A blog dedicated to helping writers and worldbuilders create consistent, plausible Science Fiction.

Wednesday, 23 March 2016

Stealth in Space is Possible

The first thing an aspiring scifi fan looking into the harder end of the spectrum is told is: No Stealth In Space. They're still high off the latest Star Wars, or vividly remember scenes from Alistair Reynold's Revelation Space or James S.A. Corey's The Expanse, but they're given the choice of giving that up or leaving.

This is ToughSF's answer to that.

To understand stealth, we have to first look at how spaceships are detected in space.

Detection relies on picking up emissions from your spaceship (passive sensors) or bouncing off radiation at your spaceship (active scanning). These boil down to thermal and optical imaging.

Thermal imaging

Jupiter in visual and thermal view.
The ease with which a spaceship can be detected depends on the difference between the background temperature and the spaceship's surface temperature.

We can get estimates for how far away a spaceship can be detected based solely on its heat emissions. This is pertinent, as objects in the 300-2000K temperature range emit wavelengths of 2-10 micrometers (near IR). Visual emissions are much less pronounced. The rest of the electromagnetic spectrum is minor compared to infrared, but can be useful for identifying the spaceship.

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.

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

Waste Heat is the power emitted by your propulsion system's inefficiency. For example, a 70% efficient 1GW rocket engine will radiate 0.3GW of waste heat. 

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.

The gigapixel CCD array inside the Kepler telescope.
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. 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 temnperatures are either very heavy or very fragile.

In the previous example, we had to radiate 0.3GW of waste heat. A solid radiator using lithium coolant at 1000K temperature would have a surface area of 5800m^2. Half of this would face a sensor, leading to a detection range of 721 million km.

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.

Establishing references

A B-52 jet bomber in infrared.
If you've followed the previous posts, you should have already settled on a travel time and the rocket engine power output you will be needing. Research into what sort of engine you'll be using will give you an efficiency rating, and therefore the amount of waste heat you'll be emitting during a full burn.

We'll be looking at a 1GW-propulsion spaceship as a reference. 

1GW spaceship:
70% efficiency: 300MW emitted
1*10^-17 W/m^2 effective sensor sensitivity
Detection range under acceleration: 4886 million km or 32.5 AU

For comparison, Pluto orbits at 40 AU.

The spaceship can be detected months before it reaches its destination. An increase in drive emissions and sensor effectiveness lead to an astounding increase in detection range. 

Now let's move onto ways of lowering or sidestepping those enormous ranges.

Cold running

Don't mind me, I'm just a... metallic, sharp-edged asteroid.
The obvious solution is to NOT accelerate at full power. This vastly decreases your waste heat output, the surface temperature of your spaceship and therefore your detectability range.

But how low can we go?

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 the spaceship is 500 tons and its density is 1000kg/m^3 (submarine-like construction), then it has a volume of 500m^3. 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.  

Since you are not running the engines, you will only drift on a ballistic trajectory through space. Your motion can be predicted very accurately during your acceleration burn, meaning, for this spaceship, the entire trip can be tracked very easily until your 'cold running' emissions make you detectable again.

Redirecting emissions

This method requires minimal investment in terms of mass, as it only requires that radiators be mounted on a swivel. It is not possible to do this with droplet radiators.

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/m^2.

10000kg radiator mass
294m^2 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. 

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.

We now try to point our radiators edge-on to the area where we expect the sensors will be. Our ability to do this depends enormously on the setting.

In a rather soft scifi setting with interstellar travel and unexplored solar systems, we won't expect to find telescopes and sensor arrays other than around the point of interest. In a setting with mature interplanetary travel and hostile world powers, the enemy's sensors would not be allowed to orbit your home planet, and can be expected to found in the hemisphere in front of you. In the worst case scenario, the telescopes are cold, numerous and positioned at every imaginable angle to watch spaceships come and go.

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

Simple 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
sin(1)*294= 5.31m^2
Under acceleration: 130000km detection range
sin(1)*50= 0.87m^2
Low power mode: 1500km detection range

10 degree visible angle
sin(10)*294= 51.05m^2
Under acceleration: 418000km detection range
sin(10)*50= 8.68m^2
Low power mode: 4800km detection range
30 degree visible angle
sin(30)*294= 147m^2
Under acceleration: 707000km detection range
sin(30)*50= 25m^2
Low power mode: 8200km detection range

60 degree visible angle
sin(60)*294= 254m^2
Under acceleration: 0.93 million km detection range
sin(60)*50= 43m^2
Low power mode: 10764km detection range

90 degree visible angle
sin(1)*294= 294m^2
Under acceleration: 1 million km detection range
sin(1)*50= 50m^2
Low power mode: 11607km 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. 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.

