# Is there any way to truly hide a spaceship?

Space is two things: empty and (mostly) dark. This makes it really, really hard to hide in it, because to do anything in it, chances are, you'll be shining a big bright light through it.

Even if you're not trying to do anything, the emptiness of it makes it very difficult to hide from some kind of scanning equipment. Either you reflect it and are found, or eventually the black box effect sets in--I imagine that a gamma ray active scanner, while impractical in an atmosphere, would eventually begin to cause your ship to radiate infrared in a very distinctive manner.

I imagine that would be expensive and rarely used, but the principle of the question still remains: how does one hide in space?

• Do you mean "hide" like in "hide from the Hubble telescope pointed at that system way over there", or "hide from people on the planet you're currently in orbit over"? – Monica Cellio Oct 1 '14 at 1:24
• In your scenario, is there any way of detecting ships other than light? Also, what is the scale of things? Is this worlds away, in orbit over a planet, what? – Vulcronos Oct 1 '14 at 1:24
• Whole planets have been hiding from us for centuries, so it can't be that hard ... – Jens Schauder Oct 2 '14 at 5:53
• @Jens: Don't forget Moore's law, no? When we have regular spaceflight, I fully expect the level of our detection/processing systems will be well beyond what could have been imagined even ten years ago. – neph Oct 2 '14 at 6:33
• Do what my kids do when they're playing "hide and seek". Hide behind something. The moon for instance. – tobyink Oct 2 '14 at 11:09

## 12 Answers

Project Rho's space war pages has a pretty thorough set of arguments on why stealth is, for practical purposes, impossible in space warfare*. The Rocketpunk Manifesto also has an essay on this subject which concludes pretty much the same thing. Therefore, to hide in space you'd have to be very creative, or do some bit of hand waving.

Some things you could try:

• Camouflage, or attempting to look like something else (a comet, or civilian ship), but of course this is highly situational
• Some sort of magic FTL technology
• Magic cloaking device
• Your spaceships are manned by exotic lifeforms that can tolerate near-absolute-zero temperatures, and hence your spaceship is as cold as space

Other than that, it's generally agreed among hard-sf folks that in space warfare, everyone can see everything, although you can still perform a lot of tactical stealth, like jamming, "smoke"-screens and the like. They'll see you coming but they might not know exactly what it is.

The arguments for why it's so hard to hide in space are too numerous to list, but in a very condensed form, they go like this:

• It is very easy to detect something hot in space, and it is very hard to refrigerate (as you need to rely on radiation). Consider:

As of 2013, the Voyager 1 space probe is about 18 billion kilometers away from Terra and its radio signal is a pathetic 20 watts (or about as dim as the light bulb in your refrigerator). But as faint as it is, the Green Bank telescope can pick it out from the background noise in one second flat.

• It is not that hard to do a full-scan of the sky. Using current technology, we can perform an entire-sky scan for magnitude-12 objects in 4 hours (that's a bit brighter than Triton or Pluto).

• You become that much brighter and easier to spot if you have your engines on. This means that, if you're invading a star system, your enemy will definitely know by the time you reach their outer planets, or you'll have to coast silent and cold for a very, very long time.

*- Space warfare being the key word; absent any tachyon weaponry, it's generally assumed that shooting will be performed at ranges well within 1AU (or a few light-minutes). Considering your fastest weapons will take minutes to hit, that's a pretty big distance for your enemy to evade. At these ranges it is indeed almost impossible to hide, but it should be much easier if you're talking light-hours distances.

