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There are several questions around regarding stealth in space, where the general conclusion is that stealth is not really feasible because (among other things) heat radiation from a spaceship doing anything interesting (except unmanned probes just coasting on a fixed trajectory) can be detected from millions to billions of km away.

One part of these answers is that high power rocket engines emit large amounts of heat radiation, so stealthy maneuvering is limited to e.g. cold gas thrusters or mass drivers, which have a low specific impulse. However that reasoning seems to assume current rocket engines that are not designed with stealth in mind. Rocket engines themselves get very hot, and the exhaust plume is very hot as well, so both emit massive amounts of thermal radiation.

Shrouded rocket engine

One item I have not seen answered in anything I have read is the following scenario:

  • Assume that only detection from one direction is relevant. If the heat signature of the ship and rocket engine can be hidden from one hemisphere of space (i.e. a 180° "cone") that is good enough. We will assume hostile sensor platforms are not all around the ship's location.

  • The ship uses large radiation reflectors (think JWST sunshield) to block visibility of the rocket engines themselves and the first few meters of expanding exhaust gas from the stealth direction. As rocket exhaust in vacuum expands rapidly, the exhaust plume quickly escapes the cover of the radiation reflectors, but at that point it has already expanded a lot. Note that the direction of firing need not be straight away from the enemy observation direction but can be up to almost a straight angle.

  • The rocket exhaust contains only gaseous species. Specifically, the rocket fuel does not contain carbon so there are no soot particles in the exhaust. (If soot-free combustion of hydrocarbon rocket fuel when run sufficiently oxidiser rich is possible, that is also an option. Current rocket engines run fuel rich.)

The exhaust gas that emerges from the cover of the radiation reflectors should then no longer emit much—if any—electromagnetic radiation, as far as I know, so this setup should allow a spaceship to remain stealthy (in one direction) while still using powerful rocket engines.

Details

Hot materials generate thermal radiation from collisions on the molecular scale, which then generate exited states in the molecules. When these exited states relax that releases thermal radiation. I would think that rocket exhaust gas in vacuum quickly expands to a pressure where there is practically no more interaction between gas molecules. Therefore the molecules can not get excited and as soon as any existing excited molecules have relaxed back to their ground state, the gas no longer emits any thermal radiation.

Soot (or other solid) particles in an exhaust plume can continue to emit thermal radiation until they have cooled down, and will also reflect sunlight. So practical rocket propellants to choose from will consist of hydrogen, oxygen, nitrogen and maybe fluorine atoms. The exhaust plume thus also consists of molecules composed of these atoms, which are all gaseous as far as I know. Assuming hydrogen and oxygen are used, there will be hydrogen gas and water vapor in the exhaust, and if nitrogenous fuels are used also nitrogen gas and nitrogen oxides. Hydrogen propellant from a NERVA-style nuclear rocket is also an option.

Fun fact: The ʻOumuamua interstellar object that passed through the solar system in 2017 exhibited non-gravitational acceleration, some explanations for that involve outgassing of nitrogen or hydrogen which would not have been detectable with Earth telescopes.

There may be ways to still detect such shrouded exhaust plumes, e.g. some gases can fluoresce from solar UV radiation, or some gases might have some phosphorescent states that I am not aware of that will cause them to emit more radiation when they have already left the radiation reflector cover. But I am hoping that if such phenomena occur, the visibility of an engine burn will still be reduced by several orders of magnitude compared to a non-stealthy engine burn.

I don't think detection of the exhaust plume by analyzing starlight from background stars would be a major issue. Analyzing the spectrum of starlight requires a lot more photons and thus observation time than just detecting regular thermal emissions, so the detector needs to get lucky to have a bright enough background star at just the right position before the exhaust plume expands too much to be detectable. Even if that happens for one observation post, that would not allow triangulating the distance to the spacecraft and finding its position or vector of movement.

This question does not deal with other ways of detecting a spacecraft, e.g. radar or reflected sunlight, but from what I have read at longer ranges (say, 100.000+ km) the main detection method that can not be prevented is thermal radiation emission.

