Stealth in Space

"Stealthiness in space was a bit like playing hide-and-seek while naked in a brier patch: it could be done, but it was a thorny business that depended upon the fact that nobody would believe you were crazy enough to actually try to pull it off."

- J. Daniel Sawyer, Free Will and Other Compulsions

Stealth, be it camouflage or outright invisibility, is often a deciding factor in many forms of combat. In fiction, this should apply to spaceship combat, too.

In reality, stealth in space is a lot harder than people think. Identification and reliable target lock may be hindered, but detection is much easier—if there is something other than dead stone, it will expose itself like a lighthouse.

The biggest problem for machines in space is heat. Most would assume that you can simply radiate excessive heat out into space, which is cold, but cooling-by-radiation is actually pretty inefficient. Plus, due to the laws of thermodynamics and conservation of energy, any attempt at moving heat energy will necessarily produce even more heat. A ship big enough to afford carrying extra expendable resources can transfer excess heat into coolant and then jettison it—but of course that coolant will glow even brighter, as it's warmer. Instead of jettisoning it, the ship could also store the excess heat in an insulated heat sink of some kind, but this has a finite time limit. Eventually your ship will have to radiate heat somewhere.

Why is this a problem? Simply because that radiation is detectable from a very long way off to anyone with infrared technology. This means that a typical spaceship with lots of machinery and electronics is going to glow like a beacon in the sky unless extraordinary measures are taken to contain its emissions. The complications rise rapidly should you wish humans to actually inhabit this ship, since you need a host of heat-generating systems to keep them alive, on top of having to fill your ship with reasonably warm gas (air). Asking this stealth ship to move creates even more problems. All propulsion methods involve generating lots of heat/energy that will radiate in a cone out into space behind you—easily traceable.

On top of all the problems involving infrared detection, there is also the rest of the EM spectrum to be concerned about. If you transmit a radio signal, that can be detected. If you wish to use radar or other active sensors to be sure you don't run into something or find an enemy, that can be detected. Stealth necessarily invokes a certain measure of "flying blind" to not give yourself away. As such you can only use passive sensors, though it should be noted this isn't as dangerous as it might sound as space is mostly empty. The opposing force that isn't trying to hide is completely free to use their active sensors, so you need to confound all their active sensors as well. Modern submarines operate under similar ideas and have for years, but in space, this is far more complicated. Earth environments tend to limit ranges and provide lots of interference compared to the vacuum of space. Perhaps worst of all, someone with a good enough telescope can just plain spot you visually. Many modern satellites are visible from the ground on a clear night with just the naked eye as little moving points of light.

Being visible to the eye can be dealt with if the concept involves an Invisibility Cloak, but expanding it to spaceship size will not eliminate its own problems, covered in more detail at that trope. The whole point of an Invisibility Cloak is to deflect EM radiation away from your ship, meaning your passive sensors won't be receiving anything, be they your sensor beams, his, radio telemetry from your non-invisible pals, or even just ambient radiation from stars. Long story short, if the Invisibility Cloak is working right and/or according to the laws of physics, he can't see you, but you can't see anything.

How do you deal with this? Most writers prefer not to bother. They either Hand Wave the whole business, pretending that ships in space are like submarines in the ocean and thus naturally stealthy, or rely on some heavy-duty Applied Phlebotinum to make thermodynamics go away. These problems can be circumvented to a certain extent by manipulating your setting's Technology Levels, which don't need to conform to modern technology; for example, a period-accurate Raygun Gothic story set in the retro-future would have rocketry and even nuclear atomic power, but not the sort of high-resolution thermographic cameras and high-capacity visual processing computer power necessary to detect and analyze a cloaked ship the way we could today. It's not common for a show that is high on the Mohs Scale of Sci Fi Hardness to have vaguely plausible stealth, but there are some.

It should be noted that while true stealth is impossible in completely hard sci-fi (read: Real Life), merely detecting IR emissions and visible light reflections does not necessarily equate to any form of useful data. Just because your opponent can see you does not mean they will, or be close enough to do anything about it when they do.

At a large-enough distance, the resolution for your optical and IR telescopes is likely to be so poor that all you see is a dot—which could be anything from a large starship to a decoy emitter drone. You might be able to shield your heat emissions by being near a hot-enough celestial body as well (IE having the sun at your back), though then there's still the problem of masking your transit to that location. In addition, consider that your image can only travel to their sensors at the speed of light: if your path is sufficiently far away, the enemy will detect your presence only after a period of time has passed, and by then all they can do is extrapolate your course and try to predict where you'll be. Naturally, this is not the same thing as knowing where you actually are. In an environment with Faster-Than-Light Travel this becomes even easier to do as you can actually outrun your image; doing so is the basis of the "Picard Maneuver" seen in Star Trek: The Next Generation, one of the few places it was exploited.

Stealth can also mean disguise, camouflaging your warship as a mining scow or delivery ship in a populated star system. It's also possible to play terrain games: you can always hide behind something big, if you have the proper velocity to hold that relative position. Or for a big enough object one could land on or even inside it.