• @TwoThe The answer mentions a "•Magic cloaking device" – Philipp Oct 1 '14 at 8:19
• @TwoThe the kind of cloaking device you describe merely bends incoming radiation around it, it does nothing to alleviate black body radiation. – congusbongus Oct 1 '14 at 9:47
• @TwoThe Assuming we can 100% reflect back/absorb the radiation (aka perfect insulation) the ship will get hotter and hotter till everyone aboard is dead – Richard Tingle Jun 5 '15 at 20:21
• Re the Voyagers, note that they're using a 3.7m dish antenna to beam transmissions directly at Earth. I doubt there would be much of a signal detected more than a few degrees off the beam. The Earth-based antennas know exactly where, and at what frequency, to look for the incoming signals. That's a far different thing than detecting the same amount of energy dispersed over a sphere, and over many frequencies. – jamesqf Jun 6 '15 at 0:12
• @jamesqf Voyager 1 is also running at -79 C and cooling, and is a probe as opposed to an interstellar ship. All these concerns are valid, but if you want to convince me at least, you'll need to address the rigorous calculations in my source, instead of my butchered summaries here. For starters: have you done the calculations for black body radiation of a typical interstellar ship with life support? What size of ship and crew are you comparing the Voyager against? Is this ship maneuvering? – congusbongus Jun 6 '15 at 1:42

I think you overestimate how hard it is to hide in space. The size of spacecraft is negligible considering what we're usually observing. Their emissions can be cut quite easily, unless you're using some kind of engine that has high emissions, which again, can probably be reflected or masked.

Additionally, celestial bodies are quite large and very effective in blocking signals and emissions. The vastness of space (it's really big) makes it very hard to scan effectively - light takes a long time to reflect, which means you're very dependent on what light is already reflecting off interesting objects. And light's the fastest signal we know. It's also the only one that will work - unless you're measuring field gradient flux or whatever, in which the effect of a ship will be less than background noise.

If we were to assume however that indeed it is hard to hide in space, it would be due to your own emissions and reflections. You can easily reduce those to background noise - lining the parts of your ship that cause these emissions with heavy materials will reduce them significantly; you can also remain stationary and pretend to be a rock, if you know where you're likely to be observed from - possibly using camouflage to imitate random rocks reflecting the sun. And always, you can plot a course through debris fields if you're desperate and use them to mask your presence behind them.

• I like this answer, with one caveat. It's very, very hard to remain "still" in space. About the best you can do is match your course and speed to some other object, which means that to an observer in the reference frame of that object (not taking the movement of their own reference into account) you appear as if you were stationary. If an object truly remains still in space, then everything else moves past it at various velocities. You would also have to contend with gravitational effects of everything else. – a CVn Oct 1 '14 at 11:18
• @MichaelKjörling Of course, thats why I said "if you know where you're likely to be observed from" :P – mechalynx Oct 1 '14 at 11:20
• @MichaelKjörling you are supposing time/space has a fixed coordinate system and everything is moving, when charts will always set a discretionary point (the center of the galaxy?) as the zero for the axes. So holding still will always be according to some referential. – Mindwin Oct 1 '14 at 18:03
• The problem I see with "lining the parts of your ship that cause these emissions with heavy materials" is, naturally, overheating. Even if you were able to mask baseline operating heat effectively, the moment you need to maneuver, you'll light up the sky in a way easily spotted by anybody looking for it. The point about debris fields is good, though: camouflage may well be the best way to avoid detection. – neph Oct 2 '14 at 6:39
• "if you know where you're likely to be observed from" - That's the one thing that people tend to overlook. How, exactly, do you know where you're likely to be observed from? If the people you're trying to hide from have equivalent tech as you, then what's stopping them from using the exact same methods you want to use to hide from them to hide passive sensor platforms from you? And unlike your ship, those sensor platforms are going to be easier to conceal. Scatter a few across the solar system, and how do you know a safe direction where you can emit? – Keith Morrison Mar 17 '18 at 17:19

ivy_lynx's answer is spot on - space is really, really big, and very easy to hide in, given some basic assumptions. I'll provide some numbers to demonstrate that:

### The scales are huge

The largest asteroid is Ceres, which is around 950km across. For perspective, Earth is a bit under 6400km across. Here's an illustration from Wolfram Alpha:

The equivalent to an average human (5' 4" = 162cm) is approximately equal to Harry Potter's glasses. They're large glasses, but they're still much smaller than Daniel Radcliffe (who is about average height).

