at 103, and draw a vertical line where it hits each of the three 700 day lines. Here it makes vertical red, gold, and green lines. See where the vertical lines hit the bottom scale.

The red line says that if the shielding is 3 gm/cm2 of aluminum, there will be a one in ten chance that the crew will receive a skin dose of over 103 rads on a 700 day mission. The gold says 10 gm/cm2 will only have a one in a hundred chance, and the green says 17 gm/cm2 will only have a one in a thousand chance.

Anti-weapon armor (lasers and kinetic energy weapons) is discussed here.


A Self-Replicating Radiation-Shield for Human Deep-Space Exploration: Radiotrophic Fungi can Attenuate Ionizing Radiation aboard the International Space Station


     The greatest hazard for humans on deep-space exploration missions is radiation. To protect astronauts venturing out beyond Earth’s protective magnetosphere and sustain a permanent presence on Moon and/or Mars, advanced passive radiation protection is highly sought after. Due to the complex nature of space radiation, there is likely no one-size-fits-all solution to this problem, which is further aggravated by up-mass restrictions.

     In search of innovative radiation-shields, biotechnology holds unique advantages such as suitability for in-situ resource utilization (ISRU), self-regeneration, and adaptability. Certain fungi thrive in high-radiation environments on Earth, such as the contamination radius of the Chernobyl Nuclear Power Plant. Analogous to photosynthesis, these organisms appear to perform radiosynthesis, using pigments known as melanin to convert gamma-radiation into chemical energy. It is hypothesized that these organisms can be employed as a radiation shield to protect other lifeforms.

     Here, growth of Cladosporium sphaerospermum and its capability to attenuate ionizing radiation, was studied aboard the International Space Station (ISS) over a time of 30 days, as an analog to habitation on the surface of Mars. At full maturity, radiation beneath a ≈ 1.7 mm thick lawn of the melanized radiotrophic fungus (180° protection radius) was 2.17±0.35% lower as compared to the negative control. Estimations based on linear attenuation coefficients indicated that a ~ 21 cm thick layer of this fungus could largely negate the annual dose-equivalent of the radiation environment on the surface of Mars, whereas only ~ 9 cm would be required with an equimolar mixture of melanin and Martian regolith. Compatible with ISRU, such composites are promising as a means to increase radiation shielding while reducing overall up-mass, as is compulsory for future Mars-missions.



     With concrete efforts to return humans to the Moon by 2024 under the Artemis program and establish a permanent foothold on the next rock from Earth by 2028, humankind reaches for Mars as the next big leap in space exploration. In preparation for prolonged human exploration missions venturing past Earth-orbit and deeper into space, the required capabilities significantly increase2. While advanced transportation solutions led by the private and public sectors alike (BFR/Starship, New Glenn, SLS/Orion) are pivotal and have already reached high technological readiness, life support systems as well as crew health and performance capabilities are equally essential. Therefore, any mission scenario such as ‘Design Reference Architecture 5.0’3 or ‘Mars Base Camp’ (with up to 1000 days of crewed usage), must include innovative solutions that can meet the needs and address the hazards of prolonged habitation on celestial surfaces.

     The foremost threat to the short- and long-term health of astronauts on long-duration deep-space missions is radiation5. Over one year, the average person on Earth will be exposed to about 6.2 mSv while the average astronaut on the International Space Station (ISS) is exposed to approximately 144 mSv, and one year into a three-year mission to Mars, an astronaut would already have been exposed to some 400 mSv of radiation, primarily from Galactic Cosmic Radiation (GCR). While the particular health effects of interplanetary radiation exposure have still not been fully assessed, adequate protection against space-radiation is crucial for missions beyond Earth-orbit, but is more than any other factor restricted by up-mass limitations10. Both active and passive radiation-shielding, the latter investigating inorganic as well as organic materials, have been and are extensively studied . In-Situ Resource Utilization (ISRU) will play an integral role to provide the required capabilities (as well as to break the supply chain from Earth and establish sustainable methods for space exploration because once underway there virtually is no mission-abort scenario). For ISRU, biotechnology holds some of the most promising approaches, posing useful for providing nutrition14, producing materials and consumables, and potentially even “growing” radiation shielding. Biological studies, however, have so far mostly focused on understanding how organisms protect themselves rather than trying to take advantage of them as biotechnological means for radiation shielding.

     Nevertheless, there exist many extremophiles that can thrive in highly radioactive environments, namely species of insects, fungi as well as bacteria. Certain fungi can utilize high-energy non-visible radiation through a process termed radiosynthesis, analogous to photosynthetic organisms turning energy from visible light into chemical energy. It is believed that large amounts of melanin in the cell walls of these fungi mediate electron-transfer and thus allow for a net energy gain. Certain melanized fungi have been found to thrive in highly radioactive environments such as in the cooling pools of the Chernobyl Nuclear Power Plant, where radiation levels are three to five orders of magnitude above normal background levels. Additionally, they have been found to populate the exteriors of spacecraft in low Earth orbit (LEO), where exposure to ionizing radiation is intensified. Here, we explore the opportunity to utilize the dissipation of radiation by melanized fungi as part of a multi-faceted solution to passive radiation-shielding for ISRU. In an additional analysis based on the concept of linear attenuation26, we determine the ability of live melanized fungus as well as the specific contribution of melanin to provide adequate shielding against cosmic radiation.

Flight-Experiment – Growth On-Orbit

     Once the hardware was powered on, the temperature rose sharply from the initial of 22°C, reaching 30°C after 4 hours and 31 – 32°C after 8 hours. Past 8 hours, the temperature remained at 31.5±2.4°C for the rest of the experiment.

     Many dimorphic fungi are characterized by slow growth and need to be incubated for 14 days (at 25°C) for sufficient growth to occur. However, in the on-orbit lab, C. sphaerospermum reached maximum growth after approximately 18 h and full maturity after 48 h, as discernible from figure 2 and the growth curve. Comparison to the preflight growth tests may indicate that the fungus could experience faster-than-average growth aboard the ISS, due to the utilization of ionizing radiation of the space environment as a metabolic support function, as has been reported for other high-radiation environments: it has previously been shown that C. sphaerospermum can experience up to three times faster growth than normal with gamma-rays 500 times as intense as normal.


     Through the design of a subtle yet simple experimental setup, implemented as a small single payload, it could be shown that the melanized fungus C. sphaerospermum can be cultivated in LEO, while subject to the unique microgravity and radiation environment of the ISS. Growth characteristics further suggested that the fungus not only adapts to but thrives on and shields against space radiation, in accordance with analogous Earth-based studies. It was found that a microbial lawn of only ≈ 1.7 mm already decreased the measured radiation levels by at least 1.82% and potentially up to 5.04%. Attenuation of radiation was consistent over the whole course of the experiment (720 h), allowing a scenario-specific linear attenuation coefficient for C. sphaerospermum to be determined. This was further used to approximate the melanin content of the biomass (~ 40%), which corresponds well with literature and served to explain the significant reduction in radiation levels by means of the fungal lawn. Based on the melanin content, the theoretical radiation attenuation capacity of melanized fungal biomass could be put into perspective at constant photon energy levels (100 MeV) with those of materials relevant for future human deep-space exploration missions: melanized biomass as well as pure melanin ranked among the most effective radiation attenuators, emphasizing the great potential they hold as components of radiation shields to protect astronauts from GCR.

     Being a living organism, C. sphaerospermum self-replicates from microscopic amounts, which opens the door for ISRU through biotechnology, potentially allowing significant savings in up-mass. Often nature has already developed blindly obvious yet surprisingly effective solutions to engineering and design problems faced as humankind evolves – C. sphaerospermum and melanin could thus prove to be invaluable in providing adequate protection of explorers on future missions to the Moon, Mars and beyond.

Storm Cellars

Storm cellars are specifically to protect the ship and crew from the natural radiation from space, specifically when the radiation suddenly increases. Much like people take shelter in a conventional storm cellar when a tornado suddenly appears. In NASA-speak a storm cellar is called a "biowell". "Sudden increases" in radiation usually means a solar storm, though occasionally it means an unavoidable pass through a planetary radiation belt.

There are also storm cellars in Orion nuclear pulse driven spacecraft, since detonating hundreds of nuclear devices for propulsion will also cause a sudden increase in enviromental radioactivity.

A storm cellar is a radiation-shielded room near the ship's center, barely large enough for the entire crew. If it can be located in the middle of dense things, like fuel tanks or cargo, so much the better. The shielding is generally a material that contains lots of hydrogen since storm cellars typically protect against particle radiation, not x or gamma rays. NASA is currently working on a new shielding material, a hydrogenated polyethylene composite. Not only is it a better shield than aluminum, it has less mass as well.

There also might be anti-radiation suits for the crew. Not as good as a full storm cellar, but much better than nothing.

Cellars might use an as yet un-invented magnetic anti-radiation field. Such fields are currently science fiction, and in any event will only provide protection against charged particle radiation, not x or gamma rays (for that you'll need an honest-to-Doc-Smith force field or ray screen). Keep in mind that almost all natural radiation hazards are charged particle, x and gamma rays generally come from human sources (such as poorly shielded fission reactors and nuclear weapons).

To protect against a significant solar storm, the shielding on the biowell should be at least 500 grams per square centimeter (5,000 kg/m2). This will give good protection against neutrons as well.

I have seen a couple of designs for Mars missions wit solid core nuclear thermal rockets try to make a poor-man's storm cellar by aiming the rocket's shadow shield at Sol and hoping it stops enough solar storm radiation so the crew doesn't die. I am trying to find some figure on how effective this would be, but it sure looks like a dangerous mass-cutting short cut to me.

A storm cellar surrounded by water tanks can be found in John Campbell's THE ULTIMATE WEAPON, Robert Heinlein's PODKAYNE OF MARS and Lee Correy's (AKA G. Harry Stine) SPACE DOCTOR. Both Heinlein and Stine call the cellar a "caisson" or "cofferdam". A caisson is actually a pressurized working area surrounded by water that is used when building the submerged pylons of a bridge, but I suppose the description is whimsically close enough to a spacecraft storm cellar.

The crew will occupy the cellar when the sun kicks up a solar storm of radiation. As these can last for days, one had better include a few crew-days worth of food, water, and tranquilizers. And a porta-potty. If you are relying upon algae for your oxygen, it deserves space in the storm cellar as well. This probably also applies to stored food too. I have heard that particle radiation can destroy a lot of the vitamins in food, especially pyridoxine and thiamine. Alas, computers and other digital electronics are also vulnerable to radiation. Don't forget repeaters for the gauges on the major ship systems, and one monitoring radiation levels outside the cellar. The latter tells you when it is safe to come out. The former tells you if there is a critical failure outside, meaning it is time to start drawing straws to decide who gets to heroically commit suicide by saving the ship. After the storm the crew can emerge and go check the dosimeters they thoughtfully left in the modules of the spacecraft vulnerable to radiation.

As a matter of fact, in many early NASA designs for Mars missions, the storm cellar is also the control room. Don't just have repeaters in the cellar, have all the spacecraft controls. It is a really bad idea to leave the control room unmanned, and the crew will be reluctant to expose themselves to an agonizing radioactive death because the blasted control room ain't shielded. Yes the control room will be a bit crowded when the sun raises up a ruckus, but you can't have everything.


“To keep you passengers comfortable, we’ll travel at constant acceleration for half the journey, go into free fall for a few minutes while I flip the liner 180 degrees, then decelerate until we are in Martian orbit. On this journey, we’ll be traveling at one-sixth Earth gravity, which is about one lunar gravity, or one-half Martian gravity. The total travel time will be seven days. I hope you enjoy your journey.”

They had the obligatory “lifeboat” drill, but instead of putting on lifejackets and reporting to their lifeboat stations, they each went back to their stateroom, packed a little emergency bag with necessities, then waited.

After about five minutes, the individual room viewers were turned on by an emergency override signal, any the captain’s face appeared.

“This is a test. Solar Storm Alert! Everyone bring your emergency bag and report to the Solar Storm Shelter. Repeat. This is a test of the Solar Storm Alert system. Everyone report to the Solar Storm Shelter.”

Johnny and Gramma Ginny took their emergency bags, and following everyone else, they headed for the center of the ship where they were ushered through a revolving airlock by the stewards into a small cylindrical room packed ten high with nested banks of chairs. Once Johnny was in his chair, he could see nothing but Gramma Ginny to his left, a strange man to his right, and the captain on the tiny viewscreen in front of him. The viewscreen was nestled under the seat of someone else stacked just above him. The captain started to speak.

