In the science fiction setting I'm working on, I had a thought on how to reduce the detection ranges in space. On a previous question, I suggested the idea of capital ships and other support ships producing a cloud of electromagnetic interference that would make it near impossible to isolate individual ships within the cloud without visual aid. Sensors would just show a giant electromagnetic cloud. Firing into the cloud outside visual range MIGHT hit something, but would more likely just miss between ships. Space is big, very big.

I came up with something else that might aid the difficulty in detecting a spacefaring vessel at long range, particularly by passive sensors. A combination of two separate ideas.

If a ship had the means to re-absorb waste heat and convert it into electrical energy, through centuries of advancements in thermoelectric cells, they could reduce their heat signatures (and get a little more power as a bonus, but I'm sure the pilots would more appreciate the signature reduction.)

Building off the electronic warfare idea previously mentioned, the ship could also emit its own interference that makes it indistinguishable from cosmic radiation to passive sensors.

The combination of a reduced heat signature and the interference signal should make a ship harder to detect by most sensors, I think.

The only concerns I have at this point are:

  • That the electromagnetic shielding that keeps this cosmic radiation OUT of the ship might be trapping the interference signal in. Even if it's not actual cosmic radiation trapped in the shield, I imagine that signal is not very helpful if it can't escape the shielding.

  • That the electromagnetic shielding that keeps cosmic radiation out may be just as obvious as a ship without any stealth measures thus defeating itself.

Are my concerns justified? Or does this system still sound plausible enough to work?

This isn't meant to be true stealth in space, just sensor mitigation that helps force ships into visual range. This setting is being developed both for games and short stories and watching your ship firing at something outside of visual range doesn't sound like the foundation of a fun game to me.

  • $\begingroup$ How is "re-absorb[ing] waste heat and convert[ing] it into electrical energy" not a perpetual motion machine of the second kind? $\endgroup$
    – AlexP
    Jan 24, 2018 at 0:56
  • $\begingroup$ I believe the second law of thermodynamics prevents any sort of stealth in space. There will always be heat entropy. $\endgroup$ Jan 24, 2018 at 1:07
  • $\begingroup$ It's not a perpetual motion machine because it can't be 100% efficient. It's just trapping WASTE heat produced by the reactor and electronic components to keep it from escaping the ship and storing it as electrical energy. And I don't think it's possible to get all of it, just most of it so the ship isn't as easily detected. $\endgroup$
    – Arvex
    Jan 24, 2018 at 1:16
  • $\begingroup$ Obscuring detection depends entirely upon how the detectors work - exactly what is getting detected? The outer hull? Reactor gamma? Occluded stars? Radiated heat? E.E Doc Smith's Lensman played with a lot of fun, clever tricks to fool detectors back in the 1930s. $\endgroup$
    – user535733
    Jan 24, 2018 at 3:12
  • 1
    $\begingroup$ @JBH: "A thermoelectric generator (TEG), also called a Seebeck generator, is a solid state device that converts heat flux (temperature differences) directly into electrical energy through a phenomenon called the Seebeck effect (a form of thermoelectric effect)" (Wikipedia). All machines which convert heat into work need both a hot reservoir and a cold reservoir. In the end, all the free energy used in the ship / by the ship must be radiated as heat, because in space there is no other form of heat dissipation. All of it. $\endgroup$
    – AlexP
    Jan 24, 2018 at 12:01

2 Answers 2


No, this will not work.

For the heat-electricity concept:

What you describe as 'converting waste heat into energy' is the naive explanation of how a thermocouple works. Heat goes in, power comes out. The problem is that thermodynamics tells us it is impossible to reverse entropy, and that means you can't convert heat into useful work. You can use a heat gradient to perform work, which is what a thermocouple actually does, but the process creates even more waste heat in the process.

You can hide that heat temporarily by stuffing it into heatsinks, and you can use that heat to generate electricity, but you cannot get rid of heat by using it to generate electricity. Your heatsinks will continue to fill until either you start radiating heat or your ship melts.

For the emissions concept:

The problem is that you can't interfere with the relevant forms of radiation. Namely, if your ship is emitting the amount of waste heat associated with onboard human life (let alone associated with gigawatt drive systems or anything of the sort), it will be visible as blackbody radiation, the kind that telescopes are routinely designed to detect. And you can't jam a telescope. As explained in this answer to your last question, the only hypothetical way to interfere with this is to spit out a whole lot of additional energy in other wavelengths, but that's neither a means of camouflage nor effective in its own right.

  • $\begingroup$ You're wrong. What the OP is describing is a thermoelectric generator. These devices actually exist and actually work. A solid-state TG only needs an electrical ground to convert heat to electricty. And as the OP is only looking to reduce the signature, not eliminate it ("this isn't meant to be...") his concept works just fine and only needs the improved efficiency brought about by future advances to be a plausible solution. $\endgroup$
    – JBH
    Jan 24, 2018 at 6:38
  • 1
    $\begingroup$ A thermoelectric generator and thermocouple are both applications of the Seebeck effect. They actually exist, and actually work to produce useful energy from a heat gradient, but they do not reduce the amount of heat in the system- they merely transfer it from the high-temperature source to the low-temperature heat sink and produce some energy in the process. You can't reverse entropy, you can only move it around. Actually using that energy then produces yet more waste heat as a byproduct. $\endgroup$
    – Catgut
    Jan 24, 2018 at 13:57

You Could Make It Work

Energy flow in your system would be like so:

  1. Waste heat is conducted through a (very, very advanced) thermoelectric generator to either a heat sink or radiator.
  2. Electrical energy generated as a result of that process is transferred to an SMES.
  3. "Stealth mode" could stay activated until either (A.) the current in the SMES reaches a critical threshold* or (B.) the temperature of your heat sink/radiator is so high that you cannot adequately cool your spacecraft. Thermoelectric generator output will simply decrease as the temperature delta narrows, which is minimal drama unless you're using it to power something important.

With a heat sink: you have a thermally massive component inside your ship, actively cooled to a low temperature during normal operation, that is then gradually heated up during stealthy operation. No energy beyond that radiated from the (presumably low temperature) skin of your spacecraft is emitted to the environment.

With a radiator: Your radiator is either directional** (to avoid radiating in the direction of the enemy) or presents with a low enough surface temperature to keep detection distance within parameters.

None of the potential roadblocks to these solutions are theoretical; they're engineering problems, and the beauty of science fiction is that engineering problems are easy to solve. It's the future--they've got materials science so good it'll write you a love poem and your spouse will run off with it.

*Exceeding said threshold would result in (A.) collapse of superconductivity, (B.) transformation of stored electrical energy into heat, and (C.) catastrophic vaporization of the SMES--sounds totally awesome if you ask me!

**The committee is aware of the "There Ain't no Stealth in Space" Atomic Rockets page. It is the opinion of the committee that the arguments presented therein are a false god.


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