Question

What I am wondering about is if this scheme would be an effective way to allow a spacecraft to maneuver while remaining stealthy. The scenario makes quite a bit of assumptions, so please tell me if you think any of them are wrong, but the main question I would like to see answered is how easily detectable the expanding exhaust plume would be compared to the detection of a non-thermally-shielded rocket engine.

In order to have a properly defined question I will add some additional assumptions intended to establish a baseline. I hope the answers generalize to different situations, so please do mention if an answer sensitively depends on these details, in which case I may need to revise them.

  • In order to set some baseline, let's assume today's technology level in sensors. In a scenario with spaceships flying around detection of both hot rocket engines and expanding exhaust plumes will be better, but without knowing which will improve more compared to the other, today's technology is a good enough baseline. Putting stuff in space is probably a lot cheaper than today, so telescopes can be bigger and more of them.

  • Any type of rocket engine is fair game. The ship is trying to stay stealthy so pick the one that is the hardest to detect. As explained above I think that excludes anything running on a carbonaceous propellant. I think hydrogen or hydrazine with oxygen would be the most promising rocket fuel types. Nuclear engines are also ok if you think they would be harder to detect. I do want the ship to be able to go somewhere rapidly so assume a thrust of at least 1 meganewton (approximately the SpaceX Merlin).

Of course anything can be detected if you have a large enough telescope and enough observation time. So the question comes down to a ratio: How much harder would it be to detect a thermally unshielded rocket engine out of the background from one with thermal reflectors as described.

My guess is that there are at least three orders of magnitude of difference between them, which would translate to a ${\sqrt{1000} \approxeq 31}$ times smaller detection range. So I guess that a telescope optimized to detect unshielded rocket engines would be able to detect the chosen rocket without thermal reflectors 31 times as far away as a same sized telescope optimized for detecting thermally shielded rocket engines which is trying to detect that same rocket with thermal reflectors in place, given the same amount of observation time for each telescope.

A telescope designed to detect expanding exhaust plumes may need to observe different frequencies of electromagnetic radiation from an infrared telescope trying to detect hot rocket engines, but there doesn't seem to be an order of magnitude difference in the efficiencies of infrared telescopes compared to optical or other wavelengths, so it appears that setting the telescope size (aperture size) of both telescopes the same would make for a good way to compare them.

The best answer would thus be the answer that is able to nail down the above ratio as much as possible, though I don't expect more than an order-of-magnitudes answer. Or, of course, an answer that can show that one of my assumptions is wrong in such a way that the question is invalidated.

Addendums

Some answers question if gasses still emit thermal radiation when the pressure is low enough that molecules are no longer interacting. As an illustration I will link this Nasa video, which shows icicles forming on the engine bell of a hydrogen-oxygen engine. The engine bell itself is cooled by liquid hydrogen, but the icicles extend quite a bit away from the bell. The water vapor exhaust does not contain any soot particles and the water vapor and hydrogen itself emits little enough radiation that it does not melt the icicles sitting centimeters away. (The test is a throttle test, but the icicles don't melt when the engine is throttled up to 100% again.) As another reference, here's Scott Manley's interpretation of that video. The exhaust gas here is still at 1 atmosphere of pressure, but even in those conditions pure gases emit far less heat radiation than solid matter (like soot) because only a limited number of excited states are available in the molecules.

As for a JWST style sunshield not being able to cope with the radiation output of a big engine, the above also disqualifies that idea. Perhaps the emissions from the engine itself require some more durable shielding if the engine is not cooled cryogenically, but the exhaust plume should not pose much of a problem. And if so, the JWST sunshield consists of several layers of aluminized polyamide foil, it would be possible to replace the inner layers of foil by something that can withstand some heat like aluminium foil. Of course the exhaust plume should not impinge on the radiation reflector, if that happens all bets are indeed off.