“That was excellent time—twelve minutes. We’ll have a good deal more warning than that for a solar storm, so when you hear the Solar Storm Alert, go to your cabins, get your emergency bags and report to the storm shelter. The shielding in this room is sufficient to protect you from the radiation of even the worst solar storm. You will notice that this room has revolving airlocks at the four entrances. This is also our shelter in the extremely unlikely event that we start to lose cabin pressure. When the Loss of Pressure announcement is made, do not go to your staterooms for the mergency bag, but report immediately to the sheler. A steward with an airmask will be there to direct you through the airlock. Above all, do not panic; even a large hole will require more than ten minutes to lower the ships air pressure by half. Now I will turn you over to the Head Steward.”

The face of the head steward appeared on the screen. “A solar storm can last up to a week,” she said. “Today we are going to pretend we have a four day storm. During a storm, no one is allowed to go to their cabins for anything. During a bad storm, one minute outside the shelter will give you a radiation dose that will make you sick. Five minutes is enough to kill you. When you enter the shelter you must bring everything you need in your emergency bag or do without. As you notice, there is nothing to do in this room but sit. There is a large reading library available through your individual consoles, provided you remembered to bring your reading glasses.”

Johnny heard Gramma Ginny groan as she searched through her emergency bag for the nonexistent glasses.

“There are four toilets and four showers in the cylindrical bulkhead at center of the room,” continued the head, steward. “We have plenty of soap, water, and even toothpaste, but did you remember your favorite razor and toothbrush?” Johnny heard mutterings from around him.

“It’s now the end of day one, and time for a refreshing shower and a change of underwear,” said the head steward. More groans.

“It’s the start of day two, we’ll be serving a cold but nourishing breakfast, and it’s time to take the morning medicine your doctor has recommended.” Johnny heard a word he wasn’t supposed to hear.

“I think the lesson is clear,” said the head steward, with an amused smile. “When you repack your emergency bags back to your staterooms, please make sure that they contain everything you could possibly need if you had to spend the rest of the trip in the shelter. Good day.”

From TURN LEFT AT THE MOON by Robert L. Forward (1995)

The SFO satellites — Solar Flare Observatories — running unmanned in a different part of geosynchronous orbit have detected activity that normally precedes solar flares. So we've been operating in the Flare Watch mode. If we go to Flare Alert, we'll have no more than ten minutes to get into P-suits and make it to the caisson, our storm cellar.

Seven hex modules in a circular honeycomb make up the caisson. It's surrounded by the water tanks of GEO Base: twelve hex modules nested in honeycomb around the caisson. There are seven half-hex modules on each end of the caisson, and they're water tanks, too. We've never had to worry about water in GEO Base for several reasons, not the least of which is the fact that every human in the station puts about seven quarts of water per day into the system through metabolism and urine. However, that's only a small part of GEO Base water supply, which amounts to over 3700 tons of water. We'll never use that much in GEO Base. It's here for the primary purpose of radiation shielding.

Dan hills had once asked me to check his figures on the mass of water required to knock down the radiation of a solar flare to something that would give a person in GEO Base less than a twenty-five rem (0.25 Sievert) exposure from the largest solar flare recorded to date — the one of 23 February 1956. Well, if we get another one like that, we'll all have to be shipped back to Earth ... and that will be the end of our space traveling. However, the average solar flare will give us less than a single rem (0.01 sv) inside that caisson.

Dan's a bright one. All the pre-Eden studies of space habitats assumed extraterrestrial materials for shielding. We aren't that far along. But we always need water, and water is handy to have around to break into hydrogen and oxygen for propellants. It's nontoxic and easy to transport. Only problem is that it weighs eight pounds per gallon. But it's easily moved around by piping and pumps.

My med team has its own duties inside the caissons if the alarm goes off. We all wear dosimeters during the Watch. In the course of the emergency, we'll be prepared to handle radiation sickness, although there isn't much we can do if a guy shows up with more than four hundred rems (4 Sieverts, LD50) in him.

"Flare Alert! Flare Alert! All personnel to the caisson! Nine minutes and counting!...

Tons of water were being pumped by computer control into the tankage surrounding the caisson.

They were on station with two minutes to spare.

It was a human sardine can.

P-suited figures were stacked in honeycomb cubicles that were just big enough to hold a single person in a P-suit — a volume of thirty-six cubic feet (1 cubic meter). The med team wasn't much better off, except that they were in the middle of the honeycomb and able to move anywhere in the caisson for medical purposes. And they had the luxury of a tiny emergency surgical volume not much larger than Tom's sleeping quarters.

Caisson stewards, chosen not only for their ability to keep cool but also because they were big and brawny, moved quickly among the hundreds of people stacking up like cord wood. They shifted P-suit supply hoses from backpacks to the caisson supply system and plugged the comm system into each person...

"These usually don't last much more than twenty-four hours," Fred volunteered.

"All personnel, this is Base Engineer Pratt.", the big man's voice boomed through their individual helmet loudspeakers. "Relax. We'll know in a few minutes if everybody made it. Sorry we don't have channels enough to allow you to talk to one another. There's music on Channel B. If you're short on sack time, I'd suggest you use this period to catch up. Under all circumstances, stay quiet and keep your activity to a complete minimum; we have limited life-support oxygen and regeneration in flare emergency mode. If you don't cool it, I guarantee one of the stewards will be around with something to send you beddy-by fast. If you're in trouble, press your call button. If you didn't get a chance to hook up your urine and fecal bags, do it now while we've still got pressure in here. If we happen to lose pressure for some reason, and if you're not hooked up, you'll just have to stew in your own juices."

There was a pause, then he went on. "This is a Class One flare. Solar protons should peak in five hours. With luck, we'll be out of here in fifteen hours. Sorry about the lack of room, but better you're alive in thirty-six cubic feet than dead with all of space to roam in. Hang in there!"

There was a click, and Pratt's voice came through the med net. "Doc, are you here?"

"Roger, Herb."

"We're missing fifteen riggers. They may have been out on the far array subassernbly. If they get here in the next thirty minutes, they shouldn't have picked up more than twenty rems (0.2 sv) outside. I'd like you to check them and their dosimeters when we let them in."

"Right. We're ready. Anybody else?"

"Don't know. LEO Base is auditing the count."

In any event, his medical team didn't have time to get bored and start crawling up the walls, since eight hundred people were crammed into space that would normally be occupied by only two hundred. And most of those eight hundred people had little to do except stare at the web netting of their honeycomb, looking at the back of the P-suit of the person "above" them'. Few had had time to bring things to read. A combination ot carefully selected music against a background of pink noise played through their helmet loudspeakers, but even this didn't keep all of them calm.

Although there were some housekeeping and support staff in GEO Base, most of the workers were involved with the final assembly and testing of the SPS itself. As such, they were the space equivalent of the high-iron men, riggers, offshore oil drillers, and electrical-transmission-line builders. They were hard and tough, and they were used to living dangerous lives. However; the space experience was new to all of them, and some reacted uncharacteristically. The strangeness of the environment, the constant awareness of death near them or around them, the invisible specter of ionizing radiation, and the relative isolation of GEO Base got to a few of them. Tom and his crew didn't have time to become bored; they were busy sedating or tranquilizing frenetic, disturbed people. They had to do it fast to prevent sympathetic reactions from others nearby who were probably just as scared. This meant that the rned team often couldn't be tactful, gentle, or highly selective. The prime objective was to quiet the person and help preserve the tenuous control of the situation.

It got worse when the fifteen riggers showed up from the far end of the assembly, where there hadn't been any fast Eff-Mu transport available back to the caisson. Tom checked each dosimeter as they filed past him into the hands of his team. "Two-ten rems ... Two-oh-fiverems ... Two-thirty rems ..."

Everyone had picked up between two hundred and two hundred sixty rems (2.0 to 2.5 sv).

They could be saved, but it wouldn't be an easy job. Nearly all of them were nauseated. Most of them had filled their fecal collection bags, and one man was nearly drowning in his own vomit-filled helmet.

Dave's primitive blood-analysis equipment wasn't really up to doing full work-ups pn all fifteen men, but it was good enough for Dave to be able to confirm the usual symptoms of acute radiation syndrome. "Leukocyte count is down. Some electrolyte imbalance."

Tom looked at the numbers on the report pad Dave had handed him. "Standard," he remarked. "Okay, let's start IV with lactate of Ringer on all of them and administer two hundred fifty milligrams of oxytetracycline through the IV channel. Give each of them twenty-five milligrams of promethazine IM; that'll make them feel a little better." He turned to one of the riggers whom Fred was cleaning up. "You're going to be all right. None of you got enough dosage that we can't treat you."

"Well, I don't much give a damn at this point," the rigger replied listlessly. "Never felt so lousy in my life."

"You'll feel better soon, and you'll all be going back to Earth on the first ship," Tom told him They would have to transport them; Tom didn't have the facilities to handle fifteen people with acute radiation syndrome. The riggers were in for two to three months of intensive hospital care.

(ed note: Warning, horrific description of terminal radiation sickness follows. Sensitive readers might want to skip this section)

     But they hadn't peaked out. Pratt himself came looking for Tom. "Doc, the seven people riding Subassembly Twenty-three up from LEO Base have arrived. They collided with the array, but the safety circuits shut down the affected portions. Uh, Doc, they're in bad shape."
     "What's the radiation level out there? Can we go out and get them?" Tom asked.
     "If you don't stay too long. You'll pick up about fifteen hundred millirems per hour (1.5 rem/hr or 0.015 sv/hr) out there right now."
     "I'll go," Tom snapped decisively. "Fred, Stan, will you volunteer to go with me, maybe pick up a rem or two in the process?"
     Stan reached for his life-support backpack. "Right with you, Doctor."
     Fred reached over and grabbed his paramedic kit. He didn't say anything.
     "Angie, you're in charge until I get back," Tom told her.
     "Doc, there're seven of them," Pratt pointed out "The three of you won't be able to handle them. I'll get some volunteers and go along with you."
     Tom turned and looked at the base engineer. "Herb, I won't ask you to risk it."
     "Have you ever handled people with heavy radiation doses before? I have. You'll need help."
     "Where the hell did you get experience with radiation-overdosed people?"

Pratt hesitated. "Shouldn't tell you, because it's still classified. The only accidental meltdown that ever occurred was Groom Lake, Nevada (Area 51). That's all I'm gonna say. I was a young civil engineer just out of Cal Tech. My first job. I went in with the medical team because I'd helped build the containment structure ..." He paused again, then went on. "Until you've done it, you don't know what's involved. And after you've done it, you hope to hell you never have to do it again. But I guess I wasn't lucky. You'll need help, Doc."

The base engineer was right. If it hadn't been for Pratt and the four men he got to volunteer, Tom and his two paramedics couldn't have done it.

The dosimeters in the personnel module of Photovoltaic Array Subassembly 23 showed a total dose of 6570 rems (65.7 sv) over a period of ten hours.

Information on human reactions, symptoms, and effects of massive doses and dose rates of ionizing radiation is sparse. Tom knew there had been only thirty documented cases of serious exposure in over fifty years; obviously, because of what Pratt had admitted, there had actually been more. Still, the number of fatalities had been less than a dozen, which is a remarkable industrial safety record. The data from Hiroshima and Nagasaki were questionable because of the amount of time that had passed between exposure and the arrival of doctors trained in nuclear medicine, which was still a very primitive field at that time. The only data of any reliability and repeatability had come from animal tests, and nobody really knew if the results could be extrapolated to human beings. But it was all there for Tom to see and eventually record: seven more cases of extreme radiation exposure.

The five men and two women had taken such a heavy dose that the cerebral syndrome had struck them full force within an hour. By the time Tom and the others reached them, the seven were already suffering from tremors, ataxia, and convulsions. There was also ample evidence of the gastrointestinal syndrome of intractable vomiting and diarrhea.

None of the seven had been in P-suits when the radiation hit, and they had neither the strength nor the will to get into P-suits afterward.

But Tom, Pratt, and the others didn't open their faceplates. In fact, Tom knew they would have to leave their P-suits outside the caisson when they returned, because there was no way any of them could avoid the human waste that floated everywhere in the personnel module.

     "Herb, they're pretty far gone," Tom remarked over the suit-to-suit radio channel.
     "They are."
     "I don't think we can save them."
     "Not with more than six thousand rems in them. Best you can do, Doc, is put them out of their misery."
     "Pratt, I won't perform euthanasia!"
     "Best thing you could do for them."
     "We'll sedate them. Fred, Stan, a hundred fifty milligrams of meperidine hydrochloride IM."
     Neither paramedic did more than acknowledge the order before starting to administer the injections while Pratt's men held the dying.
     "When we get them quieted down, we'll bring them back to the caisson and make them as comfortable as we can," Tom said.
     "I wouldn't," Pratt objected. "Knock them out with drugs and leave them here."
     "The hell you say, Pratt! Even if they're dying, they deserve to die among people, not out here all alone in a tin can! I'm a doctor with respect for humanity!"
     "I thought they drained most of the milk of human kindness out of you doctors in med school," the engineer shot back.
     "Not all of it. But we learned how to restrict its flow."

