RocketCat sez

For all you Earthworms who couldn't be bothered to read the page on Common Misconceptions, I don't care what you saw in the last Star Trek movie. You ain't gonna be able to look out the porthole and see the Klingon battlecrusier ten meters away blazing away at you with sonic disruptors. Toe-to-toe ranges only happen in Hollywood dreck, in the real world the enemy spacecraft will be so far away you'll need a freaking telescope to see them.

Planets, meteors, and spacecraft are detected by Sensors, you know: telescopes, radar, laser radar, thermal detectors. There are two types of sensors: Passive and Active aka Sneaky and Why-Don't-You-Paint-A-Bullseye-On-Your-Forehead-You-Moron.

Imagine you are a soldier with your squad, investigating a segment of the jungle on a moonless night. There is a hostile squad in the area.

Using passive sensors is like sitting quietly and using your ears. Not sending anything out, just listening. It's a pain in the posterior, but at least you are being inconspicuous.

Using active sensors is like being an idiot and turning on a flashlight. Yes, Mr. Soon-To-Be-Pushing-Daisys, it is lots easier to spot the enemy when you send out something, like a ray of light. Trouble is you've just made it ten times easier to spot you.

As the entire enemy squad empties their rifles at the target you've so obligingly made of yourself, your fellow troopers will be cursing your name due to the hail of bullets you've attracted. Later they will probably spit on your grave, at least the survivors will.

Assuming you even get a grave.

First off, there are two broad classes of sensors: passive and active. Passive sensors just detect any emissions from the target, i.e., they passively look for the target. Passive sensors include telescopes and heat sensors. Active sensors emit various frequencies and detect their reflection off the target, i.e., they actively "shine a light" on the target. Active sensors include radar and lidar/ladar.

Active sensors are much better at detection, but have the annoying side effect of virtually placing a huge flashing neon sign on your ship that says: "LOOK AT ME! I'M HERE! SHOOT ME, SHOOT ME!!" . This not only lets all hostiles (detected and undetected) know where you are, but also gives their deadly radar-homing missiles some radar to home in on.

Passive sensors, on the other hand, are more blind but are undetectable. Much better if you are trying to hide. Passive sensors also generally can vaguely detect the presence of objects at a much greater range than active sensors. But active sensors can determine the precise location of an object with much greater precision.

Why? An active sensor emits "pings" of electromagnetic radiation in order to illuminate the target, the sensor "sees" the target if the energy returned by reflecting off the target is high enough to be detected. If the target has a small dimension compared to the angular and range resolution of the active sensor, the strength of the return signal is proportional to the inverse fourth-power of the distance to the target (i.e., signal fall-off is 1/r4).

Why this fall off is 1/r4 instead of the 1/r2 you'd expect from the inverse square law is explained here and here. Basically only a fraction of the initial pulse energy is reflected back. So the target acts as if it was an active sensor emitting pings with a strength of 1/r2 of the original pulse. These pseudo-pings travel back to the original ship, suffering a further loss of 1/r2. This combines to make an effective loss of 1/r4.

But on the third hand an active sensor uses tightly focused pings while a passive sensor has to make do with whatever unfocused radiation flux the target emits.

There is one cute real-world trick. If your active radar pulses mimic radio static, enemy radar detectors will filter the pulses out as random noise and fail to see them. This will make your active radar invisible. Until the enemy catches on to the trick and redesigns their detectors.

In some SF novels, passive sensors are called "sensors" while active sensors are called "scanners."

It would be a jolly science fictional idea to postulate a break-through that could detect passive sensors, keeping in mind that there doesn't seem to be any basis for this in reality. Wave your hands real hard, and vaguely mutter about "psionics", or something based on a Schrödinger's cat-like collapse of wave function (Captain, the wave function collapsed, it means somebody is peeking at us!) or specially trained experts who feel itchy sensations between their shoulders when somebody is looking at them. But to reiterate, this is strictly science fiction.

Detection is something that's been in my family. My dear departed grandfather Charles Haney Davis was a civilian contractor for the US Navy, with a retired rank of Admiral. He worked on the USS Semmes (DD-189) in the 1940's on something that would eventually become Sonar.

Active and Passive

(ed note: the troops are surrounded inside the city, night with no moon)

A recruit turned on his hand light. The veteran beside him snarled, "F**khead! Use infrared on your helmet shield!"

The trooper on the recruit's other side—more direct—slapped the light away and crushed it beneath her boot.

From Counting The Cost by David Drake (1987)

Strategic Combat Sensors

Strategic combat sensors detect hostile spacecraft at long range, giving advanced warning of enemy attack.

Detection Range

First off, as Ken Burnside explains, there is one major way that detection in space is different from detection on Terra's surface: There Is No Horizon. Since Terra is a sphere, the curvature means if you are of average height, the fact your eyes are about 1.7 meters off the ground means anything much further away than 4.7 kilometers will be invisible. That is the distance to the horizon, anything further (that is not outrageously tall) will be hidden below the horizon.

Space don't have no horizon, nohow. The range is pretty much to infinity (or 13.798 ± 0.037 billion light years if you want to be picky).

Yes, there will be a bit of a horizon effect if you and the target are in close orbit around a planet. The target will be hidden for about one-eighth of an orbital period. For something in LEO around Terra, this means it will be hidden for about 15 minutes, max. Which is not really a militarily significant amount of time.

Secondly, there are three different ranges:

  • Detection Range: You become aware there is something out there, at that position in the celestial sphere. You may or may not know how far away it is (e.g., there is a bogey, a blip on the radar screen).
  • Identification Range: You know there is an object of a certain type at range x (e.g., there is a Blortch CL-23 "FenderBender" light cruiser at x 135.2, y 17.3, z 325.1 ).
  • Targeting Range: Your sensors have enough data for a targeting solution (Your casaba howitzers have a target lock on the enemy FenderBender, designated Target Tango 13. You may fire at will. ).

For a given sensor, these range are arranged above in order of decreasing distance.

In space, Detection (as opposed to Identification and Targeting) is basically a matter of time. You can purchase off-the-shelf software fully capable of processing a full spherical sky search and flag any bogeys. The processing power of an average PC graphics card is more than up to the task. Since it takes about three days to travel from Luna to Terra with current technology, it is not like there is any rush.

If the enemy is using torchships, then you can probably spot them with your naked eyes. At least if they are closer than a few astronomical units (1 AU = distance between Terra and Sol).

Once an astromilitary is established, a priority will be to site a sensor satellite at the Sol-Terra L1 point. This will help getting a parallax on the bogeys thus determining their range.

And thirdly, refer to the next section.

There Ain't No Stealth In Space

RocketCat sez

I know this is going start all you submarine lovers and cloaking device fans foaming at the mouth but THERE AIN'T NO STEALTH IN SPACE.

The only way ya gonna get anything close is by a strategically worthless "hiding behind a planet" maneuver, a Harry Potter cloak of invisibility large enough to cover an entire spacecraft, or something equally stupid.

Not that that's gonna stop you from trying. The only thing that cheeses you off more is that smug geezer Albert Einstein sticking a pin and popping your "FTL Starship" balloon. It's people like you that make Nicoll's Law happen. I'm sorry, if you want rubber science you've come to the wrong website.

Wargames like GDW's STAR CRUISER describe interplanetary combat as being like hide and go seek with bazookas. Stealthy ships are tiny needles hidden in the huge haystack of deep space. The first ship that detects its opponent wins by vaporizing said opponent with a nuclear warhead. Turning on active sensors is tantamount to suicide. It is like one of the bazooka-packing seekers clicking on a flashlight: all your enemies instantly see and shoot you before you get a good look. You'd best have all your sensors and weapons far from your ship on expendable remote drones.

Well, that turns out not to be the case.

The "bazooka" part is accurate, but not the "hiding" part. If the spacecraft are torchships, their thrust power is several terawatts. This means the exhaust is so intense that it could be detected from Alpha Centauri. By a passive sensor.

The Space Shuttle's much weaker main engines could be detected past the orbit of Pluto. The Space Shuttle's manoeuvering thrusters could be seen as far as the asteroid belt. And even a puny ship using ion drive to thrust at a measly 1/1000 of a g could be spotted at one astronomical unit.

As of 2013, the Voyager 1 space probe is about 18 billion kilometers away from Terra and its radio signal is a pathetic 20 watts (or about as dim as the light bulb in your refrigerator). But as faint as it is, the Green Bank telescope can pick it out from the background noise in one second flat.

This is with current off-the-shelf technology. Presumably future technology would be better.

Read the essay in the Rocketpunk Manifesto entitled Stealth Reconsidered.

Now I know you do not want to accept the fact that stealth in space is all but impossible. This I know from experience (Every day I have new email from somebody who thinks they've figured out a way to do it. So far all of them have had fatal flaws.). The only thing that upsets budding SF writers more is Albert Einstein denying them their faster than light starships. But don't shoot me, I'm just the messenger. The good folk on the usenet newsgroup went through all the arguments but it all came to naught.

If you are bound and determined to have stealth in space, you will have to postulate some sort of hand-waving technology. Popular in science fiction are "cloaking devices" and stealth as a side effect of the faster-than-light propulsion used by starships ("We can't detect the Zorg ship until it comes out of warp, sir!"). Much more rare is something like a heat radiator, where the radiator sticks into hyperspace to make the heat invisibly go away into the fifth dimension.

It is not like the absence of stealth in space takes all the fun out of things. Sometimes things are more interesting this way. For example, John Reiher shows how to incorporate this in to the tabletop role playing game Diaspora (incidentally, Diaspora has been awarded the Atomic Rocket Seal of Approval).

If you want to really argue on this topic, I'd advise you to cut out the middle man and go directly to and lay your case out before the experts. You might also want to review the section on Respecting Science.

Nicoll's Law

It is a truth universally acknowledged that any thread that begins by pointing out why stealth in space is impossible will rapidly turn into a thread focusing on schemes whereby stealth in space might be achieved.

This is true. Take my word for it, I know from bitter experience.

Happyroachs Corollary to Nicolls Law

Stealth in space discussions invariably boil down to:

  1. A: "Stealth in space is impractical."
  2. B: " But what about [something invariably impossible from a physics or engineering standpoint]?"
  3. A: "That won't work because of [reasons]."
  4. B: "But what about [something else impossible according to physics or engineering]?"
  5. A: "No, because of [reasons]."
  6. B: "I will argue the math now, though I don't quite understand it."
  7. C: "But what about this [impossible according to physics or engineering] thing I read in a SF novel?"

And so on. It can probably be done as a flow chart with no decision diamonds and a loop from C to A.

Why Not? Let me count the ways

RocketCat sez

Oh, you wanna do this the hard way, do ya?

Fine, go pound your head against the neutronium wall, see if I care. First I'll let Ken Burnside psychically predict the future and tell you each objection you'll raise. Then we'll go into savage detail on the major objections.

First off, the answer is NO, you cannot solve the problem by using a thermocouple to convert the heat into electricity.

Ken Burnside said:

Most of the arguments on thermo and space detection run through a predictable course of responses:

  1. "Space is dark. You're nuts!"
  2. "OK, there's no horizon, but the signatures can't be that bright?"
  3. "OK, the drive is that bright, but what if it's off?"
  4. "But it's not possible to scan the entire sky quickly!"
  5. "OK, so the reactors are that bright, what if you direct them somewhere else..."
  6. "What if I build a sunshade?"
  7. "OK, so if I can't avoid being detected by thermal output, I'll make decoys..."
  8. "Arrgh. You guys suck all the fun out of life! It's a GAME, dammit!"
Ken Burnside

Easily the most contentious topic in realistic space combat discussions is stealth in space, or the lack thereof.  There are two main reasons for this:

1. Space is big and dark.  That makes it the perfect place to hide, right?

2. But I want my space submarines.  Why can’t I have them?

Before beginning on the body of detection methods and supposed countermeasures, let us take a moment to consider the criteria for viable “stealth in space.”  Stealth is being able to hide from opposing sensors in a way that aids in the accomplishment of the mission.  The act of being able to hide from some sensors at some arbitrary point is not enough in and of itself.  Usually, advocates of space stealth completely ignore the practical aspects of its use in favor of increasingly complex arguments as to how it can be accomplished under highly specific circumstances, generally coasting in deep space.  

This paper will not cover the science behind detection in space.  For a discussion of that topic, go to Atomic Rockets (Here, in other words).

The typical uses of stealth ships (on the infrequent occasions when the topic of use is brought up, as opposed to proponents describing more and more ludicrous and specific circumstances under which stealth might occur) are either recon or sneaking in close to the target before attacking.  A number of factors conspire to render any stealth system for ships ineffective for either use when compared to alternative choices.

First, there is the simple issue that, even if one can make a system that renders a ship totally undetectable when not using its drive, the ship in question will become visible as soon as it begins to burn.  Not only that, it reveals its mass and velocity as well.  This provides the opponent with the vessel's destination and arrival time even if they later lose track of it, which defeats the purpose of stealth in the first place.  While there are certain types of drives that might be stealthy (low-power mass drivers and cold gas thrusters) both suffer from limitations that prevent them from being used for major course changes on reasonable mission timescales.  The highest practical exhaust velocity that can be obtained from cold gas thrusters is with hydrogen, at just below 3,000 m/s.  The low exhaust velocity, combined with the problems of handling hydrogen, limits the ability of a ship using those thrusters to make any significant changes in its velocity.  Another serious complication is the difficulty of storing hydrogen.  Gaseous hydrogen requires massively large and heavy tanks, limiting practical delta-V to less than a hundred meters per second, and liquid hydrogen is difficult to work with and very bulky, although it would be relatively easy to warm up to operating temperature using the waste heat produced by the rest of the ship.

A low-powered electrically-powered thruster can be used if one can somehow radiate away the reactor’s heat without the opponent detecting it.  If that is the case, the thruster in question will be limited to low power levels, which either will result in very low acceleration, or low delta-V.  In any case, the stealth thruster system will compare unfavorably with the prevailing engine technology, resulting in either extremely long missions (if it is used for all propulsion) or in the opponent knowing the destination (if a conventional burn is made first), which defeats the purpose of the design.  

Second, both missions can be carried out more effectively and cheaply by small unmanned vehicles.  It is far easier to stealth a drone, as it doesn’t have to keep the crew warm.  While the computer might have to be kept well above 3K, it is smaller than a human’s required living space, and can probably keep itself warm.  It is also perfectly capable of running the cameras, and the whole thing is significantly smaller, cheaper, and more expendable than a manned vessel.  

The same applies to some sort of surprise attack vessel.  First, detection is not only by IR systems.  Radar, ladar, and optical systems, including star occlusion are also key elements.  At close range, even an IR-undetectable vessel is likely to be detected by one or more of these, and removing the manned crew will reduce the signature across the board.  A long-range missile pod, launched from a merchant ship on perfectly legitimate business weeks or months previously, or from a mass driver at some obscure base, can stage a surprise attack with a significantly lower chance of early detection, and no worries about the crew’s survival.

Third, a stealth vessel is inherently provocative, and likely to defeat any advantages it has in detectability simply due to the fact that it is a stealth vessel.  Even under normal cruise, most ships will at least have a minor load on the reactor, and consequently some heat signature.  The nature of spaceflight, in both the energies and velocities involved, is going to require monitoring of all traffic, most likely by some sort of transponder beacon.  Military vessels may not have such beacons, but that only means that other powers will track them more intensely.  A vessel that disappears in deep space is going to be cause for concern, either because it’s in trouble or because it’s trying to sneak around.  A query to the relevant civil authorities should lay to rest any question on which it is, which means that any stealth vessel will have everyone watching for it anyway.  For that matter, even a supposedly civilian vessel that goes dead and is headed towards one’s territory will undoubtedly receive a visit from the equivalent of the Coast Guard.  If one detects a stealth vessel approaching one’s territory, or at least in a position to enter one’s territory, one will quite understandably take steps to track the vessel in question.  A submarine, once it submerges, can move in any direction it chooses.  A spacecraft that goes dark is still on the same course that it was before doing so, but it has revealed that it is trying to hide, rendering it of interest to any party it might be attempting to act against.