A telescope in the light blue orbit would look down on target spaceships

A telescope placed in orbits that spend a maximum amount of time far above the planetary orbital plane would have the best vision angles. Military expeditions might attempt to travel under or above the planetary orbital plane to evade the naturally more numerous in-plane sensor platforms....

Heat sinks

Liquid Hydrogen storage at Cape Canaveral
One commonly proposed method for stealth is the use of heatsinks to store waste heat. Some materials have a very high heat capacity, and could theoretically absorb large amounts of energy.

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 doesnot dissapear. The temperature would increase, but would eventually have to be radiated, either through a cooling system, or through the boiling corpses of your crew.

Let's work out an example.

Looking at the materials with the highest heat capacities, we immediately notice that water and hydrogen very commonly double as propellants. Therefore, there is no reason why a correctly configured propellant reserve can't double as a potential heat sink.

The 1GW spaceship wants to travel between Earth and Mars. To be 'stealthy', it uses a hohmann trajectory with minimal acceleration phases.

DeltaV required for a Mars-Earth-Mars with the choice between abort and insertion at the Earth end, plus margins, is about 7km/s.

A hydrogen-propellant nuclear-electric drive could achieve 30km/s exhaust velocity with liquid hydrogen. It would need 135 tons of propellant and would accelerate at 0.1m/s^2.

135 tons of hydrogen would provide 1.93GJ per Kelvin heat capacity. Heated to room temperature, it would have absorbed 567 gigajoules. Heated to 1000K, it would have absorbed 1930.5 GJ. At 2000K, it is 3861GJ.

This sounds impressive, being able to absorb potentially 3.5 hours of full acceleration, or 42 months of 'cold running', but there are several negatives.

The first and foremost is that hydrogen expands quite a lot. Following the ideal gas law, it would blow up from 44 thousand cubic meters to 22 million cubic meters. At 2000K, no material can both withstand the temperature and expand to contain that volume. The other option is to contain it under pressure, which gives us a few supermaterial candidates.

Hydrogen can also find its way through any material, even more so when hot, so you will always be leaking hydrogen into space and revealing your position.

Also, heat transfers between the spaceship and the heatsink depend on the difference in temperature. If your reactor operates at 2000K, it is very easy to remove waste heat using a 4K heatsink. But when the heatsink approaches the temperature of the reactor, heat transfer dwindles to zero unless active heat pumps are used.

Heat pump aboard the International Space Station
This is especially important for the crew cabin, which puts out very low temperature waste heat. At 350K, it can only expect to keep filling the heatsink for 396 days.

This might be sufficient for the 8.6 month Hohmann transfer, but not longer than that. Low temp (and low detectability) radiators can extend this period. The hydrogen would still expand by an incredible amount.

This is ignoring the fact that the initial hydrogen would probably be liquid to save on propellant tank mass, so the expansion would literally be explosive.

Notice how big the hydrogen leak is despite the tiny hole.
The other possibility is water.

Exhaust velocity would be halved, meaning 300 tons of water are needed.

From 4K to 273K, the water would provide 0.63GJ per K for a total of 170GJ. Between 273 and 373K, water adds another 126GJ. From 373 to 2000K, water steam absorbs 1025GJ for a total of 1321GJ.

In some ways, water is better. It is very dense for the first 373K, and expands 18 times slower than hydrogen.

On other ways, it is worse. It cuts into your exhaust velocity and makes you accelerate slower. Hot steam is extremely corrosive, too.

In conclusion, heat sinks are an option, but with limited applications. They are impractical during energy intensive operations such as full acceleration, but can be used to further reduce 'cold running' temperatures, or perform very low-energy operations.
As heat sink temperature rises and propellant is consumed, their usefulness decreases sharply.

Soyuz approaching the ISS. Notice the large, circular solar panel from a Cygnus cargo.
We'll look into more options and complications in the next post.


  1. "so you will always be leaking hydrogen into space and revealing your position."
    How easy to detect is this hydrogen, and how would they proceed best?
    I would expect fast-venting hydrogen to be hard to detect, particularly if it's not especially hot. Would it give itself away by blocking spectral lines from the background?

    Some years ago, I thought about using liquid metallic hydrogen.
    Liquid metallic hydrogen is thought to be a superfluid, so with a speed of sound in the tens of km/s, so pressure alone would give it an Isp in the thousands of m/s, without actually generating heat (though a secondary generator heating it during burn would give better Isp, IIRC I had found a theoretical max Isp of 10km/s).
    In fact, compressed so much, it would also be an powerful regenerative coolant when expending from metallic to liquid/gas state, and could be used as a great heat sink, though you may have to sacrifice its use as fuel.