"Well, now's the time to do it." Pratt paused, then put his gloved hand on the shoulder of Tom's P-suit. "Look, Doc, I may act like an inhuman slave driver, but, believe me, that's just a mask. I've been through this before, and I dislike it intensely. I feel for these seven people, but they're too far gone to realize what hit them, no matter what I do. They may be alive, they may be semiconscious, but they don't know what the hell is happening. The situation is going to get worse, and you'll have a decreasing ability to handle the total loss of sphincter control. They have maybe three hours left." He sighed, and the sigh was very evident over the radio link. "If you take them back to the caisson, they won't know it. But eight hundred people will, and those eight hundred people aren't prepared to deal with their condition. Look, we have enough problems in the caisson as it is. For God's sake, don't make it worse."

Tom had to agree with the engineer.

"I'll stay with them," Fred volunteered.

"No, you won't," Tom said as they got ready to leave the module. "We're picking up fifteen hundred milli-rems per hour here. If you stay another three hours (+0.045 sv), I'll probably have to send you back to JSP to keep your total exposure under industrial safety standards (probably 0.1 sv acute limit), Fred. And I need you."

Cycling through the lock of that personnel module as the last person to leave the seven radiation victims was one of the toughest things that Tom Noels had ever had to do. His training and education told him he should stay on and administer to the sick until they died. But if he stayed, he would pick up a radiation dose that would retire him forever to the ground, and that would mean he'd abandon eight hundred people in GEO Base—and Owen Hocksmith as well.

He couldn't resist taking a final look before closing the hatch and starting the lock cycle. Afterward, he wished he hadn't done it. The grim scene would remain in his memory for the rest of his life.

From SPACE DOCTOR by Lee Correy (G. Harry Stine) 1981.

Planetary Base Shields

To protect against galactic cosmic radiation and solar proton storms, lots of mass is required. A spacecraft has to carry its own shielding. But a planetary base can use regolith as shielding, i.e., bury the base by shoveling tons of the readily available local dirt over it. Alternatively the base can be located in artificial or naturally occurring caves and tunnels deep underground. This explains NASA's interest in Lunar and Martian lava tubes.

It is estimated that Lunar lava tubes can have a diameter of up to 300 meters and lying under 40 meters or more of basalt. In addition to protecting from galactic cosmic radiation, lava tubes will also protect against meteorites, micrometeorites, and ejecta from impacts. They will also provide a stable temperature of about -20 °C (instead of varying from -173 °C to +100 °C) and access to underground resources.

Martian lava tubes are estimated to have a roof thickness of around 30 meters. In 2010 a "skylight" (lava tube with hole in the roof) was observed in the Pavonis Mons region of Mars. The skylight was estimated to be about 190×160 meters wide and at least 115 meters deep.

Using regolith is more work, but you cannot always count on a convenient lava tube near the proposed base site.

The table below assumes that regolith has a bulk density of 1.3 grams per cubic centimeter.

Radiation Shield Mass
Equivalent Shielding
to Terra Sea Level
1,000 g/cm27.7 m
Acceptable Shielding
700 g/cm25.4 m

In the Lunar base design below, regolith is stuffed into long bags and coiled around the dome.

Radiation protection is a major concern for long-term habitation of extraterrestrial surfaces. The major hazards are from solar flares and lengthened exposure to galactic cosmic radiation (GCR). Solar flares occur sporadically and are roughly correlated with the sunspot cycle. GCR contains many more energetic particles man solar flares but at substantially lower fluxes. Solar flares can be lethal over short time periods whereas GCR presents a more long-term hazard. Shields of bagged regolith about 50-100 cm thick have been estimated to achieve a tolerable radiation environment for solar events. The shields also suffice for protection from micrometeoroids which generally penetrate only a few centimeters. Current GCR models are not yet adequate for predicting long-term shielding needs. With such coverings the habitats provide an adequate haven during a solar storm. EVA crew are at risk unless they can retreat to the habitat or some temporary haven. A regolith bagger provides for constructing temporary radiation shelters for crew when far from the base shelter such as during an extended traverse in the pressurized rover. Since the regolith bagging and stacking process can take a significant amount of time, it must be started somewhat before a solar storm.

Currently the ability to predict solar flares is somewhat limited, and warnings are best provided by surveillance of the sun. Warnings of solar storms may be as short as half an hour. Earth-based support can also be limited or nonexistent; for example, when Mars is on the opposite side of the Sun from the Earth. Improved ability to predict solar storms can reduce risks to crew since operations can be restricted during high alert periods. Radiation protection garments provide emergency partial protection when the crew does not have enough time to return to the habitat or construct a haven. The period of maximum flux of a solar storm is often on the order of a few hours. In such situations these garments give enough protection to limit exposure to tolerable levels for short periods of time. Such garments could consist of about 3 inches of multilayered carbon fiber and provide about 8 grams per square centimeter of shielding. This would reduce the dose rate of a solar flare by a factor of five to seven times that of an unshielded suit. During an event like the 6-hour peak of the August 1972 storm, one of the largest on record, they would allow for an emergency dose of about 10-15 rem as compared to 72 rem. However, they could not support an entire flare period but would give crew added time for more appropriate measures.

From Exploration Studies Technical Report FY 1988 Status, Volume II (1988)

Force Fields

Radiation shields composed of matter are quite massive, and Every Gram Counts. Researchers have been looking into using magnetic and electrostatic fields to protect against particle radiation, since such fields have no mass. Unfortunately the generators of such fields do have mass. And the field strength will have to be strong enough that the word "superconductor" will soon be mentioned. In addition, such powerful fields might be health hazards to the astronauts. It is worthless if the field simultaneously protects the astronaut from particle radiation, but also instantly kills them by being strong enough to straighten out all their DNA molecules.


      Abstract—The physics of the radiation shielding problem possess several asymmetries which may be exploited in electrostatics to obtain nearly isotropic protection without radial symmetry in the fields, a concept that has been overlooked in previous studies. Electrostatic shielding is advantageous because it solves the problem of secondary radiation generated in passive shields and allows passive shields to he smaller and more directional. This paper presents the results of computer simulations for a linear quadrupole configuration to demonstrate shielding effectiveness for protons and high-Z, high-energy particles, while at the same time driving away thermal electrons. The study indicates that this shielding method is nearly feasible with existing technology, and only modest gains may he needed to make it a reality.


     A significant obstacle to the human exploration and development of space is the energetic particle radiation. This radiation is harmful both to crewmembers (and other transported life) and to electronics. The interplanetary radiation has two major components: solar proton events (SPEs), which are unpredictable and very intense but of short duration, and galactic cosmic radiation (GCR) which is less intense but relatively constant and predictable. Life on earth is protected from both by the global magnetic field and the atmosphere. However, a significant flux of this radiation becomes entrapped along the magnetic field lines around the earth—and around other planets that have a global magnetic field—creating a localized zone of intense radiation. Crew members and electronic systems in earth orbit are exposed to all three components (SPE, GCR, and trapped radiation) to a degree depending upon the orbital parameters. Planetary bodies that have neither global magnetic fields nor dense atmospheres, such as Mars and the Moon, offer only one hemisphere of protection to astronauts on the surface, and so the total radiation dose over a mission must include exposure to SPEs and GCR while on the surface as well as in transit. This paper is concerned primarily with the interplanetary radiation environment, although the shielding concepts can be applied to other situations as well.

Electronic Effects

     Radiation particles are able to upset the digital state of circuitry because the components on integrated circuits are very small. This is known as a Single Event Effect or SEE. Other hardware such as solar cells may be degraded due to the cumulative effect of radiation. Satellites are sometimes damaged or destroyed during geomagnetic storms caused by high radiation flux.

Biological Effecfs

     Biological effects may be either acute or delayed. Acute effects include radiation sickness or bodily impairment occurring immediately or within a month or so of the exposure. It is the result of very intense albeit short duration radiation. For example, a single hard SPE could result in crew death within weeks after the event. During the Apollo program the plan should an SPE occur was for the crew to immediately abort the mission and return to earth. Since it takes only about a day for SPE particles to reach the earth and only a few more days for the radiation to tail off, this philosophy could not have prevented a possibly lethal exposure. However, the crew would at least he retuned to the earth and receive medical attention within a few days after the event, and the chance of it occurring was small considering the short duration of the missions. This philosophy is certainly not acceptable for longer missions such as to Mars, and so spacecrafi must be adequately shielded for the probable occurrences of SPEs. The goal of shielding the SPEs is to prevent acute biological effects but also to reduce the probability of delayed radiation effects due to the SPEs’ contribution to total mission dose.

     Delayed radiation effects are the result of cumulative exposure to radiation of all intensities and may include sterility, birth defects, or cancer. Unless particularly bad SPEs occur, GCR will be the largest contributor to the cumulative dose. Thus, although the intensity of GCR is too low to cause acute biological effects, it is very important to shield against it. The duration of the Apollo missions was sufficiently short that the cumulative GCR dose was acceptable compared to the dose one receives in ordinary terrestrial life. The total mission dose for long-duration trips such as to Mars, on the other hand, will be unacceptable without significantly enhanced spacecraft shielding.

Secondary Radiation

     It must be understood that the total dose received by the crew is not only due to the solar and cosmic radiation particles themselves, but also due to secondary radiation. This includes bremsstrahlung X-ray radiation which is generated when charged particles decelerate through collision with the spacecraff or with its passive radiation shields. It also includes nuclei ejected from the spacecraft’s passive shields by the colliding solar or cosmic ray particles. Other nuclei in the shields might not be ejected hut yet knocked into energetic nuclear states with short half lives, such that their delayed fission adds to the total dose received by the crew. A beneficial goal of electromagnetic shielding is to gently redirect the incoming radiation away from the spacecraft while still at some distance, thus avoiding the generation of secondary radiation. However, in the case of bremsstrahlung radiation this is difficult with electrostatics because a field which repels the energetic protons will attract and energize the much larger quantity of low energy (thermal) electrons in the vicinity of the spacecraft. It has been pointed out that a radially symmetric field sufficient to repel 3 GeV positive ions (part of the GCR spectrum) would result in bremsstrahlung radiation sufficient to kill the crew. One goal of the present study, then, is to leverage various asymmetries in order to repel energetic protons without attracting thermal electrons.


     It is beyond the present study to relate shielding effectiveness to the radiation dose received by the crew. The intention here is to present an overlooked shielding concept and to demonstrate that it merits further analysis, which is on-going. Therefore, shielding effectiveness shall be quantified simply as a percentage reduction in the number of protons and electrons striking the spacecraft as a function of their far field energy.

     In the older literature it was assumed that energetic solar protons were the result of solar flares, and so the term “solar flare” was used interchangeably with “SPE. However, more recent radiation analyses indicate that there are two categories of solar proton events, the “impulsive” and “gradual” type, and the data imply that impulsive events are due to solar flares whereas gradual events are usually due to Coronal Mass Ejections (CMEs). As explained below, the CMEs, not the solar flares, pose the most significant health threat to the crew. Here we shall briefly review some of the relevant physics, but the interested reader is directed to the referenced reviews or to the primary sources cited therein for a more complete treatment.

Impulsive SPEs

     There are thousands of impulsive solar proton events per year. They each occur over a period of just a few hours during which the radiation quickly rises then returns to normal levels. Each such event finds its source in a small region of the sun, usually correlated to a solar flare. The released particles then helically follow the interplanetary magnetic field (IMF) lines that are spiraled outward from the sun. As a result, particles from a solar flare will reach an observer only if the flare occurs very close to the magnetic field line that connects that observer to the sun. The radiation is described as electron-rich, and since electrons are stopped more easily by the aluminum structure of a spacecraft this radiation is not as dangerous as that released in the gradual SPEs.

Gradual SPEs

     Radiation analyses demonstrate another category of events, the gradual SPEs. On the order of 10 gradual SPEs occur each year but they are very erratic and presently unpredictable. They develop slowly and run their course over the course of several days, and they are almost always correlated to the visible occurrence of a CME. Hence, the crew in an interplanetary spacecraft should almost always have notification early in the event and should be able to reconfigure their spacecraft and go into “safe” mode in the most protected volume until the radiation has subsided. This is an important strategic possibility because gradual SPEs present the most intense radiation and therefore require the most weight and resources to shield. The ability to go into a temporarily non-operational mode may alleviate shielding requirements for the whole spacecraft, save spacecraft weight and make the mission viable.

     The radiation in a gradual SPE is probably due to the acceleration of solar wind particles in the CME shock wave. A CME can be very broad relative to the circumference of the sun, and so it cuts across the magnetic field lines that spiral out to a wide range of latitudes. As a result, the hulk of the accelerated particles are more likely to reach an observer when a CME occurs than when a flare occurs. The shock wave propagation through interplanetary space provides the time scale of the gradual SPE, which explains why it can occur over the course of several days.