And remember, we're comparing this to an asteroid that's almost a small planet (although far smaller than our moon). So a spaceship on that scale would cover the US from New York, New York to Cincinnati, Ohio. And even then, it's as relatively small as a pair of glasses on a person. How far away can you get before you can't tell that they're wearing glasses any more? How easy would it be for them to cover up the glasses, simply by raising an arm or turning away?

You can see that it'd be trivial to hide in the shadow of a planet or moon, if you knew you were being observed. Of course, then you'd be very visible from that planet (if it was inhabited), so you need to choose carefully.

### What about light?

So, you're able to hide if you're aware there's someone out there, but assume you don't - how hard will it be to spot your lights?

Light falls off according to the Inverse Square Law, which says that it gets fainter with the square of the distance travelled. The Luxor Sky Beam is the strongest human-built beam of light in the world. (I'm ignoring lasers for now, because they don't really illuminate anything.) It's got a brightness of 42.3 billion candela. How bright is it from the moon? Wolfram Alpha says it's approximately 2.976x10^7 candela per square meter (aka 2.976x10^7 lux). That's barely half as bright enough as it needs to be for me to be able to even see it, if it were pointed directly at my eye. If it was pointed at something else? Forget it.

Obviously, by the time we build spaceships, it'll be possible to build brighter lights... but you're still going to have to be very close (relatively speaking) to be able to see them.

For relative comparison, we could fit just under 400 of our Ceres-sized spaceships between the Earth and the Moon.

### What about heat?

congusbongus raises an interesting point about heat, but the same issues as light apply. Heat radiation is just infrared (or longer) light, and obeys the same rules as visible light does. On the scales we're talking, you'd easily miss detection on anything that wasn't specialized for space, the same way that your lights would be easily missed.

But that brings us to the assumptions I mentioned at the beginning:

### Specialized equipment can do it

It doesn't matter how far away you are, some small portion of your light (or heat) will reach whoever's watching for it. I don't have the knowledge to relate the appropriate numbers from various space projects to the tiny amount of lux I calculated above, but I can say that the longer you watch an area, the better you're going to be able to detect a photon coming from that area. But most of our current highly sensitive satellites only look at a tiny portion of the sky at a time, and vice versa. It's reasonable to assume that there are specialized detectors designed to look for these faint traces and alert when something moves that might possibly be a ship, but they're unlikely to be able to cover the whole 360° sphere of the sky at once. And if you're moving, they won't be able to tell you anything useful.

Likewise, if you happen to move your ship between the detector and a star, they'll be able to notice that if they're looking for it.

But either way, it's still very easy to get physically behind something if you have enough distance, and nothing we have or can design currently can find you through a planet.

### Beware of close distances!

All the above applies when there's some distance between you and the person looking for you. You don't even have to paint your ship's hull black to hide, just because it's so hard to see anyone if you're far enough away.

That said, if you're close enough that they can look out a window and see you, there's very little you can do to stop that. No matter what, you'll be occulding stars and radiating heat.

• Wow, that got long. This was intended on just being a supporting answer, but it grew a bit out of hand... – Bobson Oct 1 '14 at 6:19
• @stephenbayer since we don't know what dark matter is, speculating about it would be off topic on a question tagged with science-based. – congusbongus Oct 1 '14 at 22:33
• Space is big, yes, but the sky is only so many square degrees, and ever-growing processors, even with today's optics, I imagine would be sensitive enough to maintain constant vigilance for approaching spacecraft, particularly in awareness of situations such as this: worldbuilding.stackexchange.com/questions/85/… – neph Oct 2 '14 at 6:36
• @kikjezrous - Somewhat true. It'd be much harder to hide from a planet, in general. I assumed that you were talking about hiding from another spaceship. But it's very easy to hide behind something if you're just trying to hide from a planet and not trying to go anywhere. – Bobson Oct 2 '14 at 11:18
• On another question I worked out that a skyscraper-sized object (seen end on) would be one pixel on our best telescopes at well under the distance to the moon. Looking carefully for it we will find it even when it's well under one pixel but detection beyond several lunar distances is going to be awfully hard. – Loren Pechtel Jun 6 '15 at 3:43

Some people agree that it will be hard to hide spaceships with future technology. Others disagree. Your future technology may vary.

