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While there have been numerous arguments about ways of actually pulling off stealth in space, what I'm interested in is the scenario in which you can't conceal your presence in the conventional sense of hiding emissions. If that is the case, what are some ways of concealment that could still be viable?

To be clear, while there are reasonable approaches given in places like ToughSF, I'm interesting in just outright working within the limitations of the pessimistic scenario.

EDIT: It is fascinating that I just proved Nicoll's Law again "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."

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8 Answers 8

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1. Appear to be other than what you are.

You can hide in plain sight if the things looking for you do not recognize you when they see you. Use a disguise. Appear to be something that is expected or irrelevant. What that thing is will depend on what things besides yourself might occur in that space.

2. Channel emissions away from potential viewers.

It is probably impossible to conceal all radiant emissions in space. It is possible though to emit only in the directions you choose. If you have an idea about where the things seeking you might be, you can emit emissions away from there and present them with an unemitting surface, cooled by rerouting emissions to the other side.

In a (relatively) crowded space of a solar system, seekers might note your emissions reflecting off of dust behind you, or an absence of solar wind in your shadow. Seekers would need to widen their scope of what they are looking for to perceive that shimmering dust could mean a hiding ship; this would be fine material for a story.

3. Blind your seeker.

From this related question: Is it possible to blind a spaceship/space warship?

If you can produce circumstances which blind your seeker, you will not be seen. If the seeker knows you have produced those circumstances they will be on alert even though blinded. If you can blind the seeker in a way that they attribute their blindness to something other than you (other actors, mechanical failure, natural phenomena) they may not be on alert.

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    $\begingroup$ Note that "emit your waste heat in other directions" is the same as having a 100% efficient motor. If you could emit your waste heat only in one direction, it wouldn't be waste heat, it would be part of your exhaust and help propel you. $\endgroup$
    – causative
    Commented Aug 7, 2021 at 20:01
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    $\begingroup$ @causative, I've got a device in my living room that emits waste heat in one direction: it's called an "air conditioner". The reason it doesn't break the laws of thermodynamics is that it takes energy to do so: the amount of waste heat coming off the hot side of the machine significantly exceeds the amount being pulled off the cold side. $\endgroup$
    – Mark
    Commented Aug 8, 2021 at 18:53
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    $\begingroup$ @KeithMorrison The ISS cools itself by pumping heat to external radiators, so heat pumps do work in space. Reflectors can be built in the IR spectrum, so we can both move heat where we want it, and control the emission pattern. At that point, it's a question of engineering difficulty instead of physical possibility. $\endgroup$
    – jb6330
    Commented Aug 9, 2021 at 19:14
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    $\begingroup$ @KeithMorrison In what way are we violating the second law? The total entropy of the system (ship and surrounding space) is still increasing as we are converting useful energy into waste heat. If you see a specific issue say it, don't ask me to run both sides of the debate. $\endgroup$
    – jb6330
    Commented Aug 9, 2021 at 20:25
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    $\begingroup$ @KeithMorrison - I envisioned a large black circular shield carried on the front of the ship, interposed between the ship and where it thinks potential viewers are. This shield is actively cooled. Behind it I am hot. The key is the angle of approach. If there is anyone far enough off to the side, my hot ship and the cold shield will be very obvious. $\endgroup$
    – Willk
    Commented Aug 9, 2021 at 23:54
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  • Vary your trajectory randomly

The enemy knows in general where you are, but he doesn't know where you're going to be. He is millions of km away. By the time his attack gets near enough to harm you, you've changed course to be well out of the blast zone. Aircraft did this in WW2 to avoid flak.

  • Send out bright decoys

The enemy knows in general where you are, but not which of those bright dots is you. That could be enough to foil a few of his attacks.

  • At a crucial moment, pump liquid hydrogen to the surface of the hull and shut your motor off.

This will only work until the liquid hydrogen boils off, but it should mask your heat signature for a short time. Combine it with a course change so that your enemy doesn't know where you went.

The disguised course change may be more effective if you do it at the same time as you launch decoys.

  • Don't use your motor in the first place

Just enter the system in a passive ballistic orbit so the ship is cold and dark. The enemy doesn't need to see you until you shoot your weapons. Of course, there will be a defender's advantage since he can already have many ships in passive orbits in his solar system.

  • When your motor is off and you're cold, use mirrors for camouflage against sunlight

Now that your motor is off and you're no longer a bright spark, you look just like any small asteroid. But your enemy may be able to spot asteroids by the reflected sunlight. Use an angled mirror to direct reflected sunlight away from the enemy.

  • Ride on the far side of a comet into the system

Unless the enemy is in the habit of blasting every comet he sees, the ice of the comet and the gas/dust cloud surrounding the comet will help hide you.

  • Have an implausibly energy-efficient motor

The law of "no stealth in space" is a result of your motor's waste heat radiating in all directions. If your motor was 100% efficient there would be no waste heat and you could not be spotted by your waste heat, even if you're running a terawatt power plant.

  • Pass behind a planet, then while behind the planet perform a random, intense course change, then shut your motor off/use the liquid hydrogen and mirrors

If you only perform course changes while hidden behind large objects, and are in cold, ballistic coasting mode at other times, the enemy is going to have a hard time locating you.