The exact methods of stealth themselves vary, but most fall into either ignoring thermodynamics, heat sinks or directional radiation.  Other, more exotic means have been theorized. One of the most interesting is called “tailored emissivity”.  Instead of hiding the heat, this would change the radiation spectrum of the ship, hiding it from sensors designed to detect frequencies normally radiated by vessels.  There are questions about the practical utility of this technology, but if they can be overcome, this approach would prove effective against conventional IR sensors. This approach is particularly vulnerable to countermeasures, however.  Broadband scans would still detect it, and once the enemy knows the new spectrum, he will undoubtedly modify his sensors to pick it up.

The only real potential use of tailor emissivity that would give some degree of stealth is on planetary attack craft.  The atmosphere is opaque to radiation at certain wavelengths, and by concentrating its emissions at these wavelengths, a bombardment craft could make itself significantly harder to detect.

Ignoring thermodynamics is a popular method, but one that obviously violates the laws of physics.  Anyone who doubts that a method of dealing with the heat generated by the ship is needed should take an introductory thermodynamics course.  Even if it fails to convince them, they will be too busy with homework to bother the rest of the world about their pet plans to make stealthy spacecraft.

Heat sinks are impractical over any long duration due to the immense size required.  For example, an ice-based heat sink that warms the ice from 200K to 273K, and then melts it to water, would be able to absorb 467.6 kJ/kg of heat, although 333.7 kJ of that would be from melting the ice.  If used as the cold end of a heat engine, the engine would be very efficient, due to the low temperature of the sink, but this would only delay the inevitable.  For example, every kW that must be placed in the heat sink (which would include both the primary power and the waste heat from the reactor, as well as heat generated by the crew) would need 1 kg of ice every 7.8 minutes, or 184.8 kg per day.  Nor can this be vented, as that would substantially increase the visual signature of the spacecraft.  Ammonia is a slightly better choice for a stealthy heat sink, although it will have to be kept under pressure to keep it liquid at the upper end of the temperature range.  From melting at 196K to 273K (where it will be near boiling under 430 kPa or about 4 atmospheres), ammonia will absorb 662 kJ/kg, a 40% improvement over ice.  The efficiency of any heat engines using it as a sink will also be slightly improved because more of that absorption is at the lower end of the temperature range.  However, even the most basic housekeeping load will demand massive heat sinks.  A ship with a housekeeping load of 10 kW, a generation efficiency of 50%, and a 30-day mission will need 78.32 tons of ammonia heat sinks, with a volume of 122.567 m3.  All of these are rather optimistic assumptions, and it should be kept in mind that the added mass and volume of the heat sink fluid and the associated equipment will make the ship easier to detect when under thrust or by visual or active sensors.  Also, it should be noted that the above calculation neglects solar energy input, under the assumption that the ship’s exterior will be at equilibrium temperature.  Solar irradiance at Earth’s orbit is 1.361 kW/m2, although it would not be necessary to sink this entire amount.  If we assume that the ship is to be kept at 273K, then each square meter of ship surface would radiate 315 W/m2.  If we assume that 25% of the ship is exposed to sunlight, and that the heat from that is instantly conducted across its surface, then only 101 watts of heat sink per m2 exposed is necessary.  However, this translates into 13.18 kg of ammonia per m2 exposed per day, which rapidly adds up over longer missions.  Making the ship reflective would of course reduce this requirement, but it would also increase the visual signature of the vessel significantly.  On the other hand, making the ship reflective would also reduce the efficiency of the surface in radiating away heat, unless the covering were somehow tailored so that only the parts exposed to the sun were shiny.  In that case, it is vaguely possible that the ship could get away with no heat sink at all.

Directional radiation is hampered because it is not possible to shrink the radiation cone to an angle smaller than about 60 degrees.  Attempts to build a system that has a smaller angle than this tend to grow in size very rapidly, as the radiator itself absorbs most of the heat, which must then be re-radiated.  At best, the result is a very large object that radiates as an inert body.  This obviously limits its applications, and reduces the number of recon drones required to police for it to a very manageable number.  At very best, this and other suggestions can produce something that looks like a cold asteroid, with a small crew and minimal weapons, assuming that it managed to get on course undetected.  When the target detects a new asteroid heading for them, they will probably become suspicious, and try to work out why they never saw it before.  Ships will be sent to investigate, and the game will be up.

Other criticisms of “everyone sees everyone” are raised on engineering grounds.  These tend to be some of the best thought-out and the most likely to apply in reality.  However, these generally would only serve to limit instant detection, and would certainly not allow stealth in the conventional ‘surprise attack’ sense.  Telescopes and optical systems currently on the drawing board are capable of serving as system surveillance systems given military (rather than scientific) budgets.

Some point to the detection of near-earth asteroids as an example of why universal detection is not feasible.  However, asteroid hunting has never enjoyed more than a tiny fraction of the budgets that are routine in today’s militaries.  As John Schilling put it: “You might as well judge the feasibility of antisubmarine warfare by the frequency with which fisherman, oceanographers, and recreational divers stumble across submarines.”

Another, more interesting, criticism, of stealth in space is that not everyone will have access to a proper set of space surveillance equipment, which could allow stealth to work.  This is obviously true.  A civilian craft is not likely to be plugged into the space surveillance network, nor will any potential pirates or other low-level ne’er-do-wells.  For that matter, a small colony may not have a sensor setup beyond that needed for local control.  If there is only one possible point of detection, then directional radiation becomes at least partially effective, and burns might be able to be hidden from the target by careful trajectory design.  Besides not having remote sensors to catch directional radiation, the targets in these cases would not have capable sensors of their own, and might rely on transponder data except at very close range.

However, this counterexample is not true stealth.  For something to count as stealth, it must be capable of making detection difficult even for a serious opponent in an operational context.  A B-2 is stealthy because it is difficult to detect even for a serious opponent while on an operational mission.  A B-52 is not stealthy even though it could fly over your house on a day without contrails without you spotting it.  While there are many story and/or tactical possibilities in dealing with inadequate sensor networks, the lesson of this section is that sensor networks in space are both easy to create and extremely effective compared to similar networks on Earth.

by Byron Coffey (2016)

For reference purposes, here follows some brief summaries of the more common arguments and their rebuttals.

But Scanning The Entire Sky Takes Too Long

If you are hoping to lose your tiny heat signature in the vastness of the sky, I've got some bad news for you. Current astronomical instruments can do a complete sky survey in about four hours, or less. Presumably future technology can do it even faster.

Ken Burnside said:

A full spherical sky search is 41,000 square degrees. A wide angle lens will cover about 100 square degrees (a typical SLR personal camera is about 1 square degree); you'll want overlap, so call it 480 exposures for a full sky search, with each exposure taking about 350 megapixels.

Estimated exposure time is about 30 seconds per 100 square degrees of sky looking for a magnitude 12 object (which is roughly what the drive I spec'd out earlier would be). So, 480 / 2 is 240 minutes, or about 4 HOURS for a complete sky survey. This will require signal processing of about 150 gigapizels per two hours, and take a terabyte of storage per sweep.

That sounds like a lot, but...

Assuming 1280x1024 resolution, playing an MMO at 60 frames per second...78,643,200 = 78 megapixels per second. Multiply by 14400 seconds for 4 hours, and you're in the realm of 1 terapixel per sky sweep Now, digital image comparison is in some ways harder, some ways easier than a 3-D gaming environment. We'll say it's about 8x as difficult - that means playing World of Warcraft on a gaming system for four hours is about comparable to 75 gigapixels of full sky search. So not quite current hardware, but probably a computer generation (2 years) away. Making it radiation hardened to work in space, and built to government procurement specs, maybe 8-10 years away.

I can buy terabyte hard drive arrays now.

I can reduce scan time by adding more sensors, but my choke point becomes data processing. On the other hand, it's not unreasonable to assume that the data processing equipment will get significantly better at about the same rate that gaming PCs get significantly better.

Now, this system has limits - it'll have trouble picking up a target within about 2 degrees of the sun without an occlusion filter, and even with one, it'll take extra time for those exposures.

It won't positively identify a target - it'll just give brightness and temperature and the fact that it's something radiating like a star that moves relative to the background.

On the other hand, at the thrusts given above, it'll take somewhere around 2 days of thrust to generate the delta v to move from Earth to Mars, and the ship will be in transit for about 1-4 months depending on planetary positions.

Ken Burnside

Call the Belt? The Belt must know by now. The Belt telescope net tracked every ship in the system; the odds were that it would find any wrong-colored dot moving at the wrong speed. Brennan had expected them to find his own ship, had gambled that they wouldn't find it soon enough. Certainly they'd found the Outsider. Certainly they were watching it; and by virtue of that fact they must be watching Brennan too.

The Belt is a web of telescopes. Hundreds of thousands of them.

It has to be that way. Every ship carries a telescope. Every asteroid must be watched constantly, because asteroids can be perturbed from their orbits, and because a map of the solar system has to be up-to-date by seconds. The light of every fusion drive has to be watched. In crowded sectors ships can run through each other's exhausts if someone doesn't warn them away; and the exhaust from a fusion motor is deadly.

From Protector by Larry Niven (1973)
World of Ptavvs

Ships have become smaller, more dependable, more versatile, cheaper, far faster, and infinitely more numerous. There are tens of thousands of ships in the Belt.

But there are millions of telescopes. Every ship carries at least one. Telescopes in the Trojan asteroids watch the stars, and Earth buys the films with seeds and water and manufactured products, since Earth’s telescopes are too near the Sun to avoid distortion by gravity bend and solar wind. Telescopes watch Earth and Moon, and these films are secret. Telescopes watch each other, recomputing the orbit of each important asteroid as the planets pull it from its course.

From World of Ptavvs by Larry Niven (1965)
Surely Sheer Distance Will Hide Engine Burns

According to Dr. John Schilling, the maximum range a ship with its engines blazing away can be detected with current technology is:

Rd = ( 17.8E6 * sqrt( Ms*As*Isp*(1-Nd)*(1-Ns) ) )


  • Rd = maximum detection range (kilometers)
  • Ms = bogey spacecraft mass (tons)
  • As = bogey spacecraft acceleration (G)
  • Isp = bogey drive specific impulse (seconds)
  • Nd = bogey drive efficiency (0.0 to 1.0)
  • Ns = bogey "stealth efficiency", i.e. fraction of waste energy which can be magically shielded from enemy detectors. (0.0 to 1.0)

Current chemical rockets have Nd of roughly 0.95. Ion drives get about 0.50, and steady-state plasma thrusters 0.65 or so — both can in principle be pushed to 0.90 with some difficulty, but not much beyond that. For realistic rockets, Ns = 0.0. There really isn't any way to hide your waste energy from your opponents, short of science fiction.

Here they note that the assumption was a telescope with a Field Of View (FoV) of 0.8° and 0.7 seconds to scan that FoV. At 0.8° the entire sky has about 64,000 FoVs. At 0.7 seconds per FoV scan, that would take about 12.54 hours.

Dr. Schilling says the total sky scan time can be reduced to one hour at the cost of reducing the range by a factor of 3.54. Alternatively the telescope can be fitted with nine detectors instead of one (a 3x3 macro array) which would increase the FoV by three. The entire sky would then be about 7,000 FoVs. At 0.7 FoV scan, a total sky scan would take 1.3 hours.

And of course this was assuming astronomical equipment that was top-of-the-line in 1998. The state of the art has advanced quite a bit since then.


A Russian Oscar submarine has a mass of about 15,000 metric tons. Say it was accelerating at a tiny one-tenth of a g (As = 0.1). A chemical rocket has an Isp of around 450 seconds, an ion drive has 21,000 seconds, and a steady-state plasma has about 30,000 seconds.

This means the maximum detection range of the chemical Oscar is about 1.2 billion kilometers (7.7 AU), and both the ion Oscar and the steady-state plasma Oscar is 25 billion km (167.4 AU). For purposes of comparison the distance between the Sun and Pluto is about 40 AU.

What If I Run Silent And Cold?

"Well FINE!!", you say, "I'll turn off the engines and run silent like a submarine in a World War II movie. I'll be invisible." Unfortunately that won't work either. The life support for your crew emits enough heat to be detected at an exceedingly long range. The 285 Kelvin habitat module will stand out like a search-light against the three Kelvin background of outer space.

The maximum range a ship running silent with engines shut down can be detected with current technology is:

Rd = 13.4 * sqrt(A) * T2


  • Rd = detection range (km)
  • A = spacecraft projected area (m2 )
  • T = surface temperature (Kelvin, room temperature is about 285-290 K)

If the ship is a convex shape, its projected area will be roughly one quarter of its surface area.


A Russian Oscar submarine is a cylinder 154 meters long and has a beam of 18 meters, which would be a good ballpark estimate of the size of an interplanetary warship. If it was nose on to you the surface area would be 250 square meters. If it was broadside the surface area would be approximately 2770. So on average the projected area would be 1510 square meters ([250 + 2770] / 2).

If the Oscar's crew was shivering at the freezing point, the maximum detection range of the frigid submarine would be 13.4 * sqrt(1510) * 2732 = 38,800,000 kilometers, about one hundred times the distance between the Earth and the Moon, or about 129 light-seconds. If the crew had a more comfortable room temperature, the Oscar could be seen from even farther away.

To keep the lifesystem in the spacecraft at levels where the crew can live, you probably want it above 273 K (where water freezes), and preferably at 285-290 K (room temperature).

Well I'll just beam my heat the other way!

Glancing at the above equation it is evident that the lower the spacecraft's temperature, the harder it is to detect. "Aha!" you say, "why not refrigerate the ship and radiate the heat from the side facing away from the enemy?"

Ken Burnside explains why not. To actively refrigerate, you need power. So you have to fire up the nuclear reactor. Suddenly you have a hot spot on your ship that is about 800 K, minimum, so you now have even more waste heat to dump.

This means a larger radiator surface to dump all the heat, which means more mass. Much more mass. It will be either a whopping two to three times the mass of your reactor or it will be so flimsy it will snap the moment you engage the thrusters. It is a bigger target, and now you have to start worrying about a hostile ship noticing that you occluded a star.

Dr. John Schilling had some more bad news for would be stealthers trying to radiate the heat from the side facing away from the enemy.

Besides, redirecting the emissions merely relocates the problem. The energy's got to go somewhere, and for a fairly modest investment in picket ships or sensor drones, the enemy can pretty much block you from safely radiating to any significant portion of the sky.

And if you try to focus the emissions into some very narrow cone you know to be safe, you run into the problem that the radiator area for a given power is inversely proportional to the fraction of the sky illuminated. With proportionate increase in both the heat leakage through the back surfaces, and the signature to active or semi-active (reflected sunlight) sensors.

Plus, there's the problem of how you know what a safe direction to radiate is in the first place. You seem to be simultaneously arguing for stealthy spaceships and complete knowledge of the position of enemy sensor platforms. If stealth works, you can't expect to know where the enemy has all of his sensors, so you can't know what is a safe direction to radiate. Which means you can't expect to achieve practical stealth using that mechanism in the first place.

