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), and the exposure time is "chronic" (see below). This radiation comes from galactic cosmic rays, solar storms, and spending to much time in the Van Allen radiation belts. 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 (GCRs) and Solar Energetic Particles (SEPs). 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 solar plasma. And keep in mind this is with zero radiation shielding. But also keep in mind that it is incredibly difficult to shield against GCRs.

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

From NASA-STD-3000 Man-Systems Integration Standards. (ed note: for the most part, almost all naturaly occuring ionizing radiation is particle radiation. One generally only encounters gamma rays and x-rays from artificial sources, such as nuclear reactors and nuclear weapons. Naturally any NASA document will be silent on weapons.)

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.

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

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 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.

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.

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.

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.

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. 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 an astronaut on an extraterrestrial planet. For instance an astronaut on Mars will suffer 73 mGy/a from cosmic rays and proton storms, while somebody living on Terra is typically only exposed to 0.4 mGy/a from cosmic rays.

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 rule of thumb, 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

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.

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.

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.

The point is, if an unfortunate person received a radiation dose of 1 Sievert over the course of 20 years, they will NOT suffer from radiation poisoning. But an unfortunate person who received a radiation dose of 1 Sievert over the course of five minutes certainly will.

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, minor: 0.005 Sieverts0.05 Sieverts3 Sieverts
Male career limitN/A2 + 0.075 x (age - 30) Sieverts4 Sieverts
Female career limitN/A2 + 0.075 x (age - 38) Sieverts4 Sieverts
accident limit0.25 Sieverts1 SievertN/A
acute limitN/AN/A0.1 Sieverts

For a pregnant woman it is 0.005 Sievert total for the duration of the pregnancy.

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
The Weight

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)

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.


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 rule of thumb, 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 unfortunately 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.


"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. Neutron Activation happens when an ordinary harmless atomic nucleus swallows a 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. 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. Your nuclear engine will gradually turn 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.

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.

Some materials are less subject to neutron activation than others, these are good to use in reactors and nuclear propulsion systems.

Neutron activation is used 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."

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 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 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.

Neutron Embrittlement

Neutron radiation can make some materials become brittle by neutron-induced swelling and buildup of Wigner 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.

Carbon nanotubes improve metal’s longevity under radiation

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.”


Electronics are also vulnerable to radiation (including particle beam weapons) due to mechanical disruption, as you can see from Christopher Thrash's notes. Anthony Jackson says:

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

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 short 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.

However, NASA scientists were looking into something along these lines 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. One source suggested that occurred at a cable length of one kilometer with a one megawatt reactor.

Mile-Long Starship

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.


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.

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.

Philip Eklund points out that 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.

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.

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")

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.

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.

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 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.

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:

Reduction FactorAdditional cm of Lithium Hydroxide for Neutron Attenuation, (+kg/m2)Additional cm of Tungsten for Gamma Attenuation (+kg/m2)
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 = 3609 kg/m2.

So the theory is you calculate radiation flux from the atomic engine, multiply it by the appropriate attentuation factors of the shadow shield, and see of the resulting dose is within acceptable limts.

Which brings us to the problem of calculating the radiation flux from the atomic engine. This is a bit complicated, but there is a first order approximation here.

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

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.

"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.
Radiator Shielding

"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.

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. "Sudden increases" in radiation usually means a Solar Particle Event (SPE) aka "proton storm". 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. In NASA-speak a storm cellar is called a "biowell".

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, or it can be 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). 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.

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

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.

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 (proably 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.

Shields up ready for Mars shot

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.

From Shields up ready for Mars shot by Tim Dean (2014)


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.

Anti-radiation Biological Countermeasures: Amifostine

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.

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.

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. 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.

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.

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

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