    If you don't want to vent the used hydrogen for heat sink use, we could have a succession of tanks who are remodelled from tiny and thick to large and thin to hold gaseous hydrogen. But it would be simpler to just vent it, if it doesn't raise detection too much.

    Problems are that liquid metallic hydrogen requires even higher pressure than solid metallic hydrogen (at least hundreds of GPa) and is still theoretical AFAICT.
    My back-of-the-envelope calculations had given me that for a spherical tank made of plain thick nanodiamond, you'd need it to be as thick as one third of the radius, which adds some serious weight - even metallic, hydrogen isn't that dense. It may get better with new, more exotic supermaterials, but I doubt it would be by that much.
    Then I don't want to begin imagining hydrogen infiltrations at that pressure, or all the other hilarious new entries for the list of propulsion failures.
    So it may actually be impractical, but if the nuclear saltwater design is discussed, after all...

    A quick googling shows that metallic hydrogen is actually thought of as propellant, but it is about the comparatively more accessible solid metallic hydrogen, and with designs that wouldn't make for a particularly low-waste heat use.

    1. I searched the net and could not find any solid figures for hydrogen leaking through porous containers. What I did find is that you wouldn't be able to contain it for long using metals or carbon-containing materials, as they suffer from hydrogen embrittlement and Hot-Hydrogen-Attack respectively.

      AFAIK about high-pressure hydrogen in dense states, the propellant tank mass dwarfs that of the hydrogen contained. I've found figures of 100:1, up to 30:1
      The biggest issue is therefore the pressure being released by a failure of the containment system, or an increase in temperature causing the hydrogen to change state and expand explosively.

      Solid hydrogen has the advantage of being a pseudo-alkali metal, possibly superconducting, so would respond well to electromagnetic storage.

  2. With diamond+ material, you could actually get close to 1:1 for containers, ideally. Though of course, that's still not very practical. I never thought about electromagnetic storage, though, and it sounds kind of brilliant (and with even more Failure List hilarity). How would it work, and what would you need to keep something at those kinds of pressure?

    I am also curious if there are better pressure containers than a plain hollow sphere per mass. I asked around, but couldn't get more than "probably, I guess" so far.

    Some form of solid metallic hydrogen may be metastable at room temperature.
    Again, handling something that may or may not want to explosively revert state if a stray cosmic ray looks at it funnily is bound to be interesting.
    While I wouldn't use that for a heat sink, it may be useful at giving monoatomic hydrogen for propellant for a better Isp, and thus better efficiency at the same temperature.

    1. Diamond+ materials, and other carbon-based materials, have the best strenght to weight ratio, but are vulnerable to hydrogen infiltrating them.

      If it is high-temperature hydrogen, it will react to create methane. This accumulates in microscopic bubbles that weaken the pressure vessel.

      In space, you can build large volumes with low pressures cheaper than small volumes with high pressures. For example, a ballon-within-balloon-within-ballon configuration to reduce the pressure gradient between interior and exterior at each ballon wall takes up a lot of space, but can be accomplished with flexible plastic material instead of thick metal walls.

    2. https://en.wikipedia.org/wiki/Magnetic_pressure

    3. Wouldn't a system based on magnetic pressure be active and thus producing waste heat? Or did I miss something?

      Are there bulk materials that slow hydrogen infiltration down? (No need to stop it, only slow it down enough for it to survive the mission.) We could coat the inner wall of the tank with it, as long as it is not unmade by the pressure, it would rely on the then-protected diamond+ wall for structural strength.

  3. If detecting objects in space is so easy, why we didn't find every asteroid, or object in Kuiper belt? Is it that way because those objects are near 3 Kelvins?
    Sorry for my English, it's terrible sometimes :)

    1. Detecting low temp objects like asteroids only becomes 'easy' when there are many sensors such as orbital or mountain-top telescopes dedicated to the task.

      Also, in realistic science fiction, the initial acceleration of a spaceship away from its parking orbit is extremely easy to detect, giving you a rather narrow area of space to look at. Asteroids never give off the initial burst of emissions, and can be loactedpretty much anywhere in the sky.

    2. Detecting this things is easy (on paper, at least) in the far infrared part of the spectrum.

      Our atmosphere absorbs far infrared light.

      Needless to say, this rather cramps our style as far as detecting warm things. We would need to put a dedicated NEO finding IR telescope into space in order to detect all those things, and that costs a lot of money.

