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This has been considered other places on the net, but I thought it would be good for the challenge, since it is often an example of the subtle difference between soft and hard science.

Okay, so you have your spacey ship in outer space. Then it is reported that space pirates have come to pirate our space goods. Time to turn the space stealth on!

Is this possible?

  • The spacey ship needs to keep the people inside alive.
  • Ideally, the spacey ship needs to keep the people inside alive, and able to carry out their normal duties.
  • Ideally$\times 2$, the spacey ship needs to keep the people inside alive, able to carry out their normal duties, and keep their sensors on.
  • Ideally$\times 3$, the spacey ship needs to keep the people inside alive, able to carry out their normal duties, keep their sensors on, and still able to maneuver.

The more Ideally's you get, the better.

They also can't hide behind or in front of things. You never know when space pirates are going to strike. The space pirates know how stealth techs work, so if there is a sensor that sense cloaked ships, that's the sensor they'll have, within reason (they probably wouldn't have gravity sensors). The only other thing is their sensors are passive, so they can't splash paint everywhere (space pirates have stealth too!)

If any other details are needed, just ask in the comments.

This is physics and astrophysics (and for keeping alive, biology is needed.)

NOTE: I suspect the answer is no. If the answer is no, the best answer would be the most thorough explanation as to why its no.

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

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Forget it, there ain't no stealth in space.

The linked site explains in excruciating detail why stealth in space either do not work or is, when it works, extremely unwieldy or practically useless.

I will shorten the arguments in the link

  • Vacuum means the only way to get rid of excess heat is radiating it or dump hot material. Remember, you need your crew and your equipment alive and working and not frozen, so you need to maintain a temperature difference between the warm inside and the cold outside. Super insulation does not help because you need to get rid of some heat at least to avoid being cooked alive. The problem is that temperature radiation sticks out like a sore thumb before the totally black space background. Heat radiation can be detected by a distance of $13.4 \cdot \sqrt{A} \cdot T[\text{K}]^{2}$. Lets make a very thin ship with $5\;\text{m}$ radius which has a surface area of approx. $75\;\text{m}^{2}$ which results in a factor of $116$. Freezing point temperature ($270\;\text{K}$) means the ship can be still detected in a distance of $8456\;\text{km}$ which is pretty far away. Once engines fire up (and you need to maneuver !), it is getting hopeless. A single truster engine of the Space Shuttle can be detected at 15 million km range and using main engines it can be detected from Uranus. With current technology.
  • Put something between you and the observer. Could work with planets and especially suns, but only if you intend to stay out of range. Putting a cooled sunshade between you and the observer could work, too, if you know where the observer is which is pretty hard because you are essentially blind. And because you are in interplanetary space meaning you are not moving straight it means sooner or later you need to correct the course which means engines -> back to first point.

  • A strategic usage of burn outside range and coast. The problem is that the space is vast, you have no idea if the enemy has already detected you and you need endless time (months, if not years) to get in vicinity.

  • Hope that you are literally lost in space and the sensors do not pick your ship up. This is currently a valid tactic on Earth because most instruments have a very limited field of view and there are interesting regions and uninteresting regions. That will change once spaceships are used and the article makes the convincing argument that with current technology whole area sweeps are possible in hours while traveling (especially coasting) will take much longer. Both sunshades and hiding will be especially useless once strategically placed sensor platforms are used.

  • Decoys are not of much use. There must be too sophisticated, expensive and too-much shiplike to fool even a standard enemy. Not impossible, but practically useless.

EDIT: I must admit, Project Rho must be administered by a psychic the way he predicted the discussion. He already said that the second law does not allow to convert heat energy into another form of usuable energy to withhold radiation without further energy usage. No, you can't "grab" it with whatever your fantasy allows you to imagine, the idea is old and called Maxwell's demon. Given the existence of photonic metamaterials it might be possible to redirect heat radiation without active energy usage, but the metamaterials are non-perfect and will therefore cause themselves background heat radiation which will leak out.

Answer to Ryan: Yes, using a cooled shade makes you blind. Your shield must match the cosmic background radiation which is unfortunately comparable to radiation of black body with $3\;\text{K}$. It means liquid helium and for deep temperature physics in this temperature range that means that even a human entering the laboratory gives off too much heat. Building passive sensors in the shade without disturbing the temperature will be therefore a severe challenge. Analogy: You want that I don't see that you are in the same room. Unfortunately I can see in the infrared range so cloaking will not work for the reason above. You can build a wall between us. Then I see this wall and while I don't know that you are hiding behind it, I know that the wall does not belong here. So you must prepare the wall that it looks exactly like the background. If you poke a hole in the wall, I can see that immediately. So at the end we are both blind.

I think you also do not understand what these distances mean. Your space saga needs Faster-than-Light technology for travel between solar systems, else we are talking about thousand of years between star systems. Booooring. Even if we start from Pluto & Co. I presume your ship crew needs to eat, so you cannot start "billions of km/miles away". You cannot stop in space, every time you want to change your velocity it means engine usage and voila, I see you. If you start really fast and run cool after Uranus, you perhaps only need weeks but then you shortly see Earth and poof, it is already away. If you start slow, then you need some hefty stocks (the small area I used in the example will not hold anymore, increasing detection range) and a collection of board games because it will get frickin' boring.

At Mike L: I think you are mistaken because there is no such thing as a lower barrier of detection. If I point a CCD to a source, I can detect it if exposed long enough, it does not matter how weak the source is ( Otherwise it would not be possible to see faint stars/galaxies extremely far away).

immibis proposed heat sinks. That could work but stealth in space has always time as enemy. To get anywhere you need time to cover distances because they are so vast, we are talking about timeframe of weeks and months.

1 normal human creates a power of $100\;\text{W}$ comparable to a light bulb which needs to be contained. Water is an excellent heatsink with a specific heat capacity of $4200\;\frac{\text{J}}{\text{kg}\cdot\text{K}}$ in fluid form and $\approx 1500\;\frac{\text{J}}{\text{kg}\cdot\text{K}}$ in solid form. It also has an extremely high enthalpy of fusion with $335000\;\frac{\text{J}}{\text{kg}}$. If we cool solid ice down to absolute zero and get the maximum water temperature to $60\;\text{K}$, $1\;\text{kg}$ ice can take therefore $273\;\text{K} \cdot 1500\;\frac{\text{J}}{\text{K}} + 335000\;\text{J} + 60\;\text{K} \cdot 4200\;\frac{\text{J}}{\text{K}} = 992000\;\text{J}$. 1 human will therefore need $992000\;\frac{\text{J}}{100\;\text{W} \cdot 3600\;\text{s}} = 2.7\;\text{h}$ of time to overload $1\;\text{kg}$. 1 month of travel time (which is really, really fast) means $240\;\text{kg}$ of ice. And that is when the ship is completely cold, no life support on and you are able to freeze ice down to absolute zero.