     The radiation from these gradual events has been classified as proton-rich. For example, a set of measurements from two events show the ratio of the peak proton flux to the peak electron flux on the order of 103 or 106. The radiation energies and intensities vary widely from one gradual event to another. In a softer event there may he significant quantities of protons at energies no higher than 100 MeV, hut for a hard event the energy spectrum may he significant out to 1 GeV.

     There is a lower quantity of helium and heavier nuclei in a gradual event than there is in an impulsive event due to the composition of the corona where the gradual event’s shock wave propagates. (On the other hand, an impulsive flare occurs on the photosphere and apparently ejects particles from even deeper within the sun.) This lower quantity of heavy nuclei is fortunate because the heavy nuclei are more biologically damaging than single protons, having a higher rate of energy deposition into biological tissues per unit length, or “linear energy transfer” (LET). The number of protons in a nucleus is denoted by Z, so high-Zhighenergy is abbreviated HZE. With heavy nuclei, generally higher kinetic energy results in a higher LET.

     With protons the opposite is true, the lower energy protons have a higher LET and are more damaging than the higher energy protons. This is because single protons cannot fragment during nuclear collisions as they pass through the tissue, whereas HZE particles can, and it is the fragmentation which becomes more damaging at higher kinetic energies. Therefore the energy transfer from protons occurs by other means which are more effective at lower kinetic energies. The ratio of doses received from two equal quantities of protons at 1 MeV and 100 MeV is on the order of 103 or 104. An electrostatic shield will provide significant benefit against gradual SPEs, then, if it deflects all the protons at 100 MeV and below.

     The radiation from a gradual SPE has a significant directionality to it, hut some fraction of the particles have been scattered in transit through interplanetary space resulting in an isotropic component to the radiation. Observations by the ACE and Ulysses spacecraft indicate that the isotropic component may he as small as 10% of the directional component and that fact could he easily leveraged to design a directional isostatic radiation shield, repelling both electrons and protons. However, there is also observational evidence that the anisotropic component swings around to a new direction as the CME shockwave overtakes and passes the spacecraft, probably due to the orientation of the magnetic structure embedded within the shock. These observations also indicate that the isotropic component becomes much more significant as the shockwave overtakes the spacecraft, perhaps equal in amplitude to the directional component. As a result, it is probably necessary to use omni-directional shielding for protection against the gradual SPEs, although some benefit will he gained if the most effectively shielded direction is kept aimed into the peak of the flux.

Galactic Cosmic Radiation

     GCR consists of charged particles that have heen accelerated to high energies via processes that are outside the solar system, probably in supemova shockwaves. As a result they arrive isotropically and at a fairly constant rate. Their penetration into the solar system is described by a diffusion equation modulated by the IMF. As a result the intensity of GCR varies by a factor of ahout two to ten, oscillating inversely with the 11 year solar cycle.

     The GCR presents the most difficult shielding problem due to its very hard energy spectrum and significant quantities of heavier nuclei. The electron and positron component of GCR is small and contributes a relatively small radiation dose to the crew, and protons are actually a worse problem at the lower energy levels such as are commonly found in the more intense SPE radiation. The highest biological damage from GCR is due to the HZE particles. The heaviest element that can he formed in exothermal fusion (normal stellar burning) is the most stable element, iron, the heavier elements being formed only in supernovae and other exotic processes. Hence iron, 56FE+23, is the heaviest of the common ions in GCR and it will cause the highest cumulative radiation dose to interplanetary astronauts.

     The energy spectra per nucleon are modeled as approximately the same for the various GCR nuclei, peaking at approximately 300 MeV/nucleon and then decreasing in intensity through an order of magnitude by approximately 3 GeV/nucleon. Because all the high-Z elements have spectra of energies per nucleon that are about the same, and ionization charges per nucleon that are about the same (approximately 1 proton per neutron, with hydrogen being the exception), all nuclei will undergo about the same range of deflections in an electric field. Hence, all are equally well shielded in electrostatics, hydrogen being the exception and being more easily deflected by a factor of two. For the following simulations we shall therefore use 56FE+23 as adequately representative of all GCR nuclei.

     Because the issue with GCR is cumulative dose, one valid strategy to deal with GCR is to make the interplanetary transit time shorter by developing better propulsion systems. Then in situ resources may he used for shielding against GCR on a planet’s surface. Another strategy is to travel at solar maximum when the intensity of GCR is reduced. This has heen considered a zero-sum strategy because lower levels of GCR correspond to higher levels of SPE radiation. However, because SPE particles are mostly at lower energy levels compared to GCR, it may be possible to design an electrostatic shield that is far more effective against the former resulting in a large net reduction in the cumulative dose when traveling at solar maximum.


Passive (Materials) Shielding

     Passive shields stop energetic particles through a series of collisions. The effectiveness of the passive shield therefore depends upon the atomic and nuclear structure of the material and its thickness. Passive shielding includes the structure of the spacecraft and its consumables which must be present regardless of the radiation environment, so effective shielding will take advantage of these in the spacecraft design. The mass penalty to the spacecraft then is only the additional mass required to attain the desired level of protection. A disadvantage of passive shielding is the secondary radiation produced during the particle collisions, especially for HZE particles. Transport models have been developed to more accurately predict passive shielding effectiveness. With the existing uncertainties, it is thought that an effective thickness of as much as 20 cm of aluminum might be needed for adequate protection. It may be advantageous to create a “storm shelter” in the spacecraft which has additional passive shielding in case a hard SPE occurs during the mission.

Magnetic Shielding

     Magnetic shields would deflect charged particles through the Lorentz force. If the magnetic field is sufficiently strong, very fast particles can be turned with sufficiently small radius to avoid striking the spacecraft at all, and therefore most of the secondary radiation can be avoided. The effectiveness is determined by the strength of the magnetic field, which is limited by the mechanical strength of the conductive coil to withstand the induced forces. While advances in superconductivity may make this form of shielding more attainable and worthy of further study, it is argued that passive shields are still more effective per unit mass and do not carry with them the risk of functional failure bome by active systems3.

3Note that electrostatic shielding also depends upon active components, the electrostatic power supplies, and hence redundancies will be needed for mission safety However, the mass of redundant electrostatic power supplies is insignificant compared to the mass of a redundant superconducting magnetic coil.

Radially Symmetric Electrostatic Shielding

     A very strong, positively-outward electric field could repel high-energy protons. However, it has been noted that this will attract the low energy (thermal) electrons from the vicinity of the spacecraft and accelerate them to high velocity before they collide, thus creating lethal levels of bremsstrahlung radiation. It has been assumed that the only way to prevent this is to surround the spacecraff by concentric spheres of opposite charge. This could, for example, create a weak, negatively-outward electric field to repel thermal electrons from the outer shell, and a strong, positively-outward electric field contained between the next two shells in order to repel high-energy positive ions.

     This scheme is limited by the vacuum breakdown of the conductors, the limiting electric field strength at which electrons are pulled free from their surfaces into a vacuum. This is on the order of 20 MV/m. Keeping below this limit while achieving concentric sphere voltage differences of 3.02 GV (large enough to deflect 56FE+23 at 1.4 GeV) would require the sphere diameters to be many hundreds of meters, obviously too large and heavy to be practical. Furthermore, it requires voltages greater than 1 GV in order to repel GCR, and this is considered beyond the current technology for generators carried onboard light spacecraft.

Plasma Shielding

     Another proposed concept was called “plasma shielding,” which is a particular combination of electrostatic and magnetostatic shielding. The electrostatic field is used to repel positive ions, and the magnetic field is used to capture electrons into an artificial radiation belt. This maintains electrical neutrality so that other nearby thermal electrons are not accelerated into the spacecraft. It has been noted that this concept has not been studied in reference to the higher energy GCR particles. A discussion ofplasma shielding is beyond the scope of the present study.

Asymmetric Electrostatic Shielding

     We have recently noted that there are other possible solutions with electrostatics that have been overlooked. These solutions exist because the asymmetries of the physics and the asymmetries of the radiation shielding problem may be exploited to achieve nearly isotropic protection without requiring radial symmetry in the repulsive field. The idea is that an electric field can turn positive ions slightly away so that they miss the spacecraft much more easily than it can completely stop them and reverse their momentum. To do so the electric field must not be contained within a small region between concentric shells, but this raises the problem of attracting thermal electrons. However, the component of an electric field from higher order terms in a multipole expansion weakens over much shorter distances than does the component from the lower order terms, so it is possible to obtain local deflection of energetic protons around a spacecrafi while at the same time repelling the thermal electrons over a larger region around the spacecraft. In fact, there are numerous other ways that the problem might be solved exploiting the various asymmetries. The key point is that we need not make the leap to concentric shells merely because the repulsion of both electrons and protons is required. In this paper we will examine only one such spacecraft configuration in order to demonstrate the effectiveness of the basic concepts and to explore the feasibility of achieving this shielding with existing technology.


     To demonstrate that the asymmetrical electrostatics approach may in fact be feasible, we have performed numerical simulations using several variations of a particular shield geometry. The goal of the study is to demonstrate effective shielding with low-complexity designs that could he achieved using low mass, low power components and which could be implemented using existing technologies.


     Fortran code was developed to simulate the trajectories of charged particles over the region and scale of space where the electrostatic forces of the shield are predominant. The IMF may be neglected over this scale since the radius of the helical path of a proton due to the IMF is on the order of a million kilometers. Two versions of the code were developed, one in which the radiation flux is highly directional arriving through a cone of adjustable solid angle, and one in which the radiation is isotropic. The simulation uses Coulomb's law and Newton's equations in a straightforward numerical integration with selectable finite time steps. It includes the relativistic corrections but it does not include electromagnetic radiation from the accelerating particles. User selections include mass, charge and kinetic energy of the particles. Other user selections allow the initial flux of the particles to be more or less focused upon the spacecraft so that computational time is not wasted on particles going the wrong way, while at the same time including particles that might hit the spacecraft only because the electrostatic fields steered them into it.

     The shields considered in this study are clusters of inflatable spheres, advantageous because of their low mass and easy deployment in space. Because the radiation will eventually discharge the spheres, they must be conductive4 so that they can be continuously recharged. As a result, the charge on each sphere will not be uniform but will move in response to the potential field that results from the total system of spheres. This introduces computational complexity because the problem of finding the electric field resulting from the interaction of multiple charged conducting spheres is not trivial and has been discussed even in the recent scientific literature. It is important to account for the non-uniform charge distribution because, for example, on two oppositely charged spheres that have large radius and are located close to one other, the charge will gather on the near surfaces between the spheres, producing little electric field anywhere except between the spheres.

4 This study assumes perfect conductivity. Long-term degradation of this and other material properties is an important consideration for future study.

     In order to find the multi-body electric field one may use a variation of the method of images described by Jackson, in which each sphere may be replaced by an infinite series of point charges representing all the possible multiple reflections occurring in the complete set of spheres. In the zeroth order approximation of N spheres, each sphere is represented by a single point at its center so that we have simply a set of N point charges. In the 1st order approximation, each sphere is represented by its original point charge plus the (N-1) reflections of the other points representing the other (N-1) spheres. In the 2nd order, each sphere is replaced by the previous N + (N-1) points plus an additional (N-1)2 points, which are the secondary reflections of the 1st order reflections. The polarity of the reflected point charge is reversed in each reflection and its magnitude is diminished by the ratio of the radius of the reflecting sphere divided by the distance from the point to the center of the reflecting sphere. This method therefore produces an alternating series which, despite the growth in the number of reflections in each step of the series, can be shown to be convergent in Coulomb energy. Hence it must also be convergent in the electric field which represents the storage of that Coulomb energy. This series may be truncated whenever sufficient accuracy is achieved.

     The original central point charge and all of the image charges within the same sphere can be summed to find the total charge on that sphere. However, we wish to specify the voltage of each sphere, not the total charge upon it, so we must solve for the potential somewhere on the surface of each sphere in terms of the entire set of point charges, and this gives us N equations. The magnitudes of all the image charges were written in terms of the original central point charges, so there are only N unknowns and it is possible to invert the matrix and solve for the entire set of charges in terms of the specified voltages. Once the magnitudes of all the point and image charges are known, the electric field can be written for all space in the vicinity of the radiation shield. We may improve the accuracy of the truncated series by averaging the calculations over several locations of each equipotential sphere, or by using other numerical methods to adjust the magnitudes and locations of the image charges in order to find a best fit to the desired potentials specified on the spheres. We have found that as long as the diameters of the spheres are less than a third of the distance between the spheres, then a zeroth-order approximation (no image charges) is acceptable for this stage of the study. This is what is presented in this paper.

Simulated Shield Geometry

     In this study we simulated a shield with the geomehy shown in Fig. 1. No effort has been made to optimize a design yet. The overall structure is a linear quadrupole, with a negative pole at each end and a positive pole in the center where the radiation protection is provided. Each pole consists of a cluster of seven spheres as shown in the figure.