Anime and Manga

 * Starship Operators has a plausible example. The stealth ship moves by initial speed before the jump, and runs with most systems off to reduce emission. Having said that, it takes so long to close to engagement range that the Amaterasu's crew is able to find and pre-emptively destroy it.
 * And the way it is found is by
 * UFO Robo Grendizer: Big Bad King Vega's tactic during his army's offensive final depended on being stealth: He cloaked his personal Starship with a kind of reflective shield, hoping their squads of mini-ufos kept the heroes busy and distracted enough to not notice him as he descended to Earth. Of course,
 * The Shangri La in Toward the Terra can become completely invisible to any form of detection, although since it's powered more or less completely by its occupants' thoughts, the problem of engine heat probably doesn't apply. The heat of those occupants still applies.
 * Since they're using Psychic Powers, though, the physics don't apply on the other end, either—they just prevented people from noticing their ship using Mind Control and directly disrupted enemy instruments using their powers, which doesn't require actually masking their heat.
 * Oldina's ship in  Figure17: Tsubasa & Hikaru , which orbits Earth for the better part of a year, avoids detection with some sort of cloaking device despite at one point firing a gigantic laser cannon at Hokkaido while trying to blow up an alien.
 * Part of the cloaking technology, or a technology used in conjunction with it, throws up an illusion of normality over a location until it can be cleaned up, along with a with a field that makes people just... disregard it, and assume there's no need to investigate further. Thus, the crash site of DD's shuttle is never discovered when a policeman goes to investigate, and the abortive attempt at using Oldeena's ship's Wave Motion Gun as a Kill Sat is considered nothing more than a particularly weird, atmospheric light show after attempts to investigate turn up nothing.
 * Robotech features Shadow Devices, which prevent long range detection by protoculture emissions, but do nothing in visual range.
 * The Macross universe features passive stealth systems (design shape and materials to absorb and scatter sensors away from the enemy) in the VF-17S Nightmare from Macross Plus and Macross 7. However, in Macross Plus, the VF-19X and the VF-22X both feature an "Active Stealth System", which bends electromagnetic waves around the fighter, effectively making it invisible to all but optical targeting systems. The VF-25 in Macross Frontier does not appear to have this system, but Macross Frontier is a little squicky on the technical details of their mecha.
 * Keeping in mind that Valkyries are Transatmospheric Superiority Fighters, meaning they're designed for combat in atmospheric conditions as well as in space. In fact, the one time the 'Active Stealth' system is specifically demonstrated in Macross Plus, the YF-22 is well within the atmosphere of Planet Eden.
 * In Dragonball Z, an alien ship keeps itself hidden by being mirror coated. So very wrong.
 * Gundam gets off somewhat easy by Minovsky Particles that scramble electromagnetic sensors, reducing detection to heat sensors and the old Mk. I Eyeball. It doesn't have too much in the way of actual stealth, but it is possible to sneak very close to an enemy if you know what you're doing.
 * Gundam Seed and Gundam Seed Destiny have radar-jamming Phlebotinum (N-Jammers rather than Minovsky particles), but also has the Mirage Colloid system, which uses microscopic prisms contained in a magnetic field to bend light around the ship/mobile suit, rendering it invisible to the naked eye and radar. Heat coming from the life support systems is contained or redirected by the field, but if the ship or mobile suit fires its thrusters, the hot exhaust trail will give it away.
 * The Girty Lue class assault carrier (the first of which was captained by Neo Roanoke) gets around this problem by equipping both Mirage Colloid technology, and a stealth propulsion system that uses jets of pressurised gas to move the ship without emitting a thermal signature.
 * Gundam Wing has Deathscythe, whose stealth systems include radar jammers and (for the Mid-Season Upgrade) optical camouflage. It might also get a leg up thanks to its use of Gundanium for armor, which is stated to be electrically neutral, highly heat-resistant and radar-dampening.
 * Gundam 00 has GN particles, which at the very least have been shown to frizz all conventional sensors in a large area (rather than actually preventing detection). This is effective against both new and old enemies as few if any designs allow the pilots to use their Mk. I eyeballs and rely on CGI-augmented cameras instead. While they do gain outlandish properties in the second season, the particles haven't been shown making objects invisible.
 * A particularly iconic example is the Throne Drei 'stealth' unit, which releases a tremendous and visually striking cloud of bright red false GN particles to conceal its allies. Much like with some forms of modern ECM, this makes it obvious that enemies are present but leaves you with no idea exactly where or how many there are.
 * Martian Successor Nadesico had an interesting version of this. In order to bait a Jovian ship, Yurika used... fishing tactics. Which meant shutting off the Nadesico completely and having the Aestivalises manually launch and just sit and wait. Using pressurized gas to move just enough to throw the Jovians off, they waited until they got close and got the drop on them. The Jovians were actually impressed by their captain and wished him the best... which was an insult to Yurika.

Film

 * Kirk manages to avoid detection by Khan in Star Trek II: The Wrath of Khan by directing the Enterprise into a nearby nebula which was flooded with sensor-jamming electromagnetic radiation and severely limited visibility.
 * In The Last Starfighter, the Gunstar does not have stealth but avoids detection.
 * In Star Trek VI: The Undiscovered Country, the Enterprise rigs a torpedo to follow a Klingon Bird-of-Prey's exhaust trail, seeking it out despite its being cloaked.
 * In the novelization at least, this is explained as that particular ship's schtick (firing while cloaked) requiring a massive power plant, which exceeded the cloak's ability to hide/contain the exhaust.
 * In Wing Commander, the preferred method of stealth typically involved staying close to other objects (asteroids, bigger ships, etc.) in hopes that it would prevent the enemy sensors from getting a clean look at them. At one point in the movie, an entire fleet hides by staying in low orbit over a planet. The effectiveness of the strategy typically depended on how close the enemy got.

