Stealth In Space
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Basic Information

Cloaking Devices, Stealth Technology, Running Silent, using only Passive Sensors to limit your signature. These are common elements of sci-fi battles, because sci-fi writers think space combat is like Old School Dogfighting or that Space Is An Ocean.

In reality, that's all hogwash. Stealth in space is just not worth the trouble.

Why There Is No Stealth In Space

The background temperature in deep space is just a few degrees Kelvin. Against that backdrop, you have a bright flaring torch of an engine and a hot burning nuclear reactor. While those are online, you may be detectable from light years away. From the magnitude of your engine exhaust, they can tell how much thrust it's producing. Speed deduction is a simple act of measuring how quickly that bright point on the sensors is approaching. Knowing energy and speed means you can calculate mass, so they know how big a ship their facing as well. Simply powering down isn't likely to fool them, either. There's no atmospheric convection currents to reduce your heat - even if you turn off the engine, it either stays hot or bleeds infra-red out into the cosmos. Fire your maneuvering thrusters just once, and you'll be visible for several astronomical units in every direction.

What this boils down to is this - unless there's FTL or other clear violations of physics - stealth in space is wasteful, impractical, and ineffective. You might be able to hit an unsuspecting peaceful planet with a hidden missile if you plan the attack several years in advance and never declare war, but the odds of you ever getting the jump on an enemy fleet are next to zero. If you're the same species as your foe, Terrorism and Sabotage are probably much easier (and more cost-effective) than military sneak attacks in space.

Maybe for a very rare and specific circumstance, your ship might be able to radiate reduced waste heat away from the target, or behind a sunscreen, and sneak up on someone with your engines off. In order for it to happen though, you'd probably have to start moving towards the target while some object is intervening, then cut your engines before that object's orbit clears your path. Then you coast silent and cold for several months, hoping that the enemy will be kind enough to stay on it's current bearing that you based the intercept course off of. If they do that, and you manage to avoid occluding any stars while they're looking in your direction, and you have a ship shaped to baffle their active radar, then maybe, after a few months, you might manage to catch up on them unawares.

This is rather unlikely to work for human crews, though, because even if the engines are off, the crew compartment is still 280 degrees above the background temperature. So yeah, if you're a robot or alien species that really likes the cold, and you have reliable data about where the enemy will be six months from now, you just might be able to pull off one sneak attack.

Unless, of course, the target is also receiving telemetry from a space station, probe, or picket vessel that's at some other angle or position where it can notice the last burn on your engine.

Probably better to just assume they're going to see you coming, and spend all that money and effort on weapons and defenses instead of stealth.

FTL and Stealth

Now if there is tech that lets you move Faster Than Light, the whole thing changes. If it's just possible for your ship to arrive before the light bouncing off of them (or the heat they're radiating) does, then stealth in space is not only possible, it becomes the default. That is, unless they have some sort of FTL Sensors. Such sensors would be a likely outgrowth from FTL Communication.

It's worth mentioning, as well, that Relativity equates FTL and Time Travel, so causality is negatively affected by such a star drive. You may complete your ambush before you launch it. If that sort of thing becomes common-place, Collapse of Causality will result.

If you need a jump gate, or if your drive actually warps a large amount of space around it, than you may find the whole "stealth ain't worth it" statement to remain true even with FTL.

Caveat

The material above pre-supposes that all of the target information that a spacecraft is providing is sucessfully picked up, processed and correctly interpreted, which on the face of it is a reasonable assumption but is also like saying that you can't be stealthy underwater because the noises you make will always be heard. To a certain extent it's true - you'll never have a completely silent submarine, nor will you have a spacecraft looks the same temperature as the background - what you need to rely on is not sounding like a submarine. Stealth in space - as far as it can be achieved - is likely to depend on knowing what the enemy are looking for and then looking like something else. Currently we think of space as big and empty, but once you are dealing with a spacefaring civilsation and the resulting high volume of stuff moving about there should be enough "noise" in the background to hide in until someone starts actively looking for you. If your opponent knows there's a ship out there, he'll probably find you - if he doesn't, you may stand a chance.

Sources

Bibliography
1. Atomic Rockets - Detailed explanation of why stealth in space is unfeasible.

Game and Story Use

  • Really light and soft Space Opera campaigns can get away with bending physics to match the Rule of Cool, but games set Twenty Minutes Into The Future probably need to bow to science. See Mohs Scale of Sci Fi Hardness.
  • In a hard sci-fi setting, in the rare situations where stealth works, it's likely to crush the opposition. When you're used to detecting the enemy 48 hours before they hit effective weapons range, chances are a good that sneak attack is devastating. Especially since they'll be packing nukes.
    • That said, there's ways of pulling off a sneak attack in space:
      • An enemy who knows where the players are going might lay down a space minefield.
      • The enemy might use hacking to produce false sensor readings.
      • Opponents flying a false flag (say, posing as a merchant or an ally) could easily launch a surprise attack.
  • Obviously, TV shows like Star Trek get a lot of mileage out of hand-waving this sort of stuff.
    • The best hand wave may be to just invoke super science. Make stealth tech expensive or difficult or tightly controlled, but still possible. A properly equipped ship can pull it off, but the default assumption is that any normal ship will be detected while its still hours away.
      • Tracking down a stealthed ship (whether immediately as in a sub hunt or trying to find its port of origin) could be the focus of a session or a story arc.
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