On the "it's easy to hide" side of the argument, space is also very large, and it would take a lot of looking and data-crunching to continuously watch it all to find what you might be interested in finding, amidst all the other input from looking at space in all directions. Another consideration is hiding technology, for example:

http://www.theblaze.com/stories/2014/09/27/scientists-have-invented-a-real-life-cloaking-device-and-it-could-cost-less-than-a-new-computer/

Another consideration is the situation. What size area is under observation, what are you trying to do there, what other noise is usually there (i.e. can you hide by looking like something other than what you don't want the lookers to know you are), and what are the accelerations and times involved?

This problem is a major topic of David Weber's Honorverse books - pretty much each book has a lot of discussion of different situations with different technologies for ships playing hide and seek in space (and then doing violent things), with an attempt to be realism-y about the technology and physics math.

Edit: I meant to add, but forgot, that even looking for asteroids that might be on course to wipe out our planet in our own solar system, with decades to do it, we on Earth in the 21st Century are aware that we have not been able to do a very thorough job of even managing that. So, it's not like it's a trivial problem to detect and be aware of everything around you in space.

Too long for the comment.

First, the space is far from being dark. There is a lot of background radiation, it is not visible, but it is there.

As for hiding, it depends very much on the size of the ship, could you expand on that? For example, a small blacked-out spaceship would be impossible to find without any hiding equipment, just because the space is so vast. On the other hand, a spaceship big as the sun might have to use different techniques, like active camouflage (but also can do much more, because of its energy supplies; it also could have a giant heat sink inside to keep the heat signature to minimum). Perhaps it could manipulate gravity to bend space and in some way appear smaller to the external observer.

Also, big distances make it impossible to accurately pinpoint an object, as any info you can have is outdated by minutes, hours, days, years, ... In other words, a fast Brownian motion might just happen to be a perfect long-distance hiding technique ;-)

I hope this helps ;-)

Edit: As for the heat, it's impossible to conceal indefinitely, but we could hide it for some time, for some nice ideas see this question at physics.SE.

# Decompose the ship

I'm not sure if this matches your requirements, but one possibility overlooked up to now would be to make the ship cease to be a ship for some time. I mean, it could disintegrate into smaller parts, which would be not that easily identifiable as parts of a large, dangerous spaceship. The parts could travel together as a loose swarm of space debris or dust, or even travel over different trajectories, and assemble again at the target location.

Of course, you likely cannot do that with a ship carrying a biosphere and supporting human life, as the parts would still be large and identifiable as ship parts. However, this could work perfectly if your ship is robotic in nature. The parts themselves could be a kind on nanomachines or grey goo, able to function on their own on a limited scale. They might even be very small, in the micrometer scale. This would also make it impossible for the opponent to identify what kind of ship would result from them assembling again.

It would even be thinkable that a smaller, but still functioning ship could be assembled from a subgroup of the machines, should some of the others be destroyed. The nanomachines could also be self-replicating, which has further interesting implications beyond the scope of this question.

For more ideas of this kind, you can see https://en.wikipedia.org/wiki/Grey_goo (also the related fiction book section) and https://en.wikipedia.org/wiki/The_Invincible .

• Very good unique answer. Hiding in plain sight, as it were. – neph Oct 16 '18 at 20:48

Since space is very cold, the thermal radiation of your ship will give it away. If there is a military reason to be looking for spacecraft, then there will certainly be enough scanners and number crunching computers to do so, given that even a "slow" moving body in space is moving far faster than a rifle bullet or artillery shell, so would pack an immense amount of kinetic energy (an object orbiting the Earth would be moving at @ 11 Km/sec, while the fastest unpowered bodies in the Solar System would be moving at a mind boggling 72 Km/sec.