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  • $\begingroup$ Changing your course requires firing thrusters is akin to repeatedly announcing exactly where you are and where you are going. Someone tracking you then merely has to tell the monitoring system to keep tracking where you must be and provide an update if you fire your thrusters again...which since they know exactly where you must be they will be able to see immediately (lightspeed lag aside). $\endgroup$ Commented Aug 8, 2021 at 5:42
  • $\begingroup$ @KeithMorrison they can only track where you must be after a course change if they were able to tell in what direction you accelerated. Decoys can help with that so they don't know which accelerating dot is you. And maybe your motor can provide a lot of burst acceleration so all they see is a very bright dot for a moment that then goes dark before they could detect which direction it was accelerating in. $\endgroup$
    – causative
    Commented Aug 8, 2021 at 5:46
  • $\begingroup$ For a decoy to work it needs to have the same engine moving the same mass, otherwise it can be distinguished from the original object. And unless you're proposing magic propulsion, if it relies on Newtonian motion it will be more than a momentary bright dot. $\endgroup$ Commented Aug 8, 2021 at 16:44
  • $\begingroup$ @KeithMorrison no, the decoy doesn't need the same mass. It needs the same acceleration and the same heat signature. This can be achieved with a small decoy, a weak drive, and a powerful waste heat generator, that only needs to last a few seconds. Regarding how momentary the thrust needs to be for the enemy not to know which way you're going - that depends on how far you are from them and how sensitive their telescope is. If you are very far and blurry you can be a "bright dot" for a longer period of time (just a few seconds) before they are able to calculate your exact acceleration. $\endgroup$
    – causative
    Commented Aug 8, 2021 at 18:38
  • $\begingroup$ no, that's not how it works. Action has to equal reaction, and if the signature of the stuff coming out of the back, which can be detected, doesn't match what would be necessary to move the proper mass with the observed acceleration, then the mass is wrong. So you need the same mass. $\endgroup$ Commented Aug 9, 2021 at 15:53
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BE LOUD

MAKE SOME NOISE, IN STYLE! THEIR SENSORS CANNOT DETECT IF YOU THEY ARE OVERWHELMED!

ARE THEY SCANNING THE SKY THROUGH INFRARED? LASER-PAINT THEIR TELESCOPES! RADIO? JAM ALL CHANNELS! VISIBLE LIGHT? X-RAYS? USE ENOUGH TO STERILIZE THEIR PUNY PLANET AAAAAAAAAAAAAAAA

or use a decoy

Find a loud guy that likes the sensorial overwhelm strategy and send him a bit ahead of yourself. No one will notice you for a while.

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If your story allows faster-than-light travel, or hyperspace travel:

FAST--if your ship is faster than what ever medium your enemy is using to track you, they can't track you. Radar wave travels in light speed, then if your ship is faster than light, the radar wave can never get to you. No radar wave reflecting back to the enemy's receiver means there won't be a blip on the enemy's scanner.

DIVE, DIVE--use the hyperspace itself to hide, like a submarine. If you can't hide in this universe, hide in somewhere else. Just like UX-01 in Space Battleship Yamato, they hide in the fault line between dimensions, so their emission dont travel into the dimension their enemy is in.

If your enemy needs to obey some sort of cosmos Geneva Convention and you don't. Consider hiding behind, below, or inside a civilian vessel. If the enemy is not careful, they would just assume this is a normal civilian transport. But if they do detect you, let's see whether Captain Goodguy would rather blow up a ship full of innocent civilian to destroy your ship than having your evil organization accomplishing your objective.

Similar trick can be used even if you are the good guy. The enemy might be detecting you, but they don't know who you are. Disguise your ship to be something else--for example, a space freighter, or a passenger vessel. Deliberately use unscrambled radio to create the illusion that you are just a civilian passing by. As long as you discard your disguise and raise your color before engaging your enemy, this is a legal trick and not a war crime. This has been done by an British vessel when exercising with a US carrier battlegroup, as well as the Q-ship during WWII.

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  • $\begingroup$ The Picard Maneuver grants +1. Unless your universe explicitly has FTL sensors, a ship travelling at >1.0c will appear to be moving in the opposite direction, and a ship that uses hyperspace will appear to teleport between locations. $\endgroup$
    – papidave
    Commented Aug 8, 2021 at 14:17
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An Agent in Place Have someone on the other side slow walk their data, or otherwise sabotage the search.

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Stealth does work in space, as further down it is a matter of good shaping and the right materials.

There are lots of military satellites out there whic hare difficult or impossible to find because the combination of low-observability and the bigness of space*.

(*space is really big).

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Many decoys

A cheap engine and a disguise on an asteroid and you have a fake space ship. Prepare a big fleet of them, break up a comet to project a lot of fragments towards the enemy, mix the fake space ships with the swarm of rocks and the enemy would be quite busy.

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  • $\begingroup$ You'd be better off putting a pricey engine on smaller rocks and sending them at 100G toward me. In fact, skip the rocks and send the engine at 200G If I can get away from that, you probably should run. That seems less like stealth and more like a preemptive strike. $\endgroup$
    – chiggsy
    Commented Aug 10, 2021 at 10:35
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"Stealth does not work in space" is inconsistent with basic logic. If stealth does not work in space then everybody knows position of enemy sensors and what stops stronger side from destroying enemy sensors in some region and then just turning radiators of their stealth ships toward that empty region?

You need stealth at least for launching sensors and it can be used for some other purposes.

Stealth in space is mostly about masking heat of you engines. Build giant ice spheres and tunnels in asteroids and launch your ships from there. And secret maps of those tunnels can be great plot driver.

Many ships can gather together and then run in different direction - it would be hard to get details of orbit of specific ship. They can use space tethers/magnet sails/cold hydrogen for slow maneuvers.

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  • $\begingroup$ Sensor floating in space at 3K. How'd you find it? You are not at 3K, so it can find you, but your best bet might be to drift a waldo, or some dude over to it, hack it and spoof the output. You might be able to do it via radio, although that is as active as it gets, so best of luck. $\endgroup$
    – chiggsy
    Commented Aug 10, 2021 at 10:32

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