     In most simulations, the spheres are at potentials of about +/- 10 MV relative to their neighbors, and the largest potential used in this study was 30 MV which can he achieved by daisy-chaining three spheres with electrostatic generators between them. Much larger voltages could be achieved by this simple chaining approach which should work well for the space environment with large distances between spheres. For the nominal case, the distance between the centers of the clusters is 100 m, and the distance between the center sphere in a cluster and any of its immediate neighhors is 30 m. The sphere radius is 5 m. In the following simulations we vary these parameters from the nominal values in order to demonstrate the effect.

     The voltages on the spheres are chosen so that the spacecraft has an overall negative potential so that the long-distance monopole term in the electric fields will drive away thermal electrons. Energetic ions will penetrate more deeply than thermal particles into the local fields surrounding the spacecraft where the quadrupole structure becomes apparent. This structure repels positively charged particles from the central region of the spacecraft where the crew is located. This can be seen in Fig. 2. Highly energetic electrons are stopped by passive shielding (such as the protons, and so these are allowed to penetrate the electrostatic shield if they have sufficient energy. These will he decelerated by the overall negative charge of the spacecraft and then accelerated again by the local stmcture near the center, resulting in just a little overall reduction in their energies. Striking the spacecraft they will create some bremsstrahlung radiation. However, these highly energetic electrons are relatively few compared to thermal electrons and so the bremsstrahlung radiation may be kept to a very low or perhaps even negligible level. It may be noted that the highest flux of energetic electrons relative to protons occurs after an impulsive SPE, but this is the most directional source of radiation and so if it were necessary the spacecraft could be oriented axially toward the sun, or an additional directional shield could he employed to repel energetic, directional electrons.


Protons with Nominal Shield Configuration

     Simulations were performed for low-energy electrons, high-energy protons, and very high energy iron ions. An example of the resulting particle trajectories for protons is shown in Fig. 3. This is a projection of 3D trajectories onto the plane of view. The particles are 20 MeV protons, the shield configuration is as shown in Fig. 1 (rotated so the long axis is vertical) using the nominal geometric dimensions and V0 = V1 = V2 = 10 MV. Most of these particles had been aimed into a region wider than hut centered upon the protected zone, which explains the high flux in this region. It can he seen that the flux is repelled from the protected region by the local structure of the fields. Fig. 4 shows 50 MeV protons tired at the same shield configuration. As expected, higher energy particles are not deflected as efficiently as those at lower energies. Nevertheless, a significant reduction in radiation occurs in the protected zone, even for these particles at energies much greater than the shield potentials.

     Figure 5 compares the protected region for 35 MeV protons with the shield disabled on the left and the shield enabled on the right. When a particle strikes the protected zone, its location is noted by a dot in the figure. This demonstrates how the particle flux is greatly reduced. Simulations are repeated for particles of different energies, and the total strike count is obtained for each. A computation is made for each energy of the fraction of particles which would have struck the protected zone but were deflected away from it. The results for this shield configuration are plotted in Fig. 6. A good fit to these data, shown as the dashed line in the figure, was obtained using the purely empirical form,

with a = 100 MeV and b = 50 MeV.

Electrons with Nominal Shield Configuration

     Simulations were then performed in the same way for electrons. Examples of their trajectories are shown in Figs. 7 and 8. The goal is to drive the electrons away with the monopole term in the electric fields so that they do not penetrate into the region where the quadrupole structure is apparent. Therefore, simulations were performed over a larger scale and electrons were considered to have penetrated the shield if they entered a 135 m sphere centered on the spacecraft. All electrons were repelled from this region up to and above 1 MeV, and the effectiveness decayed for higher energies demonstrating anisotropic penetration around the equator of the quadrupole, as would be expected.

The anisotropic pattern of high energy electron penetration is shown in Fig. 9. If it were necessary to reduce the number of electrons penetrating at these high energy levels, an advantage could be.taken from their directionality so that aneven smaller-scale locally-repulsive structure could repel them from around the equator of the spacecraft. The shielding effectiveness for electrons without any such finetuning is shown in Fig. 10.

Iron Ions with Nominal Shield Configuration

     Finally, simulations were performed in the same way for 56FE+23. The efficiency in deflecting HZE particles is not shown because it is identical to the deflection of protons with one exception: the energy scale per nucleon is about half the energy scale of Fig. 5. This is because there are about as many neutrons as protons in the stable isotopes, so the energy per nucleon is ahout half the energy per proton. As a result, HZE particles are not deflected as well as single protons by a factor of 2 on the energy scale.

Shielding Eficiency as a Function of Voltage

     To determine how the shielding efficiency varies as a function of the voltages on the spheres, simulations were periormed with all the voltages doubled. The result is what one would expect: the plot has the same form as in Fig. 6 except that the fit is obtained by doubling the parameters a and b. Shielding efficiency is shown in Fig. 11 (with both axes logarithmic) as a function of proton energy for three different voltage scales. For example, ~10% reduction of 2 GeV proton flux can be obtained by increasing voltages by a factor of 10 as shown by the dotted line.

Shielding Eficiency as a Function of Geometry

     Simulations were performed with three geometrical changes: (1) the distance between clusters of spheres doubled to 200 m, (2) same but also the cluster dimension increased to 40 m, (3) same hut also sphere radius increased to 7 m. The results are shown in Fig. 12. Changing the long axis had a greater effect on a, the scale exponential decay, but changing the sphere radius had a greater effect on b, the scale of the power law. Changing the cluster dimension had little effect in this zeroth order approximation. A greater effect may be expected if the simulation were using the exact charge distribution on the spheres.


     These simulations have demonstrated the hasic concept, that electrostatic fields which are not radially symmetric can provide isotropic protection from positively charged nuclei without attracting thermal electrons. This makes electrostatics appear more useful because effective shielding can be realized without the burden that had been assumed in previous studies, the heavy concentric grids and support structure needed to stabilize them. What this scheme requires are only lightweight balloons that self-inflate under the Coulomb force, lightweight tethers, and a minimal amount of rigid structure such as thin rods.

     Four important questions must be addressed. First, can sufficient voltage be achieved to deflect GCR particles and also the highest energy protons in the hardest SPEs? Second, can the voltage source supply sufficient current to offset the incoming radiation current? Third, can the spacecraft maintain its overall (monopole term) charge state? And fourth, what effects will electrostatic shielding have on other aspects of spacecraft design?

     First, in order to deflect 100% of the 56FE+23 ions at 1.4 GeV per nucleon (78.4 GeV) and below, the voltages io the nominal shield configuration would need to be increased by two orders of magnitude. This is not possible using current technology. However, the effective voltage could be increased by several methods: by increasing the number of spheres; by increasing their diameter; by daisy-chaining electrostatic generators between multiple spheres; and by using other geometries. Further study is needed to see if these are feasible. Even if the effective voltage is only increased by a factor of 40 this will result in a 50% reduction of 56FE+23 ions at 1.4 GeV per nucleon so there may be significant benefit to using electrostatics in conjunction with passive shielding to reduce the total dose. Electrostatics may result in a net mass reduction by reducing the required passive shield mass including “storm-shelter” shielding for hard SPEs.

     Breakdown voltage, which was limiting in the concentric sphere design of electrostatic shields, is not an issue in this design since long distances between conductors are easily achieved with low mass. The design presented here bas electric field strengths 1 to 2 orders of magnitude helow the vacuum breakdown level, and geometric changes could easily make that lower if required.

     Second, simple electrostatic devices such as Van de Graaff generators are capable of delivering hundreds of microamperes at hundreds of kilovolts to as much as 10 MV. Also, since ripple is not an issue, an alternating current could he rectified to produce several megavolts at 30 mA. The latter is 18.75 × 10+15 electrons per second, or 2.4×10+11 particles/sec/cm2 on a 5 m radius sphere. This is on the order of the solar wind. It should he relatively easy to extend the existing electrostatic generator current capacity by a factor of 10 if the need existed.

     Third, if solar wind and thermal electrons are being repelled, then the corresponding low-energy protons are being attracted. They will not have sufficient energy to penetrate the inner structure of the quadrupole and create a bremsstrahlung hazard to the crew, but they will strike the outer (negative) poles of the spacecraft and make it necessity to eject a net current back into the plasma. Also, particles passing by the spacecraft will radiate as they are accelerated through the interaction, and as a result they will lose energy. Electrons will be repelled no matter how much energy they lose, but some fraction of the protons can accumulate as a cloud around the spacecraft. This should not degrade shielding because any thermal electrons which manage to penetrate that cloud will be repelled from its core where the spacecraft is located. However, as the protons coutinue to lose energy by radiation and spiral inward, they will increase the positive current into the negative poles of the spacecraft. Occasional pole reversal may be needed to blow these orbiting particles away (with only a small additional integrated GCR dose to the crew). The net negative potential might be achieved through fine-tuning of the neutralizer keeper current in an ion engine (if the overall spacecraft charge need not be too far from neutral) or by extending an additional positively-charged sphere away from the spacecraft on a tether.

     Fourth, if electrostatics is used in a spacecraft then they would need to be integrated with the overall vehicle’s structure and operations at a fundamental level. This is because the vehicle’s metallic structure will interact with and affect the fields. Also, deploying the balloons via tethers and flexible rods would be an important part of the mission which must be accomplished successfully before committing the vehicle to a Mars trajectory, lest after the point of no return the mechanism are found to be jammed. Perhaps most importantly, the use of ion engines would need to be carefully integrated with any electrostatic shielding system because the fields may deflect the jet of ions and affect thrust efficiency. It may be necessary to avoid a monopole term and instead rely upon other asymmetric structure when ion engines are a part of the spacecraft.


     Further advances need to be shown before electrostatics becomes a viable shielding method, However, these advances are not out of reach. Only a factor of 40 improvement is needed in the effective voltages to get a significant reduction in GCR, and of this a factor of 4 may be easily achievable by geometric changes in the overall shield and by increased surface area of the spheres. It is not unreasonable that a factor of 10 improvement may be made in electrostatic generators when the need is presented to industry. Furthermore, some of this improvement could be achieved simply by placing generators in series and in parallel to obtain the needed voltages and currents, which is equivalent to making the belt longer and wider on a Van de Graaff machine. This is especially easy in the space environment where long distances can be maintained between conductors of different voltage.

     The basic reason this new electrostatics shielding concept is more feasible than concentric sphere electrostatics is that radiation particles are gradually steered away over long distances instead of abruptly reversed close to the spacecraft. This is possible because the statement that concentric spheres are needed to solve the thermal electron problem is too restrictive. The thermal electrons are at much lower energies than the particles that cause biological harm and this disparity can he leveraged to find a more finessed solution. The shield desiga shown here is not a final solution, but only a first attempt to demonstrate the concept. The ease with which a viable design was found implies that there may be much better designs waiting to be discovered.

     If a design is found which integrates well with a spacecraft structure, it will have the benefit that it will not cause secondary radiation. It reduces the needed mass of passive shields and in some cases may allow the passive shields to be more directional. Electrostatic shielding might also be useful for spacecraft operating in the radiation belts of planets. We conclude that further study of asymmetric electrostatics is warranted witb the goal of moving toward a possible technology demonstration flight in the near future. Ongoing work is addressing these issues.


It takes a couple of years for a crew of astronauts to sojourn to Mars and back. In that time the team would be exposed to enough radiation to significantly increase the chances of each of them dying of cancer, says Roberto Battiston, Professor of Physics at the University of Trento in Italy. With a crew of five there is a 20% probability that one will die of a cancer caused by radiation damage from the trip, he says.

So Battiston and his colleagues are developing a remedy that sounds like something from the starship Enterprise. It’s called the Space Radiation Superconductive Shield (SR2S). It is effectively a superconducting magnetic energy shield that mimics the protective effect of our planet’s own magnetic field, deflecting cosmic rays away from the crew’s precious cells.

A magnetic shield to protect spacecraft is not an entirely new idea. It was first proposed by German rocket scientist Wernher von Braun at the dawn of the space age. As von Braun and his contemporaries well knew, running an electric current through a wire creates a magnetic field around it. So he proposed looping electrically charged wire around a spacecraft to deflect charged cosmic rays away.

But there’s a problem: a magnetic shield drains precious energy that needs to be conserved for other uses in space flight.

This is where superconductors step in. Normally if you run an electric current through a wire some of the charge is lost due to resistance as electrons bump into atoms on their way through. Superconductors, on the other hand, allow electrical current to flow utterly unimpeded – the electrons just keep on going. So the beauty of a superconducting shield is that, once charged, it doesn’t consume any energy. “The coil can be charged using solar arrays, only needing tens of kilowatts, so nothing very dramatic,” says Battiston. “And once it’s charged, it stays charged for years due to the superconductivity.”