      Also, the Kuiper belt objects are really cold, so they don't emit a lot of infrared light to begin with.

  4. Your formula on detection ranges neglects to mention the aperture of the scope used to collect the infrared light. A 10 meter scope, for example, will collect 100 times the infrared emissions as a 1 meter scope with the same CCD detector array, allowing detection at 10 times the range.

    1. I don't really know how parameters other than minimum sentivity and signal/noise affect the detection range.

      I bundled it into 'effective' sensitivity.

    2. The theoretical limit of sensitivity would be a detector that can detect a single photon. An emission source create a flux of radiation that spreads photons out more or less uniformly in an expanding semisphere (or a full sphere). The aperture of a telescope determines how much surface area of that semisphere you are sampling. If you double the size of the apperture, you increase the sample area by 4x. On the one hand, this means that you have that much more area to find the single photon. On the other hand, if you don't have a theoretically optimal detector, it multiplies the number of photons collected exponentially.

  5. You should source your equations for detection range. Besides, they are strange as they don't seem to take into account exposure time or sensor aperture.

    1. It is a simple comparison between power output spread out over a sphere with a radius equal to detection distance, over the effective sensitivity of the sensor.

      'Effective' sensitivity covers all factors relating to how much much watts per square meter are needed to detect a spacecraft.

  6. There is a pretty good reason propellants have limited use as thermal mass for heat storage... they generally are used to cool their engine in operation. Preheating your fusion fuel by cooling the exhaust bells or control baffles is standard for liquid rockets; hot fuel reduces that. Further, hot fuel expands. Water expands 28:1 in phase transition to steam if uniform pressure is maintained; if not, pressure goes up proportionately.
    Even before that, water is most dense at 4°C, heating it expands it; from a density of 1 to a density of 958 at 100°C... a 4% increase in volume. May not seem much, but it adds up. Especially if the pressure has dropped due to fuel use. the 28:1 expansion crossing to vapor is potentially traumatic; the tanks are probably not rated for 28x normal pressue....

    1. Well I do know that cryogenic fuels are used as coolant in chemical rockets, but I very highly doubt that the same would be performed for a fusion engine. For one, the majority of fusion rocket designs use a magnetic 'bird cage' as a nozzle, which might be incompatible with liquid hydrogen cooling.

  7. Minor correction regarding life support: humans actually produce a considerable amount of heat. Most modern spacecraft (and spacesuits), however, now have very good insulation. The old apollo and soyuz spacecraft were esentially thin metal cans with no insulation, so some heating was necessary (for the most part, however, they just rotated the craft to allow the sun to heat it evenly, and maintain a relative equilibrium). Now, though, the insulation is so good that no heating is required ( or even installed). Instead, the energy on life support is spent COOLING the craft.
    If you needed to heat the craft, you could just radiate your waste heat back into the craft, and thereby reduce your emissions. But the reality is you need to cool the air to maintain a comfortable environment, and this is why it adds to the overall emissions.

  8. '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.'
    Thats pretty much the exact same solution I happened upon. Of course, only certain shapes will be able to take advantage of this method. They aren't going to look like the andromeda ascendent. Function over form.

    'In a rather soft scifi setting with interstellar travel and unexplored solar systems, we won't expect to find telescopes and sensor arrays other than around the point of interest.'
    Very trueconsideration. The number and location of hostile sensor platforms will determine how long your ship can remain hidden for. An intruding ship cannot use active sensors or communications, nor can it use the main thrusters: The exhaust plume would be seen from across the solar system!

    'Let's calculate some values. We assume thin radiators, so they only radiate from one side.'
    You suggest that at the lowest aspect ratio (while under thrust), the reference ship can remain hidden beyond 130,000 km. That may be true with regards to the heat radiators, but what about the exhaust plume itself? John schilling estimated that a maneuvering thruster on the space shuttle could be detected from 15 million km!
    I'm unsure whether he was referring to the primary RCS or the vernier RCS, which have significantly different thrust ratings. A cold gas rocket should be the stealthiest drive theoretically possible, more than a chemical rocket and much more than a plasma thruster.

    '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.'
    To be honest, remaining hidden even at 1 million km would be an impressive achievement and pretty damn important at the strategic level. It would give the enemy space fleet much less time to deploy and react to your own ships. I don't think you should discount it so quickly.

    1. The restriction of having to use different radiators comes mostly from the coolant fluid that you can use and the temperature of the heat being generated. A nuclear reactor with walls at 1400K will happily accept molten lithium, but a hab at 300K can use water (or like on the International Space Station, ammonia, that has a lower freezing point).