Literature
"Jaina's Astromech: "A STEALTHX EMITTING COMM WAVES IS NO LONGER A STEALTHX. IT IS JUST A POORLY ARMED, LIGHTLY ARMORED X-WING SAYING COME GET ME.""
 * Alastair Reynolds' Revelation Space series has has a model of computer that cools down when it computes, explicitly in violation of the second law of thermodynamics. It gets used by spaceships to stay at background-radiation temperature, and is the way most of the Invisible Aliens stay invisible. Since entropy and computation are both information-theoretic-ish things, this almost makes sense if you don't think about it too hard. Early versions can break/overload though, performing calculations so fast that the entire ship begins to drop down to absolute zero; including the life support systems and cabin
 * ...a computer that cools as it computes? That is, LITERALLY, Fridge Logic.
 * Before the magical cooling computers are perfected, there's a scene that almost perfectly delineates the extent to which stealth in space is possible. The protagonist is in a small, maneuverable ship. Its thrust is a reasonably-tightly-collimated photon beam, which therefore can't be observed unless you're almost directly aft of the ship. He's being pursued into a solar system rather than out, making it more difficult to notice the heat from his life-support systems. The system has a number of other ships, and his pursuers don't have governmental control over the system so they can't necessarily track them all. All of this means that he can stay hidden for roughly a day, after which his only recourse is to blow up the ship, fake his own death, and trust that someone will come pick him up; even that isn't really good enough as his pursuers can eventually analyze the wreckage for traces of human particles, but it buys him enough time to get to a major enough population center that he can disappear for real.
 * CJ Cherryh's Alliance Union universe has an interesting take on stealth: anything in Hyperspace is totally invisible (or is at least going faster than anything coming from it, so you only know it's there after it's gone) and is going so fast when it drops out that it doesn't really matter, so one tactic that is mentioned as something even the Kif won't use due to MAD is to drop out near a target and drop off a large bomb as you go screaming through the system before going back into hyperspace. Of course, this is more of a problem when dealing with the species that can stop instantly out of hyperspace and are unable to understand the concept of traffic laws.
 * It's also played straight in the Chanur Saga, where the main character's starship went quiet while floating through an Asteroid Thicket in order to hide from kifish hunterts, and spy ships can sit invisible at the edge of a star system while gathering information on passive scanners.
 * Star Wars is pretty true to Real Life when it comes to this stuff. It has cloaking devices that generate a field around the ship that bends light, sensor beams, and (it would be imagined) other radiant energy around the ship... but just as no one can see in, no one can see out, so they're rarely used. Thrawn got around the two-way cloaking problem by using the cloaking field to hide booby traps within an uncloaked, innocent-looking freighter. The Republic could see that the freighters were carrying something by measuring their mass, but they didn't know what until the trap was sprung. Later, he got around the no-communication thing by means of careful timing and Joruus C'baoth's Battle Meditation.
 * In a lower-tech example, at one point, Talon Karrde hides from Thrawn by hiding behind an asteroid, on the understanding that if they so much as turn on their engines while the Star Destroyer is still there, they will be noticed. Of course, Thrawn already knew he was there given his psychological profile, but he was not aware of the presence of Mara Jade and her Force-sensitivity ...
 * Admiral Ackbar also notes that any cloaked ship under power could be tracked by its emissions. This is later tried, and only fails because it is attempted on a cloaked asteroid.
 * The sequel duology starts with Supreme Commander Pellaeon testing a computer that's designed to read enemy movements and predict what they'll do next, the idea being that the computer would get hooked up to the turbolasers and they could finally get some use out of cloaking. Unfortunately, though not at all to his surprise, the Predictor isn't as good as advertised, though later he is able to use it to determine something about attackers.
 * Later in the duology a cloaked Star Destroyer lashes onto a comet and rides it to be in position near Bothawui, planning to wait until both sides of the conflict had weakened each other so they would be easy pickings. It's mentioned that the isolation and lack of outside light or input is driving the crew a little bit crazy. The debris and emissions discouraged anyone from taking a close look. They still ended up being spotted, though admittedly that was a fluke.
 * There's also Nightcloak technology, which draws in all light into its satellites... which results in a not-at-all-stealthy chunk of missing starscape. It has a very few, very complicated uses.
 * Like planetary deep freeze and a complete biological holocaust.
 * In general, altogether more effort in the Star Wars verse is put into disguising a warship as something else, rather than hiding it. Warships, especially pocket carriers, disguised as freighters are quite common. Fiddling with IFF projectors and specific gravity (to fool dovin basals) also comes up regularly. On the less... orthodox end of the scale, one favorite X-Wing tactic is to wait for something to drop into atmo - a meteor shower, say, or debris from a destroyed ship - affix ablative plates to the fighters (which plates would burn up on entry), and drop in alongside the perfectly innocent debris, only pulling up underneath effective sensor altitudes.
 * "Stealth-field generators" in Knights of the Old Republic appear to use chameleonic techniques rather than outright invisibility (and no one has tried using them out of atmosphere, at least in canon), but Goto's Yacht still has some explaining to do... (granted with all the traffic at Nar Shadda, hiding in plain sight isn't hard)
 * It probably had more to do with G0-T0's mass robot controller. Considering how robots are essentially just a different type of computer system, it was most likely something along the lines of "I will control your ships' computers so that they don't report that they detect me, and I will camouflage myself so that you don't see me with your own eyes." When
 * Thrawn probably also deployed smaller ships to flit in and out of the cloaking fields at one point, so that the Star Destroyers wouldn't have to expose themselves. He may also have used small cloaked drones, basically a turbolaser with an engine, to simulate Star Destroyers firing through planetary defense shields, by placing them so that it appeared the turbolaser beam fired straight through when in actuality the beam impacted on the shield then the cloaked drones fired from their meticously planned positions.
 * In The Empire Strikes Back Han managed to be stealthy in a low-tech manner. He docked to the back of a capital ship, where his hull and heat signature would easily be missed. When they jettisoned waste, which would have residual heat from being onboard, he drifted away in it under low power. Boba was not fooled.
 * One of the Imperial officers notes that it should be impossible for them to have just disappeared, as "no ship that size has a cloaking device". While not technically true as later Expanded Universe and Star Wars: The Clone Wars material would show, cloaking field generators that could be mounted on a Millennium Falcon-sized ship were extremely rare and highly classified.
 * In Legacy of the Force and Fate of the Jedi Book series, Jedi regularly use Stealth-X fighters. These use things like emissions discipline as opposed to "Cloaking Devices", and are used more like Real Life stealth planes. In Allies, this is Discussed:


 * In case it wasn't obvious, this works well, because the Jedi pilots of the Stealth-X fighters have their own built-in and incredibly effective set of sensors, thus avoiding the "flying blind" problem noted above for craft attempting to be stealthy by not using active sensors.
 * Perry Rhodan: just to be confusing, its cloaking devices are known as 'deflector fields'. Frakking Germans.
 * Specifically, deflector fields mostly just warp visible light to provide invisibility. Their inability to handle other wavelengths in the EM spectrum has been used as a plot point in the past, and they're generally useless against advanced sensors—in fact, the field's emissions can be detected in turn. That said, starship stealth in that universe still seems to be mostly a matter of shutting down any systems that generate 'noise' and hoping the ship itself doesn't get pinged by an active scan.
 * Just change infrared with Hyperenergy radiation. Most space faring civilization in the perryverse can detect five dimensional wavelength.
 * Another method for ships to cloak is to send them 5 seconds into the future. You cannot detect something that is not here yet. It even works with whole planets, as done with earth itself.
 * The tsunami division uses this concept but it needs two ships to be effective, the one hiding under the CTF field is blind.
 * Arthur C. Clarke's The Last Theorem subverted and lampshaded this trope. The Grand Galactics ordered One Point Five's armada to immediately slow down, and to remain undetected from . They immediately decelerate, but wonder how are they suppose to keep themself hidden if gigajoules of energy were poured out from 154 spaceships at once. They, of course, then get detected.
 * Clarke also used another space-stealth rationale in his short story "Superiority". A space-warping field literally put light-years of extra distance between the generating ship and the rest of the universe, thus putting it far beyond detection range. Of course, this worked both ways, and the device turned out to have other problems as well.
 * In the Honor Harrington books, this is a key element of the space combat. All ships rely on gravity-themed Applied Phlebotinum for propulsion, but gravity drives can be detected for a great distance, regardless of lightspeed limitations. However, a ship with the gravity wedge turned off is essentially invisible, and the ships also have stealth systems which dampen or alter the signature of an active drive. The series' surprising adherence to certain elements of physics means that, in at least one interesting example, a small, patchwork force took out one far greater numerically without even being fired upon simply by using chemical thrusters to get moving and then opening up with all the weapons at point-blank range against an opponent who had no clue they were even physically in the system.
 * In a later installment Dame Honor confronts a hero-worshiping cadet attempting to praise her ingenuity for the aforementioned gambit by explaining just how stupid and desperate the tactic was. All Honorverse ships include numerous conventional sensors, radar, lidar, passive EM and IR detectors of all kinds; any one of which could have seen Honor coming from light-minutes away. Her trick worked simply because her enemy's sensor-techs were too lazy and trusting to check anything but their gravitic sensors, and apparently they had no automated monitoring systems.
 * Of course, the reason they use gravitic sensors is that gravity sensors are FTL as they work in the universe, and all other sensors are speed-of-light. Not to mention the distances involved are usually several AU, which is rather extreme to expect any other sensors to work in time to be useful in combat. She just closed close enough to fire on them before they noticed she existed, which resulted in their instant destruction because ships have to 'roll' to protect themselves or people can shoot straight down their unprotectable weak spot between their gravity wedges (which also means that Honor's ship was also completely unprotected, because she had them off).
 * Ships in the Honorverse also make extensive use of sensor jamming and decoys (more broadly known as Electronic Warfare). While this is the opposite of stealth in literal terms, it has most of the same effect—it's obvious you're out there somewhere, but since space is big, if an enemy doesn't know exactly where you are, they can't hit you.
 * Averted in Mission of Honor, where a small number of extremely stealthy ships using a unique drive system are deployed. One of them has a near miss with some unexpected enemy warships, during which the captain is praying that noone gets into a position to see his far side, from which all the heat is being dumped.
 * Kimball Kinnison acquires a stealth space speedster in the Lensman series. It's stealthy by dint of being visually undetectable apart from obscuring the odd star thanks to being painted with near-perfectly-absorbent black paint, being made with no iron alloys to avoid magnetic detection, and having a "detector nullifier" to scramble his universe's equivalent of radar (and presumably IR detection as well).
 * There's also constant reference to 'baffles' being used to reduce engine signature (and the consequences it has for a ship's performance and the time needed to get home).
 * In the Night's Dawn series, stealth can be achieved by dumping your heat away from the enemy and staying out of visual range. It won't work if the enemy has sensors on more than one side of you, so it's only used for tailing suspected smugglers.
 * Ships attempting stealth tracking of smugglers also get coated in a special foam which holds in any heat the ship might produce. Obviously, this means that most operations against smugglers have to be short-term missions, otherwise the crew would be cooked inside the ship.
 * In "The Nano Flower" by the same author, a scientist in a Twenty Minutes Into the Future world is asked whether an alien could have made First Contact with humanity yet. He responds that there's no possible way an alien vessel could have arrived in the solar system without being detected. As it turns out, the alien actually evolved within the solar system.
 * The Imperium's attempts at stealth in space for the Warhammer 40,000 universe is touched on in the novel Cain's Last Stand. Lacking the technosorcery of the Tau or Necrons, the Imperium uses Q-ships which are merchantmen or barges (the novel uses an ore barge) refitted to performance levels of military-grade craft of a similar class. Given that an ore barge is the size of a light cruiser....
 * In the Halo Expanded Universe novels, there are "Prowler" stealth craft that minimize their emissions and dump their nukes before returning to realspace, so as to avoid the Cerenkov radiation giving their reentry away. They're not so much cloaked as just hard to detect.
 * Broken Angels by Richard Morgan. A UN force fighting a planetary war puts its most important assets on space platforms in far-flung elliptical orbits so the enemy can't find them. Space, as the protagonist points out, is very big.
 * In So Long And Thanks For All The Fish, Ford Prefect refers to the possibility of "clos[ing] down all external signalling and radiation from the ship, to render it as nearly invisible as possible unless you were actually looking at it". To be fair, this is an empty ship, so he really could shut everything down without worrying about life-support or steering.
 * And then we consider the possibilities of the Somebody Else's Problem field, which could fool the perceptions of the people operating the sensors (and maybe even automates if Douglas Adams' usual sentient robot theory applies).
 * Tomorrow War has "X-Cruisers", essentially space submarines. Instead of passing through hyperspace X-matrix into normal space again they can "hang" on the border, so they aren't here for locators, but close enough to sniff out ships with mass-detectors. The good news is that they drop out of this state slowly enough to prevent micro-telefrags through the whole volume which plague normal jumps, up to popping up in the atmosphere, but can emerge, fire and "dive" back fast enough to prevent a strong retaliation. The bad news is that this mode continuously wastes FTL fuel, about the most expensive matter known. Also, they can't move fast there and even if they re-emerge, they aren't going to accelerate well, because engines that keep the ship a proper 3D object where it shouldn't be take up to 9/10 of the ship's volume, so the normal equipment, even propulsion and defense, is severely limited.
 * In the Void Trilogy by Peter F. Hamilton, almost all spaceships basically hide in hyperspace, or manipulate of quantum states.The more advanced the technology, the better they are at hiding. This makes the Void Trilogy significantly softer than the Commonwealth Saga, where there is almost no stealth.
 * In Anathem there is a a rare hard science fiction example without hyperspace or phlebotinum. It requires doing all the assembly on the other side of a planet from the ship they are sneaking up on. Then orbiting around hidden behind the 'cold black mirror' and hoping that the ship doesn't notice the reflection of other stars. It also involves decoys to simulate a catastrophic mission failure so they believe that those sneaking up on them are dead.
 * The in Jack Campbell's hard-sf The Lost Fleet have superb stealth capabilities: their ships can become completely undetectable at will, and even if they're visible, no one can get a detailed look at them. This is because.
 * In John Hemry's A Just Determination, there is a great deal on the devices used to keep ships more or less invisible in space. Both messages and changing course are avoided for the chances they offer for detection.

