With the lack of windows and an endless void between you and any other ship, the usage of sensors is crucial for survival and navigation alike. On a naval destroyer you can always use your eyes and binoculars if all systems fail (not very helpful in smoke or at night). On a starship you are actually a blind mole all the time unless you can use a whole set of electronic eyes. Detecting large bodies like planets is probably not too difficult, but what about killer asteroids crossing your course or a tiny enemy spaceship more than 100.000 kilometers away?

Spaceships practically will not be able to hide in space at all. At least as long as they are moving and emitting energy of some kind. More details why there is no stealth in space can be found here. So how do ships pick up signals and detect other ships?

First of all there are two types of sensors – active and passive. Passive sensors analyse external signals (a telescope using the light falling into it) while active sensors send out a signal and analyse the reflection or returning signal (radar, lidar/ladar). Active sensors are much better at detecting ships and objects, but their active signal can be picked up by the target and used by their missiles to home in on the searching ship. Passive sensors also generally can vaguely detect the presence of objects at a much greater range than active sensors. But active sensors can determine the precise location of an object with much greater precision.

There are methods to hide the visibility of active sensors. If your active radar pulses mimic radio static, enemy radar detectors will filter the pulses out as random noise and fail to see them. This will make your active radar invisible. Until the enemy catches on to the trick and redesigns their detectors.

Sometimes active sensors are called “scanners” while all passive systems are referred to as “sensors”, but these are not technical terms.

Detection Range

There is no horizon in space like on Earth. So enemy ships will not be able to hide or “go below” your radar.  There is no atmosphere, magentic field and other things to block or disturb your sensor signal.

There are three important types of ranges:

• Detection Range: You become aware there is something out there, at that position in the celestial sphere. You may or may not know how far away it is (e.g., a blip on the radar screen).
• Identification Range: You know there is an object of a certain type at range x (e.g., there is a Farragut-Class light cruiser at x 135.2, y 17.3, z 325.1 ).
• Targeting Range: Your sensors have enough data for a targeting solution (Your kinetic kill cannons have a target lock on the enemy Farragut, designated Target Tango 13. You may fire at will. ).

Planets and moons with stations in their orbit will most likely install a lot of satellite sensors that are capable of detecting any bogey at great ranges. No way of sneaking in, unless you use an inconspicuous small craft that hides in a large mass of other craft.

This isn’t so bad at all. Just think about how deadly the exhaust of any larger ship is. Accidentally running into one is not like bumping a car, it means instant death. If your system sees frequent travel of several hundreds or even thousands of smaller and larger space craft, just think about how crowded places like planets or orbital stations will be. Try to run a major airport on Earth for day without detecting every vehicle all the time and guiding them through tower control – this would cause thousands of losses within minutes. Just as airplanes are not allowed to fly, launch or land as they wish, spacecraft must also submit to a strictly coordinated control system. Military vessels may get special rights or privileged treatment, but basically have to follow the rules, unless they wish to be treated as hostile or errant vessels. That’s one reason why friendly fire, communication and maneuvering errors lead to losses during military operations.

“Going cold”

What if a ship switches of engines and power generators and sets all systems to passive to emit less energy? Remember, you are still in space. Which is absolutely freaking cold. Humans and electronics do not function well at 0 K. Even if you decide to turn off all heating and life support systems to the point that your crew is close to freezing to death, your ship surface will still be hot compared to surrounding space (water freezes at 273 K, comfortable living conditions aboard will heat the ship up to 285-290 K). Any passive heat sensor will easily detect a difference of almost 300 degrees K over dozens of million kilometers. So this “tactic” is actually totally useless and no ship commander would even consider using it.

Beaming away waste energy in the other direction will not work either. The refrigerator concept requires power, which means you would need to fire up your nuclear reactor (800 K+) again. There is no way of knowing the “away direction”, fractions of a degree would make the waste heat beam visible again to the detecting ship. Heat radiators can only emit a certain amount of energy per surface size and time. You simply can’t pump out Terawatts of energy through a tiny hole (you actually created a powerful energy weapon by doing so and should probably direct it at the detecting ship instead).

Burn and Cost

On cruise speed many vessels will only fire their engines for a short time to reach a certain velocity and the coast the rest of the way without burning reaction mass. During that phase the engine will not be detectable from distance. But every ship will use apogees and auxiliary drives frequently to make course corrections (other ships, particles, other reasons). Those will be detectable from far way. You will not disappear from an enemies sensors just because you suddenly switch of the main engine. Computers will be able to calculate your course, detect your speed from the spectrum of your drive emissions and will not get you away just because you choose not to fire your engines constantly. Actually you will just be slower by that and the enemy ship will be able to close in. Civilian tenders towing cargo might use this tactic, as it is ok that they will take months to get to a place where they could go in weeks under constant thrust.

Using decoys

Decoys can be used to fool enemy missiles or small craft at very close ranges and when short time frames are applicable. To hide a ship for a prolonged time one would need to use decoys that actually are ships. It is quite easy to see wether a plasma emission or similar is a decoy or a real ship drive under thrust. Decoys would have to be used in great numbers and so powerful that the ship trying to hide would not be able to load anything else than decoys (and even that would not be sufficicient). On Earth you can use smoke and radiation created by decoys to cover up large areas because you have an atmosphere and distances are relatively short. In space you would need to create clouds of obscurement over insanely huge areas. Your own ship is typically flying so fast that you would negate the effect of any local distraction.

A large warship hiding in a fleet of smaller civilian vessels and throtting it’s engines could be taken for a civilian ship, at least for a while.

Conclusions

• Carefully scanning the entire celestial sphere takes 4 hours or less.
• Thruster burns of any drive with reasonable power can be detected all the way across the solar system (billions of km away).
• Even with engines cold, the heat from radiators attached to life support will be detectable at tens of millions of km away, which is still far too large to get any sort of surprise.
• Radiating heat in a single direction (away from the enemy) is easily defeated by fielding a number of tiny detector probes which idly coast about the system. Additionally, the narrower of a cone in which you radiate heat, the larger and larger of radiators you need to field. A 60 degree cone of radiation is roughly 10% as efficient, and it only gets worse the tighter of a cone you have.
• Making a huge burn and then trying to stealthily coast for months to the target is do-able, but as long as your enemy can track your first burn, they can very accurately predict where you’ll be as you coast across the solar system. And you still have to worry about radiating your heat for months.
• Decoys are only really viable on really short time scales, such as in combat. Over the long term, study of a decoy’s signature over time will reveal it’s true nature. It would need a power source and engine identical to the ship it’s trying to conceal, as well identical mass, otherwise the exhaust plume will behave differently. This means your decoy needs to be the same mass, same power, same engine as your real ship, so at that point, why not just build a real ship instead?
• Hiding behind a planet to make a burn is not really feasible. All it takes is two detectors at opposite sides of this planet to catch this. In reality, a web of tiny, cheap detectors spread across the solar system will catch almost all such cases.
• A combat-ready ship will require very hot radiators for its nuclear powerplant for use in combat. If these radiators are going to be completely cold for the journey, they will suffer enormous thermal expansion stress when activated. In order to avoid this, very exotic and expensive materials for your radiators will be needed to get from 10 K to 1000 K without shattering. Not only that, your radiator armor will need to be similarly exotic, which means it will likely not be very good at armoring your radiators anyways.