Time is a friend for observers because the longer they can scan the sky with their sensors, the better they can even detect extremely weak signals. Using heat sinks means internal room which is also desperately needed for delta-V fuel (accelerate and brake). Increasing capacity will also increase the area and therefore the detection range.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Vincent Aug 27 '15 at 0:04
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    $\begingroup$ There is a lower barrier of detection: It's when the signal is weaker than the noise. Every device inevitably has noise, and if your signal is drowned in noise, you don't see it. $\endgroup$ – celtschk Aug 28 '15 at 20:21
  • $\begingroup$ Can you reflect heat radiation with mirrors? $\endgroup$ – PyRulez Oct 23 '15 at 22:08
  • $\begingroup$ I like this thread, and I think you're right, but I loooove playing the devil's advocate. Ok, so we can't hide our heat signature, and our heat signature is what gives us away, but can we try to not hide our heat signature? That is, can we just distort the hell out of it? Some stealth doesn't just not show up on RADAR, it shows up all over the place, in arrangements that don't make sense. Pirates don't know which is the ship because they all could be. I get this alerts people to your presence, but at the same time it does hide you until they can work out which signature is yours. $\endgroup$ – Lord Farquaad Aug 19 '16 at 20:28
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    $\begingroup$ @LordFarquaad It is actually depressing because it is exactly the next argument predicted by the Project Rho side. Please read the section "All Right! I'll Use Decoys!" It is incredible how every stealth argument is repeated ad nauseam. $\endgroup$ – Thorsten S. Aug 21 '16 at 17:51
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I think the answer depends on the situation.

  • Space is big. There are millions of asteroids bigger than 1 km. We're not even close to having mapped them all, and they're not actively hiding. How do you tell a 100m ship radiating internal heat from a 1000m rock reflecting the sun? Spectroscopy, maybe.
  • Sensors can be active like radar or passive like telescopes. The energy received with passive sensors scales with $\frac{1}{distance^{2}}$, the energy received with active sensors scales with $\frac{1}{distance^{4}}$. That means active sensors have to be really big.
  • Passive sensors tend to have a very narrow field of vision. Not a problem if you're mapping natural objects, more of an issue if you look for a ship.
  • It will be impossible to reduce the radiation from the ship to zero, and the background is not suitable for blending in.
  • drives will use lots of energy, and radiate it freely.

That suggests to me it is very hard or impossible to break contact after initial detection, but making that initial detection is also very hard unless the ship radiates lots of energy towards the observer.

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    $\begingroup$ "Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space." - Douglas Adams, The Hitchhiker's Guide to the Galaxy $\endgroup$ – Magic-Mouse Aug 25 '15 at 7:57
  • $\begingroup$ Good idea and not covered by the current top post, however the question states "They also can't hide behind or in front of things". So asteroid cover is probably not possible. It should probably be mentioned in the accepted answer, however, since it's kind of a grey area (not covering behind one but cloaking as one). $\endgroup$ – Sebb Aug 25 '15 at 15:07
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    $\begingroup$ @Sebb, I wasn't suggesting to hide behind an asteroid. The idea was that the ship might look like an asteroid to cheap and/or wide angle sensors. For all practical purposes the ship or an asteroid are a point source, and does a heated 100m ship emit more energy than an unheated 1000m rock? $\endgroup$ – o.m. Aug 25 '15 at 15:16
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    $\begingroup$ @o.m. Sorry, got that wrong ;) $\endgroup$ – Sebb Aug 25 '15 at 15:22
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    $\begingroup$ Another thing to note is that even the passive sensors are subject to inverse square law. This means that distance is your friend; in order to be detected, your emitted energy needs to stand out from both background noise (which has a low temperature, but much larger form factor relative to the sensor) and sensor noise (spurious energy emitted by the sensing device itself). Passive sensors can discern a lot of detail if they're built the right way, but even just at interplanetary distances they need to be looking squarely at the target for a lengthy period to be able to discern anything. $\endgroup$ – Mike L. Aug 25 '15 at 21:10
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While the other answers explain very well the issues that stealth would have to overcome, I'm going to have to take a slightly different tack and say it is possible.

Heat: Okay, purely using heatsinks seems to be out, unless the ship is very large. However, it is possible to collimate the IR output of a ship, and not by using a laser either (which would seem to violate thermodynamics). Instead, simply have a hot core which is exposed from just one narrow opening/tunnel/exhaust port. Anyone looking from the direction of the beam will see it; but anyone slightly off-axis won't. IR lenses are difficult, but IR mirrors are quite easy. Actively cool the port (or baffles along the edges of the beam), and also the outside of the ship. Use a blob of material that heat is actively dumped into (for example with peltier effect or similar), at the focus of a system of mirrors which collects and collimates the emitted IR. I think the IR can be collimated to about the same degree as a laser, and thus the odds of it hitting a sensor platform are quite low. This is not perfect stealth, but it is stealth for all practical purposes.

Drive: The easiest stealth drive is something like a railgun, only used for propulsion. It can produce cold exhaust chunks (macroscopic particles, not gas - if needed, pre-chilled), and any residual IR would be hard to see because of the velocity. I don't know why everyone here is thinking hot exhaust...

Occlusions: Okay, this is a tough one. Aside from some handwaving about metamaterials, I'd say the practical approach is to stick to a natural object that is travelling in the right direction. The exact size of a small object ought to be much harder to gauge from occlusions than whether it is there at all. Thus, practical stealth would involve brief runs in free space between convenient camouflage asteroids.

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    $\begingroup$ Exactly this - I also don't see why you can't dump heat and exhaust in one direction.... Sure you don't know where the enemy is, but as long as he isn't exactly in that direction, you are safe! The trusters could also fire particles with lightspeed. There are not many particles to react with in the near vaccum of space, so this concentrated particle beam will be hard to detect from another angle... $\endgroup$ – Falco Aug 26 '15 at 11:41
  • $\begingroup$ Heat doesn't work that way. The narrower the angle you want to emit the heat, the more heat you are attempting to release hits its own radiator, which makes the radiator hotter, which increases the amount of heat you need to get rid of which makes the radiator hotter which increases the amount of heat you need to get rid of which hits the radiator... $\endgroup$ – Keith Morrison Jan 24 '18 at 9:03
  • $\begingroup$ @KeithMorrison Consider mirrors. In a very simple 1D example, an omnidrectional point IR source with a 98% reflective mirror immediately to one side of it will radiate 99.5% in one direction and 0.5% in the other direction. (50% hits the mirror and is reflected, 1% is absorbed by it, 0.5% of that is re-emitted behind the mirror). For metal mirrors 98% seems very possible. $\endgroup$ – Alex I Jan 24 '18 at 15:36
  • $\begingroup$ @KeithMorrison Also, nothing in principle prevents the construction of a mirror stack, it just has to be done so the path of IR reflected by each successive mirror (mostly) misses the earlier mirrors in the stack. The only limit on the performance of a system like that is the weight/size of the mirror array that can be used. Picture parabolic reflector collimators like this static.wixstatic.com/media/… only nested so each one is at the focus of the (much larger) next one $\endgroup$ – Alex I Jan 24 '18 at 15:39
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You can't hide the engines

If we're assuming hard science no-magic-warp engines and ships that go places in years, not millenia, then the most visible part of any ship is going to be it's exhaust. In most natural planetary environments, there is nothing that could be confused with it. Some answers here are arguing that you might make a ship so "quiet" that can be confused with an asteroid - but that can be done only while not maneuvering at all.