Live-Action TV

 * Star Trek has cloaking devices, mostly used by the Romulans and Klingons. They can be detected by someone who suspects they're there and has good enough sensors, though.
 * The original example, from "Balance of Terror", was essentially so the show could do a wartime submarine drama In Space, with the cloak substituting for submersion. The source movie is The Enemy Below(1957) and during the initial contact the dialogue between the ship's captain and the sonar man is almost identical to the spaceship allegory, including the initial bearing of the shadowy contact, and the ship's course change to determine whether the contact is genuine or a sensor malfunction.
 * In Star Trek: Enterprise, the Suliban have a different cloaking technology which seems to be particle/radiation-based, as when Trip is trying to reverse engineer one, he ends up accidentally triggering it and rendering his hand temporarily invisible.
 * In the Star Trek: The Next Generation episode "The Emissary", an old Klingon warship awakens and cloaks to attack. However, as a case of Technology Marches On, in the 70 years since the warship was constructed, the Federation's sensors have advanced so they can detect the old warship.
 * In Farscape they frequently reference ships flying on a "Stealth Vector", presumably flying on the other side of planets and other large objects as often as possible. It rarely, if ever, works.
 * The new Battlestar Galactica has the Blackbird, which use carbon composite plating to avoid DRADIS detection, and is painted black to reduce the sunlight deflected. Both it and Stealthstar (different craft) got destroyed pretty quickly, though.
 * In the miniseries Boomer escapes detection by the Cylons simply by turning off her Raptor's engine until they reach Caprica. Of course, Raptors also have their own Electronic Countermeasures equipment.
 * Possibly justified, in that the area they are flying through was recently the site of a LARGE battle, and there would be lots of ships and fragments of ships emitting all kinds of waste energy. Tough to tell one glowing piece of space junk from another.
 * The Blackbird's actually not too bad, comparatively. It was only stealth when everything was shut down, so it had to coast on the scouting run it made.
 * In Babylon 5, the Minbari have their own take on this trope by having advanced jamming technology that allows them to play havoc with enemy sensors, making it impossible for them to get effective lock-ons on their ships. Although this makes it easy to know that there are Minbari present in the area, this technology is treated as effective stealth since locating the exact positions of ships—to say nothing about aiming weapons at them in a combat scenario—is literally impossible without sensor data.
 * One wonders why Earth Alliance hadn't just used visual targeting, though.
 * Sinclair did. You need to be close though...
 * Space is big. Really, really big.
 * As stated in one of the books, "The Shadow Within" Earth Alliance warships do have manual targeting options for close work. In "A Late Delivery from Avalon" we see a gunner manually firing the guns that damaged the Grey Council's ship.
 * Compounding their problems was the fact that the Minbari gravimetric drive systems were much faster and more maneuverable, making them very hard to track manually, and that the Minbari main weapons could essentially one-shot even the biggest earth ships. So they rarely got enough time to acquire visual targeting. The Minbari were also fully aware of their advantage and used various means of blinding Earth Alliance pilots, by attacking with the sun/star behind them, or the glare of a jump engine, etc.
 * The difficulty of stealth in space is acknowledged in the pilot episode of Firefly when the crew's attempt to avoid detection from the Alliance by hiding among the wreckage of a larger ship is foiled by the heat coming off their ship. They only get away by the use of a "cry-baby" - a device rigged to send off the signal of a freighter full of passengers that needed help, which they knew (hoped) the Alliance would go help rather than them.
 * In a later episode, the crew does manage to sneak up on a space station by flying toward it with their engines off and systems powered down, and pumping out electromagnetic interference to disguise the ship as a radar glitch. This wasn't a military station, so presumably did not have the full array of advanced sensors an Alliance warship would have.
 * The series favors misdirection over outright stealth. In The Movie, Serenity slips past Alliance ships twice in one go; the first time, they descend toward a planet, presumably losing themselves in the traffic above it, but the Alliance follows their "pulse beacon" and locks onto it with a missile....only for Mal to reveal that he'd removed said pulse beacon and was carrying it with him to keep the Alliance ship from launching a missile at Serenity. Later, as they escape the planet and slip out in the orbital traffic again, the Operative tries to track them via their navigation satellite's trajectory, as Serenity is a registered transport, only to discover seven different nav-sat trajectories belonging to Serenity, six of them being decoys (we were treated to six barrels with blinking lights and thrusters being jettisoned from Serenity as it broke orbit).
 * Shows up again later, but in a not-so-obvious manner. The crew attempts to approach Miranda, but in order to get there, they have to pass through a Reaver fleet. Since they're disguised as Reavers, they can attempt to pass through. The not-so-obvious part comes from the fact that they can't go around the Reaver fleet; since there's no Stealth in Space, any attempt to go around the Reaver fleet would be spotted, and be taken as a surefire sign that they're trying to avoid the Reaver fleet.
 * When coming off of Miranda, they do the same thing...only once they're through, they shoot at the Reavers, eliciting the response they want...an all out dash to Mr. Universe's planet. Said planet has a whole mess of EM interference, so much so that the Alliance fleet gathered to head Serenity off doesn't know it's being chased by Reavers until the first Reaver ship emerges.
 * The RPG states that it is possible to hide, but reliable heat-masking requires you to be hiding in the flame from someone else's engines. This requires two things: an insanely good pilot, and a degree of regard for one's personal safety that makes mere Suicidal Overconfidence look cowardly.
 * Shown in "Objects In Space", where Jubal Early was able to sneak up on Serenity by hiding in their heat wake. Wash noticed something funny about the sensor readings, but he dismissed them, probably for that exact reason.
 * In an early episode, the Serenity crew stumble across a derelict spacecraft, not actually detecting it until it's nearly close enough to be seen out the window with the naked eye. While a derelict would be "cold" to thermal-infrared sensors, such an object would still reflect light from the local sun and would still show up on radar sweeps. Perhaps they knew something was there, but guessed that it was a small asteroid until they got close enough to see surface details.
 * In that situation, though, it's because nobody on the ship was paying attention—presumably if Wash was in the pilot's seat instead of playing...whatever game that was, he'd have seen it sooner. The proximity alert which still notified them in plenty of time to avoid a collision.
 * The ''Stargate Verse has all kinds of cloak generators. For example, the Sodan personal cloak is said to shift the wearer out-of-phase with this reality. Some Goa'uld ships can also cloak themselves but they can't hide reentry heat. Asgard ships can mask their presence from just about everything except visual detection. The kings of this field were obviously the Tau'ri who figured out how to invert Ancient shields, exchanging protection for total undetectability.
 * The ''Stargate Verse has all kinds of cloak generators. For example, the Sodan personal cloak is said to shift the wearer out-of-phase with this reality. Some Goa'uld ships can also cloak themselves but they can't hide reentry heat. Asgard ships can mask their presence from just about everything except visual detection. The kings of this field were obviously the Tau'ri who figured out how to invert Ancient shields, exchanging protection for total undetectability.