From here, "Atmospheric Ultraviolet Remote Sensing", Robert E. Huffman, in International Geophysics, 1992, chapter 5:

Cold gas thrusters are used on many spacecraft to change the attitude of vehicles in pitch, yaw, and roll. This cold gas, usually nitrogen, cools and condenses into particles near the thruster. The particle cloud is an excellent scatterer of sunlight, which has lead to unwanted interference in measurements. An account of this scattering by thrusters using argon gas and its elimination by the use of neon for the thruster gas is given by Kolb et al., 1983, 1985. The locations of all thrusters on the spacecraft should be known and planned for by principal investigators.

So it seems that nitrogen cold gas thrusters are not as stealthy as I had expected. I'll need to figure out if this also applies to hot rocket exhaust and water vapor.

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  • $\begingroup$ Pure hydrogen-oxygen flames are visible in the UV, not visible or IR. Once the plume hits other things all bets are off… $\endgroup$
    – Jon Custer
    Commented Dec 23, 2022 at 0:26
  • $\begingroup$ (a) Posts are allowed to have one question. Thus, you cannot expect the "subordinate" questions to be answered. Whether they are or are not is at the whim of the respondent and it would be very bad form for you to select as a best answer one that is only "better" because it answered the subordinate questions. (b) No information travels faster than the speed of light. That sets your hard minimum limit. The maximum limit depends strictly on the quality of the sensors - which you set. While answers might be interesting, this Q's only answer is, "whatever you, the worldbuilder, wants." $\endgroup$
    – JBH
    Commented Dec 23, 2022 at 3:35
  • $\begingroup$ @JonCuster hydrogen-oxygen flames emit UV at atmospheric pressure, do you have any references on what happens in near vacuum? $\endgroup$
    – JanKanis
    Commented Dec 23, 2022 at 10:56
  • $\begingroup$ @JBH Of course any question can be answered with "whatever the worldbuilder wants". I'm interested in what is hard or easy according to the laws of physics with respect to stealth in space. See the linked "Stealth in space" question for more context. $\endgroup$
    – JanKanis
    Commented Dec 23, 2022 at 10:57
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    $\begingroup$ So, this is an old question and most people only assert that rockets are bright and therefore stealth is impossible. Few people look at sensor capabilities, as those are generally really complicated. ToughSF's Stealth in Space us Possible Series is probably the best source on that that entire subject. The conclusion: It is possible, but you need specialised designs (which he provides in later posts). Dark propulsion like mass drivers, slingshots and momentum exchange teathers help a great deal as well. $\endgroup$ Commented Dec 23, 2022 at 14:21

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Atoms spontaneously emit energy even in the absence of intermolecular interactions

Atoms are bouncing around and vibrating and rotating, and have loose electrons which can spontaneously decay into lower energy things and emit electrons, and this happens even when they're not touching other things because when charges accelerate they emit electromagnetic radiation and space has lots of magnetic fields, such as the sun's and the galaxy's or dipoles within the atom itself. You can look up dipole radiation to see more on this, or the Larmor formula

If you're dumping mega or giga joules of energy into your exhaust, they'll dump a lot of photons because that makes them vibrate and rotate and go to higher energy electrons a lot. So, your exhaust plume will obviously have a huge amount of energy in it that will easily be detected by any decent sensor.

As the other question notes, you know that it's easy to detect extremely hot exhaust plume. That doesn't stop if the gas is spread out. This is a basic property of atoms, they get into excited states and decay, emitting radiation.