Even our current technology allows us to make an exhaustive list of any ships firing their engines in our closest star systems - I mean, there aren't any; but if someone used any engine capable of moving a sizeable ship (as per the requirement of some people inside, not a tiny probe; plus a capability for maneuver not the current practice of a single burn and airbrake because we don't have enough fuel to stop) in story-meaningful time - say, no more than a few months for interplanetary distances or less than a century for interstellar distances - then we already would know it.

If you're seen, we know that you're not natural

We can't do it yet, but there should be no issues for a spacefaring civilization to have a detailed list of every asteroid of significant size (say, ship-size) in their planetary systems, in the same manner that we currently track every baseball-sized piece of junk in earth orbit. Yes, there are many of them, that's why we have computers. We don't frequently get any new asteroids, and when we do, we can observe the collision that made it. If a ship that's otherwise indistinguishable from an asteroid arrived in some soft-science way, say, from hyperspace - then we'd still could get an immediate and automated warning from passive sensors that hey, we have a new asteroid that wasn't there yesterday.

Once you are detected, you stay detected forever

Let's assume that some pirate successfully robs a ship, gets some loot, starts running away and has the quietest most hidden ship possible. Well, we still know where exactly it is, and and rather accurately where it will be in next month or next year. The only thing that a perfectly stealthy ship can do is float in a straight line, and the moment it wants to adjust it's course, then once again everyone will know where exactly it is and where it's going to now.

This also means that "You never know when space pirates are going to strike" is not really true - every space station would have an exhaustive list of every ship that is currently on route "nearby" (for very, very large values of "nearby" - passive detection of engines would work at longer ranges than ships with those engines will fly in a lifetime) as well as their exact routes, since you don't need their cooperation to obtain that, you just need to look. If you don't want to fly past anyone, then you don't. If someone fires up their engines to intercept you, then you notice that well in advance, and everyone else does as well, and is able to track the other ship to wherever it goes to.

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    $\begingroup$ "The only thing that a perfectly stealthy ship can do is float in a straight line" It's a small detail, but since we are in hard-science territory here: The ship would be travelling along its current orbit, not in what we would normally think of as a straight line. Compare Why does a spacecraft use a curved orbit to go to its destination, instead of traveling in a straight line? on Space Exploration. $\endgroup$ – a CVn Aug 26 '15 at 12:11
  • $\begingroup$ Yes! for the last bullet, but NO of the "stay detected" thing. I can fling some not so huge chunk of ambient temperature rock into some direction and will go in the other way. While that will change my course only slightly, it should be enough to not be where you expect me, a month or so later. Then its time to fire the engines. - So, yeah most of all ships will not be able (or desire to) evade the sensors of the stations. But its not impossible. $\endgroup$ – Angelo Fuchs Oct 9 '15 at 11:08
  • $\begingroup$ @AngeloFuchs yes and no - yes, inaccuracies and small adjustments will mean that you can change the course and in a month or so you'll be 'somewhere within a million miles' and not easily findable; however, space is very vast and very empty - if you're seen moving in a rough direction of a planet/large asteroid/space station/mothership/whatever, then you can't go to a different interesting point without a significant, visible maneuver. Assuming that you intend to go to a particular place and not live in empty space forever, your movements between such points can be traced. $\endgroup$ – Peteris Oct 9 '15 at 11:59
  • $\begingroup$ @Peteris Yes, but it doesn't matter. Because as the mighty pirate captain I am, I have a second ship thats larger then my attack vessel. I only need to steer to it. Dock there ("That exhaust was a malfuctioning something, we got it under control now." when somebody asks) and now I have laundered my traces to a ship thats legally there and have the stolen fright with me. Hiding "A bit" is far enough. (Compare the Cosa Nostra in NY in the early 20th century, that operated with legal firms as cover) $\endgroup$ – Angelo Fuchs Oct 9 '15 at 12:06
  • $\begingroup$ "once you are detetected, you stay detected forever" is not necessarily true. There are some detected asteroids (coming really near to the Earth) whose orbits could not be reliably determined and that got lost in space again. $\endgroup$ – jknappen Feb 10 '17 at 17:44
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I hate to rain on the parade, but stealth in space is an absolute cakewalk. Generally speaking, if you put it up in space, it's stealthy.

The most important detail about space, never to be forgotten, is that it is big. Really big. Really really big. If you can imagine how big it is, you're wrong... it's bigger.

We do a pretty good job of tracking things around our planet. We track roughly 19,000 objects with ground radar and lidar to make sure we know where they are. However:

Most debris remains unobserved; according to the ESA Meteoroid and Space Debris Terrestrial Environment Reference 2005 (MASTER-2005), there are more than 600,000 objects larger than 1 cm (0.4 in) in orbit.

So, if you look like an uncontrolled object (i.e. not maneuvering), you have a 3.2% chance of being on the detected list today.

Now, consider that Low Earth Orbit extends out to 2000km. That sounds like a lot, but space is big. For distances within our solar system, we use Astronomical Units (AU). 1AU is equal in length the orbit of the Earth around the sun. Phrasing that 2000km in those terms, it's 0.0000133 AU. Not a very large region, in astronomical terms.

But surely we can see you if you move. Then we know you're not an asteroid. Or do we? Consider that we are having trouble detecting asteroids large enough to cause extinction events. Space is so big that we have a hard time seeing things, even when they are a concern on the extinction-of-the-human-race level.

Why is it so hard? Space is big. Passive sensors like IR sensors find their ability to sense falls off with the square of the distance. In IR sensing, the intensity (how much energy is being emitted) is not actually the part that matters. It's the irradiance (how much energy hits the sensor surface). For realistic space borne systems, it is not uncommon for the irradiances to be measured using femptowatts per square centemeter (fW/cm^2). At long distances, this can drop well below that unit, leaving it very hard to determine the brightness of an object compared to the thermal noise from the sensor. Multi-color systems can estimate temperatures, and better find "interesting" regions of the sky, but they're still fundamentally limited by noise.