Tabletop Games
"So which one is you, Joey, and which are the aluminum balloons? (Seven dots grew on the screen, all had slightly different vectors.) Now you know my heater can take you in one flash and you also know that one zap is all I'm going to get. And if I take it you've got a perfect excuse to blow me up for the honor of the company rather than recapture valuable property for the accountants. So what's it going to be? I think you shot off too many balloons too early Joey -- cause the other ones aren't making the course correction you just did. Ain't that you, Joe? Ulans squinted and tapped his foot. (firing guns)"
 * In Traveller Tabletop RPG, it is impossible hide a ship. There's always a blip. That blip, however, might be a bathtub with engines or a fleet of warships. Electronic masking can make ships stealthy, and the rules address the thruster issue. The trope kicks in however with sensors, stolen wholly from the submarine warfare playbook.
 * Also of note is the black globe generator. Looking to make Deflector Shields "realistic," what they came up with was barrier that absorbs all energy. Hence, no traces and total stealth. However, it also means the ship is surrounded in a field of darkness, so while you couldn't see the ship, the mysterious black sphere would occlude the stars behind it, so it could be detected by sight, if you knew what to look for.
 * Eldar ships in Battlefleet Gothic use a variant of this called a holofield. Rather than try to completely hide the ship, it makes it almost impossible to pinpoint. You know it's there, but only its general area. The most effective method of attack is straight up saturation bombing.
 * The other fun thing about the Eldar is that they use solar sails for propulsion. This effectively eliminates the problem of detection by exhaust.
 * And replaces it with the problem of being a big, glittery mirror.
 * And Necron ships are stealthy, due to being crewed by cold metallic creatures, and having inertialess drives.
 * Not to mention being made of a material that is able to instantly react to outside effects to absorb / deflect them, which may include sensor sweeps. On the other hand since Necron technology remains very obscure, it's possible that their vessels are radiating all sorts of exotic particles that no one can detect (or knows to look for).
 * Explicitly averted in Ad Astra Games' Attack Vector: Tactical, the first chapter of the rulebook of which is available here. Ships are visible long before they reach effective guns range. In other Ad Astra titles, stealth in space is allowed, in large part because they're not aiming for as hard science an impression.
 * Hiding a starship in GURPS is essentially impossible without superscience, instead you have to rely on being so far away that sensor locks are nearly impossible.
 * But making a spaceship harder to find is quite possible without superscience.
 * But making a starship requires superscience anyway so what are we complaining about?
 * Redmond Simonsen's Battlefleet Mars gave us this little gem:


 * Star Fleet Battles. Being based on Star Trek, Romulans vessels have this, of course.