Sixty degrees has been suggested here as a reasonably "narrow" cone to hide one's emissions in. As a sixty-degree cone is roughly one-tenth of a full sphere, a couple dozen pickets or drones are enough to cover the full sky so that there is no safe direction to radiate even if you know where they all are. The possiblility of hidden sensor platforms, and especially hidden, moving sensor platforms, is just icing on the cake.

Note, in particular, that a moving sensor platform doesn't have to be within your emission cone at any specific time to detect you, it just has to pass through that cone at some time during the course of the pre-battle maneuvering. Which rather substantially increases the probability of detection even for very narrow emission cones.

(Somebody suggested using a continuous blinding barrage of nearby nuclear detonations in order to hide thrusting.)

The timescale of the radiant emission from a nuclear detonation in vacuum is measured in milliseconds. The recovery time of a good CCD array is measured in microseconds. You'll need to detonate nuclear explosives at a hundred hertz, minimum, to cover an accelerating ship. That's going to get expensive.

It also rather clearly indicates where the enemy should start looking...

Dr. John Schilling

The problem with directional radiation is that you have to know both where the enemy sensor platforms are, and you have to have a way of slowing down to match orbits that isn't the equivalent of swinging end for end and lighting up the torch. Furthermore, directing your waste heat (and making some part of your ship colder, a related phenomena) requires more power for the heat pump - and every W of power generated generates 4 W of waste heat. It gets into the Red Queen's Race very quickly.

Imagine your radiators as being sheets of paper sticking edge out from the hull of your ship. You radiate from the flat sides. If you know exactly where the enemy sensors are, you can try and put your radiators edge on to them, and will "hide". You want your radiators to be 180 degrees apart so they're not radiating into each other.

Most configurations that radiate only to a part of the sky will be vastly inefficient because they radiate into each other. Which means they get larger and more massive, which reduces engine performance...and they still require that you know where the sensor is.

The next logical step is to make a sunshade that blocks your radiation from the sensor. This also requires knowing where the sensor is, and generates problems if the sensor blocker is attached to your ship, since it will slowly heat up to match the equilibrium temperature of your outer hull....and may block your sensors in that direction as well.

Well I'll Just Make A Burn Then Coast

If you are actually trying to apply thrust, the upper equation comes into play, and they can see you all over the solar system. What's worse, they can measure the spectrum of your drive to estimate the thrust and use a telescope to observe your acceleration. Simple division will reveal the mass of your ship.

"Well fine!", you say, "I'll just burn once and drift silently"

But now you will be months in getting to your target. The extra time increases the chance that the enemy will spot you. It will be harder to keep your directional radiator aimed away from any enemy observers. And if you are spotted, so much of your ship mass will be radiators instead of weapons, so that the enemy ships will out-gun you by an obscene margin.

Not to mention the fact that once your initial burn is spotted, the enemy will be able to calculate your future position anytime in the future. They can set a computer controlled telescope to track your current calculated position, and will quickly spot any future course correction burns.

(Somebody suggested a ship shutting down and stealthly coasting into enemy range from a billion kilometers away)

That's nice if you can plan your tactical operations six months in advance. Not very likely, at least against a maneuvering foe. Sometime between when you boost and when you arrive, he'll redeploy and you'll have to correct your course accordingly. Which will give you away.

And you can't beat that effect by coasting in really, really fast so as to cross a billion kilometers in a week. Boosting to such a speed in the first place will require so much energy that you'll be detected even from a billion kilometers away. You can back off to twenty billion kilometers, of course, but then you're dealing with that six-month planning cycle again...

Distance cancels out of the math on that one. The detection range scales as the square root of the target spacecraft's drive power, and the drive power required to cross a distance in a given time scales as the square of that distance. No matter how far away you start, you find that there is an irreducable mimimum of time that must be spent on boost-and-coast to avoid detection. Which is generally measured in months. Fine for strategic planning, but not for tactical operations.

Only if you can predict the strategic positions well enough to plan the tactical deployment of your forces during the attack months in advance. Otherwise your space fleet will have to chose between correcting its own course and blowing its cover, opening fire from the wrong position, or aborting the attack entirely.

Accelerating to a proper vector while beyond detection range runs into the fundamental problem of how you figure out what the proper vector is. Even granted that you know the present location of the enemy fleet, you're going to be coasting for a very long time, and you've no way of knowing where they will be months in advance. So you'll probably have to adjust your course somewhere along the line, which means lighting up your engines, which means giving yourself away.

Dr. Schilling

So much for being ambushed by a space pirate appearing out of nowhere. And everybody on a cruiser would know that the hostile bogey would be within combat range in two months, three days, five hours, and thirty-three minutes. You might as well take it easy and get your rest before the battle. You know the cliché: long stretches of boredom punctuated by brief moments of stark terror.

All Right! I'll Use Decoys!

And to forestall your next question, decoys do not work particularly well either. More specifically, a decoy capable of fooling the enemy would wind up costing almost as much as a full ship.

Just to make sure that we are both on the same page here, I am talking about time frames of weeks to months. Such as found when a task force weeks or months away from their target, attempting to fool the enemey observers into thinking that your are a force of twenty warships, when you are actually a force of one warship and nineteen decoys.

I am not talking about time frames of a few seconds. Such as found when a combat spacecraft, with a hostile heat-seaking missile attempting to fly up its rear, dumps off a couple of decoy thermal flares hoping the missile will be confused.

First off, a decoy needs to emit a similar amount of radiation and heat as the ship it is pretending to be. This means each decoy needs a power source comparable in size to a full ship, the same goes for radiator area.

If the decoy and the real ship thrusts, it becomes worse. The exhaust plume has to be the same, which means both the decoy and the real ship has to have the same thrust. This means the decoy has to have the same mass as a real ship, or it will accelerate faster, thus giving itself away. If you down-rate the decoy's thrust, the dimness of the exhaust plume will give it away.

So if each decoy needs a spaceship sized engine in a spaceship sized hull with a spaceship sized mass isn't much of a decoy. Why not add weapons an make it an actual spaceship?

And you'd better add defenses as well. Otherwise the decoy is nothing more than an unusually expensive, unusually easy to destroy missile.

Isaac Kuo points out that all of this assumes that the decoy and the warship are using rocket propulsion. It does not apply if they are using solar sails, laser light sails, magnsails, or other non-rocket propulsion.

But I repeat: while it is more or less impossible to use decoys to fool distant observers, it may be possible to use something like decoys in a dog-fight to protect your ship from enemy short-range antiship missiles. In the latter case, you are not trying to make a fake image of your ship so much as you are trying to break the target lock the hostile missiles have on your ship's vulnerable posterior.

Dr. John Schilling discusses why the exhaust plume of a decoy will have to have the same thrust as a real ship:

Problem is, the rate (i.e. velocity) at which the plasma is coming out, manifests itself as a doppler shift in the characteristic emission lines of the plasma. As soon as a dedicated tracking sensor focuses on the target for a second or two, the game is up. If the plasma is coming out fast, it can't help but produce thrust proportional to mass flow rate (manifested as luminosity) times velocity (doppler). If the plasma is coming out slow (or fast but in opposing directions), it will be seen to be coming out slow and thus be recognized as not a real engine.

Conservation of momentum doesn't leave much room to hide thrust, or lack thereof, in a visible exhaust plume. If you know how much exhaust there is and how fast it is moving, you know how much thrust is being produced, period. Thrust estimation by observing plume properties is in fact a common procedure in laboratory testing of plasma thrusters, and while it's no substitute for a direct mechanical thrust measurement it will certainly provide the sort of order-of-magnitude values needed for decoy discrimination.

Dr. Schilling

The final step for most people comes when they say "OK, so it will always be detected. I'll just launch decoys."

Unless your decoy has roughly the same mass of the ship it's duplicating, and the same engine, it'll be easy to discern. If it's lighter, and has the same acceleration, the decoy's engine signature (which is a function of the mass being pushed) will be dimmer. If it's lighter and has the same engine signature, it'll be thrusting a heck of a lot faster.

Your best decoy is to run with commercial traffic. He may be able to ID it as 20 ships pushing 0.005 gs with a drive output of 25 GW each, giving a rough mass of 5,000 tons each, but he'll have some difficulty (until they get closer) telling which ones are the freighters and which ones are the warships...

Children of a Dead Earth

Zane Mankowski, creator of Children of a Dead Earth, had a few notes about stealth in space:


Few concepts of space warfare have inspired as much controversy (and hate mail) as discussing stealth in space, so I figured it’s time to have an article about that.

For starters, though, I’d recommend checking out Winchell Chung’s website, Atomic Rockets, which has an excellent discussion on this topic, aptly titled There Ain’t No Stealth in Space. I will summarize the main points about stealth here, but for an in-depth discussion of them, see the above link.

  • Carefully scanning the entire celestial sphere takes 4 hours or less.
  • Thruster burns of any drive with reasonable power can be detected all the way across the solar system (billions of km away).
  • Even with engines cold, the heat from radiators attached to life support will be detectable at tens of millions of km away, which is still far too large to get any sort of surprise.
  • Radiating heat in a single direction (away from the enemy) is easily defeated by fielding a number of tiny detector probes which idly coast about the system. Additionally, the narrower of a cone in which you radiate heat, the larger and larger of radiators you need to field. A 60 degree cone of radiation is roughly 10% as efficient, and it only gets worse the tighter of a cone you have.
  • Making a huge burn and then trying to stealthily coast for months to the target is do-able, but as long as your enemy can track your first burn, they can very accurately predict where you’ll be as you coast across the solar system. And you still have to worry about radiating your heat for months.
  • Decoys are only really viable on really short time scales, such as in combat. Over the long term, study of a decoy’s signature over time will reveal it’s true nature. It would need a power source and engine identical to the ship it’s trying to conceal, as well identical mass, otherwise the exhaust plume will behave differently. This means your decoy needs to be the same mass, same power, same engine as your real ship, so at that point, why not just build a real ship instead?

There are a few more points that are not mentioned but I get messaged about them a lot, so I’ll put them here.

  • Hiding behind a planet to make a burn is not really feasible. All it takes is two detectors at opposite sides of this planet to catch this. In reality, a web of tiny, cheap detectors spread across the solar system will catch almost all such cases.
  • A combat-ready ship will require very hot radiators for its nuclear powerplant for use in combat. If these radiators are going to be completely cold for the journey, they will suffer enormous thermal expansion stress when activated. In order to avoid this, very exotic and expensive materials for your radiators will be needed to get from 10 K to 1000 K without shattering. Not only that, your radiator armor will need to be similarly exotic, which means it will likely not be very good at armoring your radiators anyways.

Now there are plenty of dissenting views (as Atomic Rockets is good to point out, as well as rebuttals to the rebuttals). Certain partial solutions, such as using internal heatsinks, and so on, are pointed out, but they all are very limited.

Ultimately, stealth in space is somewhat possible, but current proposed solutions are either ridiculously expensive, impractical, or require you to accept limitations that defeat the purpose of stealth in the first place. Indeed, rather than consider it a ‘yes-or-no’ question, it’s simply a matter of how close you can get to the enemy before they detect you.

In practice, ‘how close’ generally means halfway across the solar system, with expensive stealth solutions reducing that distance only partially. Given this, Children of a Dead Earth runs with the assumption that stealth is not a reasonable military tactic for near future space warfare.

But let’s look at an example of possible stealth: replacing your main engine (nuclear rocket or combustion rocket) with a solar sail. Your exhaust plume is now nonexistent, but now you have to take decades to centuries deliver a military payload anywhere (troops or weaponry). Your best bet is to keep your payload very small if you want to get anywhere in reasonable time. And you still have to worry about your radiators.

Suppose replace your crew module with basic electronics, and do away entirely with the crew and their hot radiators. This is reasonable for any short term space travel, but over the course of months where things can and will go wrong with the ship or the strategic situation, having a human element is necessary. Alternatively, if Strong AI can be developed, this is another possible solution, but this assumes that such an AI won’t require lots of power and heat to radiate as well.

Given a solar sail and crewless ‘dumb’ ships with miniature payloads, you can build ships that can sneak across the solar system and do very little. Such ships would be unable to respond to complex and unexpected tactical decisions, and would be very easy to outsmart, as well as easy to spoof with electronic warfare. They could perhaps be used as mines, given a tiny amount of a delta-v and a small nuclear payload.

Ironically, this specification of tiny, ‘dumb’ stealth crafts is exactly what you need to build a web of detectors scattered about the solar system. This means the field of cheap detectors you want spanning the solar system can be created stealthily.

Defensive stealth in space exists in full force. When you enter orbit of an the enemy’s planet, they might have an inordinate amount military hardware and spacecrafts hidden beneath the surface. But as soon as they launch, the secret is out.

This idea plays a major role in Children of a Dead Earth, as when the enemy drops into orbit around your planet, one must always be wary that the enemy fleet is simply trying to draw out your forces to get a tally on what you actually have. This constantly requires balancing of launching just enough firepower to deal with the enemy without revealing too much about one’s own reserves.

The easiest way to conceal a large amount of military hardware for a long distance invasion is to hide it amongst commercial traffic. Of course, this requires complicity with the civilian traders, either bought with money or intimidation, but it is possible. And such perfidy also plays a key role in Children of a Dead Earth.

With that all in mind, I will admit that at the beginning of my project, I was dead set on getting stealth to work in space warfare. Ultimately, I came to the conclusion that while stealth in space is certainly possible, it is not feasible given mass, cost, and time constraints. If you want stealth, you need to pay the price of decades-long travel times, enormously massive ships, vastly reduced military effectiveness, or all of the above all at once.

At the beginning of the project, I did explore some more exotic solutions to stealth, but I ultimately wasn’t keen on implementing technologies that were not heavily reviewed and published in scientific articles. At some point though in future posts, I will go over all of the more ‘out there’ technologies I considered for all aspects of space warfare (like a hypothetical nuclear rocket which generates an exhaust plume at 30 K, for instance). Stay tuned!

From STEALTH IN SPACE by Zane Mankowski (2016)

A Dissenting View

Matterbeam, author of the always worth reading Tough SF blog disagrees with the "No Stealth In Space" concept. Specifically he is of the opinion that it is possible under certain circumstances.

Actually, I too agree it is possible under certain circumstances, any disagreement is over where one draws the line. Matterbeam is not talking about a Romulan cloaking device that will let that dastardly Romulan Warbird from unexpectedly appearing a couple of meters behind the Starship Enterprise and shooting a plasma torpedo up her tailpipe. He states that a spacecraft is eventually going to become visible to its enemies, but there are strategies that can put that off as long as possible.

It appears that Matterbeam and I mostly differ on our assumptions about sensor platforms. My opinion is that a full-sky scanning sensor capable of detecting a hostile stealthy spacecraft at absurd distances will be so inexpensive that any astromilitary will fill their entire solar system with the little darlings, while Matterbeam says there are plenty of valid reasons that ain't necessarily so. Such reasons can be used by any science fiction author or game designer who wants more stealth. The number of sensor platforms is important because the prime stealth technique is jettisoning waste heat in a direction not seen by any sensor platform. The more platforms, the fewer the safe directions.

He had run a four article series on the topic on his blog, but asked permission to write a couple of specific article for inclusion here. Which I instantly granted. I am a strong upholder of the scientific method, especially the part about it being self-correcting by peer review and data from new experiments. His two articles are below:


Once technological and mechanical factors are accounted for (such as having a large enough telescope lens or having a low enough signal-to-noise ratio), all that matters is the energy output and the energy per square meter received by the telescope. The telescope's sensitivity is the minimum difference between background and target radiation required to create a signal. Sensitivity is a property of the CCD used by the sensor, measured in watts per square meter. In an ideal case, it is be as low as 3×10-19 watts per square meter, or with future technology, lower. This is nearly a hundred times better than sensor technology in the 90's, so expect this figure to become lower and lower over time. However, a realistic sensor has to deal with quantum inefficiencies, signal noise, electromagnetic interference, internal thermal emissions and so on. This can reducing effective sensitivity by a lot.