Active systems, like radar and lidar, face even greater issues. The return from an active sensor falls off not by distance squared, but distance to the fourth power (d^2 going out, d^2 coming back). It is nearly physically impossible to detect these return signals unless you know what you are looking for. By that, I mean you need to keep a continuously running clock with nanosecond precision while the signals are being listened to, so you can compare the signal received against the signal emitted. Clock drift limits the ability to do this.

There are only two ways I can think of to have trouble with stealth in space:

  • Get close enough to reach out and touch someone. At that point, the stealth challenges are identical to those faced by stealth aircraft, because the distances are similar.
  • Use an engine that is especially noisy. For instance, if your engine sheds hard gamma rays, you can bet that result will be interesting enough for someone to take the time to narrow in on your position.
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  • $\begingroup$ Can you speculate on just how close a stealth ship might be able to get before being detected by a not-so-specially-alert target? Assume stealth ship is radar absorbing, stores its waste heat in some temp heatsinks, emits nothing, and coasts towards a target.. obviously visual observation might pick out an occlusion of a background star, but besides that could stealth ship get to within hundreds of meters before correcting its course? $\endgroup$ – Innovine Mar 22 '17 at 16:09
  • $\begingroup$ @Innovine Such an estimate would be very difficult because of the wide range of numbers we can use. There's probably a 4-6 order of magnitude range depending on the equipment. As a general rule such a ship could evade detection by a civilian craft because we typically cut the costs of our sensors down to just the right amount, making it easy to make craft that could hide from those sensors. Military sensors are a much more difficult question because their job is to pick up such stealth craft. $\endgroup$ – Cort Ammon Mar 22 '17 at 16:56
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Edit: @ThorstenS. convinced me that this is not a feasible solution (especially within the hard-science tag). So please do not take this answer into account anymore.


Desperate times call for desperate solutions. Let's take space-time to hide our craft.

You may have seen one of these graphs, that show how a 2D black hole would look like:

We assume now, that there is a device, that also allows to bend space-time, but instead of just piercing a hole in it, it creates a pocket. (The concept is similar to those of pocket universes.) This pocket is still connected to the rest of the universe, but only through a very small opening. Inside the pocket the ship rests in an area of almost no distortion of space-time.

The pocket can move (and the ship with it), and some minor interaction with the space around is possible. However, the larger the pocket opening is (to enhance interaction) the more probable is detection. In the best (most-hidden) case the opening between space and the space-time pocket is of sub-atomar scale, so that most traces, that the ship generates, will not leave the pocket.

Is this hard to create? Definitively. And the amount of energy would be enormous, if no other means are found. But is it unrealistic? I think not. Space-time does some really strange things, like creating vortexes around earth. This makes the idea at least in principle feasible.

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    $\begingroup$ Wouldn't any such pocket be really bright, because of all the entropy leaking out. $\endgroup$ – PyRulez Aug 25 '15 at 20:24
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    $\begingroup$ That depends entirely on the time span the ship remains in the pocket... If they have heat stores, they could take it for a time. Assume, they have tanks of water, that they let freeze in the vacuum. A human produces ~350000 Joule per hour, which means, it heats a kg of water by ~84 Kelvin in that time (1J heats 1g H2O by 0.239K). So 1kg of vacuum-frozen water would last to compensate for a human for 3-4 hours until it starts boiling. $\endgroup$ – Boldewyn Aug 25 '15 at 20:58
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    $\begingroup$ @Boldewyn We are talking about hard science. This 2D rubber fabric analogy is sometimes really garbage because a black hole can be detected with sensors from all sides, there is no small opening to one side! $\endgroup$ – Thorsten S. Aug 26 '15 at 9:04
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    $\begingroup$ @Boldewyn If the hole is more and more closed, the Einstein equations predict (being that the connection must be smooth) that the curvature of space-time grows enormously. That means every object is accelerated towards the opening, disintegrates and begins to radiate. You have a "hidden" object behind a maelstrom which warps the light and radiates like hell. $\endgroup$ – Thorsten S. Aug 26 '15 at 11:40
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    $\begingroup$ Ah, curse you, differentiable manifold! You are right, that the energy needed to create the almost-closing opening makes the answer leave the hard-science tag realm. Also I forgot about vacuum fluctuations, so that at any time something will be close to the pocket (as small as it can feasibly become) and give away its position by lighting up like a Christmas tree. Thank you for your persistence! $\endgroup$ – Boldewyn Aug 26 '15 at 12:31
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The Revolation Space universe is at baseline the real universe with emphasis on Hard SF. That means no FTL.

Then, Reynolds can add made-up breakthrough physics in moderation, careful to keep it "hard" by not doing things that would mess up real physics wholesale. One item he has is for steathing like you ask about:

Cryo-arithmetic engines are a specific class of quantum computer discovered by the Conjoiners. When certain algorithms are executed on processors of this architecture, it leads to a local violation of the Second law of thermodynamics: the computer gets colder instead of hotter. Consequently, cryo-arithmetic engines have massive industrial (as opposed to computational) ramifications for Conjoiner manufacturing; such engines abound in Conjoiner asteroid factories, where their calculations can drain away the heat of starship construction.

Cryo-arithmetic engines are also used by the Conjoiner's modern 'stealthed' lighthuggers; they cool the exterior of the ship to the temperature of ambient space, making the starships difficult for the Inhibitors (or other foes) to detect.

This points out the big reason it is hard to be invisible in space: heat.

More realistically, there are ways to stay cool for limited time periods. Endothermic reactions use chemestry to trade local entropy (in other forms) for thermal energy, and can work until you run out of un-reacted chemical. Superfluid and non-superfluid helium solutions can cool a chip to near absolute zero: if the entropy increase can be stored internally in a compact form (a black hole is the limit, and it's way up there) you can keep cooling the exterior until your sink is full.


To hide camaflaughed against empty space means keeping cold. But now we have images like the Hubble ultra-deep field. A big enough telescope can see whatever is behind you (waaaaay behind) and note the eclipse. So look at the physical limits of that detection. If the exposure was short enough, the aperture would be very large. The ship, besides being a superabsorber in optical frequences, could mimic the appearance of faint red protogaxaxies in the background, but this requires knowledge of the observers position, both bearing and distance.

The ship could be shaped or have thingies attached to make light diffract around it, being invisible enough to an observer with an aperture large enough to see the background pattern before you move out of the way. This might entail secondary structures keeping station thousands of miles away from the ship, in the same line as the observer. Such an extended arrangement could give general invisibility against a more normal, background of nebula and haze in a specific direction.