      The maneuvering thruster on the Space Shuttle is about 80kW, burning MMH and NTO. Chemical thrusters put all of their waste heat in the plume, so it is hot. They also expand into vacuum quickly, so it has a very large cross-section. It's basically a hot flare several times the size of the shuttle. An ion drive puts its waste heat into the radiators. The actual components that accelerate the ions are extremely efficient, the ions never get very hot. They expand slowly, so by the time they have expanded, they are already cold.

      If your opponent only finds you at 1 million km, and you are in a setting where interplanetary crossing is done at 80-100km/s, they have less than three hours to mobilize. Even a low-tech Hohmann will give your enemy a window of two days or less.

    2. You can use the same radiators for everything, though, by using heat pumps. It actually cuts down on radiator size, because while more waste heat is created in the process, the hotter radiators are much more efficient at radiating it.

    3. 'The maneuvering thruster on the Space Shuttle is about 80kW, burning MMH and NTO.'
      Is 80 kw for the primary thrusters or vernier thrusters? You are right about them being MMH, I mistakenly thought they were cold gas. No wonder they are observable from such long ranges...

      'Chemical thrusters put all of their waste heat in the plume, so it is hot. They also expand into vacuum quickly, so it has a very large cross-section.'
      I got the impression from projectrho that chemical rockets were actually stealthier. The author claimed that chemical rockets have Nd of roughly 0.95, ion drives get about 0.50, and steady-state plasma thrusters 0.65 or so.

    4. It is the vernier thrusters.

      Honestly, I don't know the full details on how stealthy different drive types are. The devil is in the details.

  9. Even if you don't care about being stealthy you will put your radiators edge on to the sun so you aren't picking up heat from the sun while you are trying to dump heat to space. So you automatically make yourself less conspicuous to sensors in the vicinity of any planet & most asteroids.

    So anyone who is trying to monitor who is doing what in the solar system, whether for space traffic control, or to watch for hostile activity, will put a bunch of telescopes with visible & infrared sensors in orbits well out of the plane of the ecliptic. Something similar was already done to study the solar poles with the Ulysses spacecraft.

    So in any scenario in which there are hostile powers in different parts of the solar system there will the monitoring telescopes scattered well out of the ecliptic & trying to put your radiators edge on to one set of sensors will make them more conspicuous to another.

    Jim Baerg

    1. Hi Jim!

      The position of the radiators had to be mentioned specifically for several reasons:
      -Some designs have fixed radiators.
      -The further away from the sun, the less important your radiator angle is.
      -Some solar panel designs have the radiators on the rear face of the panels, so angling the radiators is counter intuitive.
      -Many readers may not have thought that it is an important issue.

      You are correct about the extra-elliptic position most interplanetary spy satellites will have. From a practical standpoint, however, it becomes harder to conceal the satellites themselves, if not the very long mission to put them there, and ultimately the station-keeping burns they have to do to maintain their unstable position.

      To give a few numbers, a satellite at Earth orbit (1AU) will have to rise at an 'altitude' of 1.53 AU above the ecliptic to gain 20 degrees on spacecraft radiators at Jupiter's orbit (5.2-1= 4.2AU distance). It will orbit at a true altitude of 1.83AU and a solar inclination of 56 degrees.

      In the worlbuilding section I added in the following parts of this series, I mentioned that the most important peacetime activity of space militaries could be observing the skies above and below the plane to try and locate these spy satellites. In the case of war being declared, they will be the first to go and the hardest to replace.

    2. I don't see that the spy satellites would need station keeping burns. There would just be a lot of them put in orbits at a high inclination to the ecliptic so each one would spend most of its orbit well off the ecliptic, resulting in most of them being off the ecliptic at any time.

      How conspicuous the spy satellites would be would depend on how much power they need & how tightly they can beam their data to the organizations that launched them

      Jim Baerg

    3. By station-keeping burns, I mean the deltaV required to periodically correct for unstabilities in the orbits. An above-plane elliptic orbit will be perturbed by the gravity of the planets as they change position over the course of years. After all, it is not a Lagrange point where gravity cancels out.

      These burns are small, but they would starkly contrast against the cold, black background of interstellar space, high above any inteprlanetary or solar interference.

      I've come to appreciate the unstabilities of orbits better after trying my hand at Children of a Dead Earth.

      I think the biggest factor in spy satellite stealth is how they deal with solar heating. It will dwarf any waste heat produced on-board, especially if advancing electronics creates more and more efficient processors.

      The Hydrogen Steamer concept I posted on recently might be a solution to this problem.