The best way to hide, paradoxically is to move even faster, at high fractions of c, in fact. A ship moving at .9*c* will be just behind it's own "light", for most purposes, a ship or object moving that fast would not be "detectable" in any practical manner, since it would have moved a considerable distance between the time it emitted a signal (heat, reflection from the sun, rocket exhaust) and the time you saw it. The kinetic energy of even a small object accelerated to that speed would be enough to destroy continents, if not render entire planets uninhabitable, which is a pretty frightening thought: you literally won't see it coming.

• If its moving towards you the blue shift will make it much brighter, and visible from further – Donald Hobson Aug 30 '16 at 21:22

A suggestion to hide thermal emissions:

Paint your ship with Vantablack. Use a heat pump just below the hull (peltier device, A/C unit, whatever) to lower the surface temperature of your ship to ~3K. Of course, that heat has to go somewhere (because physics), so exhaust that heat in a concentrated beam away from you. Then, only someone who is actually in the path of the beam will be able to see you.

# Spaceships will probably be very difficult to detect over planetary distances

Here is an example of how Hubble sees Pluto:

Image: Adapted from NASA and Universe Today

When Hubble is seeing Pluto, it is not seeing anything else. There is "all sky surveys" like Wise and Plank that have mapped the whole sky, but in a level of detail that will not even detect Pluto. Of course we can build better instruments in the future, but I doubt we will have a device capable to make all sky surveys in weeks capable to detect even small planets in the Kuiper Belt. Even if we did, spaceships are way smaller than planets. A building size ship would not even be a pixel on Hubble resolution.

Wise all sky survey full resolution atlas has an pixel resolution of 6 arc seconds. At an earth-moon distance this means each pixel has 73km. At earth-mars distance this means each pixel has about 8560km. And it took wise 6 months to capture the data to compose this atlas. Not even close to a early warning system.

This is both a problem of the computational capacity and of the optics of the detection instruments. Computation power still have a lot to increase. Our computers are still too primitive when compared to the computational capability of the human brain. But I do not know if is possible to improve the optics part enough to make a big difference.

# The engines will generate the most visible energy when the ship is deaccelerating, but still may be too faint to be detected

Space is not so empty as people think. There is a lot of dust, asteroids and comets, stars and galaxies in the background. A dust closer to the telescope can be bigger than a star in another galaxy. To detect ship would be needed to differentiate the ship signal from these natural sources. That could be simple if the radiation generated by the engines was very different for most natural energy sources and probably will not be. Fusion drives will work with fusion (duh) like any star. Antimatter drives will generate gamma rays as many galaxies, neutron stars and black holes. Cosmic rays generate false signals in the detection devices.

A stealth destroyer may have thick armor to avoid internal radiation to escape, be black to not reflect much light, decelerate tangent to its target to minimize the luminosity of the engine pointed to the target, calibrate the engine to generate radiation closer to the background stars and line itself with a natural radiation source closer to its signature.

# A possible scenario

Let suppose a interstellar war. People from Proxima Centauri wants to invade Earth and both civilization have fusion drives, but not warp drives.

When the Proxima fleet comes to Earth they have to accelerate and the radiation of their fusion Drives will be pointed to the Proxima system which makes it undetectable from our solar system, but at some point the fleet have to decelerate. This means to burn the engines in the direction of our solar system for a few days, maybe even for a month. Their fleet will appear as very faint points. The light they emit will probably be comparable to a asteroid. Which is almost undetectable if the burn happens outside Pluto's orbit.

To better hide themselves they can make the bulk of the deceleration tangent to our solar system. That way they may reach much closer discretely between Mars and Saturn orbit. If they reach Pluto's orbit change their direction and make a 10G burn over an hour, they can reach Earth in 166 days. With that speed they need only one hour of deceleration. They can do the same trick as before, decelerating tangent to the Earth to minimize the chance of detection. Of course is a game of chance. A satellite in the right position can detect them fast.