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The conventional wisdom has been for a long time that stealth in space is impossible, simply because it is impossible to hide your thermal signature. Which makes sense: after all, a spacecraft inevitably produce lots of heat: varied machines running even when the ship is idle, body heat from the crew (if any), heat from the electrical generator, heat from the engine plume - and the last two can be gigantic, particularly if you want your ship to go anywhere in a reasonable time. All this heat has to leave your ship at some point. Even if you insulate it as best as you can, it will still end up leaving your ship. And given the time scales involved with spaceships, delaying it a little bit with insulation will not change anything.

However, a fellow named Isaac Kuo recently discovered a way to bypass this, with a concept dubbed the Hydrogen Steamer.

Basically, while searching for some half-serious way to give brief tactical thermal stealth, he decided to try and use hydrogen, due to its fantastic heat capacity and heat of vaporisation. What he discovered, however, is that liquid hydrogen is not just that good at absorbing heat. It is much better. With the right design, you can keep a spaceship hull cooled down at 20 K for years, long enough for it to go anywhere in the system. And an object at 20 K is pretty much undetectable. Cover it with superblack material (like the commercially available Vantablack) and it can only be detected by occlusion, but due to the nature of light, this only works at very close range.

But what about the engines?

Indeed, such craft would be detected as soon as it light its engines, wouldn't it? Assuming it has conventional engines, this is true. It could still use low-efficiency manoeuvring thrusters to make small corrections to its trajectory, making interception more difficult. This would for example give a missile or attack drone considerable advantage as the defenders would have a harder time getting a proper firing solution, even knowing it is coming.

However, it is also possible to have relatively decent "cold" engines. The trick is to take a conventional hydrogen engine (for example nuclear-thermal) and give it an enormous expansion ratio - meaning the throat of the nozzle is tiny while the nozzle is gigantic. That way, the hot hydrogen that is coming out of the heating chamber will expand considerably before exiting the nozzle. By expanding, it will cool down (while retaining its velocity, so it will still push the craft anyway). With a huge enough expansion ratio, it will cool down to be practically invisible, like the craft itself. As it is hydrogen, you cannot detect it by watching stars for new diffraction lines: most of the Universe is hydrogen already. Compared to conventional engines, it will be heavy and have a pathetic thrust-to-weight ratio, but it will have good enough specific impulse (aka efficiency), and you don't need a big push to move your craft in space, only a long one.

So yes, to the general surprise, stealth in space is actually possible!

More details on the concept of Hydrogen Steamer on this ToughSF article: http://toughsf.blogspot.com/2016/10/the-hydrogen-steamer-stealth-spaceship.html

The only way to detect such a ship is with active measures like radar, but those can be defeated with conventional radar stealth techniques. They are not quite the same than with stealth aircrafts, but the principles are similar.

Note that such stealth, while making detection considerably more difficult, is not perfect. But this is the nature of stealth, the same way stealth aircrafts can still be detected under proper conditions.

A Hydrogen Steamer would be akin to a modern diesel-electric drive: limited autonomy, rather bad performances for a warship, relying on stealth and surprise for both offence and defence, and once detected, it is pretty much dead if any conventional weapon platform is close enough to engage it ("close enough" potentially meaning interplanetary ranges).

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  • $\begingroup$ you may want to add a link to the source $\endgroup$ – PyRulez Sep 4 '17 at 17:33
  • $\begingroup$ This is a good solution EXCEPT that the OP seems to want to stealth a cargo ship or something similar that space pirates may want to pillage. The hydrogen steamer is a not a viable option for almost anything other than a first strike platform. It's almost a better choice for the pirates :) $\endgroup$ – Jason K Sep 5 '17 at 14:51
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Yes.

Here is a list of what you need to hide from others

  • Heat Stuff (engines, people, appliances) produce heat. Even copper wires.
  • Gravity You ship has a mass, possibly large, therefore it creates a gravity field that can be detected.
  • Electronic waves (communications, leaks from that µwave oven, engine)
  • Engine Nowaday engines shoot burning gas. That's visible on many levels

How you hide it

  • Heat your hull must be completly insulated. You need to capture the heat in the ship, and either store it to use it somewhere, or, when there is too much, release it. One way of doing that is using a single laser. That laser will be visible, but only from a very small angle. It will also push your ship forward a little.
  • Gravity not much to do here. Lighter ships. Decoys (heavy stuff in a barel dumped somehwere).
  • Electronic waves Very insulated hull, avoid broadcast communications (directionnaly antennas or even lasers give much less to be seen).
  • Engine Ion drives (or even laser drives) and solar sails are better suited for long term space maneuvering. Shut them down when your course is set or when you risk being detected (no deceleration in space). They also have a much smaller signature, and capturing the heat produced back will limit it even further.

Then there are active sonars (not sonars in space, obviously). The other ship would broadcast a wave, and see what hits back. You can detect that, and you can use the same principles used on earth to counter it (absorbing materials, mostly, but beware of not being absorbing too much stuff, as you'll gather energy from cosmic radiations that you'll have to evacuate).

Finally there is simple optic detection. Obviously, not reflecting light helps, but if you are in front of a star, short of being invisible (which is possible, using sensors and projectors), you'll be in the spotlight.

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    $\begingroup$ How do you reverse entropy and convert heat into a laser? $\endgroup$ – PyRulez Aug 25 '15 at 18:03
  • $\begingroup$ I don't know. I assume there has to be a way to convert absolute heat to electrical power. I was thinking piezoelectricity from heated molecules bouncing all over the place. Technically, the ship is radiating heat, which basically is emitting infra red photons. if those could be channeled in a coherent beam on one side of the ship instead of everywhere, that would work $\endgroup$ – njzk2 Aug 25 '15 at 19:13
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    $\begingroup$ The laws of thermodynamics means you can't convert absolute heat into electricity. $\endgroup$ – PyRulez Aug 25 '15 at 19:15
  • $\begingroup$ damn. I should have paid more attention during those lectures. $\endgroup$ – njzk2 Aug 25 '15 at 19:16
  • $\begingroup$ @PyRulez but doesn't a hot gaz radiate within itself (or toward vacuum, which can be obtained) ? is it not possible to capture the infrared photons and turn them into electricity? $\endgroup$ – njzk2 Aug 25 '15 at 19:18
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The closest you could get to stealth would be using metamaterials to confuse active sensors. Metamaterials are materials with properties engineered into them to change the refractions index in ways that are not natural. In theory, you could design a spherical cloak of metamaterial which would "bend" the light around in such a way that if you followed the photon, it would simply "slide" along the surface and continue on its way in a straight line. As far as the observer is concerned, there is nothing there, you would see everything on the opposite side and there would be no shadows or distortions to signify that anything strange is happening.(Interestingly enough, this principle can work with ALL wavelengths, a submarine can be protected from sonar and a building could be protected from the shockwaves of an earthquake with a suitable series of pilings driven into the earth around the foundation to reflect or deflect the shockwaves in the earth).