To obtain the detection range for a point source (such as a poorly collimated exhaust plume emitting in all directions) we use this equation:

Detection range = (0.07958* Waste Heat / Sensitivity) ^ 0.5

If you are using radiating surfaces and a very tightly collimated exhaust (such as high exhaust velocity ion engines), and know the temperature the radiators are operating at, then you can use this estimation:

Detection range = 13.4 * Surface area ^ 0.5 * Temperature ^2

The surface area is that of the radiating surfaces facing the sensor. In flat-panel radiators, this is half the total radiating area. In an angled radiator, it is determined by cosine rules.

(ed note: multiply half the total radiating area by cosine of angle radiator is angled away from the detector. Directly facing: cos(0°) = ×1.0. Turned half away: cos(45°) = ×0.71. Edge on: cos(90°) = ×0)

In a liquid droplet radiator, it is a section through the droplet cloud.

We can immediately see that when using radiators, the configuration with the least detectability has a very large surface area and a very low temperature. However, this leads to very inefficient radiators. Radiators optimized for low temperatures are either very heavy or very fragile. The equations assume that an entire fleet of sensors will be pointed at the accelerating spaceship's position for extended periods of time, and will always maintain optimal sensitivity. This means that the figures you calculate will be the upper limits of detection ranges.

Cold running

If your spaceship is manned, you'll need power input for the life support. You also need to run the various electronics, and re-radiate the heat you get from sunlight hitting your hull. Modern lifesupport requires about 7kW per crewmember for a closed life support system, but a military spaceship during combat would have an open life support system (consumables and filtered water and air), so would only need to heat the compartiment and run the pumps, bringing that figure down to maybe 300W per crewmember. Estimating the power consumption of future electronics is an entire field of study in itself, so a figure of 10-100kW, drawn from modern data center consumptions, down to 1kW in low power mode, can be expected.

A minimal power draw of 2kW for such a small spaceship is to be expected. This can be supplied by a 20% efficient nuclear reactor, producing 8kW of waste heat.

If the dry mass of an example spaceship is 500 tons and its density is 1000kg/m3 (submarine-like construction), then it has a volume of 500m3. We will assume that it absorbs sunlight instead of reflecting it, so it will be optimised for a narrow cross-section. It can fit 5m in diameter and 25m in length.

Facing the sun, it will absorb up to 25kW near Earth orbit, up to 15kW at Mars and lower beyond.In total, the waste heat to get rid off is 25-35kW.

Detection range is between 52 and 44 million km. An improvement, but still an enormous distance.

Redirecting emissions

Let's assume that a whole 20% of the example spaceship's dry mass is devoted to radiators, equalling 100 tons. Most likely, it has a very small, low-temperature circuit for dealing with regular waste heat, and a large, high-temperature circuit for dealing with propulsion heat. The increased temperature allow for better waste heat radiated per square meter. The hull's exterior is insulated and cooled, meaning radiators have to handle the entire waste heat load.

Various radiator designs exist, with various masses per meter squared and maximum temperatures. For the propulsion radiator, we have to deal with 300MW of waste heat. To lower our radiator temperature and reduce detection range, we will use a microtubule array radiator at 34kg/m2.

10000kg radiator mass
294m2 radiating area, or about 30m wide and 10m long.
300MW waste heat
Radiator Temperature = (Waste Heat / (Area * Emissivity * S-B constant)) ^ 0.25
Temp = 2086K

It would have to be constructed from refractory materials such as metal carbides to support such temperatures.

The low temperature circuit only has to deal with 25-35kW. This can be dealt with by a 500kg system of 50 square meters, operating at 350K to remove up to 50kW of waste heat.

The problem can be reduced to the radiator's visible angle.

Simply put, it is the angle between the current and optimal position of the radiator panels. The optimal angle is being pointed edge-on at the sensor platform. With multiple platforms, there might not even be an optimal angle. Let's calculate some values.

We assume thin radiators, so they only radiate from one side:

1 degree visible angle
Under acceleration: 130,000km detection range
Low power mode: 1500km detection range
10 degree visible angle
Under acceleration: 418,000km detection range
Low power mode: 4800km detection range
30 degree visible angle
Under acceleration: 707,000km detection range
Low power mode: 8200km detection range
60 degree visible angle
Under acceleration: 0.93 million km detection range
Low power mode: 10,764km detection range
90 degree visible angle
Under acceleration: 1 million km detection range
Low power mode: 11,607km detection range

We can conclude that this method is extremely effective at low angles, but is essentially worthless as the sides of your radiators become more visible.

Tactically, this means that if your opponents are very far away and are limited in the positioning of their sensors, your initial acceleration will not be detected. As you get closer to enemy positions, the sensor platforms will start seeing the sides of your radiators and your detection range sharply increases.

Strategically, it becomes vital to position sensor platforms at an off-angle from the opponent's likely approach routes, or above the orbital plane (ed note: in positions opponent will figure you do not have sensor platforms, i.e., directions you opponent will direct their radiators). A sensor platform trying to hide near the opponent's planet could have consequences as dire as uncovering nuclear missiles in Cuba, as it threatens every military expendition heading out.


Heat capacity is measured in J/g/K, or the number of joules required to increase the temperature of 1 gram of material by 1 Kelvin.

Water 2.1/4.18/2
Ammonia 4.7
Hydrogen 14.3

Anyone familiar with thermodynamics would know that the heat in the heat sinks does not dissapear. The temperature of the coolant increases, and eventually has to be radiated, either through a cooling system, or by expelling the coolant (ed note: or by the heat sink exploding).

Let's assume that our example spaceship has a 1 GW drive that produces 300MW waste heat. In low-power mode, it has to contend with 35kW. If it uses its entire reserve of propellant (135 tons for 30km/s rocket for Earth-Mars) as a heat sink, it will only absorb about 3 hours worth of waste heat before it has to vented. For a 'cold run', it can cool the crew compartiment for 8 months using a heat pump. This is plenty for a Hohmann transfer.

Using water propellant gives you more mass for a heatsink (lower exhaust velocity — 300 tons), but the lowered heat capacity means it can only hide the spaceship for 2.3 months.

In both cases, open-cycle cooling using high heat capacity materials, usually the propellant reserves, are very effective for small spaceships or manned spaceships drifting through space. An optimized 'hydrogen kettle' could rely on this method entirely, instead of using radiators.

Cold Plate

Instead of insulating the spaceship and drawing the heat away to be disposed of using radiators of open-cycle cooling, the spaceship can be hidden from view using a plate between the sensor and the spaceship. If it is cooled to background temperature, it will render the spaceship invisible to a certain portion of the sky.

The advantage is that the 'cold plate' presents a large surface that is easier to cool and handle than the complex shape of a spacecraft with multiple protruding elements. The total surface area is also lower, meaning it can be very a lightweight solution.

Tactically, the spacecraft is less sensible to pointing errors and non-directional leakage from radiators when trying to redirect emissions away from likely positions of enemy sensors.

The simplest configuration is a multi-layer 'cold plate', with the cold face absorbing sunlight and the hot face reflecting radiation from the spacecraft.

A coolant flow is established to move the absorbed sunlight to the spaceship's radiators. An occlusion angle is angle formed between the plate's edge and the spaceship's rear-most component. Anything within this cone should not be detectable.

Our example spaceship of 500m3 can be reconfigured into a cylinder of 8m diameter and 10m length. A cold plate 10m in diameter placed 1m in front of the long end of the spaceship will cover the spaceship from sensors in a 90 degree cone.

Disadvantages do exist. The spaceship's own sensors would have to be mounted on periscopes with cooled heads. It is hard to design a spaceship that can change the position of the cold plate without moving the entire spaceship. This can be done with a detached plate, but then it would have to be able to cover the spaceship from off-axis angles, where it may be wider and require a larger plate. A spaceship designed to hide behind a cold plate would have an optimal 'short and fat' shape, which contradicts with the requirement of reducing exposed area to sunlight ('long and thin' shape) when not using the cold plate.

Finally, the simple cold plate only cover the spaceship from sensors in one hemisphere. The spaceship is completely exposed to detection from the sides and rear. The solution to that is to extend the edges of the cold plate around the spaceship, increasing the occlusion angle and the volume of space it is undetectable in. However, this reduces the volume of space it can radiate waste heat into proportionally, meaning larger or hotter and heavier radiators pointed directly rear-wards.

Projectile and missile stealth

In some settings, it might not be possible to avoid detection for any practical amount of time. There might be sensors everywhere, or the size of the spaceships and power levels used for travel might be too hard to hide from the prevailing technology used for detection.

Stealth projectiles have numerous advantages.

At tactical ranges, they allow the firing spaceship's projectiles to evade detection for longer from the target's defensive fire. This increases average lifetime of the projectiles and therefore the number that survive the trip and reach the target intact.

At strategic ranges, stealthed projectiles can be used as a deterrent or last-resort weapon. Streams of missiles sent into heliocentric orbits, accelerating using low-thermal-impact propulsion systems or burning against the backdrop of the sun, then positioning themselves around the target planet would be the equivalent of nuclear weapons today. With a tiny deltaV maneuver at their apoapsis, they can be sent screaming down onto the target at incredible velocities, instantly destroying orbital installations, low-orbit spaceships and with appropriate shielding, ground targets too.

The easiest way to cool down a projectile is through open-cycle cooling. They would be too small to carry an onboard cryogenic cooling and waste heat management system. They need to dissipate heat absorbed from sunlight, as they cannot afford to reflect it away and into the target's sensors. Here's an example projectile, designed to catch a target accelerating at 0.1m/s2 from an initial distance of 10000km.

10kg kinetic impactor
Launched at 20km/s at target
Transit time 500 seconds — deltaV needed 50m/s
Propulsion provided by a cold gas thruster with exhaust velocity 700m/s
Mass ratio 1.074, so total mass is 10.74kg
Average density 8000kg/m3 (less than iron)
If spherical, surface area exposed to the sun is 0.014m2
Energy absorbed is 18W at Earth orbit

A liquid hydrogen reserve at 4K could be heated to 20K to achieve about 228 joules of waste heat per gram ejected. At a rate of 87 milligrams per second (43 grams in total), the projectile could be kept extremely cool for the entire trip. The detection range equation, inputting 4K temperature and 0.0014m2 surface area, gives us distances of a handful of kilometers.

At longer distances or with 'hot' propulsion, a missile might not be able to stay entirely cool. However, it can still use the directional tactics discussed before, on a smaller scale.

This might necessitate that defenders launch sensor drones at the start of every battle to watch for the hot sides and rears of accelerating missiles, and losing those sensors would open up the defenders to attacks from projectiles invisible from the front...

Bright backgrounds

It was noted that not all space combat occurs in 'deep space', where the background is uniformily black and cold. With no terrain, no atmosphere and standing hot against a cold background, it is the worst place to be for a spaceship trying to hide. However, the situation changes when the spacecraft is in low orbit, hiding its thermal radiation against the brightness of a planet or moon.

Earth's flux (the proper name for the watts per square meter measurement) is between 66 (cloud cover) and 380 (hot oceans) watts per square meter. A spacecraft accelerating across the face of the Earth would still stand out to nearby sensors, but the 'hot' background it traverses makes distinguishing it harder from long distances.

We substract the Earth's flux from that of the spaceship to determine the new detection range. Let's work under the best possible scenario, with 380W/m2 behind the spaceship.

1GW spaceship
300MW waste heat
Spaceship flux = 300MW / ( 4π * Distance^2 )
Planet flux = 380W / (( Distance / Planetary Radius ) ^2)
At 100000km, the spaceship's flux is reduced by 99.9999985%
At 100 million km, the spaceship's flux is reduced by 99.99999984%

We can conclude that accelerating with a planet behind or in front of you leads to practically the same results: the enemy will know that something is emitting energy, by analysing the total flux of the planet, but cannot gather more information than that...

Active defense

One suggestion is to actively respond to sensors by shooting lasers at them. The idea is that the laser beam gets bounced into the telescope's optics and onto the sensor. The problem is, at ranges where your spaceship is only a few pixels wide on the sensor array, the laser beam will only reach those few pixels. Overall, it would take a massive coordinated effort from a huge number of angles to burn through a significant number of pixels on a sensor array. The alternative is to heat up the entire sensor platform so as to increase the operating temperature, lowering the signal-to-noise ratio and decrease sensitivity. The problem is that doing so adds more waste heat to your spaceship than it does to the target. The sensor platform can have cooling systems of its own that could handle the heat load. Also, increasing your waste heat load increases your visibility to other sensor platforms, both visible and invisible to you. Active defense against sensors is not a realistic choice for achieving stealth, at least against a moderately competent opponent.

Active detection

The concept is simple: produce your own energy, send it out into space, and listen for echoes. This can be RADAR, using radio, LIDAR, using light, or various other radiations.

In space, the biggest problem with RADAR and active detection in general is the inverse-square law. It states that energy per square meter is divided by the square of the distance. The return signal you are hoping to pick up goes through this twice.

Return signal = (Output * RCS * Antenna) / (157.9 * (Distance) ^4)

Return signal is measured in watts per square meter. Output is the power you put into your outgoing signal. Antenna is the aperture of the radio receiver, measured in square meters, dependent on the frequency used and the antenna gain. RCS is the radar cross section, and determines how much of the radar signal is bounced off the target back towards you. For example, the RCS of a flat steel plate is about equal to its area. A spherical ball of steel of the same width would have an RCS of about 6% of its visible area. Radar absorbent materials can further reduce this figure, as well as shapes designed to bounce the signal into other directions. In space, the shortest frequencies will be used, because there is no interference from atmosphere or clutter. Due to the distances involved, a high-gain antenna will be optimal.

LIDAR uses laser light instead of radio. It has the advantage of directing its energy very effectively onto the target, with a corresponding increase in return signal strength.

LIDAR obeys the inverse-square law, so the equation for return signal strength is very similar to that of RADAR systems. The difference is that RCS is replaced with its visual equivalent, and the Antenna factor now relates to the sensitivity of the photodetector. It is likely that a combination of the two methods will be used to detect a target. While RADAR will likely return a weaker signal compared to LIDAR, it will penetrate through features designed to defeat LIDAR and better identify the target. The actual detection range for RADAR and LIDAR methods is the following:

Detection range = (0.07958 * Power Output * Radar Cross Section * Antenna Gain / (157.9 * Sensitivity) ) ^0.17

The sensitivity is a factor determined by the receptors used. Photodetectors are generally more sensitive than IR receptors. The general rule is that return signal strength drops very sharply with distance, leading to an extremely short detection range in comparison to a passive sensor. Additionally, the power output can be detected by the target craft before the return signal is strong enough, giving it time to deploy decoys or reconfigure itself in radar stealth mode.

Course correction

In some scenarios, the spaceship will have to change its trajectory after the departure burn.

The most obvious method is using an inherently stealthy maneuvering system, such as a cold-gas thruster. However, propulsive performance is directly tied to the temperature of the reaction chamber. So, a cold-gas thruster would have very low exhaust velocity, and would require very large amounts of propellant to achieve good deltaV. For example, a nitrogen gas thruster has an exhaust velocity of barely 700m/s.