Superconductors only work at very low temperatures, which makes space the ideal place to use them. “The average temperature of the Universe is very low, less than 10 Kelvin [-263° C],” says Battiston.

But all it takes is for one tiny spot on the superconducting coil to get slightly too warm as it catches the Sun or from heat trickling from the crew’s quarters and it can suddenly lose its superconductivity, a phenomenon called “quench”. That spot can rapidly heat up to dangerous temperatures, burning out the coil. So Battiston’s team is working hard to develop lightweight, low energy cryogenics to keep the coil cool.


A Deep-Space Radiation Shield

The single largest obstacle to manned interplanetaly space flight is not efficient drives or long-term life support, it is the radiation that floods interplanetary space. This comes in two major varieties—the relatively soft protons and electrons streaming out from the sun (the solar wind), and the very hard interstellar cosmic rays, some of which have the impact of a major league fastball packed into a single heavy nucleus.

Earth’s magnetic field and atmosphere protect us from most of this radiation and in low earth orbit the doses are tolerable. Once you get beyond the Van Allen belts the situation changes drastically. The Apollo flights were possible only because they were short enough that the radiation risk was considered acceptable. Any long-duration flight, such as one to Mars or the asteroids, would be out of the question without shielding.

There are five basic possibilities for shielding a spacecraft—mass, plasma volumes, electrostatic fields, confined magnetic fields and unconfined magnetic fields.

None of these are perfect solutions, particularly against the heavy ion component of the cosmic ray background, however, the one with the most promise is unconfined magnetic shielding this is what works for the Earth, after all. The Earth’s magnetic field is tenuous here on the surface, but the dynamo that drives it is the entire iron inner core of the planet and the total energy in the field is tremendous.

The 1000 Tesla field mentioned in the story is about twenty times the strongest constant field yet produced on Earth (about 45T, at the National High Magnetic Field research facility at Florida State University). Given the material properties of the buckytube components used in the story, this is a conservative estimate, and 5000T might not be out of the question. A 1000T field is starting to get within reach of the required field strength for a reasonable shield, although much depends on the details of the design and on the characteristics of the cosmic radiation profile, both of which are sketchily understood right now.

One interesting side effect of a magnetic radiation shield is that it would capture particles from the solar wind and produce its own miniature version of Earth’s Van Allen belts. The drag caused by this capture would exert a small but steady force on the bubble which would over time produce tremendous velocities, and magnetic sails like this have been considered along with light sails as a potential interplanetary drive mechanism.

There are important differences though—a light sail uses photon pressure from the sun acting on a huge reflective sheet and can be positioned to “tack” a spacecraft up or down in the sun’s gravity well. A magnetic sail of this type would only be able to accelerate away from the sun, like a surfer riding a Wave in to shore. A ship which used a magnetic shield and a light sail would be able to accelerate rapidly out from Earth but would still be able to make it back down the gravity well on the return leg of the mission. This raises the rather beautiful image of space captains laying courses balancing the solar wind and photon pressure just as clipper captains balanced trade winds and ocean currents.

From THE SCIENCE IN THE STORY (of the Cutting Fringe) by Paul Chafe (2004)


There are some medications that can offset the harmful effects of acute radiation exposure, but there is a limit to the protection they can offer.

If there is nuclear fallout or a release of radioisotopes/fission fragments into the air, people in the area should immediately take a potassium iodide tablet. While none of the fission fragment elements are particularly healthy, Iodine-131 is particularly nasty. This is because ones thyroid gland does its level best to soak up iodine, radioactive or not. Your thyroid will quickly become saturated with deadly iodine-131 and thyroid cancer will ensue. Potassium iodide pills load one's thyroid with safe iodine, so it be sated and thus ignore any deadly iodine-131 that passes by.

Obviously potassium iodide tablets provide zero protection against any of the many other radioisotopes.

The body's hematopoietic (blood forming) tissues are seriously damaged with exposures of 1 gray or more. This is mainly the bone marrow, causing damage to blood-cell production and the immune system.

The standard treatment is Granulocyte colony-stimulating factor (kicks the surviving bone marrow into overdrive), but G-CSF must be administered as soon as possible or it does not help. A new treatment combines thrombomodulin with activated protein C, this will help if administered within 24 hours of exposure.

The gastrointestinal tract is increasingly damage with exposures above 5.5 grays. The regenerative peptide TP508 (rousalatide acetate) will significantly increase survival and delay mortality by activating stem cells (though I am curious as to the exact definition of "significantly"). TP508 may also activate stem cells in other organs besides the GI tract, helping them recover from radiation exposure as well.

The drug dimethyloxalylglycine helps protect the GI tract from radiation damage by blocking PHD proteins. It should be administered within 24 hours of exposure.

Radiation damages a cell's DNA. If a cell discovers too much DNA damage, it commits suicide (to avoid the risk of becoming cancerous). If too many cells suicide, the person dies. Unfortunately the mechanism is set too conservatively, it will kill the cell even if the damage is slight enough to be repairable.

The drug 2-[4-(1,3-dioxo-1H,3H-benzoisoquinolin-2-yl)butylsulfamoyl]benzoic acid (mercifully abbreviated to DBIBB) delays cell suicide and speeds up DNA repair, giving the cells a fighting chance to heal themselves.


Whenever human spaceflight comes up, inevitably someone mentions radiation. Personally, I think the radiation risk is WAY overblown. “Compound conservatism” is rampant, I believe, and gets worse as time goes on and people keep recycling the same sources, adding some safety factor each time. (see here for a slightly longer explanation) Being extra conservative with radiation risk assessment eventually can cause an estimate for the tolerable risk that’s completely detached from reality, leaving very little budget left to deal with the other, much bigger risks if there’s even any money left to do the mission at all!

If we followed EVERYONE’s conservative advice for radiation risk, we’d be asking astronauts to fly in a giant sphere of polyethylene with no windows, hardly any room, and no EVAs ever (no “one small step” moment because of the risk of radiation, let alone a colony). We certainly wouldn’t be flying to ISS as we are now.

That aside, we can look at what IS a reasonably feasible and low-mass approach to dealing with radiation. Instead of the usual water or polyethylene or regolith shielding or magnetic shielding, I will look at a somewhat over-looked option: biological countermeasures. Radiation is, of course, often used to treat cancer. As such, there is a sizable body of work and several possible treatments that limit the toxicity of radiation to normal (non-cancerous) cells (thus allowing a higher dose to be used against cancerous cells, which are protected less). The most studied drug is, I believe, Amifostine. “Amifostine is the only approved radioprotective agent by FDA for reducing the damaging effects of radiation on healthy tissues.” (Cakmak et al)

While most such studies look at the ability of Amifostine to protect healthy cells from cell death and other damaging effects of radiation (such as damage that may lead to neurodegeneration), which seems to be effective (according to Cakmak and friends), what is most relevant to us in this discussion is the effect on a specific type of radiation-induced toxicity: carcinogenesis. People have suggested that stopping cell death may actually increase tumor-related toxicity (I see their argument, but it is much more likely that, due to Amifostine’s free radical scavenging, the total damage to the DNA is reduced) but is that actually true? No. No it’s not:

Paunesku et al:

Amifostine protected against specific non-tumor pathological complications (67% of the non-tumor toxicities induced by gamma irradiation, 31% of the neutron induced specific toxicities), as well as specific tumors (56% of the tumor toxicities induced by gamma irradiation, 25% of the neutron induced tumors). Amifostine also reduced the total number of toxicities per animal for both genders in the gamma ray exposed mice and in males in the neutron exposed mice.

(note: neutrons have a high quality factor, sort of like GCRs)

However, there is the argument that long-term use of a radioprotectant is not very effective, since it could reduce the body’s natural defense mechanisms.

As an aside, these very natural defense mechanisms are exactly why I think the threat posed by long-term chronic low doses of radiation is actually quite low… The body adapts to the constant radiation by increasing its natural repair/scavenging mechanisms… But with a short, very large acute dose, the body does not have time to adapt and its repair mechanisms are over-whelmed. It is these large acute doses that the general risk of cancer is actually based off. I find that extrapolating down from acute doses is incredibly unrealistic (on the ultra-pessimistic side). Aside over.

So, it may be that Amifostine and similar drugs are really most effective against acute doses of radiation. You might want to inject a little Amifostine when you learn a flare is on its way (once you get inside your radiation shelter). BUT I am not entirely convinced that there’s no benefit at all to Amifostine for chronic low-dose radiation. Even so, this whole field has tremendous potential. Imagine, you can potentially reduce the tumor toxicity of a really bad solar flare event by 25% with just a few grams of extra mass! And that’s on top of the benefit you might get from shielding and fast transit. One a per-mass basis, biological countermeasures are essentially unbeatable. This is why I think that if we’re going to spend any resources on solving the radiation problem, it probably should be to maximize whatever benefit we can get from drugs like Amifostine and, say, finding out if we can maximize our bodies’ built-in repair mechanisms through, say, targeted gene therapy. There are examples of extreme radiation tolerance and gene repair in nature that put even some rad-hard electronics to shame, so the ultimate potential (on the physics side) of biological countermeasures is pretty high as well. Biology may be a lot messier and frustratingly complex, but the potential gains make this path toward radiation mitigation worth it. Once developed, a drug or treatment would be very cheap, while shielding your transit craft with tens of tons of polyethylene or something will always be fairly expensive (even with space mining) or at least cumbersome.


The problem of space radiation does not have a well-known techno-fix.  The Sun, particularly at times of solar flares, spews out floods of fast electrons and protons that make the Northern Lights on Earth and represent radiation hazard for space travelers.  Moreover, galactic and extra-galactic cosmic rays include a population of very energetic highly-charged atomic nuclei, frequently iron nuclei, that create a radiation shower in shielding.   These bare nuclei ionize so strongly that, if they encounter the DNA of a cell, will almost always break both bonds of the double helix, making natural DNA repair effectively impossible.

The standard international unit of radiation dosage is the sievert (or Sv), defined as one joule of ionizing radiation energy deposited in one kilogram of mass.  A sievert represents a seriously large exposure to radiation.  A dose of about 4.5 Sv is enough to kill about half of a population of humans in 30 days.  More typical exposures are measured in millisieverts (or mSv), one thousandth of a sievert.  For example, the average human living on the Earth's surface will receive a yearly dose of about 3.6 mSv, a CAT-scan delivers a dose of about 8.5 mSv, a Department of Energy radiation worker is allowed a yearly dose of 20 mSv, and a Mars colonist would receive a yearly dose of about 234 mSv on the surface of that planet.  A 234 mSv dose is not lethal, but it greatly increases the likelihood of mutations in children produced by colonists and the incidence of cancer in later life.  To put it another way, unshielded life on Mars will deliver a dose of ionizing radiation that is 65 times larger than that of the average Earth resident and 12 times larger than that allowed for a DOE radiation worker.  That is enough for a great deal of concern about the health of Mars colonists.

The cells of a living organism damaged by radiation take three paths: cell repair, cell senescence, and cell death.  For very large doses the dominant effect is cell death, bringing with it the symptoms of acute radiation poisoning: immediate hair loss and low blood pressure, nausea and bloody vomiting in 10 minutes, bloody diarrhea and fever in 1 hour, headache in 2 hours, and ultimately death.  For milder exposures the outcome depends on the character of the radiation.  Electrons, muons, x-rays, and gamma rays tend to produce single breaks in DNA strands that, if they don't pile up, can be fixed by the ever-present internal cell repair mechanisms.   Protons and heavier nuclei have high ionization densities that produce more damaging double DNA breaks.  The radiation damage might be in a "junk" DNA region where it would have little effect, but some of the double-break radiation damage will inevitably render the cell non-functional.  Then the cell will either die or go senescent.

In the average human intestinal cells die and are replaced every 10 days, skin cells are replaced every month, red blood cells are replaced every 4 months, and liver cells are replaced every year.  In case of mild radiation exposure, cell death is preferable to cell senescence, as long as it does not add too much to the normal rate of cell replacement.

When cells do go senescent, there is a problem.  They shut down their normal functions and express the protein p16, thereby warning cell-reproduction machinery not to cause this cell to divide and reproduce.  However, because of their internal malfunctioning they also become "zombie cells", sending out harmful chemical messages to their cellular neighbors that create inflammation and disrupt operation.

In a recent test with mice, a quantity of fat cells were withdrawn from test-subject mice and externally exposed to x-ray radiation until the fat-cell population became about 80% senescent.  Then the treated cells were re-injected into the test subjects.  The effect of the presence of senescent cells was compared with a control group that had the same treatment without the radiation exposure and induced senesce.  It was found that when as little as 0.1% of the mouse fat cells were made senescent, this produced observable degradation of the motor-activity and fitness of the test subjects.  The conclusion was that even a small fraction of senescent cells present in living organisms degrades health and fitness.