Video Games

 * Homeworld has cloaked ships, and the second game has distortion probes. Cloaked ships can only be spotted via proximity sensors unless they reveal their position by opening fire.
 * The Kadeshi used a rather clever form of stealth: instead of modifying their ships, they set up camp in a nebula where the massive amounts of gas and energy obscured their ships from sensors. By the time the victims got a visual on the dozens of Swarmers bearing down on them, it was already too late.
 * Mass Effect has a prototype system on the SSV Normandy that works by storing the waste heat into its internal heat sink, which can only be done so for a limited period of time, or else the personnel will be cooked. The ship's own mass effect drive is then used to create mass effect fields that the ship "falls into", thus propelling it without needing to generate engine emissions. The ship is thus rendered "heatless" and invisible to scanners, but still visible to anyone peering out a window, and because the stealth system is extremely new and extremely secret, no one has yet developed countermeasures against it.
 * However, the stealth systems only work while not moving at FTL speeds since any energy passing through the ME field automatically gets blue-shifted. A sudden flare of blue-shifted energy indicates the presence of an FTL mass effect field.
 * At the beginning Mass Effect 2 however,  Note: This is all in the opening movie.
 * The stealth is put to use during 's loyalty mission. Since the target station has no windows (structural liability), the Normandy can dock without opposition.
 * The Normandy SR2's stealth system is put to use any time Our Heroes are going to interstellar locales that enemy forces don't want them to be; Geth space, Rogue Terminus states, Batarian space, you name it.
 * As of Mass Effect 2 the Systems Alliance has built several more stealth frigates based on the Normandy's design. An SA strike team actually uses them to destroy a Batarian solar mirror array that can be used as a WMD if Shepard  at the end of the first game.
 * As of Mass Effect 3, the Normandy has been improved to the point where it can mask the arrival in a system via FTL with it. This allows the ship to move unhindered through Reaper-controlled systems, though scanning can still alert them.
 * The same aversion can be seen in I-War 2: Edge of Chaos video game. There are special "stealthy" precooled internal heatsinks used to reduce the ship's IR profile, making it more difficult to detect. You can also reduce your ship's visibility by switching off non-essential systems.
 * StarCraft has cloaking that renders units invisible to the naked eye and to non-specialized sensors. The Terran Wraith and Ghost can cloak temporarily, while the Protoss Observer and Dark Templar are permanently cloaked (they can cloak indefinitely, so they don't bother to put an off-switch into the game). The Protoss Arbiter can cloak other units, but not itself; this tends to strain Willing Suspension of Disbelief - who the heck would make a unit that can't cloak itself? Yeah, yeah, gameplay mechanics, we know - but still. Also, almost all Zerg ground units can burrow underground (even if that ground is the hull of a space station), which has the effect of immobile cloaking. The Protoss explain cloaking with psionics and reality warping. The specifics of Wraith and Ghost cloaking are never fully explained.
 * Actually as I recall it's "All There in the Manual" as it's explained that what the Arbiter does is just creates a reality warping field that makes their allies invisible, but to make such a big field it has to anchor itself in space-time making it immune to the fields effect.
 * The sequel offers an explanation on the Zerg. Part of their uber-evolution included an ability to vibrate their bodies at incredibly high-speeds, allowing them to quickly dig through any surface. While this would work for loose terrains, it doesn't explain hiding their burrows when digging into rock or structures.
 * Probably because they could hide under loose rocks or plating (or rip it up and hide), but making that animation that's only for very specific tilesets would likely be a waste of time and processing power.
 * Wing Commander 2's plot was based around the main character being demoted for supposedly lying about the Kilrathi having cloaking devices. Of course, halfway through the game stealthed fighters are all over the place wrecking Earth's shit. You still don't get your old job back, though, thanks to a traitor destroying all the evidence.
 * The third game (And the film) introduce the Skipper Missile, a powerful, very annoying and thankfully rare anti-capital ship weapon. It travels under cloak, only becoming visible for a few seconds to check and correct its course before cloaking again, causing it to 'skip' in and out of sensor detection. This is actually a plausible way of using a cloaking device given the assumption that the technical issues behind making one have been solved.
 * Freelancer has stealth ships, mostly used by Rhineland.
 * To be fair, a lot of people think cloaking tech is just a myth - it's been experimented with but no faction managed to design it on their own.
 * Which is funny when you remember that, in the prequel Starlancer, the Coalition has Basilisks flying around everywhere. The game's "Attack on Pearl Harbor" is carried out exclusively with them (of course, the premise of Freelancer is that the nations in the Coalition didn't go with them to Sirius, so there is a Hand Wave, but...)
 * It's mentioned in game dialog that cloaking is perfectly possible, it just requires way too much power ("the power of 10 battleship cores"). Presumably the coalition came up with some way to cut that down, possibly the same way as the
 * It's also possible that the old Coalition cloaking method is simply obsolete with more modern sensors capable of seeing right through it. Technology Marches On quite a bit in 800 years.
 * EVE Online has cloaking as an ability. Pretty much all ships can do it with the right equipment (although it cripples the combat abilities of everything but specialized covert ops and recon vessels). It is the subject of much controversy in the community, as it is widely used by pirates and carebears alike.
 * Even without cloaking modules a ship can be immune to combat probes if fitted with the right modules. The ship still shows up in the probe scans, but the actual location is impossible to pinpoint.
 * Not to mention that while the cloaks don't cripple the Covert Ops and Recon classes of ships, you still can't lock, fire, or really do anything but move while under cloak.
 * Star FOX Assault: The first mission features Stealth Fighters... which take too long to fire to act on the surprise element.
 * Technically, they aren't fighters. In the pre-mission cutscene, they were depicted as being used as long-range missile launchers, and may have also been attacking from the flank. They seem purposed to attack the Cornerian cruisers instead of smaller, interceptor sized craft.
 * Averted in Spore: you can't use the stealth tool while outside of a planet's atmosphere.
 * The manual for Frontier: Elite 2 advised switching off one's engines, as the trail of bright blue plasma made one's ship relatively easy to eyeball at a distance. Like much of the manual's advice it was pure fiction (the game had trivial AI).
 * Averted in Civilization 2: Test of Time - the most advanced stealth unit can move unseen through the oceans of an Earth-like world, the dust oceans of a Mars-type planet and even the clouds of a gas giant, but not through the fourth map, which uses space as the equivalent terrain type of Ocean. Its Civilopedia entry explicitly states that "it cannot move in space; there's nothing to hide beneath."
 * Sword of the Stars features cloaking devices, which are hand waved as a series of counter-sensor arrays and a light-masking system that render ships with a cloaking section undetectable except from ships with deep scan arrays. Unfortunately, it only takes one deep scan ship to detect a whole fleet of cloakers and cloaked ships are extremely expensive and cannot fire while cloaked. In addition, obtain cloaking is all up the Random Number God and Hivers, Humans and Tarka will do so less than average. Still, cloaked ships can be very devastating in the right situation, especially when used as a first strike against someone who lacks cloak-detecting capability, as cloaked fleets cannot be seen on the tactical map.
 * The even rarer Improved Cloaking allows for shooting while cloaking, albeit with a 50% increased cooldown on all weapons. This gives cloaked fleets a tremendous advantage against fleets without deep scan arrays. However, the game's AI is quite adept at researching and building spotter ships after the initial strikes, and can also spot where the fire is coming from and dumbfire weapons at "empty space" to try and hit you (missiles and torpedoes will not lock, though).
 * However, without evidence of the destruction of cloaked ships, the AI can continue to fire at empty space after the cloaked vessel is long gone.
 * Unfortunately for the AI, they will fire even if their own ships are in the way - you can get the enemy to destroy more of its own fleet with just a couple of cloaked ships. Improved AI AM Liir cruiser dancing circles around heavily armed and slow Hiver ships? Hilarity Ensues.
 * A few space vessels in Star Wars: Empire at War Forces of Corruption can cloak. The Vengeance-class frigate and hero ship Merciless for the Zann Consortium, and the TIE Phantom for the Empire. As well, the original Empire at War mentions cloaking in the profile for one of the planets.
 * Averted in Free Space. The stealth fighters are very similar to stealth fighter planes of today - designed to be nearly invisible to radar, but not to normal vision. Hence they're very effective scouts while in the nebula, but presumably would be rather less so in open space.
 * Much like the F-15 vs F-22 note below, whilst it is quite possible to see the GTF Pegasus and GVF Ptah yet be completely unable to acquire a lock on them. In fact, you cannot even target them, which means you must gun for the fighters on manual rather than relying on the guidance/lead reticule. Of course, you can still hit them with your main guns and unguided secondary weaponry such as rockets.
 * Don't forget that the testing (and thus presumably use) of those takes place in a nebula, which means plenty of background noise (including, judging from the game's nebulae, thermal) to hide your ship in AND you have a hard time even seeing it with your own eyes.
 * The GTF Pegasus is still tough to hit even outside of a nebula. The ship has mostly black paint, meaning it's hard to see against the background of space. They are also razor-thin, meaning that they're fragile, but really hard to hit except from above and below.
 * Conquest: Frontier Wars: Has this an a borrowed alien technology, while the humans get to use this for two of their ships the aliens can even cloak themselves and other friendly ships.
 * Star Control has the Ilwrath, whose warships can turn invisible. We're never told how it works, but nobody can see Ilwrath ships until they fire, except by stars they block. Fortunately for everyone else, their attack range is pathetically short. And since the positions of combatants are directly opposite relative to the center of the screen, its general location is obvious, which reduces the net effect to "somewhat harder to hit" and having to aim with normally homing weapons.
 * Battlezone 1998 had the RED (Radar Echo Dampening) Field that masked the tank's radar signature, hiding it from non-visual detection and making radar-tracking missiles useless.
 * The sequel added the Phantom VIR which makes you invisible and makes image-locking missiles worthless.
 * Also, a vibrating topographic map is the sign of a Scion Jammer operating within radar range; it messes with every enemy that can detect it so that their radars only show friendlies.
 * In A Final Unity the Chodak have a "chameleon field" that sends out false sensor readings. A 20-meter-long probe appears to be a part of a space station on sensors and tricorders but is clearly alien in nature upon visual inspection (One wonders what Geordi saw when he looked at it). It's only visible on sensors when one cycles randomly through sensor frequencies so the chameleon field can't anticipate how it's being scanned. When the probe makes its escape the Enterprise follows it, but loses it in an asteroid field when it changes its sensor profile to appear as one of the asteroids and cuts its engines to have the same motion as an asteroid. Not quite a stealth system since the chameleon field has to appear to be something but still appears to be anything but a Chodak ship.
 * Master of Orion 2 has Stealth Field making the ship undetectable at interstellar distances, Cloaking Device (also available in the first game) that does the same and makes the ship harder to hit as long as it doesn't fire, and Phasing Cloak that does the same and grants invisibility on Tactical Map, but only for 10 turns (or until the ship shoots).
 * Star Ruler has stealth technologies, but all they do is make the ship harder to hit.
 * Allegiance has cloaking devices equippable for certain ships. They don't make you fully invisible; instead they temporarily reduce your detection signature, meaning enemies must be at a closer range than normal to see you. With a cloaked, stripped-down ship and good piloting you can be right on top of them and they won't know you're there until you start shooting.
 * Star Crusader has an unusual way of handling stealth in space, at least for video games. There is no cover-all cloaking device. Stealth missions are only possible in vessels specifically designed for stealth, and staying stealthy is a matter of keeping your distance from enemy vessels while minimizing the energy emissions you produce. Thrust, steering movements and firing weapons all cause said emissions. The end result is much like trying to crawl through a field of waist-high grass patrolled by guards who stand up straight.