Another approach is low-energy propulsion. This relies on using an efficient, high exhaust velocity but low total power engine over long periods of time, with the waste heat generated dealt with using low temperature radiators or manageable amounts of open-cycle coolant.

For example, our 1GW spaceship has 294m2 of radiators. The area/temperature detection range equation tells us that if it wishes to remain undetectable up to 10,000km, then it can operate its radiators up to a maximum of 208K. This gives us a waste heat removal capacity of 28kW. If our 'stealth' propulsion system is tailored to be more efficient at the cost of thrust, then 90% propulsive efficiency and 60% reactor efficiency is reasonable. This gives us an output of 151kW. Using liquid hydrogen as propellant, we can expect an exhaust velocity of 20km/s and a thrust of 15 newtons.

1GW spaceship
151kW stealth drive
20km/s exhaust velocity - 15N thrust
500 ton dry mass, 635 ton at launch, 558 tons during transit
Acceleration: Force/Mass = 0.02mm/s2

Although it seems incredibly low, it can be operated over the course of the entire Hohmann transfer. Between Earth and Mars, it is 8.6 months. Over the course of one week, the spaceship would have deviated its trajectory by 12m/s. In a month, it is 52m/s. Over 6 months, it is 311m/s. While it sounds small, you have to realize that a few dozens of meters per second can mean the difference between an interplanetary attack and a flyby mission abort. If it detects an approaching enemy force or a dangerous area, the spaceship can hide within a volume 6.5 million km wide in a day.

Stealth in Space is Possible

You must understand that stealth is not an absolute. That means that 'stealth' is actually a smooth transition between low and certain detection. This leads to sorting a detected spacecraft into one of four categories:

  • Soft Detection
  • Hard Detection
  • Identification
  • Target Lock

A soft detect happens when a spacecraft emits enough energy in the direction of a sensor that the signal generated rises above the noise floor. This sort of detection is generally the job of wide-angle scanners that sweep the entire sky, searching for above-average levels of photons. Looking at a planet and measuring a spike in brightness, or watching empty space and detecting a handful of high-energy photons, will reveal that something is emitting energy. However, the same characteristics that allow a soft detect by a sensor prevent it from establishing a precise location or velocity of the emitter. They can only say that 'something in this direction is hotter than empty space'. Cross-referencing the data from several sensor platforms can narrow down the location of the stealthed spacecraft, but it will still encompass billions of cubic kilometers.

Once the wide-angle sensors have piked up a statistically significant signal, the defenders' next step is to try to obtain a hard detect.

A hard detect is a precise and certain localization of the stealthed spacecraft. This is achieved with narrow-angle sensors that focus on a small slice of the sky. Once they narrow down the source of the energy emissions to a small enough area, the amount of data obtained on the spacecraft rises quickly. You could reasonably say that the spacecraft is not 'stealthed' anymore. By watching a time-lapse of the spacecraft's location, the velocity and heading can be obtained. Even more sensitive sensors can be set to track the spacecraft instead of scanning huge areas of the sky, leading to a 'hard detect'. However, transitioning from soft to hard detection is not a simple feat. The wide-angle sensors and the soft detect only provide a cloud of likely positions of the stealthed spacecraft. Over time, the cloud becomes smaller and denser. A narrow-angle sensor would still have to be run over millions of cubic kilometers, if not billions, of potential positions before the emissions are caught in its field of view. Our reference 1GW spacecraft with its cold 208K radiators and 'stealth' 151kW propulsion could change its position by up to 6.5 million kilometers in a single day. This is a volume of 1.15 million million billion cubic kilometers to hide in, even after a soft detect has been achieved.

(Identification) After a hard detection has been achieved, and your spacecraft is being tracked with great accuracy, there are still ways to fool the sensors.

One method is to hide your spaceship inside a voluminous shroud. Once visual surveillance becomes available, you will be hard-pressed to hide the exact size of your radiators, the shape of your propulsion bell and the width of your primary laser lens... Hiding all this in a metamaterial cloak that shrouds or obscures the exact features of your spaceship probably won't hide your purpose (an attack fleet would probably be travelling along deltaV-expensive or otherwise unusual trajectories), but it will reduce the accuracy of your opponent's estimate on the composition and strength of your forces.

The downside is that if this technique is permanently deployed, it will interfere with your stealth (catches incoming sunlight and outgoing waste heat), and if deployable, requires you to know when a passive sensor has detected you... which is impossible.

Another option is to bundle several spaceships together. This way, your opponent's mass estimates cannot be relied on. Yet another is to place your radiators on extremely long booms, so that they do not correspond to the position of your spaceship. If they move or rotate, it will further confuse opponents into over or under-estimating your forces.

Following Jon's law, spaceships will be tightly regulated and would have to report their positions at all times. A spaceship cannot therefore switch between stealthed and posing as a civilian ship. Due to design constraints, it would be difficult to disguise it as civilian, and it would be less effective than a dedicated warship, giving the worst of both.

In practice, identification will be performed using active scanners. Once your position is established, the power output of a RADAR or LIDAR can be focused on your position for good return signals. This creates a requirement for a set of countermeasures quite different than those for thermal imaging. RADAR countermeasures include radar-absorbent surfaces and cool-looking angular shapes. LIDAR defenses include meta-materials that can modify the light bounced off. These techniques can help fool identification, but immediately flag your spaceship as a hostile target.

(ed note: and of course a target lock is when the active scanners have pinned the location of the target close enough to give ship weapons a targeting solution; that is, you can see the target clear enough to shoot it.)

Strategic movement

Stealth on its own does not achieve anything. Your spaceships WILL be eventually detected, and the enemy will not jump in surprise. The thermal signatures increase in number, become statistically significant, are narrowed down then identified as spaceships, with sensors attached to track each of them days, weeks or months before the come close. The point of stealth is that it allows for strategic movement. If spaceships are launched on an 8 month trip, and are only detected in the last week, then you can launch multiple fleets from several directions, and have them insert into various orbits for a multi-pronged or staged attack, before any are detected. Similarly, stealthed spaceships can choose to engage or break off from an upcoming encounter. Stealth allows for first-attack advantage. In its purest form, a fleet can fire upon an opposing fleet twice its number, and immediately destroy half of it. This means that even if your are immediately spotted, identified and targeted after firing, you'll be able to wield a decisive advantage going into any engagement. Stealth also ties into the capabilities off various weapons systems. If lasers are effective from a distance of 100,000km, and you are spotted incoming from 80,000km, then you can strike first. You can launch missiles from closer ranges, too. This means your missiles will not need as much deltaV to reach the target: as a result, they can be smaller, and you pack more of them into the same ship, which is important when facing laser defenses.

The home advantage

Home advantage is an extension of how battles are won: an objective is set, and two opponents fight to complete it or stop the other from completing it. In interplanetary space war, the attacking fleet's objective is to destroy all space defenses so it can move onto pressuring ground objectives. To do that, it approaches along a Hohmann trajectory, during which it drifts through space after a departure burn. The second step of a Hohmann trajectory is an insertion burn. The attacking spaceships perform a retro-burn that puts them in orbit around the destination planet. The spaceships defending the planet can win by destroying the incoming spacecraft. However, they can also perform their own departure burn, and attempt to meet the attacking fleet in deep space. If they can stop the attacking fleet from performing a retro-burn, they will force them to be flung back out into interplanetary space. This is a second win condition, and constitutes the home advantage.

In practice, the defenders don't really have to send out their own spaceships. They can shoot projectiles, launch missiles or send off drones into the path of the attacking fleet, and home to defeat them weeks or months before they approach the planet. If the attacking fleet is then too damaged to face the remaining defenders, or expends too much propellant dodging the projectiles and so on, then it will be forced to abort the mission and perform a fly-by.

If the attacking fleet completely forgoes stealth, then the defenders will be able to fire projectiles and missiles at it for months. Sending a missile into the path of an incoming spacecraft is much cheaper and faster than sending another spaceship, so defenders will have a great advantage in terms of resources and efficiency.

With stealth, the attacking fleet is detected closer to the planet. This reduces the amount of weapons fire that it has to dodge, and considering the fact that a soft detect only gives a fuzzy location with lots of room to hide in, the defenders would have to shoot huge volumes of fire to hope to catch and destroy an attacking spaceship from far away. With stealth and stealthy propulsion, the attacking fleet can come from a variety of trajectories that are close to the Hohmann trajectory, but can deviate by millions of kilometers from the most efficient route. This vastly reduces the 'home advantage' of defenders.


How stealth affects your setting depends on the technology level of the setting, its level of development and ultimately, where you want the balance to lie.

Remember, this is ToughSF, where we give options, not restrictions.

If you want to recreate submarine warfare in space, you can. Restrict the sensitivity of sensors, increase the effectiveness of stealth techniques and the mass devoted to them, and you'll have spaceships traversing the solar system unnoticed until they attack. You have to realize the consequences, though: If 'space submarines' are capable of invisibly launching missiles and streams of kinetic projectiles without being detected, then your opponents will try to counter it with more sensor platforms, and in return, you'll build sensor hunters to keep your 'space submarines' undetected and safe. Similarly, you can try to find a sweet spot that gives stealthy spaceships some level of effectiveness, but make the requirements great enough that fleets are regularly composed of both stealthy and unstealthy spaceships. For example, you might build a setting where the solar system has been explored and settled for a long time, and tension between the warring parties have been building up gradually. Sensor platforms will litter the solar system, above, below and around your planet. In such a situation, the only way to escape detection is with a 'hydrogen steamer' — a spaceship with large volumes of liquid hydrogen that it boils off to reduce its emissions to zero. However, such a spaceship could not compete with armored, high-powered warships in direct combat. As a result, you'll build some of both.

Sensors are what really make or break stealth.

If you want spaceships to accelerate into faster trajectories than multi-month Hohmann missions, then you'll need directional stealth: cold plates, angled radiators and so on. For that to work, you'll need the enemy's sensors concentrated into one area of the sky — so maybe during peacetime, opposing factions will spend their military budget creating spaceships equipped with powerful sensors, LIDARS and small lasers. Their only job is to hunt down enemy sensor platforms and shoot them down at the start of the war, paving the way for the main fleet to attack undetected.

Replacement sensors take time to reach the far-away but advantageous watchpoints, and those who try to do it quickly will be detected, so as the war goes on, sensors will be concentrated near the enemy, where they can be replaced faster than they can be shot down.

Or instead, military spaceships could spend their entire time tailing each other. If one fleet breaks off and enters an attack trajectory, the tailing fleet will attack it well within detection range. To complicate things, you can have a fleet of stealthed ships tailing the visible fleet tailing your visible fleet, with the opponent's stealth fleet trying to hunt it down at the same time....

On the opposite end of the spectrum, you can apply stealth techniques to the sensor platforms and make the impracticably well hidden. In a setting where you'll always get detected, there is no need for stealth. Since it is cheaper to shoot down a spaceship than to build one, the defenders might simply build orbital defenses to counter fleets rather than using their own. The attackers would then trade in their fleets for massive, interplanetary lasers that require re-focusing mirror drones that are also much cheaper than spaceships, and easier to hide too....

In conclusion, you cannot ignore stealth in space as being possible. If will affect how your fleet is build up, how spaceships look like and even the grand military strategy pursued by opposing factions. At the very least, you must give strong arguments as to why it is not feasible and even then, consider the fact that like many modern military technologies (tank armor, air drones, aircraft carrier fleets...) it will enter into cycles of development and proliferation that have to be matched or countered.

by Matterbeam (2016)

(ed note: naturally there is some controversy about the above article. I am going to present select comments and let you sort it out.)

     Ken Burnside:
     Um, none of that qualifies as stealth. Detection of a main exhaust plume at 32 AU (16,000 light seconds). Detection of a "cold running" ship at 0.2 light seconds under optimum circumstances (one with no onboard power supply and an open cycle life support system). Using an onboard power supply puts that "cold running" detection range at about 2 AU.
     He does the standard debunking of directional radiation.
     His math is correct, his title doesn't match his writing.

     Ian Mallett
     Just a note on radiometry (my field): for Lambertian emitters (blackbody radiators), radiance doesn't decrease at angles (directed radiant exitance decreases, but not radiance). Since sensors are (to a 1st order) sensitive to radiance, angling radiators doesn't work to reduce your effective heat signature. Basically, angling your radiators reduces their projection on the sensor, but not the intensity of the signal they produce, unless you get it perfectly edge-on.
     Ken Burnside I was about to say the same thing. A couple of special cases isn't general-purpose stealth. Special-purpose is great, though. In my universe, the rare ships that are capable of stealth dump heat into internal heat sinks for short interplanetary hops. They only do burns and dump heat near planets, which are industrialized. The reason this works as camouflage is that even though detection threshold is low, angular error for thermal sensors is still pretty high.

     Matter Beam (article author)
     Ian Mallett, I didn't mention radiance. Also, reducing their projection on the radiator is exactly the same as reducing your effective heat signature, at least as deducted from the Area-Temperature detection formula. Temperature is the same, area is lowered

     Ian Mallett
     Matter Beam, I know you didn't mention radiance, but I'm bringing it in anyway because it's relevant. And this is precisely what I'm saying--again, sensors are sensitive to radiance, not radiant flux. So no, reducing your projection on the sensor does not help unless you reduce it to exactly 0.
     Here's an analogy (since cameras and eyes are also, to first order, sensitive to radiance). Take a picture of a candle. Now step closer and take the picture again. The candle doesn't get brighter. You receive more energy from it, but its brightness stays the same. Specifically, if you look at the sRGB values of the flame, they're the same, regardless of how close you are. There are more of those pixels when you're closer (more energy), but the irradiance and radiance are the same (the sRGB values are roughly constant).
     Heat sensors and brightness detection in general are (effectively, and for binary classifiers, exactly) thresholding on images, which means that projected area doesn't matter, so long as it's positive.

     Matter Beam (article author)
     Ian Mallett, so you are saying that a candle seen through a pinhole is as easy to detect as a wall of fire with the same brightness?
     I find it hard to believe that the visible area has no effect on detection range.

     Ian Mallett
     Matter Beam, close, but yeah (aperture has a multiplicative effect, so pinholes are out). This is why IR fire detectors freak out when you walk into a room with a candle. They really do think it's a wall of fire. Since these are binary classifiers, they literally can't tell the difference.
     It's a bit of a simplification, since there are issues with noise and most objects aren't perfect blackbodies, but to a first order, that's the way it is. The math is pretty easy to work out (I'll show you if you care), but I think RGB-pixels-not-changing-by-factors-of-billions-with-distance is pretty intuitive.

     Matter Beam (article author)
     Ian Mallett, that's another thing I don't understand. What's an object's colour got to do with how far away it can be detected?
     Wiki page on infrared signature says that apparent temperature difference and contrast radiant intensity do depend on distance and/or apparent surface area.

     Ian Mallett
     Matter Beam, Looking at the wikipedia, they "take the difference in average radiance of the object and that of the immediate background and multiply this by the projected area of the object". This gives you watts per sterradian, which is distance-invariant. The tricky thing is understanding all these units. I've sortof been tossing them around, but you can get formal definitions e.g. from my webpage here. In particular, radiance and radiant intensity are distance-invariant. It's easy to think that radiant intensity isn't, because of light's inverse square falloff. The key insight is that the sterradians intercepted by any constant area as you move further away does fall off. What that tells you is that irradiance falls off, not radiant intensity.


     But by now, though, we're pretty well ratholed on this. Might as well go full-hat. Here's the simplest technical explanation I can muster.