In practice, metamaterials are made of very precisely manufactured "optical lattices" which can be thought of as something like a diffraction grating to bend the light the way "you" want. This is rather expensive so far, and also with current technology only works on a narrow frequency band. For example, you could make a cloak of metamaterial to deflect or bend radar, but the object would still be visible in optical wavelengths.

It should be assumed that something so useful will be fully developed in the fullness of time, and eventually you could build a fully spherical metamaterial shell around your spaceship, and be invisible from optical and radar searches.

Sadly, any working spaceship needs sensors, antenna and an engine, so there would have to be breaks in the metamaterial envelope. This would render parts of the ship visible to optical and radar searches. The metamaterial cloak would also have no effect on the issue of thermal radiation, your ship will still "shine" against the 3K background of space, and indeed the cloak might make things more difficult for the crew as heat builds up inside the cloak with no way to be radiated away (until the cloak itself reaches equilibrium with the ship).

Since localizing the ship will be difficult on optical and radar frequencies, it would be useful to confuse the thermal signature. Having extendible radiators which can be moved would make localizing the ship a bit more difficult (think of the radiators as something like the bullfighter's red cape). Using a heat sink to store the heat in an insulated container would also work for a short while. If you carried a number of them, you could launch the hot container away from the ship in a random direction while heating up the next one. The downside of that plan is you will be carrying around a lot of extra mass for the heat sinks which you probably wanted for other uses, and once you run out of heat sinks or the heat sink material is saturated, then the game is up.

So for some very limited cases, you could make it much harder to see the ship, but you could never become totally invisible.

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"It is not possible". Or perhaps is it??


(1) How enemy might detect us

(1.1) Active. This is the active way of detecting. Enemy will emit some signal. This signal will interact with the hull. Iff hull reflects, then the enemy will pick up the signal. Analysis of the signal will identify our position, our speed, and possibly more data about us.

(1.2) Passive. This is a passive way of detecting. Enemy will not emit a signal, at all. Enemy will just listen for signals. If we emit a signal, enemy will detect us. If we reflect from signal emitted by someone else (say.. sun.. or another ship), enemy will detect and gather data about us.

(1.3) Cast Shadow. Other sources emit signals continuously at all directions. Say.. The sun. If the enemy is looking at sun, and their sensors detect the sun stopped transmitting its light in some small dot! Since the source emit continuously by hypothesis, conclusion is something is in between the sun and enemy ship. Something has been detected. Detection of cast shadow might be done with light of stars, sun, continuous radio emissions, etc.


(2) Detection by EM Spectrum

(2.1) Radio and microwave. RADAR is a system that actively scans regions of space, by emitting electromagnetic radiation mostly in microwave range, and hoping they will hit our ship and reflect back. Also, they might not emit any radiation at all, instead, they will listen for any emissions we do. If we try to contact someone using radio, we give away our position. Other radio sources that is emitting radiation might hit us, reflect from us, and enemy might listen.

(2.2) Passive infrared detection. The enemy will try to look infrared signals. They are often released by black body radiation in normal temperatures. Basically, a thermal detectable signature because we have non-zero temperature. Also, other infrared sources might hit us, reflect and go to enemy, making us detectable.

(2.3) Passive visible light detection. Looking.. maybe with naked eye. Or telescopes. Or whatever. We might be emitting visible light (lamps, engines, or whatever). Or, any other visible light sources (say.. sun, etc) might be emitting, hitting us, and reflecting back to enemy, which in turn it will detect.

(2.4) Detection by radiation of higher energies. Cosmic radiation interacting with matter might emit photons, which can be detectable (to a ultra amazing high ridiculously accurate detector). Maybe we should ignore this situation. Or postpone to a future version.


(3) Effective cloak.

There are 4 possibilities of interaction of classical light and us: Reflection, Transmission, Absorption. In addition, there is the possibility we are emitting such things.

  • (3.1) Emission. We cannot emit any of these signals (radio, microwave, infrared, visible, higher energies). If we do, they will detect us. Its easy to avoid emitting radio, microwave. Infrared is tricky. Visible is easier. And higher energies is special.

  • (3.2) Reflection. We cannot reflect any of these signals. Its easy to avoid reflecting radio, microwave. Somewhat hard with infrared and visible.

  • (3.3) Transmission. This is the ideal. We must transmit everything.

  • (3.4) Absorption. This is good, but not good enough. With careful looking and amazing algorithms, the enemy ship might detect that some stars are disappearing and appearing back (because we are absorbing their light). The same apply with natural radio emission sources, infrared sources, maybe even X-Ray sources).


(4) Microwave and radio cloak

(4.1) Emission. Trivial to avoid.

(4.2) Reflection. It is not that hard to prevent reflection of microwave and radio sources. The reason for this, its because their wavelengths are sizes we are costumed to (from meters to centimeters). By choosing the right geometry in your ship, to reduce RADAR cross section at maximum you can, you will avoid reflection.

(4.3) Transmission. This is ideal, as we said. Thus we need to transmit the maximum we can. But it is somewhat hard to transmit radio signal, precisely because their wavelength is big. There are few ways of doing this.

  • (4.3.1) Making a ship transparent to this kind of radiation. This means, in all practical purposes, making a ship made fully of dielectric. Metallic or electrically conducting surfaces reflects this kinds of signals. Hardly feasible. And even pose some problems: By Snell law, the transmitted signal will shift angle and thus if enemy is far enough, we will be effectively blocking the source. Conclusion: (4.3.1.1) Offers good protection against active radar. (4.3.1.2) Offers good protection against natural sources. (4.3.1.3) Offers bad protection against shadow cast. (4.3.1.4) Unfeasible.

  • (4.3.2) Redirection. We can redirect all incoming radio signals, such that their Poynting vector remains unchanged from what it was before hitting us. (4.3.2.1) Using wave-guides, this is extremely hard precisely because their wavelengths is too big. A wave-guide that guides EM Radiation of wavelength $\lambda$ needs to have a size of approximately $\lambda$. Your ship would have to be huge and covered with wave-guides. (4.3.2.2) Using nice engineered materials (say.. meta-materials), this task becomes somewhat "simple" precisely because the wavelength is big. Some methods have already been developed

(4.4) Absorption. Since transmission (the ideal one) is unfeasible, we shall place our hopes here. And thank for us, absorption is possible.

  • (4.4.1) Geometry. We can make the ship in such geometry to reduce active radar cross section at maximum we can. Then the signal of enemy radar will mostly be absorbed and be made undetectable.

  • (4.4.2) Material. There are absorbing materials that we can place in the hull, such that it will absorb incoming radiation. There also exists engineered materials (like meta-materials) made to absorb radiation in this range of frequencies.