Web Comics

 * Part of Fructose Riboflavin's shtick in The Inexplicable Adventures of Bob is that he invented a spaceship cloaking design years ago which he has carefully kept secret and which he can cobble together out of stray bits in a short time when necessary. It's a modification to the ship's artificial gravity system (much of Nemesite technology seems to be based on artificial gravity), creating a gravitational lensing effect around the ship. He's amazed when Galatea deduces how his design works before he can explain it.
 * In Schlock Mercenary while Stealth in the sense of cloaking is not common, this trope has been touched upon when the company wanted make its warship look like a freighter. Given the vast difference in the size (and thus power output) of a warship and a freighter's power plants this only worked when the warship was more or less totally shut down inside a freighter several times its size.
 * Later there was a ship masked with hull coating. Of course, this could matter at all only because it had shields down, and its shadow didn't matter much, because it was coming in at 0.41 c. And neither using up heat capacity or time needed to start up powerplants for a fight was a problem because at this point it was only a projectile - which is why this was all about being detected too late to deflect: even if blasted into sawdust, it would still deliver about 400,000,000 megatons worth of "slam", only over a wider area.

Web Original

 * The difficulty in doing this is acknowledged in Orion's Arm. During the Interplanetary Era the only options were hide and seek with bazookas or using tons of ships. By the modern era a large empire can usually acquire a few of the reactionless drives developed by the AI Gods, making stealth practical again.
 * Averted in a section of the podcast Novel "The Crypt". A stealth ship is described as only being able to go a few weeks undetected (which seems like a long time, except they're observing a science vessel and it's escort looking for something in space, which also would take a long time), during which the heat rises to uncomfortable levels and almost all systems (including most of the AI, and even their holographic display) must be shut down to prevent that time from lowering even more. When a.

Western Animation

 * Exo Squad had the cloaking technology invented by Space Pirates based on dark matter. And that's pretty much all we're told about it.
 * On Challenge of the Go Bots, Cy-Kill's spacecraft has a "cloaking device" in the first miniseries (which only makes it invisible to radar), and a "stealth device" for the rest of the series (which makes it completely invisible).
 * The Clone Wars brought in a cloaked vessel that was apparently visual cloaking only. Anakin was unable to see it at all in the hangar from a distance of 10 feet or so, while the villain was able to fire torpedoes at it by having them follow the magnetic signature.
 * The ship was unable to fire while cloaked, due to power reasons I believe.
 * Also, the crew was able to see and scan through their own cloak.
 * Parodied on Tripping the Rift; Chode is so cheap, the escape pod's stealth mode simply produces a starprint curtain that surrounds it.

Real Life

 * Decoy balloons used in ballistic missiles - since all objects fall at the same speed in a vacuum regardless of mass, a balloon made of aluminum foil looks just like a warhead until it hits the atmosphere, making it harder to either predict the target or shoot down. That is, when and if anyone will spend outrageous amounts of resources on orbital weaponry - such things were already possible in 1980s, but how to make them efficient is a much more complex issue.
 * The Misty series of spy satellites are thought to use a variety of stealth technologies to deflect ground based radar, lasers and sunlight away from observers on Earth.
 * Unfortunately these technologies are not perfect and are only effective against observers within a specific directional cone allowing astronomy geeks in the free world to use occasional observations and orbital mechanics to keep good track of the stealth satellites' position.
 * This also happens naturally. Earth-crossing asteroids are frequently discovered. These are capable of The End of the World as We Know It, so we have some reason to pay attention, but we've mapped only a small percentage of them. They don't use any fancy technology—just hunks of cold rock in very very big space.
 * Note that most of the problems with Stealth in Space are heat related, and asteroids are generally cold. Also, asteroids are really, really small compared to all the other stuff we could be picking up on. It's like trying to find a frozen needle in a stack of burning hay.
 * Akin to the asteroids issue, just about anything in space has a good chance of going undetected from the simple scale of the problem so real life still subverts this trope, at least until we significantly upgrade our electromagnetic detection capabilities and actually deploy the technology. It should be doable, but hasn't happened yet.
 * Of course the reason why the technology hasn't been deployed is that there isn't (barring belief in UFOs) any need to: there's no one out there running around the solar system to track.
 * This page shows very well why there can not be stealth in space and how even decoys wouldn't work either.
 * There is no such thing as total stealth on Earth either unless fairies have it, so in a way the argument only goes so far. Even submarines can be detected by sound and if you are close enough, magnetic anomaly.