     1: Radiance from a Lambertian emitter (or reflector) is independent of angle. This happens because radiance falls off with a cosine factor WRT angle, but the differential area you see at that angle increases by secant (exactly canceling). Easy example: look at a sidewalk. The squares under your feet are the same brightness as those stretching out in front of you. Blackbodies (radiators emitting IR) are Lambertian emitters.

     2: Therefore, the brightness of a Lambertian emitter is independent of the viewing angle. So your radiators look the same brightness no matter how you look at them.

     3: Sensors are (to first order) sensitive to brightness (i.e., radiance). The measurement equation defines the signal of a camera. It is essentially the integration of radiance over the hemisphere and the sensor area, times the sensitivity. The insight is that for long focal lengths (i.e., most cameras, and especially telescopic sensors) the cosine4 falloff in the hemispheric integration can be neglected because the angle is small (if it couldn't, then you'd get vingetting in your images, like in old daguerreotypes). This means that irradiance (the power incoming per unit area) is roughly proportional to the aperture. The final insight is that irradiance is power/area, but sensors produce electrical response/area. So, the incoming light produces the same electrical response, just over a smaller region. This is the-pixel-values-are-the-same argument I gave before: the energy of the signal is less, but its maximum value isn't.

     4: Binary IR detectors check for any electrical response over a threshold. So in particular, it doesn't matter how much area produces it (total current); the voltage (signal value) will still trip the threshold. Intuitively, this means I don't care how many white pixels there are, just so long as there's at least one. Except in this case, the "pixels" are quantum wells producing bucketed electrons. So yes, there is a spatial limitation, but not really because noise dominates in this regime anyway. This is also why, as I mentioned in my OP, that directional acuity for IR detectors is bad.

     5: To sum: Radiators emit radiance, which is distance-invariant (and angle-invariant, since it's Lambertian). Detectors are sensitive to radiance. Therefore, I don't care what angle your radiators are at; I can detect them equally well. Just so long as you can rule out quantum noise and diffraction limits in your optics (which you nearly well can because far-IR emitted by low-temperature radiators has a really big wavelength relative to visible radiation, and the whole thing scales linearly with aperture area, so you can halve any of those problems by making your lens sqrt(2) larger).

     Whew. Makes at least some kind of sense?


     You know what, actually, that's a stupid way to explain it. Let's try an explanation based on Physics instead of Math:
     1: Detectors trip when a high-energy photon hits them.
     2: Photons don't lose energy as they travel.
     3: The further away I am, the fewer the photons that hit my sensor. But I don't care — so long as one (or a few) photons hit, I detect it anyway, because I'm detecting based on the maximum energy, not the total energy.

     Matter Beam (article author)
     I see now. You were trying to explain to me a concept I was aware of in a way unfamiliar to me.
     Yes, if we look at it on the photon-per-photon level, every single spaceship anywhere will be detected.
     Detection range then becomes a misnomer, and has to be replaced with detection time.
     The maximal signal to noise ratio, as I know it, is N-rootN, ie, for N photon strikes, there will be rootN false positives. Therefore, ONE photon strike is indistinguishable from noise.
     Also, the binary detectors cannot distinguish between starlight and photons from the device's own heat. I don't know how they would be useful in a scenario where there is more than 1 emitter in the entire sensor cone.

     Ian Mallett
     Matter Beam Yeah; sorry about that. I think of everything in terms of radiometry because of my research, so I sometimes forget about easier techniques.
     I'm not familiar with the n-limit you gave, but I suspect that's only relevant for statistics, and it decays to something like a T distribution instead of Gaussian when n=1. You can definitely detect a single photon. You might not be too sure about it, but it's enough to slew your observation array around and take a closer look at that patch of sky with wider-aperture scopes.
     Note that the goal is not to resolve an image. The way I imagine it is you have some IR telescope trained on a suspect planet. If the IR spectral radiance increases, you sound an alarm because some enemy ship just did a departure burn. You can't really tell where it's going, but you can see it go.

     William Black
     I tend to be of the opinion that stealth would be very hard in any highly developed solar system setting such as The Expanse, or my System States setting, where there is interplanetary commerce and trade between large scale habitats, and points of industrial infrastructure at Earth, Mars, and among the moons of Jupiter and Saturn.

     1. There will be many senor platforms above and below the plane of the system as a matter of course. With a high density of sensor platforms around habitats and around areas of high activity such as cislunar space, Mars orbit, Ceres, in and among the moons of Jupiter and Saturn.
     If there is significant political friction (i.e. if any of the powers are hostile to one another) this is especially the case.

     2. All commercial spacecraft will carry transponders and radar beacons. It would likely be a significant legal infraction not to have working transponders and radar beacons.

     3. No one is going to let you near their propellant depot or cargo docks without working transponders, because, point 4.

     4. Regardless of national origin there is a culture among commercial transport operators: everyone reports hazards to navigation. It’s likely there would be a specified communications channel just for this purpose and everyone will listen in. The reason being, for a commercial operator, the standard of practice is that if you hit something and damage the vehicle and/or cargo, you bought it. It comes out of your pay and you’ll be looking for a new job with that black mark on your record.

     In such a setting rogue operators will not remain anonymous for very long, and would soon find their operation unprofitable, see point 3.
     The investment in sensor platforms, hazard mitigation, and traffic control operations will be of increased value and priority if mass drivers are in use, or if moving asteroids around becomes common practice in the setting.

     Isaac Kuo
     The only way open cycle hydrogen cooling makes sense in the first place is if the vented hydrogen is cool (and thus practically invisible). The laws of thermodynamics dictate that the only way you could concentrate low temperature heat from the sunlit face to "hot" hydrogen is if your refrigeration process itself has a heatsink cooler than the sunlit face. Which makes the hydrogen part of the system redundant.
     Basically, you can't magically get around the requirement for a cooler heatsink than the sunlit face. That means either a big radiator (which can actually be the sunlit face itself if has a steep angle to the Sun), or open cycle cooling venting coolant which is cooler than the target temperature.
     In practice, the bulk of the heat drawn by LH2 coolant will be heat from the phase state transition from liquid to gas. That gives about 450kJ/kg of heat, keeping the sunlit face about 20K.
     Now, you can also take advantage of the fact that radar is easy to detect far beyond the range at which it can detect things. The sunlit face could normally be a highly reflective mirror, with a hinged absorptive lid swung open against the side. This mirror surface would only absorb a fraction of the sunlight, while reflecting most of the photon energy into a narrow cone. You only need to swing the radar absorbing lid when you get near a radar sensor.


     Stealth Space Carrier thoughts
     I was pondering a Star Raiders type game for the VIC-20, when my thoughts became less arcade-like and more simulator-like. Less air combat inspired, and more submarine combat inspired. When I started crunching numbers, I was shocked to find stealth space carriers seem viable. Some relevant numbers:
     100 watts — the amount of heat a pilot generates. This boils about 13 grams of liquid hydrogen per minute, or about 19kg per day.
     50 watts — the amount of solar heating a 99% reflective 2m diameter sun-facing mirror system suffers. This boils about 7 grams of hydrogen per minute, or about 10kg per day. Adding this to a human pilot, this translates to around 30kg per day.
     44 kilowatts — the amount of solar power a 2m diameter concentrator mirror could give to a solar thermal thruster. Divide that by 0.5 * 9000m/s * 9000m/s, and we get...
     60kg/day — the hydrogen consumption of a solar thermal thruster using a 2m diameter concentrator, assuming the hydrogen is boiled by some other heat source. Note that this exceeds the 30kg/day of hydrogen boiled by the human pilot plus heating of the solar concentrator itself, but I figure various system inefficiencies can make up the difference. In particular, any heating of radar shrouds and a tail boom will boil additional hydrogen. The thruster can be a stealthy pulsed thruster, flash heating a drop of almost boiling hydrogen by squirting it through the heating element, which then expands into a large exhaust cone. Performance is quite good, with an exhaust velocity around 9km/s.
     2m x 200m — The dimensions of a stealthy space fighter shaped like a spike. There is sufficient internal volume for up to 14,800kg of hydrogen, or about 8 months of continuous thrusting flight. Some internal volume will be used for other equipment, of course, but the bulk of volume will be hydrogen due to its low density.
     The interesting thing is that the presence of a human pilot does not directly impact how stealthy this space fighter is. The waste heat generated by the pilot merely preheats the hydrogen propellant a bit before solar heating does most of the work of boiling and accelerating the propellant.
     If we scale things up to a space carrier, things get even more stealthy. Let's consider a 10m x 1000m carrier. This boosts solar heating by 25x, but it also boosts internal volume by 125x. The bottom line is that it could perform 40 months of continuous thrusting flight, plenty of time to go on an interplanetary patrol and return for resupply.
     Scaling things down, we can see that a stealth missile still can have significant endurance. A 20cm x 20m missile only has internal volume for around 14kg of hydrogen, but that's still good enough for 3 weeks of continuously thrusting flight.
     The interesting challenge is designing a solar thermal thruster which can take in light from most of the front profile and also be stealthy to radar. This is fundamentally similar to the way stealth aircraft use serpentine intakes to prevent the jet turbine to be visible to radar.

     Pictured is one idea I have. There are two sets of mirror slats, each covering half of the nose "intake". The angle of these slats deflect the incoming sunlight toward two parabolic concentrators, which each focus light onto the heating element. The heating element is surrounded by photon absorbing walls, such that any incoming photons will either hit the heating element or a wall. One issue with this design is that it has a square profile rather than a circular profile (which is what's most efficient for the LH2 tanks). But I'm sure an arrangement of mirror slats could work for a circular profile. This is just an example design to show the basic principles involved.
     This solar thermal thruster is basically at the nose tip, so it requires an auxiliary tail thruster to counteract torque. The ship is very nose heavy, so the tail thruster doesn't need much thrust.
     So how does this space carrier work? Well, attacking a non-stealthy target is dreadfully easy. The carrier can be stationed in a co-orbit nearby any planetary system, sending a fighter toward the target with the target none the wiser. Depending on the mission, the fighter might attack with stealth missiles or it could simply shoot some sort of gun from point blank range before slipping away.
     Fighting other stealthy forces, though, is a game of hide and seek. Long range detection is sporadic and a matter of luck. As a carrier passes between a sensor and a star, it may see the star wink out or see a tell-tale diffraction pattern. Over time, this may give strategic information about where a carrier is lurking. This carrier likely has a CAP of stealth missiles constantly roving around looking for any incoming approaching carriers, so a fighter may have a better chance of getting close without detection. The fighter itself looses missiles to hunt for the target carrier. Basically, the more eyes in the area, the better the chances of occultation events detecting the enemy carrier. The fighter then remote commands the missiles closer and closer to the carrier based on those detections, until one actually directly hits it. Conversely, the enemy carrier may notice an attack based on occultation events; it may try to counterattack with its own missiles while trying to get away. Note that a carrier has a much lower acceleration than a fighter.
     Another idea is to try and attack the logistics, but a carrier has an endurance of perhaps 3 years. It could take a long time to have a decisive effect even if you take out all of the enemy resupply bases.
     Now, I don't actually see any compelling reason to have human crew on board. I'm just noting that the numbers for human heat generation are not such a big deal.
     Either way, I think this qualifies as "stealth in space", according to popular imagining. The stealthy carriers roam interplanetary space like submarines, performing significant maneuvers continuously rather than just coasting on ballistic trajectories. When they attack "surface vessels", the results are one-sided surprise attacks. A non-stealthy spacecraft just gets "blown out of the water" without warning. When they fight each other, it's a game of hide and seek, using lucky sporadic detections to hunt the enemy and/or trying to slink away with the occasional turn to try and throw off the pursuit.

     Note that this analysis assumes a distance from the Sun around 1AU. Endurance numbers scale upward proportional to the square of the distance from the Sun—assuming the spacecraft is not lengthened to take advantage of the smaller Sun cone.
     If you do take advantage of the smaller Sun cone by making the spacecraft even longer, endurance numbers scale upward with the cube of the distance from the Sun.
     But if you want a human crew, the heat generated by the crew comes to dominate as you move the setting to the outer system.

     +Elie Thorne It deals with sunlight hitting the sides by not having sunlight hit the sides. The shape is a pyramid or cone, with the sides sloping inward from the nose. That's what makes it so nose-heavy. At 1AU, the inward cone angle is around half a degree, resulting in the overall length being about 100 times the base diameter.
     I am unaware of a robust analysis of occultation sensors, but I do know what sort of telescope would be used — a Shmidt telescope, similar to Kepler. Kepler, of course, stared at a patch of sky continuously monitoring a set of stars for transit events. These telescopes are "wide angle" in an astronomer's sense; it would still take hundreds of them to cover the sky all around. So, you'd still scan around with them.
     One thing that's somewhat counterintuitive is that occultation detection is subject to diffraction range limitations. If you're sufficiently far away, the light from a star actually diffracts around the target and you don't see any occultation event at all. But if you're not too far away, you will still see a weird symmetrical dip/increase as you see the effects of a diffraction pattern (rather than a sharp shadow).

     +Matter Beam I don't think a thin cylinder works well with the idea of using the sunlight for solar thermal propulsion.
     Also, I consider it important to have radar stealth, because of that tactic of a radar beacon. The concept I'm proposing involve stealth spacecraft with all around radar stealth. I'm assuming extremely black materials suitable for radar stealth are available (similar to Vantablack). Photons go in, but they don't come out.
     I think I've figured out an elegant stealth solar concentrator suitable for a circular profile.
     Instead of two rectangular halves, the face is split into three pie shapes. These deflect incoming sunlight toward three parabolic concentrators.
     And instead of a radar shroud surrounding the rim of the face, it's better for the radar shroud to actually be the internal divider planes splitting the face into three slices.

     John Reiher
     Well, unless your pilot has a VR environment to live in for that 40 month mission, he will slowly go insane. Long distance pilots need lots of breaks and they never stay in the seat for more than 8 hours. That's the weakest part of your design, the human element. And since it's a "stealth" craft, there's no direct communication between the pilot and anyone else.
     It's a case of why a pilot when a missile bus would do the same job and cost less to keep in space?
     I imagine that you'll coat the ship in Vantablack™ as well as the solar collector element. The real question is how hot your solar unit will get. It should be hot in the IR spectrum and mirrors work both ways, so from the front when you're soaking up the sunlight, the ship will be an IR flashlight forward.
     Now Vantablack can work as a radar absorbent material, as it traps the energy in its structure and turns it into heat. But, if your ship makes a maneuver that exposes one of the sides of the ship, let's say at 1 AU, it will capture and absorb sunlight at 1367 watts per square meter. While Vantablack doesn't reflect light, it can and will emit light in the IR spectrum. Suddenly your ship is not very stealthy at all.

     Isaac Kuo
     +John Reiher The carrier, which is what has a 40 month endurance, has a diameter of 10m and a length of 1000m. The crew compartment could be just a 10m length of that, and provide an extremely roomy environment.
     The ship never makes any maneuver to expose the side of the ship. It always points its nose directly at the Sun, at all times.
     The question of the forward facing IR flashlight is more interesting. Yes, the optics work both ways. But the heating element can be hidden behind a cooled IR filter. This will absorb IR from both the Sun and the heating element, but it lets through the peak sunlight optical frequencies. It basically hides the heating element within a greenhouse.
     The bottom line is that there isn't an IR flashlight pointing forward.

     +MatterBeam You have not had the "aha" realization that the front cone can be stepped like a pagoda. It is made out of a bunch of inverted cone sections, progressively getting larger with distance from the Sun. The overall shape is similar to a sun-facing cone, sort of, even if each section has an inward slope.
     Basically, instead of a single circular base facing the Sun, there's a series of ring shapes facing the Sun.
     The stepped "pagoda" cone is pointed toward the sun, with a normal inverted cone pointed away from the sun. The overall volume is roughly a double-cone, giving you almost twice the internal volume of a single cone for a given amount of absorbed sunlight.