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  • $\begingroup$ This doesn't really answer the Q at all. OP asked "How far away can rocket exhaust be detected?" Forget the hard-science tag requirements, all this answer really says is "hot gases cool by radiating heat," and assert detection of gases is trivial. $\endgroup$
    – BMF
    Commented Dec 23, 2022 at 0:45
  • $\begingroup$ OP linked an answer that noted that rocket exhaust can be detected from an absurd distance, they just assumed they had a clever workaround to that, and I noted they don't. They didn't define what gas they're using, so I can't really link to a paper which explains the mechanisms of photon emission for whatever gas, so I am just noting the common fact issue. OP themself noted that thermal emission is absurdly hot. $\endgroup$
    – Nepene Nep
    Commented Dec 23, 2022 at 1:08
  • $\begingroup$ Hmm you're right, OP doesn't say anything about their engine. Not sure the Q can actually be answered as it is lol. The exhaust plume is going to lose energy fast as it expands. Hiding the brightest portions of it from certain angles might actually work, but then you have to deal with the extra waste heat and cryo/heat rejection system mass for the "reflectors". Nothing's 100% efficient. The better question might be whether the concept is feasible for a given engine and reflectors. $\endgroup$
    – BMF
    Commented Dec 23, 2022 at 1:31
  • $\begingroup$ This is the closest to a hard-science answer here. That it's a frame challenge is perfectly fine. $\endgroup$
    – JBH
    Commented Dec 23, 2022 at 11:04
  • $\begingroup$ I don't think this answer is correct. Molecules have vibrational, rotational and electronic excited states, which emit radiation when they decay back to the ground state. Once that happens, in pressurized conditions collisions with other molecules re-excite the molecule so the emissions continue. When there are no more interactions with other molecules there is nothing for a molecule to 'bounce around' against, the excited states decay and after that the molecule just continues in a straight line unless it interacts with something else again. $\endgroup$
    – JanKanis
    Commented Dec 23, 2022 at 11:15
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Will expand on this answer later to comply with the hard-science tag.


The JWST sunshield doesn't reflect anything like the output of a decent mega/gigawatt scale rocket. If the shield is near the nozzle you'll have to deal with extra heat rejection systems to actively cool it. If the shield is near the front of the ship you'll have to deal with potential backscatter (possible hard radiation and neutron flux) on the fuel storage (probably kept cryogenic), payload bus (where the crew resides), thermal management systems (TMS), etc., straining those systems greatly with additional TMS and shielding mass.

Possible compromise is to push the engines away from the cryogenic remass tanks on trusses and place the reflectors amidships, just "below" the propellant tanks and at the "top" of the truss work. The shield's distance from the engine helps alleviate its active cooling by spreading the radiation exposure over a larger area. Going by some rough assumptions and guestimates, the truss length and shield area will have to be large for a gigawatt-scale engine. Possibly pretty taxing. (Absorbing large fractions of your exhaust's radiation is not a great idea; most engines are designed to minimize exposure as it represents more waste to pump through TMS heat rejection.)
Engine radiators can be angled perpendicular to the shield, but being 3d objects some aspects of the radiators will be bathed in more waste heat, inflating the TMS mass budget.

Perhaps another option is to project a large, ultralight sail far ahead of the ship and keep it "balanced" against ship acceleration by a forward laser array. Aluminized sail materials can be extremely light and reflective. Being so light, the laser power may not need to be great to achieve the same acceleration as the main drive.
If power requirements are somewhat large, the laser array might draw power from the main drive. It only needs to be powered when the ship is accelerating, anyway.