It could be possible to use some space-time warping to appear in one position and almost immediately disappear to another, non-overlapping position, so that any human operators and many computer programs would miss you, since you're so fast.

It wouldn't work for low frame-rate technology, but advanced technology could be fooled like that. If the scanner is like a radar, with a beam that slowly changes direction to cover the whole area, then you could transport yourself as soon as you detected a scanning beam (assuming your sensor density is high enough) so that you would seem like a dot on the screen, fooling most light scanners (which also pick up stars).

If you want to avoid some kind of gravity/space-time scanner, then you might need a space-time field generator that countered all effects from the ship and whatever was inside it.

You could also somehow warp the light around your ship like military scientists are trying to do, so that it seems as if there's nothing where you are, but then you wouldn't be able to see out either.

There is also a possibility of creating a space 'pocket' or moving through a fourth spacial dimension, with a camouflaged scope (looking like debris, a dead body, etc.) being the only part of the ship sticking out in normal space so that you could see.

However, you couldn't move when somebody else was in sensor range, because objects moving like that doesn't exactly seem real. One-way-mirrors or video screens covering the whole ship would work, but you would still be visible to more observant people.

Hypotesis: Assuming the side of the ship you want to hide, is fighter like (20m long maximum while a battleship will be at least 400m), you could use modern warfare technics to hide fighter planes. I suppose too that scanning technics doesn't much evolve (IR scanning, and radar). I purpose methods just to hide not totally disapear. The engagement distance should be long too to avoid visual spotting.

Like a fighter plane: Fighter planes of today have several ways to avoid detection. I see two main way : flying a low altitude, to be in the radar's dead angle, and flying in close proximity of a larger civilian plane, the two planes, if close enough, have only one radar signature. So your ship can hide behind large stellar object if it knows the direction the scan come from, to place itself in the ennemy radar dead angle. It can hide too with a large civilian transport or cargo ship (and even land on it if space cargo is as large as today sea cargo).

Like a ship: Have you ever heard of spy trawler ? You basically disguise your military ship in a civilian ship, and you hope no one learn the truth. A cargo-like ship of the Kriegsmarine even manage to sink an Australian cruiser.

Like both plane and ship : In desperate case, when you're spotted, you've one last way to prevent the deadly blow: flares. It's a little device you throw to create heat and/or radar signature looking like yours. It will not hide you but it will confuse the ennemy targeting system, buying you time to escape.

This is an interesting question and the answer will always depend on what is the scenario. In most cases, using active sensing (e.g. radar, the beam you proposed, etc..) is essentially rendered redundant because passive sensing is usually enough to blow almost any cover...almost.

One of the easier ways of hiding is to be behind a massive object that's mass is at least 4-5 magnitudes larger than your spaceship, such as a planet. In a game of hide and seek in space, being behind a planet – or a star! – is a common way of staying undetected. However, staying behind an object like that means that your mobility is limited to your host (and your enemy). You also need to know where your opponent always is, which can also reveal yourself.

If you go high tech enough, your ship can be the planetoid itself. However, in solar system scale scenarios, significant undetectable movements could take millions of years. On the other hand, with a bit of planning and adjusting e.g. albedo or mass distribution, you could attempt to destroy planets quite stealthily.

If you want active stealth for a standard space ship, then I suppose you can design surface that looks like background radiation to one direction, while projecting heat to another direction rendering you invisible to specific direction. Obviously this would not work at close ranges against high tech opponent where they would most likely have detection capabilities from every direction.

Of course, all of the above scenarios assume that you're not radiating excessive amounts of detectable particles, such as neutrinos, which would be a dead giveaway even behind a planet.

In interstellar combat, there's really only one guaranteed way to be stealthy. That's by being fast. If you can accelerate fast enough to a speed that is very close to the speed of light relative to your target, you can take it by surprise. Down side: It's essentially a bombing run. I suppose you could take generational approach e.g. by riding an asteroid but hiding heat signatures is still complicated.