(5) Infrared cloak

(5.1) Emission. This is terribly hard to avoid. Our ship emit infrared radiation because of black body radiation, which mostly fall into the infrared, using Wien's law. ) According to Wien's displacement law: $\lambda = b/T$, where $b$ is Wien's constant. If we decrease the temperature to shift from infrared to microwave, your temperature would need to be unfeasible low. In addition, you must radiate waste heat, if you plan the ship inhabitants/computers/equipments to survive. To effectively cloak our selves, we need to make sure enemy does not receive our black body radiation.

(5.2) Reflection. Hard Hard Hard. We cannot reflect from other infrared emissions if we plan to be undetected.

(5.3) Absorption. If we absorb, we must be careful with the shadow cast problem, so you cannot be in between infrared sources and your target. This is very hard in star systems. And hard in open space (since stars of course do emit infrared, and they are everywhere). Absorption only is dangerous and risky. If we plan a good cloak, we cannot absorb. There is only one left: We must transmit.

(5.4) Transmission. Here we are. Its a must. If we transmit, we avoid active and passive detection. Our biggest problem is cast shadow: There is continuously emitting infrared source $S$, and enemy ship. We are in the middle. If the transmission is perfect, light from source will pass thru us if we didn't existed. If there is a delay, may be detectable if we are moving at an certain speed or higher. If it is not perfect (likely), there might be distortions in the transmitted light, and aberrations. If enemy calibrate their scans, they might detect this distortions/aberrations, and move to investigate (or perhaps to simply shoot at it, just in case).


(6) Visible Light.

(6.1) Emission. Trivial to avoid. Just shut down all the lamps in the outer-hull.

(6.2) Reflection. We cannot reflect from other visible light emissions if we plan to be undetected

(6.3) Absorption. Same argument for infrared. If we plan a good cloack, we cannot absorb visible light.

(6.4) Transmission. Here we are. Its a must. And same argument of infrared applies to visible light.


(7) What we need.

(7.1) Geometry. Making things in the right geometry to reduce radar cross section and be invisible by radio and microwave. Maybe a fractal pattern. Or something to make the waves cancel out. I don't know.

(7.2) Infrared and Visible light transmission. We need to perfectly redirect the infrared and visible sources as if we are not there. This is proving to be quite difficult with our current technology level.

(7.3) Black body Radiation. Following (5.1), we need to make sure enemy does not receive our black body radiation. If you have hypothetical material that nicely does (7.2), then it will not heat up (since there is no conduction, no convection, only heating by irradiation in space, and (7.2) transmits everything, not heating it up). After some time, it will be as cold as environment. Since at principle you know where the enemy is, you can transform inside-ship black body spectrum into unidirectional beam, and waste it away not pointing the beam at enemy. That way, you remain undetectable, and you have successfully dissipated your waste heat.


(8) Additionals.

(8.1) Keeping people inside alive. (7.3) solves the waste heat. Life support system is not disrupted by any of this. Then, you can keep people inside alive.

(8.2) Carry out the normal duties. Yes. There is no reason why not. As long as the ship is shielded internally (so radio signals from inside does not reach outside). And shielded infraredly and visibly (easy). Be careful as to your power source. It may release some kind of detectable radiation that might make its way to the outside ship (maybe neutrons, or gamma rays).

(8.3) Keeping your sensors on. Of course!! You will only need to shutdown your active sensors (for obvious reasons). And leave only the passive sensors online (which is more than enough to detect an enemy ship). Enemy thermal signature alone might do the trick. And if enemy ship has active radar online, you can detect its position by the incoming radio signals. =).

(8.4) Still able to maneuver. Tricky. Your engines cannot release any radiation at all. The only only way I can think off to accomplish this, is a special propulsion to operate in cloak only, which shoots projectiles. Very cold projectiles (temperature of background universe), which does not reflects radio, microwave, infrared, visible. Hard, isn't it? Perhaps you could engineer such projectiles to be such way: To cover the projectile with the same hypothetical material that keeps your ship cloaked in (7.2).


Let me know where this answer can be improved. =).

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  • $\begingroup$ > If you have hypothetical material that nicely does (7.2), then it will not heat up It will be heated up from the inside to the temperature you maintain inside (~290K) unless you continiously burn energy to maintain temperature gradient. Well, if you have layered outer hull with layers separated by vacuum and refrigerated it can be tolerable, but you would be pretty limited in outwards-looking sensors. $\endgroup$ – Daerdemandt Dec 4 '16 at 19:08
  • $\begingroup$ As for controlled emission of heat, it can be done if you have a safe direction to emit to, and that depends on what economic tradeoffs are. If sensor platforms are cheap then they're abundant and you don't have safe directions if you're close to anything of interest. Otherwise, you might have a chance. Take a look at link in the beginning of the accepted answer, there are some options. $\endgroup$ – Daerdemandt Dec 4 '16 at 19:28
  • $\begingroup$ @Daerdemandt Yes. Something like that. Perhaps (also) insulating hull from inside. Time constants of heat diffusion in insulators can be made in days range. Perhaps even months in special cases. As for emission of heat and sensor platforms, space is vaaast. Very vaast. All you need to do is release your heat with extreme power density (compress full beam in, say, $1mm^2$ of area). Furthermore, with platforms, enemy cannot have your position, rather, only a oriented line of where you are. $\endgroup$ – Physicist137 Dec 4 '16 at 19:46
  • $\begingroup$ It's not the area that matters, but a cone. If extremely-hot 1mm^2 spot radiates to half of the outer universe (180-degrees cone), you are going to be detected by sensors in that half. You can aim for 60-degrees cone as reasonably narrow, but even then spamming space with detecting drones could be cheap enough to make that pointless. As for enemy having only a line of where you are - no, with platforms enemy can know the location too. Even if the enemy knews trajectory of his own platform and observes your ship for some time, there's much more info than just a direction. $\endgroup$ – Daerdemandt Dec 4 '16 at 19:50
  • $\begingroup$ @Daerdemandt I said "unidirectional beam" in (7.3) which by definition has your "cone angle" of 0 degree, and yes, its a line. Of course, you won't have exactly 0, but very near. And this ensures enemy only will know a line-location (if there happen to be a platform in front of your beam (you won't aim in a plataform!!)). Its like a laser. $\endgroup$ – Physicist137 Dec 4 '16 at 19:54
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The most likely sensor of space ships will be radiation detection. There is radiation everywhere in space, but closer objects will not produce random noise. It will be more predictable and more intense. Even if technically a space ship has no moving parts, it still can't exist at 0 kelvin, so heat radiation is still released albeit in small amounts. You can heavily insulate the ship so that the outside will get as cold as possible to reduce this signature, but even a reduction in radiation received is an indicator.

You'd of course need an onboard computer capable of analyzing radiation received in real time and notify of signatures coming from anything relatively closeby (within 1000 kilometers).

To fight this, your ship would have to emit background radiation to mask the radiation that the ship is emitting. The crew could operate onboard without interruption and anyone detecting the signal would see only background radiation.