At present the only remedy for space radiation exposure that has been seriously considered is the use of shielding, which requires lots of mass to be transported into space.  It's envisioned, for example, that if a Mars mission carries with it a large quantity of water for consumption and propulsion reaction mass, the crew must be housed behind water-filled walls to reduce space radiation exposure, and there may also be smaller extra shielded regions to which the crew can retreat in the event of a major solar flare.  Such shielding requirements greatly complicate manned-mission design.

Therefore, I have a suggestion for an alternate and complementary way of dealing with the problem of space radiation.  In my Alternate View column "Can We Cure Aging?", published in the May-June 2018 issue of Analog, I described a radical new DNA-based technique developed by the Seattle-based startup Oisin Biotechnologies for reducing the effects of human aging and for treating cancer.  Oisin has sequenced a plasmid DNA-ring that, when deposited inside a cell wall by a bubble-like liposome, detects whether the cell has become senescent and is expressing the protein p16, and if so triggers a suicide gene that causes the senescent cell to neatly disassemble itself and go away.  Oisin has also developed an alternate plasmid that detects the expression of protein p53, which is a signal that a cell has become malignant and cancerous.

Ionizing space radiation is normally not so intense as to produce massive cell death in exposed humans in space.  Rather, cell damage accumulates over a period of time, with the rate of damage accumulation much larger than in environments on Earth.  That accumulated damage mainly takes the form of senescent and pre-cancerous cells.  With the added body-burden of senescent cells, the space-traveling humans will have reduced fitness, premature aging, and a much greater cancer risk.

The good news is that by applying the Oisin treatment, damage from space radiation at moderate exposure levels producing senescent and pre-malignant cells can fixed.  The damaged cells will be swept away, to be replaced by healthy ones, potentially restoring space traveling humans to optimum fitness and providing the ability to better withstand the effects of space radiation.

I should also mention a slight downside to the Oisin treatment.  It has been discovered that senescent cells play an valuable role in the healing of wounds, by sending out chemical signals that promote the wound closure and healing.   Therefore, an astronaut in a hypothetical situation in which he is both wounded and exposed to radiation should be treated for the wounds first and for the radiation exposure only after the wounds have healed.


Space Radiation: The NASA Space Radiation ebook: nasa_space_radiation_ebook_0.pdf

Senescent Cell Clearance in Transgenetic Mice:
Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders", Darren J. Baker, et al, Nature 479, 232-236 (10 November 2011); see also
"Ageing: Old cells under attack", Daniel S. Peeper, Nature 479, 186-187 (10 November 2011)

Oisin Biotechnologies web site:


Genetic Engineering

Tardigrade are microscopic animals that do not grow larger than 1.5 millimeters or so. Ordinarily they would be very forgettable creatures, were it not for the disconcerting fact that the blasted things are almost indestrutable, or at very least invincible.

They can withstand pressure of 6,000 atmospheres (about six times the water pressure at the bottom of the Mariana trench). They can withstand the zero pressure of outer space. They can survive a temperature of −20° C for about thirty years. They can survive a temperature of 151° C for a few minutes. They can officially survive being dehydrated for ten years, though there was one report of a 120-year-old dehydrated specimen waving one of its arms.

One wonders if laminated tardigrades would make good combat armor.

But more to the point, those little adamantine monsters can withstand 1,000 times more radiation than other animals. 10 grays is certain death for a human being. The median lethal dose (LD50) for a tardigrade is 5,000 freaking grays of gamma rays or 6,200 freaking grays of heavy ions. This means you could irradiate a bunch of tardigrades with sixty times the radiation that would instantly put a human into a coma and kill them in 24 hours and half of the blasted tardigrades would survive!

Naturally scientists were interested in [a] how the heck do they do this? and [b] can we teach humans to do this as well?

Scientist Takuma Hashimoto et al figured it out, and published their results in a paper Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein

Using tandem mass spectrometry they discovered a previously unknown protein that they gave the boring name of Damage suppressor (Dsup). The stuff stays inside the nuclei of tardigrade cells and apparently wraps itself around the nuclear DNA.

Using standard HEK 293 cells as experimental vectors, the researchers did genetic engineering to give the experimental cells the gene for Dsup. Then they subjected the cells to 10 grays of radiation. The Dsup cells had only 48% of the single-strand break radiation damage, 40% of the double strand break radiation damage, and only 25% of the reactive oxygen species radiation damage; as compared to ordinary HEK 293 cells. Which is an amazing increase in radiation resistance, expecially just from a single new stupid protein.

The bad news is that apparently the only way to obtain this radiation protection is to do genetic engineering on the astronaut's cells. Which has quite a few ethical problems. But study of the Dsup protein will eventually reveal the mechanism of its protection, and may lead to radiation protection that is a bit less invasive than mutating your cells.

Dsup is probably not the only anti-radiation measure in the tardigrade's genome. For example, its genome contains more copies of an anti-oxidant enzyme and a DNA-repair gene than any other animal.


For astronauts preparing to spend a long summer vacation on Mars, hats and umbrellas might not be enough to protect them from the sun’s harsh rays.

And just like beachgoers slathering on sunscreen, explorers on the moon or Mars may one day shield themselves using creams containing a new bioengineered material called selenomelanin, created by enriching the natural pigment melanin with the metal selenium.

Outside the Earth’s protective magnetic field, humans are exposed to many types of dangerous radiation, according to NASA. This includes damaging ultraviolet radiation, X-rays and gamma rays from the sun, as well as superfast subatomic particles called galactic cosmic rays that originate outside our solar system. 

The invisible accumulation of DNA breakage caused by these space rays can lead to cancer and, in high doses, radiation poisoning and death. Traditional countermeasures, such as lead or water shielding, tend to be heavy and greatly increase the cost of a space mission.  

Enter melanin, a broad class of naturally occurring pigments found in animals, plants, fungi and bacteria. Some types of melanin provide humans with their wide range of skin, hair, and eye hues and help protect us against the sun’s ultraviolet light

“Melanin is ubiquitous and enigmatic,” Nathan Gianneschi, a biochemist at Northwestern University in Evanston, Illinois, told Live Science. "We don't fully understand it."

Animal melanins are divided into two main forms, eumelanin and pheomelanin, according to a 2014 paper in the New Journal of Science. Eumelanin is responsible for conferring black or dark brown colors like that of skin and hair, while pheomelanin contains the element sulfur and tends to be reddish or yellowish, and is found in red or auburn hair and in human lips. 

Pheomelanin also absorbs X-rays much more efficiently than eumelanin. Knowing this, Gianneschi and his colleagues took some pheomelanin derived from red rooster feathers and tried to see if they could make it even more protective. 

Instead of sulfur, the researchers swapped in the element selenium, which sits just below sulfur on the periodic table of elements and is known to play a role in cancer prevention in organisms. By doing so, they created selenomelanin, which has never before been seen in nature. 

During lab experiments, skin cells treated with selenomelanin were able to shrug off doses of X-ray radiation that would be lethal to a human being. The selenomelanin was absorbed into the cells and formed what Gianneschi called “microparasols,” or tiny shields around the cells’ nuclei, where DNA is stored. The cells took on a naturally brown or tanned color when they absorbed selenomelanin, Gianneschi said.

Additional tests demonstrated that engineered bacteria fed selenium could produce selenomelanin, meaning the substance could be manufactured in space. The results were published July 8 in the Journal of the American Chemical Society.

“I love it,” Radamés J.B. Cordero, a microbiologist who studies melanins at Johns Hopkins University in Baltimore, Maryland, and who was not involved in the work, told Live Science. “The authors present a neat example of how biology can inspire the design of a melanin analog.”

The material will still need to be tested on human beings and in space to see if it confers the same protection, he added. Gianneschi has already been contacted by other groups interested in studying his team’s intercellular sunscreen. 

The fact that it is lightweight and can be created from basic organic chemicals during a space mission rather than lugged from Earth makes it quite attractive, Gianneschi said. Selenomelanin could even be incorporated into clothing, such as the lining of a spacesuit, where it could work much like the microparasols to confer continuous protection, he added. 

He and his colleagues are now wondering if the chemical they synthesized might already be present in nature, perhaps in fungi that live in high-radiation environments. 

Example: Atomic Engine Design

This section is sort of a case study on the many ways atomic radiation impacts on atomic engine design. This is taken from Radiation Effects on a Nuclear Rocket Engine Systems (1961)

The report is about the Kiwi-B2 experient of the Los Alamos Scientific Laboratory. It goes into great detail on the design challenges created by the radiation enivronment. The report details what it would take to alter the Kiwi-B2 to create a practical engine.

Obviously the radiation affects everything. Neutron heating and Gamma heating puts thermal stress on the engine components, structure, and propellant tanks. Neutron embrittlement damages sensitive materials and electronic equipment. Ionizing radiation disrupts electrical systems. The grim spectre of neutron activation will transmute once-safe engine components into radioactive isotopes spitting death in all directions. This will present a severe radiation hazard to the crew tasked with maintenance, disassembly, and inspection.

If that wasn't enough, allowances must be made to ensure the blasted engine will actually operate in the space environment. Passing tests on the ground with flying colors is no guarantee it will work in space. On the ground there is an atmosphere and the ground. Both can backscatter neutrons back into the reactor core, acting like additional neutron moderators that will not be present in space. Real embarrassing if the reactor won't react once you get it into orbit. Plus the atmosphere will give additional convective cooling to the reactor, which will also be absent in space. Melting in orbit is even more embarrassing.

The priority focus of the report is trying to optimize the balance between "reactor — tank" separation and radiation shield weight. Because the rest of the design hinges on it. Remember that radiation shields have outrageous high weights but are compact. Distance is anything but compact. In theory distance has no mass, but in reality you at least need a structural spine whose mass increases with distance. By using graphs one can determine the sweet spot: that perfect balance of shield mass and distance mass which adds the lowest additional mass for a given radiation protection level.

Secondary focus was on specifying components such that they were light-weight, resistant to radiation damage, and resistant to neutron activation.

Kiwi-B2 Description

As per the design parameters the engine assumes the use of a LASL Kiwi-B3 reactor, a Rocketdyne Mark 9 pump + turbine, and a modified Kiwi B nozzle. The diagrams above show the nozzle with lines for expansion ratios of 50:1, 75:1, and 100:1.

The turbines can be energized in any of three different ways

  1. Generator: turbine driven by hot gases created in a separate gas generator
  2. Bleed Cycle: turbine driven by hot gasses tapped from the reactor exhaust
  3. Topping Cycle: turbine driven by hot gasses created by pumping stolen cold propellant through special heat exhanger pipes passing through nozzle and reactor

Kiwi-B2 design chose to use Bleed Cycle because it has a performance advantage over the gas generator method, and because the topping cycle lacks over-all engine flexibility.

The reactor power is 1500 megawatts, propellant mass flow through reactor is 83 lb/sec, chamber pressure is 816 psia, and propellant temperature at reactor outlest is 4500° R.

In the design process it became glaringly obvious that the propellant tank was going to need protection from reactor radiation. Or the propellant would start violently boiling and the tank would explode, this is a bad thing. This is the reason for the focus on balancing the distance separation with the radiation shield weight: if the boiling is a show-stopper you must prevent it, but every gram counts.

Radiation Shadow Shield

True, the primary job of the radiation shield is to prevent lethal radiation from destroying the ship and killing the crew. But there are other concerns besides how much radiation they stop.

The shield should be as light as possible because every gram counts. It should be strong enough to withstand rocket acceleration, requiring additional supporting structure is adding more grams. Since it is stopping radiation it is going to grow hot, so it will need adequate cooling. This also means it has to be constructed of materials that have good heat transfer properties and high-temperature strength.

Obviously it will have to stop both neutrons and gamma rays. Just stopping one type will be a deadly mistake.

An important point is it should not reflect neutrons. Otherwise it will increasing the amount of free neutrons in the core, and could cause a runaway reaction.

In the report, they figured that the best shield material that fulfilled all the requirements was boronated graphite.

Balancing "Reactor — Tank" Separation with Radiation Shield Weight

In the three graphs below, Reactor — Tank Separation is displayed on the bottom scale ( abscissa ) and Radiation Shield Weight is indirectly displayed at the "Attenuation Factor" curves (the attenuation implies the thickness of the radiation shield). The side scale (ordinate) is the system weight change. The idea is to find the lowest value for system weight change.

There are three graphs because there are three mathematical propellant flow models: Potential Flow Model, Completely Mixed Flow Model, and Recirulation Flow Model.

The minimum system weights for each of the three models is:

Fig.Flow ModelPower
4Completely mixed150012001665200

which you can see is the lowest curve point of all the curves, in the three graphs below.