     (why not a cylinder instead of a cone?) Because Sunlight isn't parallel from a point-like source. It's coming from a solid angle. If you have a "cylinder" shape, you are wasting the conical volumes at the front and back. The cone in the front adds nothing to the shadow. The cone in the back is already entirely within the shadow.
     That said, there could be various design considerations that make a modular "bamboo" design good. A bunch of identical modules latching together in a linear stack could be good for standardization and logistics. You have flexibility to add and detach modules like train cars or cargo containers. It might not be the 100% optimal double cone shape for a unified spacecraft, but it might be a better system.
     Anyway, all the potential advantages of a cylindrical pagoda don't matter if the less efficient design make it impossible to perform the spacecraft's mission — reach a target without being detected.
     The volume-to-sunlight ratio is the thing which makes this concept work, and a double cone gives you twice as much endurance as a single cone, which gives you more endurance than a cylinder (for a particular shadow profile). That's the difference between, say, making it to Mars and back vs just making it to Mars.
     But this ratio is scale dependent. You can upscale a spacecraft design in all three dimensions until the endurance level is what's desired.

From a G+ thread by Isaac Kuo (2016)

Sparked by an idea in a discussion on Stealth in Space, Isaac Kuo formalized the concept of a ‘stealth carrier’ spaceship relying on liquid hydrogen boiloff to maintain Cold Stealth. Here is a further development of that concept.

To stay undetected, a spaceship must be very hard to distinguish from the background. For example, a spacecraft at 3 Kelvin temperature cannot be distinguished from the background radiation in space by any physically possible sensor. Even at a more moderate 22K, it is extremely difficult to detect. This is Cold Stealth.

To maintain this level of stealth, the spacecraft cannot use radiators, nor can it leave a trail of bright hot exhaust.

There are three parts to solving these problems:

-A cryogenic heat sink

-An insulating hull

-A stealthy propulsion system

The heatsink:

A cryogenic liquid heatsink is boiled off to remove a minimal amount of waste heat. The resulting gas is fed to the propulsion system. The only two candidates for this are liquid hydrogen and liquid helium.

Both boil at temperatures very near absolute zero, but helium boils at 4K while Helium stays liquid up to 22K. Helium can be used to directly cool the spaceship to background temperature, but hydrogen has twenty times the latent heat of vaporization (the energy required to move it from liquid to gaseous form). This means that we'd need twenty times less hydrogen per second to stay cool, and the difference between 4K and 22K at long range should be negligible.

The choice between the two depends on the setting you are building. In a sensor-poor environment, hydrogen offers much better performance and higher endurance per kilo. Helium is 76% denser and very conductive to heat in its superfluid state, simplifying design. Helium might be the only solution to a sensor-rich setting (such as around the enemy's home planet), where short ranges or large numbers of sensitive satellites can detect even 22K temperatures.

If hydrogen is selected, a 1kW source of waste heat will require 2.24 grams of liquid hydrogen per second, or 8kg per hour, to cool down to 22K. If the spaceship is discovered or decides to end its stealth mode, it can further heat the hydrogen gas. This removes 14kJ of heat per kg for every Kelvin above 22K. If it is used to directly cool the crew quarters, and allowed to reach 300K before it is dumped overboard, one kilo of hydrogen will take away 3.94MJ of waste heat with it. If it used to cool a 700K piece of equipment (about 430 degrees Celsius), it can take away more than 13MJ per kilo.

The insulating hull:

The cryogenic heat sink is of no use if hot spots on your spaceship leak Infrared radiation and reveal your position. Constructing a spaceship that can uniformly cool its exterior is hard, which is why the simplest designs uses nestled shells.

Like a submarine, the stealth spaceship will have a very cold exterior shell, with liquid hydrogen running along pipes on its inner surface, and a 'hot' pressure vessel inside that holds equipment and crew. The gap between the shells is used to evacuate the hydrogen gasses produced.

The exterior surface of the outer shell is of particular concern. It is what the enemy 'sees'. A regular metal surface, even if cooled to cryogenic temperatures, is quite reflective. RADAR and LIDAR will bounce off it and produce a strong return signal. Sunlight will turn it into a bright beacon.

The solution is VantaBlack. It is one of the products of research into the optical properties of carbon nanotubes. It can absorb 99.9%+ of all incident light. Sunlight, the biggest problem, will be completely absorbed, and no reflected light will reach enemy sensors. If the exterior surface is coated with this material, it can become 'blacker than black' across most of the electromagnetic spectrum.

The overall shape of the outer shell is important too. The stealth spaceship will want to minimize how much energy it receives, minimize the amount of reflected energy, and reliably contain liquid hydrogen for a long period of time.

A shape which corresponds to these requirements is a very thin cylinder with an opening for nozzles in the middle. The rounded shape disperses reflected signals across a much wider area than flat sides. A cylinder is the second-best shape for containing cryogenic fuels after a sphere, but a sphere would absorb much more sunlight than a thin cylinder pointed end-on to the Sun. As the position relative to the Sun has to be kept fixed, the engine must be able to swivel around the centre of gravity to allow it to accelerate along different axis. To allow acceleration along multiple axis, a swivelling nozzle is kept in the middle of two long tanks of propellant.

The propulsion system:

There is no point in flinging a cold shape into space if it has no way to move afterwards.

If the enemy detects the initial boost, they can calculate the trajectory for months in advance. If the enemy's forces change position, the stealth spaceship will miss them entirely.

The primary requirement of a stealth propulsion system is that it does not shoot hot gas into space. A secondary requirement is that it does not consume a lot of electricity. Producing electricity creates waste heat, and that heat must be removed by boiling liquid hydrogen, in addition to what is absorbed from the Sun.

The solution comes in the form of a solar-thermal pulsed rocket. This rocket engines takes in sunlight into a spherical solar furnace with a heating element.

A small amount of sunlight escapes through the opening into the furnace. Most of it eventually heats up the heating element to a very high temperature (3000K+). Tungsten is a suitable material for this element. Hydrogen propellant is injected into the chamber in bursts. It heats up, and pressure in the chamber increases. A shutter to the nozzle releases the hydrogen at high velocity. A de Laval nozzle allows the propellant to expand before it leaves the engine.

Using perfect gas laws (PV=nRT), dropping the propellant from 3000K to 20K temperature requires a decrease in pressure or volume of a factor 150. An isobaric nozzle keeps the pressure constant, so volume must increase for the temperature to drop. A hydrogen pulse is roughly spherical. A 150x reduction in volume entails a 5.3x increase in diameter. Therefore, the nozzle must be at least 5.3 times wider at the opening than at the throat.

The propellant used is hydrogen gas, boiled from the liquid reserve by the cryogenic cooling system. Pulsed operation allows for the hydrogen to reach temperatures very near that of the heating element, maximizing efficiency. Exhaust velocity can reach 8km/s, and it can generate 0.34N of thrust per square meter exposed to the Sun. Performance varies depending on where the ship is in the Solar System. A large Fresnel lens made up of super-cooled VantaBlack can be deployed in front of the spaceship to collect more sunlight.  

For increased stealth, the inlet uses cooled optics to direct the sunlight onto a lens, which focuses it through a pinprick hole into the furnace. Sunlight reflected from the inside of the furnace can only come out through this hole. It will create a new narrow cone of light going from the spaceship to the Sun. Coincidentally, the hardest way to detect a spaceship is to have the Sun at your back and only a pinprick of light to pick up.

The cone of light could compromise the spaceship, but it would be very difficult to do so.

For maximum stealth, the sunlight inlet can be pulsed. A shutter opens, allowing light into the furnace. It closes before light can bounce back out.

Mechanical shutters are unable to spin or move quickly enough to be useful. A 300m distance between inlet and furnace would require a spinning circular shutter to reach 180,000,000 degrees per second (30 million RPM). LCD shutters, with transitions between opaque and transparent measured in nanoseconds, must be used. A shutter time of 50ns allows for inlets to be as short as 15 meters.

The shutter material would absorb half the sunlight, so it has to be super-cooled by liquid hydrogen so that it does not emit infrared radiation. This would halve the overall propulsion system's efficiency, but allows for extreme endurance.

A spaceship equipped with such a propulsion system will have nearly all the sunlight touching it going into the solar furnace. The heat capacity we use is that of hydrogen at 3000K. This is possible because the sun's surface is at 6000K, and we are operating on the same principles as a looking glass focusing sunlight on an ant. The hydrogen is boiled by waste heat from several sources, such as unavoidable sunlight, a shutter system, crew heat or the furnace's imperfections. We do not break the laws of thermodynamics, as the hydrogen is moving the waste heat, not eliminating it. Essentially, the engine is a heat pump: it concentrates the absorbed sunlight into a point, and cools off that point with cold hydrogen.

At the nozzle, the hydrogen absorbs 60MJ/kg or more (heat capacity rises with temperature, from 14kJ/kg/K at 100K to 20kJ/kg/K at 700K and so on).

A design:

Here, we will design a stealth spaceship to work out the capabilities and uses it might have.

The mission is to travel from Mars to Earth, and stay there. It will depart from Mars on top of a conventional booster, which will impart 2.94km/s. It then follows a Hohmann trajectory to Earth.

The trip duration is 260 days. The deltaV requirement is 2.65km/s for the insertion burn, and another 3.5km/s for manoeuvres (enough to drop from geostationary to low orbit for an attack run). It is required to stay around Earth for 2 years, enough time for a replacement to be sent at the end of the Earth-Mars synodic period.

Between Earth and Mars, sunlight averages 980W/m2. Around Earth, it is 1370W/m2. This design does not use a shutter system, so when the spacecraft is accelerating, all sources of waste heat are dumped overboard as propellant.

We want the spaceship to carry 1000 tons of useful payload to Earth. The liquid hydrogen tanks carry ten times their mass in propellant. A 1mm thick VantaBlack external shell masses about 2.3kg/m2.

Energy generation relies on a 30% efficient nuclear reactor paired to an MHD generator. It masses 1 tons and can produces 1MW. Most of the time, it is powered down to the minimum level required by on-board systems. This can be as low as 1kW, with 2kW of corresponding waste heat.

The deltaV requirement translates into a mass ratio of 2.15. This works out as a total mass of 2150 tons. 115 tons of the dry mass is now considered to be the hydrogen tanks.

If the payload has a density of 1200kg/m3, like in a submarine, it fits inside a cylinder 3m wide and 117m long. Liquid hydrogen has a density of 70.8kg/m3, so the propellant fits inside two cylinders 3m wide and 1148 meters long.

Total length is 2.4km. The VantaBlack cover takes 52 tons out of the payload.

The final shape is needle-like, with a width-to-length ratio of 800. However, with most of the mass concentrated in the centre, the spacecraft can turn around without difficulty. This is important when it comes to maintaining the nose pointed at the Sun. With swivelling pairs of nozzle in the middle, it does not have to turn to accelerate in any direction.

The nose is 3m wide and has a surface area of 7.07m2. Its entire surface is an inlet for the solar thermal pulse rocket. Without a deployable Fresnel lens, such as during the transit between Mars and Earth, the engine only produces 2.4 Newtons of thrust.  It consumes barely 0.16 grams of liquid hydrogen per second. Over the course of 260 days, it expends 3.64 tons, dropping deltaV by 11m/s.

Around Earth orbit, it deploys a 100m wide Fresnel lens. This lens focuses sunlight into the inlet. It can be very lightweight if inflatable technology is used. This powers the engine with 10.7MW of solar energy, allowing the stealth spaceship to produce up to 2.7kN of thrust.

Initial acceleration is 1.25mm/s2 (0.00125 m/s2 or 0.00000125 km/s2). It rises to 2.68mm/s2 when the tanks are emptied. At an average acceleration of 1.97mm/s2, it can burn through its deltaV reserve in 36 months.

Without the deployable Fresnel lens, it can run the engine for as long as 161 years...

What can it do?:

Once it has inserted itself into Earth orbit, the stealth warship has hide several hundred tons worth of ammunition for years, decades or more if it has no reason to move.

It needs to change an orbit, it can add 170m/s per day to its velocity, and do so for a year and a half.

The ammunition can be a massive amount of shrapnel to wipe out an orbit through the Kessler Syndrome, a fleet of missiles to devastate an enemy fleet before it even sorties, or a large laser to back-stab targets and slip away again.

On shorter missions, it can handle a crew without any significant increase in the amount of liquid hydrogen expended. It even serves as the perfect platform to mount a telescope on and detect other stealth ships.

Further enhancements:

-IR filter

Sunlight is actually a wide spectrum of wavelengths. Most of it concentrated between 400 and 600nm wavelength. Placing an infrared filter between the inlet and the furnace will allow most of the sunlight to go in, but the returning radiation will be absorbed.

At 3000K, the pulse engine's furnace radiates in between 750 and 1500nm (Infrared). This would remove the requirement to have shutters, and also eliminate the cone of light a 'naked' engine would bounce back to the Sun.

-Serpentine nozzle

 Before the exhaust expands and cools down, it is very hot. It radiates strongly in the infrared, and is very visible. If the exhaust nozzle was straight, the hydrogen would shine brightly before expanding. With a pulse engine, it would appear to the enemy as a rapid series of bright flashes: easy to pin down and detect.

A serpentine nozzle obscures the hydrogen while it cools behind a bend. It is already used on aircraft today to reduce their thermal signature.

by Matterbeam (2016)

Cloaking Device

A Cloaking Device is handwaving technology that violates physics and gives you your ardently desired stealth in space. In fact, it goes a step further and makes the freaking ship utterly invisible. Be warned that if you use this in your novel RocketCat will hunt you down and give you an atomic wedgie.

The concept seems to have originated when Star Trek screenwriter Paul Schneider rewrote the WW2 submarine movie The Enemy Below as the Star Trek episode Balance of Terror.

The tension of the submarine movie is because the captain of the US surface destroy escort ship knows there is a deadly German submarine lurking somewhere like Jaws, but you can't see where the blasted thing is hiding. Since there is no ocean in space to hide in, Schneider postulated incredibly advanced stealth technology (but probably thought of it as a high-tech cloak of invisibility). Another Star Trek screenwriter, D.C. Fontana, coined the term cloaking device for the 1968 episode "The Enterprise Incident".

For a good treatment of this theme, read PASSAGE AT ARMS by Glen Cook. For some handwaving about using the Fourth Dimension for cloaking purposes, read this quote.


CLOAKING DEVICE. This is the usual TECHJARGON for stealth technology used in SPACE WARFARE. When used, it converts Space combat from a stand-up drag-out into submarine-style lurk-and-shoot action more like "Das Boot." Cloaking Devices, alas, are almost pure HANDWAVIUM, whereas hardly any of that costly stuff is needed for the other guy's SCAN gear that is searching for you.

The closest you can get to a Cloaking Device without piling on too much Handwavium is to surround yourself with some sort of opaque mini-nebula, like a sea destroyer laying smoke. The enemy's Scan will still give your approximate position, and how much energy you're putting out, but at least this will fuzz things up a bit and make it harder for him to score a hit. Of course, if the enemy can't see in, you can't see out, either. This is inconvenient.

In any case, Cloaking Devices never work as advertised. Someone on board — either a particular dumb ensign, or the Science Officer — invariably hits the wrong button, sending out a transmission that pinpoints your location to the centimeter.