So a secondary sensor might be an optical scan. The ship would use a telescope, and when used in combination with the radiation sensor, the ship could focus on anything minutely off. The optical scan would be able to determine the existence of an object by deliberately putting a star behind it in the line of sight. If something is there, the light will be blocked and you'd know that there is a ship emitting background radiation and likely a pirate of some sort.

Of course you cannot observe everywhere at once, so you'd first have to suspect a strange radiation signature (even minute) before you can investigate further. Suffice to say that detection and stealth would depend largely on the capabilities of your onboard computer to emulate background radiation and the other ship to detect anything strange, but my guess is that a ship could get fairly close before being detected (within a kilometer) if it is trying to be stealthy.

My guess is that if the technology existed, every ship, not only pirates would equip a stealth drive to emulate background noise. The trick is knowing where to aim it, since I don't think it would be possible to emulate it in every direction simultaneously. This means that the first to detect the other is the one that has the biggest advantage, and so the name of the game is having the best detection system in order to stay alive.

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  • $\begingroup$ 1000 km distance wouldn't buy you much. Even for the puny tin cans we used to hurl people toward the Moon, that's still only about two minutes warning. (Translunar is about 11 km/s relative to the Earth.) $\endgroup$ – a CVn Aug 25 '15 at 9:09
  • $\begingroup$ @MichaelKjörling You're right, in the scheme of things, probably not. $\endgroup$ – Neil Aug 25 '15 at 9:35
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Yes it's possible, but not in the way most of the responses I've seen are thinking.

As some users have outlined, it's impossible to truly hide your existence in space. So what do you do when you need to essentially hide in plain sight?

You blind the enemy without letting them know they are blind.

The way I see stealth in space being a thing is that you would have to have extremely complicated and sophisticated hacking that essentially reaches out to any ship within range of sensing you and it ('it' being said program or effort) tells all of the observer's sensors that there isn't anything there.

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

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The primary issue, as many posts have noted, is the challenges of thermodynamics. In order to cool something, you must make something else hotter. When you radiate heat into space, you make space hotter and your ship cooler. Of course by releasing this radiation, you will be able to be detected. You could try to store the energy on your ship, but this has drawbacks that others have noted.

Alternatively, one very good method would be to power your ship with negative energy! If you assume that negative energy is possible (you need it for gravity drives and stuff anyway), then you could store a large amount of negative energy and use it to provide power for your ship. In thermodynamics, energy flows from high to low, so by using negative energy your ship would actually be absorbing energy from the surrounding vacuum! There would be a net energy flux into the ship, rather than out of the ship. As a result, your ship would not radiate heat, but would instead be absorbing radiation from the surrounding space and be colder than the surrounding space. However, it could still be visible as a shadow against the background radiation. In this case, in order to be stealthy, you would just have to heat up your ship to 3 kelvins with an auxiliary positive energy power system, which you would need anyway to keep your crew from freezing. With this system, your sensors should work just fine. Whether or not your maneuvering would make you visible would depend on what propulsion technology you use. Assuming we are bending space, this would not give off radiation but could be detected with gravity sensors or by observing gravitational lensing around your ship. In general though, using a gravity drive would be much more difficult to detect than something like an ion drive.

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

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    $\begingroup$ This question is tagged with hard-science. As the panel under the question states, "All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed." To keep your answer around, please supply references. $\endgroup$ – Azuaron Apr 13 '17 at 19:42
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I am a passerby. Yet it seems likely that the question and answers above are misleading by using the term "stealth" as something identical to "cloaking."

They are different. F-22 Raptor is visible in IR and even with state-of-the-art active radar systems in close distances.

Yet F-22 is 'less' likely to be detected and 'less' likely to be hit by a IR-guided missile. Thus it suffices to be a stealth aircraft.

(F-22 is seen as a size of a bird through rgular radars. No bird can fly faster than the speed of sound. Then, you might think it will be easy to point out the aircraft and track it. Sadly, the answer is no.)

If any spacecraft successfully reduces its radiation to a degree that is indistinguishable from space debris, mechanical noise, or Asteoid. It is good to be regarded as stealthy.

Also there is a issue with Rayleigh's criterion. To run passive and active radars large enough to see throuh the 'vast' space.... is nothing but impractical.

Let's say a sapcecraft is equipped with a IR-detector with the diameter of 2.4m(equal to Hubble Telescope.) Rayleigh criterion says its can detect a heat source with an angular resolution of 0.1 arcsec at best.

If the 2.4m-diameter detector uses the shortest wavelength IR, it will see a 100m-long object as a small dot until the obeject gets as near as 200,000km.

200,000km is closer than the distance between the moon and the earth. Whether it is far enough or not is a relative question.

In addition, an asteroid with a diameter of 1km, traveling 2,000,000km away, look virtually identical. So do space debris with a diameter of 1m, traveling 2,000km away.

Since it is possible to cool down a part of the spacecraft that faces the observer, while emitting heated-up exhaust chunks to the space. Such dust may go invisible immediately thanks to the limit of angular resolurion.

How big will exhaust chunks be? Are IR radars capable of detecting such small things overcoming the vastness of the space? I am not sure...

(English is not my mother tongue. Sorry for the grammatical errors.)

Please correct me, if my caculation is wrong.

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    $\begingroup$ Because this question has been classified as hard science, it requires answers backed up by equations, empirical evidence, scientific papers, other citations, etc. You'll need to provide support for your statements, or your answer risks deletion. $\endgroup$ – Starfish Prime yesterday
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    $\begingroup$ Hi Chin. It looks like you might have created two different accounts, which would be why you cannot edit this post directly. If that is the case, then you can contact Stack Exchange and request that your user profiles be merged. $\endgroup$ – a CVn 19 hours ago
  • $\begingroup$ @aCVn I just flagged it before I noticed your comment, apologies. $\endgroup$ – Chickens are not cows 19 hours ago
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Stealth in space cannot exist unless you can achieve invisibility. Stars in space do not twinkle and every spot in the night sky contains stars.

The simplest sensor system would consist of high def cameras recording 360 degrees around the ship and a computer system watching for changes. Should stars suddenly disappear or reappear, it means something has moved in front of it and you can focus a telescope on it.

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  • $\begingroup$ There would always be a certain threshold for how much change was needed to be detected, you could easily miss a ship in the vastness of space even with computerized systems as long as it doesn't get too close. $\endgroup$ – Braydon Sep 5 '17 at 4:33
  • $\begingroup$ Last time I checked, it's difficult for space pirates to steal your stuff from a long way away as per the brief. Considered we can already detect planets from the wobble of the star thousands of light years away, the space pirates will need to be a really really long way away. $\endgroup$ – Thorne Sep 6 '17 at 4:11

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