The report is vague, but I gather they went with figure 5: separation distance of 17 and attenuation factor of 6. These were used in the radiation flux plot below. Figure 3 was thrown out because an attenuation factor of 2 is not enough to prevent severe radiation damage to the engine and crew.

Radiation at the "top" has been attenuated by a factor of 6 by using a 14-inch 20% void boronated graphite shadow shield. This protects the crew, but also the feed system and propellant tank (and the propellant, if it starts boiling that will be very bad).

The Kiwi test engine included some explosive devices for emergency break-up and scattering of the reactor core. Alas the reliability of these drastically degrades as they are exposed to gamma radiation (neutron radiation has little effect because explosives have relatively small numbers of hydrogen atoms). There are two radiation effects. First the explosive force is diminished as the radiation breaks down the molecular structure. This can be measure by the amount of gas evolved by a unit dose of radiation, to allow selection of the most resistant explosive type. Second, the radiation can make the explosive unstable and sensitive, so it might "predetonate" (explode whenever it damn well pleases instead of waiting for you to push the button).

A glance at the graph shows that the obvious winner in the explosives contest is good ol' TNT. Its gas evolved line is practically flat, meaning that the radiation has little effect. The maximum dose at the horizontal centerline of the engine is about 1011 erg gm-1 (C) (i.e., way off the right edge of the graph) but the TNT line is flat enough that it seems safe to extrapolate.

Atomic Maintenance


The various controls, tongs, and remote control "waldoes" will reach around or penetrate the anti-radiation shadow shield, and there may be auxiliary lead baffles. Peeking around the baffles is how Rhysling lost his sight in Heinlein's "The Green Hills of Earth".


Remember that the shadow shield will be in the floor, with the engine below that. Closed-circuit TV monitor will help Astro see what he is doing, but if they are damaged, he'll have to make do with mirrors and/or doing it by touch. What he really needs is one of Tom Swift Jr.'s Giant Robots, which were designed to do maintenance inside nuclear power plants. There is more about robots here.

For external repairs, the chief engineer might use something similar to the amazing Canadarm 2, which is currently on active duty on the International Space Station. Unlike the first Canadarm, this one is not attached at either end. Instead, either end can plug into special sockets ("power data grapple fixtures") built at strategic spots on the surface of the station. Canadarm 2 can literally walk on the surface of the station to where it is needed, moving end-over-end like a giant metal inch worm. The main limitation is that each "step" must end at a socket, but this is due to power and control signal issues. A more advanced version might be self contained enough to not require sockets, just hand-holds or other protrusions that it could grab.

Canadarm 2 is quite large, 17.6 meters (57.7 feet) long when fully extended. It can move payloads with a mass up to 116 metric tons.

On your atomic rocket, one would use arm(s) long enough to reach any spot on the radioactive engine.

Hot Soup Wagon

Refueling and maintenance on radioactive spacecraft out on a landing pad will need something with lots of waldoes and probably treads. In olden days (the 1950's) they figured these things would be controlled by men inside lead-lined control cabs, using TV cameras. Nowadays it would make more sense for the vehicles to be remotely controlled drones.

In the old Tom Corbett Space Caded novels, such vehicles were called "hot soup wagons", because the spacecraft in the novels used liquid core nuclear thermal rocket propulsion. Though in reality I doubt that a landed rocket would keep the plutonium fuel molten just for ease of pumping it into the wagon.

The Beetle

Length19' 0"
Width12' 7.5"
Height - cab down11' 7"
Height - cab up26' 7"
Weight170,000 lbs
Ground Pressure35 lbs/in2
Elevation max15'
Rotates360° at 0.8 rpm
Lead wall thickness12"
Hatch opens1 minute
0% grade forward8 mph
0% grade reverse5 mph
10% grade both5 mph
Max reach
from operator
17' 9"
Max weight lift
straight down
2,000 lbs
Max weight lift
with arms
100 lbs
Window thickness23.24"
Flood light250 foot-candles at 15'
Operating ambient
-30°F to 130°F
Drawbar pull85,000 lbs
8 hours

One of the more interesting examples of a hot soup wagon is the Beetle. It was built in 1961 by Jered Industries on contract for General Electric's Nuclear Materials and Propulsion Operation division. It was going to be used in the US Air Force Special Weapons Center to service and maintain a planned fleet of nuclear powered Air Force bombers. The bombers were never constructed and Beetle was scrapped.

You can find all the details in the report AD0402748 USAF Shielded Cab Vehicles Test And Evaluation. This includes the layout of all 120-odd buttons on the control panels, which I didn't bother to include.

On the Beetle, the engine and transmission are located in the front of the chassis, while the operator cab and manipulators are mounted on the rear. The cab walls are solid lead 12 inches thick, clad with a one inch steel shell on the outside and a 0.5 inch steel shell on the inside. The five operator windows are 23.25 inches thick made out of seven panes of leaded glass (same radiation shielding level as 12 inches of solid lead). The cab can lift up to a height of 26 feet off the ground since nuclear bombers are quite tall. Each arm can lift 2,000 pounds yet are delicate enough to pick up an egg without breaking it.

General Atomic Remotes

This is from General Atomic Company - ANL THE-NEXT-STEP Scoping Studies - Vol VII Remote Maintenance Systems

The Aerojet Manufacturing Company got a contract from the General Atomic Company to develop a remote maintenance system for the Ignition Test Fusion Reactor. The neutron flux from the fusion reaction is going to neutron-activate the heck out of the reactor and everything near it. The equipment proposed will be similar to a hot soup wagon used to service future nuclear powered rockets.

The equipment is to have a service life of 10 years, and the replaceable sub-components are to have a service life of 5 years.


     The BMSCM is a manned shielded cab equipped with two manipulators. The cab is suspended from a bridge crane via a telescoping mast and provides maintenance coverage for the upper portion of the reactor. The cab has sufficient space and radiation shielding for two operators. The BMSCM weight is 50 tonne.

     The cab is an octagonal shape, 183 cm across the flats and 267 cm in height. The wall thickness on three forward sides and the bottom is 25.4 cm (2.5 cm of steel and 22.9 cm of lead). The top and five remaining sides are 12.7 cm thick (2.5 cm of steel and 10.2 of lead). Three shielded windows (45.7 cm x 45.7 cm x 45.7 cm thick) are located in the 25.4 cm thick walls at a 15° downward viewing angle. A hinged personnel access hatch with a 20.3 cm diameter shielded viewing port is provided at the rear of the cab.

     Two electromechanical manipulators are mounted on the lower portion of the cab and telescope in a horizontal direction. Manipulator arm length and telescope reach are 138 cm and 165 cm respectively, providing a total reach of 306 cm. The telescoping tube also provides rotation of 360°. The manipulator arm consists of three main elements (upper arm, forearm, and wrist) with an extendable hand which is capable of continuous rotation at the wrist in either direction. Load capacities at the hand are rated at 181 kg with the manipulator elements in any position.

     Closed circuit television (CCTV) is provided to supplement viewing through windows. One CCTV camera with zoom lens, pan/tilt head and illumination is mounted on a telescopic mast. The TV mast is attached to the lower portion of the cab and is capable of projecting 3.8 meters.

     A three bay control console is located within the cab. The center bay contains controls for the bridge, cab mast extension, BMSCM trolley, 45 tonne auxiliary crane trolley, cab rotation, CCTV system, cab lighting, air conditioning system, intercom system, alarm system, status monitors and radiation monitor. The left-hand and right-hand bays provide controls for the manipulators. Each motion of the manipulator is controlled by a knob, spring centered to the "off" position. Velocities in both directions are proportional to the displacement of the knobs. Rotary motions are controlled by rotary knobs and the others by sliding knobs. All are flush to minimize accidental movements. Color coding on the console and on the manipulator provides direct correlation between motions of the two. Thus, movement of a pivot knob toward the "yellow" bar on the controller will move the corresponding manipulator element in the direction of the "yellow" side.

     The control panel provides the necessary controls for grip force, power on-off, and tool power.

     The CCTV monitor is mounted above the center window for convenient viewing by operator(s). The air conditioning unit and the lighting fixtures are mounted on the cab ceiling. A fire extinguisher, first aid kit, and life support equipment are stowed within easy access of the operator(s). Tools and personal effects are stowed in cabinets. Two adjustable chairs are provided and may be folded and stowed when not in use.

     A three bay control console is located in the master control room with controls and monitors identical to those within the BMSCM cab. Thus all BMSCM functions may be controlled remotely and viewed with the CCTV system from within the master control room.

     The BMSCM bridge span is 50 meters with the bridge rail elevation at 16.8 meters above the operating floor. The BMSCM trolley has a load capacity of 50 tonne and supports an extendable cab mast assembly with a 8.2 meter vertical travel distance. The BMSCM floor clearance is 12.5 meters with the mast retracted and 4.3 meters with cab mast extended.

     An auxiliary 50 tonne capacity crane runs on the same bridge as does the BMSCM trolley. This crane controlled from either the BMSCM cab or the remote control console in the master control room.

     In the event of a failure in the BMSCM bridge crane drive system, the 150 tonne facility bridge crane may be used to move the BMSCM to a safe position. Overhead Bridge Manipulator

     The bridge, trolley, mast and auxiliary manipulation are the principal components of the Overhead Bridge Mounted Manipulator (OBMM). This system capable of servicing the hot cell during assembly and disassembly operations. The traveling bridge spans the width of the hot cell (18.28 meters). The trolley travels the length of the bridge and supports the 3 section telescoping mast (7.62 meters retracted, 16.76 meters extended), which has an azimuth drive for the lower section. The auxiliary manipulator is mounted within the mast. Bridge and trolley movement allows the vertical axis of the mast to be positioned over any set of horizontal coordinates within its range of travel. A control located in the operating gallery provide for the remote operations of the bridge, trolley, mast, manipulator, and positioning heads.


     The VSCM is a manned shielded vehicle equipped with two manipulators. The cab is mounted on an industrial truck chassis and provides maintenance coverage for the lower portion of the reactor. The cab has sufficient space for two operators. The VS CM weight is 50 tonne.

     The cab is a rectangular shape 213 cm x 213 cm x 229 high. The thickness of the three sided front wall is 25.4 cm (2.5 cm of steel and 22.9 cm of lead). The top, bottom and three remaining sides are 12.7 cm thick (2.5 cm of steel and 10.2 cm of lead). Three shielded windows (45.7 cm x 45.7 cm x 45.7 cm thick) are located in the 25.4 cm thick front wall. A hinged personnel access hatch with a 20.3 cm diameter viewing port is provided at the rear of the cab.

     Two knuckle-boom cranes are mounted in front of the cab. These cranes have 180° rotation capability at the base and provide a telescoping boom. One electromechanical manipulator is mounted to the end of each boom crane. The manipulator arm consists of three main elements (upper arm, forearm, and wrist) with an extendable hand capable of rotating 300° at the wrist. Load capacities are rated at 181 kg at the hand with the manipulator elements in any position. Manipulator arm reach is 138 cm. Total reach with boom and manipulator extended is 719 cm.

     Closed circuit television (CCTV) is provided to supplement viewing through the windows. One CCTV camera with zoom lens, pan/tilt head and illumination is mounted on a telescopic mast. The TV mast is attached to the shielded cab between the two knuckle-boom cranes and is capable of projecting 3.8 meters.

     A three bay control console is located within the cab. The center bay provides controls for the CCTV system, cab lighting, air conditioning system, intercom system, alarm system, status monitors, radiation monitor and vehicle drive functions.

     The left-hand and right-hand bays provide controls for the knuckleboom cranes and manipulators. Each motion of the manipulator is controlled by a knob spring centered to the "off" position. Velocities in both directions are proportional to the displacement of the knobs. Rotary motions are controlled by rotary knobs and the others by sliding knobs. All are flush to minimize accidental movements. Color coding on the console and on the manipulator provides direct correlation between motions of the two. Thus movement of a pivot knob toward the "yellow" bar on the controller will move the corresponding manipulator element in the direction of the "yellow" side.

     The control panel provides the necessary controls for grip force, power on-off, and tool power.

     The CCTV monitor is mounted above the window for convenient viewing by the operator. An air conditioning unit and lighting fixtures are mounted on the cab ceiling. Fire extinguishers, first aid kit, and life support equipment are stowed within easy access of the operator(s). Tools and personal effects are stowed in cabinets. Two adjustable chairs are provided and may be folded and stowed when not in use.

     The vehicle is a modified industrial lift truck, 549 cm long, 386 cm wide and 350 cm high (top of shielded cab). The truck is powered by a LP-gas engine. The truck has a flat area behind the cab suitable for transporting items. The power control center, containing individual panels for motor control units, is mounted on the truck chassis adjacent to the cab. A cable reel is mounted on the rear end of the truck to provide power, communication and monitoring.

     In the event of a failure of the VSCM, an electric cable winch mounted on the rear of the truck, may be used to move the vehicle to a safe position.

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