From: Capt. Isvieve Kalyn, Procurement, Resplendent Exponential Vector
To: Adm. Gilad Tsurilen, Bureau of Innovation
Subject: FAT NINJA progress report

Development on project FAT NINJA itself is essentially complete. Our research contractor has successfully demonstrated a prototype design capable of using intense paragravitational fields to distort the fabric of space-time in such a manner as to place the prototype within an enclosed polypoid volume of distortion, connected to the original location of the prototype by a narrow “neck”. They have further demonstrated limited communication capacity through this “neck”, suggesting that it would be theoretically possible to monitor events outside the distortion using a small drone vehicle, rendering FAT NINJA a non-double-blind device.

Unfortunately, no progress has been made on the fundamental problem of sustaining the distortion in the light of fundamental thermodynamics: necessarily, an enclosed polypoid volume suitable for preventing detection retains all radiation, including waste heat, emitted by the objects within it. Were the distortion to be handwaved into existence, this would be merely an irritating limitation; however, given the extremely high energies required to create the distortion, even with the most efficient power generation and paragravity equipment available, FAT NINJA is able at best to sustain a cloaked state for a matter of milliseconds before undergoing a catastrophic thermal excursion leading to complete vaporization of the prototype and immediate reversion of the cloaked volume.

While the experiment was worth doing, I must conclude that this is a physical limitation of all FAT NINJA type devices, and in the absence of some new fundamental breakthrough with regard to the thermodynamics of the case, FAT NINJA is a dead-end – at least as a cloaking device.

It does, however, make a rather splendid, if outré, bomb.

In service and glory,

Isvieve Kalyn (Capt.)

From CLOAKING by Alistair Young (2016)

Strategic FTL Sensors

If you have faster-than-light starships in your science fictional universe, and also want to have starship combat (and instersetllar empires), you have to have a carefully crafted set of limitations.

The Alderson Drive or "jump point" drive has been used in many SF starship combat games, for the same reason Niven and Pournelle used it: unlike most other FTL, it allows the possibility of interstellar battles.

Most other FTL is a "fly anywhere" kind of propulsion, which generally does not allow battles to occur except by mutual consent. Often a planet cannot even detect an enemy invasion fleet until it suddenly pops out of hyperspace. Interstellar wars only last long enough for your hyperspace bombers to fly to the enemy's planets, then a brief emergence to spit out a hellburner, a planet-wrecker nuclear bomb, a planet-sterilizing torch warhead, a planet-cracker antimatter warhead, or a planet-buster neutronium-antimatter warhead. Then they fly home, only to discover that the enemy's bombers were on a similar mission. Go to The Tough Guide to the Known Galaxy and read the entry "SLAG"

These start-anyway go-anywhere drives play merry Hell with concepts like 'distance', 'remoteness', 'proximity', 'adjacency', 'line of communication', 'border', and 'defence', while reinforcing such concepts as 'trade', 'concentration of force', and 'first strike'. Give me a setting in which the map still matters.

Please note that there is a second FTL situation that can allow interstellar combat. You need two things.

[A] Ships travel faster-than-light taking some time to travel the distance (i.e, travel is NOT instantaneous).

[B] There must exist some kind of faster-than-light radar that can detect the invading ships far enough in advance that the defenders have time to do something about it.

In other words you have to postulate Strategic FTL Sensors.

This will create something like wet navy combat in the Pacific ocean in the period after the time the navy was equipped with radar, but before the advent of orbital spy satellites that can see every ship on the ocean. This is more or less the situation in the Star Trek TV show(s).


Combat scoutships are spacecraft whose primary function is military reconnaissance and military surveillance. They may also be used as military artillery observers and forward air controlers. Scoutships are a part of the Intelligence, surveillance, target acquisition, and reconnaissance system (ISTAR).

They might be only lightly armed with self-defense weapons and "stealth", or they might be heavily armed for reconnaissance-in-force and killing enemy scoutships.

In a realistic solar-system-based no-FTL ain't-no-stealth-in-space universe, there is little or no need for scoutships. Constellations of observer satellites scattered over the entire solar system and the occasional drone recon missile will suffice. The astromilitaries of all space-faring nations will spread their recon satts near anything they consider to be of strategic importance.

In a science-fictional universe with either cloaking devices or faster-than-light starships, they may or may not be useful. It depends upon the limitations of the scifi technology.

Offhand, scoutships would be a useful anti-cloaking device measure if the cloak had a range limit. Meaning that a scoutship might be able to detect a hostile cloaked spaceship if the scout can get close enough.

Currently radars can be jammed if the enemy sends back fake radar returns on the same frequency. The radar can "burn through" the jamming by increasing the strength of the radar beam. Instead of turning up the power to the radar emitter, one can also increase the strength by reducing the distance between the radar emitter and the target (that pesky old inverse-square law strikes again). Thus a scoutship in advance of the task force would be closer to cloaked ships, and thus might be able to burn through the cloak while the distant task force was still blinded. If you make the assumption that the handwaving cloaking technology operates like radar jamming.

Obviously it would be useful for a task force to send scoutships ahead, if the scouts could spot cloaked ships and give advanced warning.

With respect to faster-than-light starships, the handwaving FTL technology would have to allow the existence of strategic FTL sensors in order to make scoutships worthwhile.


Fifteen million miles from Jupiter they (the alien FTL starship invasion fleet) slowed below the speed of light—and the IP stations observed them. Instantly, according to instructions issued by Commander McLaurin, a fleet of ten of the tiniest, fastest scouts darted out. As soon as possible, a group of three heavy cruisers, armed with all the inventions that had been discovered, the atostor power system, perfectly conducting power leads, the terrible UV ray, started out.

The scouts got there first. Cameras were grinding steadily, with long range telescopic lenses, delicate instruments probed and felt and caught their fingers in the fields of the giant fleet.

At ten-second intervals, giant ships popped into being, and glided smoothly toward Jupiter.

From UNCERTAINTY by John W. Campbell (1966)


Remember the light-speed lag. Light moves quickly, but not at infinite speed. It takes about eight minutes to travel one astronomical unit. So if you are in orbit around Terra and you observe a spacecraft near the Sun with a telescope or radar, you are actually are seeing where the ship was eight minutes ago. By the same token, if you change course it will be eight minutes until the Sun-grazer ship will know.

In C.J. Cherryh's Company Wars universe, ships use both radar and something called Longscan for detection and tactical information. Longscan helps cope with the lightspeed lag of radar.

Ships have two kinds of radar: the ordinary sort which operates sublight; and longscan, which is part guess and part radar.

The way it works is this:

It takes the original information of the jump range buoy and identifies every ship and object in the system, how fast they're going and in what direction. (ed. note: a jump range buoy is a satellite parked where ships emerge from hyperspace. It gives incoming ships an instant update of the location of all known ships.) It calculates a likely track and shows it on the screen as a four colored line. Red is what track the ships will take if they keep on as they bear. Yellow is what they will do if they veer as much as convenient: this is a cone-shaped projection. Blue is their position if they decide to stop.

Human operators rapidly intervene and as the computer priorities them the the fastest-moving ship data, they decide, on the basis of emotional human knowledge, what those ships are likely to do when the informational wave they have just made entering the system hits them (i.e., when the ships learn that you just popped out of hyperspace). If a warship, for instance, it may turn towards them as fast as it can. An operator is assigned for each ship under consideration while the computer handles the slow craft and the other which for various reasons do not need constant monitoring.

In the meantime two things have happened: Their ship has changed course and speed either following or not following the buoy lane assignment; and the other ships one by one pick up their presence in the system and react accordingly.

But this radar image changes constantly, so when the action begins to conform to one of the projections, the computer changes the color codes, assigning red to the most probable and so on down to blue as least. So it is part radar, part computer, and part human guesswork.

The data in the bank is the best information about the mass and engine capacity and turning ability and hostility or friendliness of each ship whose computer number is on that chart; and all ships know to be in space are in that computer memory.

Now, military craft (particularly Earth Company warships) are always making adjustments and honing their turning abilities if only by the smallest degree; this fouls up the enemy's longscan guesswork and can provide surprises. Mallory's Norway for instance, has not recently tested her adjustments to the extreme, and therefore the captain herself does not know just what Norway might do if she has to. And those refinements are only tested to the fullest, of course, when it comes to a situation where a ship either turns tighter than it is supposed to, or breaks apart -- or dies in impact.

From the COMPANY WARS universe by C.J. Cherryh

Detection and Stealth

Before you can engage the enemy, you must first detect the enemy. Paradoxically, this is both extremely easy, and rather difficult.

To begin with, detection itself is easy. There is, to sum up many an armchair strategist’s lament, no stealth in space. Running the life support alone makes a starship stand out 300K hotter – for warm-blooded oxygen-breathers – than the background of space. Using power plant, thrusters, weapons systems, or anything else aboard only makes it more visible. Starships stand out plainly against the near-absolute cold of space, even across entire star systems, and this is inescapable.

Stealth, such as it is, would be better described as masquerade. One cannot avoid being detected; but one may be able to avoid being identified, or identified correctly. Performing such masquerades by altering one’s sensor signature is an important part of the function of a military starship’s defense drones.

It is difficult, on the other hand, because light, that sluggard, imposes an absolute limit on the currency of the data available. No sensor yet developed is capable of detecting objects in real time at a distance: at best, one can see what the situation was when light left that region.


The answer to this is longscan.

Shortscan is what one’s own starship’s sensors, passive and active, are reporting.

Longscan, on the other hand, is the informational gestalt of that shortscan information along with all informational available from other sources (other starships in one’s formation or elsewhere in the system; tactical observation platforms; civilian navigation buoys or stargates, when available; and so forth), along with AI predictive extrapolations of what each starship or other object visible in longscan has done since the last update and/or will do, based on further extrapolations of what their longscan is telling them – and projections, likewise, of what they can know about your actions.

(Establishing this is in turn complicated by the nature of the tactical networks that provide that informational gestalt; modern navies provide their ships with tangle channel FTL communications between themselves and their own observation platforms, but since tangle-channel relays are point-to-point, this does not apply to most civilian sources except, in wealthier systems, as relays between STL EM communications buoys. Determining the “shape of the information wave” – who can know what, and when – is one of the most complex problems a warship’s tactical department faces.)

All of this information is displayed upon the tactical display, along with probability and reliability estimates, in graphical form. Learning how to read these tactical displays at a glance is, in itself, a significant part of naval officer training.

Observation Platforms

One of the greatest advantages one can have, therefore, is expanding one’s informational gestalt. Thus, virtually all military starships carry observation platforms with them for ad hoc deployment; and indeed, most navies routinely seed their own systems (and neutral systems in which they may operate) with dormant, concealed observation platforms awaiting activation when necessary by starships on the scene.

It is, of course, much harder to sneak concealed observation platforms into the sovereign systems of other polities, current enemy or not, and as such the information advantage in invasion scenarios is almost always with the defender.

Information Warfare

The nature of this data environment highlights the importance of information warfare in naval operations. One of the most valuable things it is possible to achieve, when still maneuvering for engagement, is to successfully infiltrate the tactical network of the opposing force. While direct stealth in space is impossible, the ability to distort one’s sensor signature, inject fake signatures, and otherwise falsify the information upon which one’s opponent is basing their tactical decisions is extremely valuable.

As a result, any major naval engagement is invariably accompanied by high-intensity information warfare, as each side attempts to corrupt the tactical networks and other data systems of the other.

An even greater coup, of course, is to penetrate the internal networks of an opposing starship and, having established a degree of computer control, simply order it to drop its kinetic barriers, shut down its point defense, vent its fuel, disable its life support, or otherwise change sides. Although remarkably difficult to achieve at the best of times, such a victory is almost always complete.

James Huff is experimenting with plotting something similar to a Longscan display. He is trying to make a "probability plot" of where to aim your guns, given the target's acceleration, maneuvers, and lightspeed lag due to the range to the target. Mr. Huff generated these plots with a custom C++ program he wrote for generating iterated function systems.

It's a lot simpler if rotation is done independently of burn, but somewhere I recall reading that Outsider ships use differential thrust of their main engines for hard maneuvering, so there will be a significant linear component in addition to the angular component of their acceleration. For a 180 degree turn, the start-turn/stop-turn accelerations cancel out, leaving the ship offset sideways somewhat from its original course, but with the same velocity. For rotations less than this, there will be a linear acceleration left over after the turn. Also, there is a maximum rotation rate that a ship can handle, but I'll ignore that.

For simplicity, I'll assume that the turn is completed before the linear burn is started, no nudging the ship into a turn and going into a full-forward burn during the turn. In addition, I'll confine the movement to a single plane, though in reality, ships would be able to change their axis of rotation mid-turn. I've already got most of the code for this written, in an IFS renderer...

All of these plots are for a period of time equal to twice the time it takes to flip the ship 180 degrees, halting the rotation... burning one thruster to rotate the ship during the first half of the flip, and the other to cancel the rotation during the second half.

James Huff

Tactical Combat Sensors

Tactical combat sensors work at close range in a battle, guiding your weapons to the enemy targets (a "firing solution"), detecting incoming enemy weapons, and analyzing the enemy for weakness.

  1. DRI
    1. Detection: ability to distinguish an object from the background
    2. Recognition: ability to classify the object class (metor, asteroid, spacecraft...)
    3. Identification: ability to describe the object in detail (HMS Repugnant battlecruiser of the royal Callistonian navy). Specifically is this an enemy spacecraft or other military asset?
  2. Tracking:
  3. Firing Solution: enough information to calculate where to train your guns so you can shoot the snot out of the target

Firing Solution

Merely detecting the presence of an enemy spacecraft is not enough. If you want to actually shoot it, you need to obtain enough sensor data to get a firing solution. These sensors are part of the ship gun fire-control system

The enemy will naturally be doing all it can to degrade your firing solution. This includes evasive maneuvers, electronic warfare (i.e., trying to jam your sensors) and handwaving cloaking devices.

Detecting Incoming

These sensors give advanced warning to the space warship about incoming hostile weapons fire. Some will be mounted on point defense systems so said systems can shoot down incoming enemy missiles. Some will feed data displays in the ship's combat information center; suggesting to the captain that they might want to order the ship to, you know, dodge the missile?

These sensors are very similar to ordinary non-combat tactical sensors. Except they are much more sophisticated. As a general rule, meteors do not use stealth technology nor do they home in on you. Hostile weapons do.

Looking For Weakness

When the enemy spots your ship by the exhaust plume, it not only knows that a ship is there, it also knows the ship's exhaust velocity, engine mass flow, engine power, thrust, acceleration, ship's mass and ship's course. Not only can it tell a warship from a cargo freighter with all that information, but it can also tell the class of warship, and maybe make a good stab at determining which particular member of that class it is.

In more detail: as mentioned above, propulsion system's exhaust velocity is revealed by the Doppler shift in the emission lines, mass flow is revealed by the plume's luminosity, the thrust is exhaust velocity times mass flow, acceleration is revealed by watching how fast the plume origin changes position, ship's mass is thrust divided by acceleration, and ship's course is revealed by plotting the vector of the plume origin.

This means that painting the ship with camouflage in an attempt to disguise its identity is pretty pointless.

During a battle, sensors also give "intelligence". That is, for example, if you fire your lasers at the target, and suddenly two of the target's nuclear power reactors have a drop in temperature, you've probably scragged them and the target's power budget has been substantially reduced. Your ship's captain will alter their battle tactics accordingly.

In a similar manner, a spectroscope can be used on any plumes of gas vented by the stricken target. If it is hydrogen, you probably punctured a propellant tank. If it contains oxygen, you probably holed the habitat module. If the target is antimatter powered and you suddenly detect a drastic increase in 511 keV gamma rays, turtle up quick cause she's gonna blow!

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