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  • $\begingroup$ I don't see why something similar to the JWST sunshield would not be able to reflect the thermal radiation of a big rocket engine, see also the addendums I added to the question. The radiation reflectors specifically reflect thermal (infrared) radiation, so I don't see what hard radiation and neutron flux has to do with it. While nuclear rockets is an option I mentioned I am also happy with a chemical rocket engine in which case there won't be any neutron or ionizing radiation. But I will attempt to clarify what exactly I'm asking about. $\endgroup$
    – JanKanis
    Commented Dec 23, 2022 at 12:41
  • $\begingroup$ @JanKanis I was assuming a high-performing Nuclear Thermal Engine (mega- to gigawatt scale thrust power), something you'd expect to find on a space warship. Such engines have high Isp yet low thrust, and I see now that you specified your engines are chemical thrusters with high thrust and low Isp, the opposite. $\endgroup$
    – BMF
    Commented Dec 23, 2022 at 15:15
  • $\begingroup$ @JanKanis NTRs can achieve high thrust by "after-burning" the exhaust. It involves injecting a stream of propellant into the nozzle, say hydrogen, allowing the very hot exhaust to heat up the hydrogen and increasing mass flow. It decreases exhaust velocity and efficiency a lot, but skyrockets the thrust. $\endgroup$
    – BMF
    Commented Dec 23, 2022 at 15:23
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    $\begingroup$ @JanKanis it's your decision what engines to use on your spacecraft ofc, but personally I think heat of the exhaust is the least of your problems when it comes to detection. Reflectivity of chemical engine exhaust will be the major issue. Search "S-IVB plume Apollo 8 TLI" (trans-lunar injection) for images. If your spacecraft is larger than Apollo 8, you'll be seen much much farther than the Moon. $\endgroup$
    – BMF
    Commented Dec 23, 2022 at 15:38
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    $\begingroup$ @JanKanis Apollo 8 TLI. Notice how the plume is many times the size of the spacecraft itself. The camera is just a camera, sampling the visible spectrum broadly, not designed to detect the plume. One designed specifically for detection would likely see a lot more. $\endgroup$
    – BMF
    Commented Dec 23, 2022 at 15:43
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As was noted in another answer, the molecules emit energy even without colliding. [see eg. The Emission of a Single Photon from a Single Atom or Molecule - An Experimenter's View https://onlinelibrary.wiley.com/doi/pdf/10.1002/1438-5171(200204)3:1%3C19::AID-SIMO19%3E3.0.CO;2-Z]

So, the only problem is not the initial heat of exhaust plume but the whole plume: the gases that have cooled down absorb energy from radiation sources such as sun. [does this need source?] And alas, the exhaust plume itself is a radiation source, too -- it only can lose its heat through radiation. This (reabsorbed) energy will get emitted as well.

With sensitive sensors the gases should still be detectable long after they have left the nozzle. It takes lot of space and lot of time until they have expanded enough to lose any potential heat signature into background noise. You would need an enormous shield to hide it all. But then the bigger shield becomes another problem by itself: either it is visible on a radar or it can block something behind it.

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  • $\begingroup$ Reflection (or fluorescence) of sunlight might be an issue, it would be nice if someone could quantify that. But gasses need interaction between molecules in order to convert their heat to radiation, see also my comments on other answers. $\endgroup$
    – JanKanis
    Commented Dec 23, 2022 at 22:47
  • $\begingroup$ @JanKanis radiation is emitted when electrons jump from higher-energy orbitals to lower-energy ones; the energy difference is the emitted photon. No collisions are needed at this step. The collisions in the hot gasses of chemical rocket engine already have made the electrons jump to higher-energy orbitals from where they jump back, emitting the photon. Also radiation can make the electrons jump to higher-energy orbitals. $\endgroup$ Commented Dec 24, 2022 at 14:29
  • $\begingroup$ The jump back to lower energy orbitals generally happens very quickly, before the exhaust has time to move out from under the radiation reflector. There are exceptions, which is called phosphorescence, and depends on the molecule species. If there is a high level of phosphorescence radiation from the exhaust plume that would invalidate the approach for the involved fuel types. If you have information that would help quantify the amount of radiation from phosphorescence for some common exhaust species that would be very useful. $\endgroup$
    – JanKanis
    Commented Dec 25, 2022 at 22:53
  • $\begingroup$ @JanKanis true, generally happens very quickly, but if you want stealth, then generally is not enough -- you need to aim at always. Thst is where the problem lies in. The shield must be able to hide everything that could be detectable, otherwise the rocket can be detected. $\endgroup$ Commented Dec 26, 2022 at 3:28
  • $\begingroup$ You don't need to hide everything as detectors need a certain magnitude of signal to distinguish a ship from the background. The (edited) question is phrased as the ratio of how much harder it would be to detect a shrouded rocket from an unshrouded one. These kind of delayed emissions usually represent only a very small fraction of the fast emissions. If some of the exhaust gases had substantial delayed emissions that could be the core of a good answer but that would require more specifics. (Which transitions/emission lines, and what kind of delay/half life do they have?) $\endgroup$
    – JanKanis
    Commented Dec 27, 2022 at 23:20

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