Crossroads Inc. wrote:
It is a bog standard for every Sci-Fi show out there. Any ship out among the stars, sees something odd, and someone will say "Bring up Sensors!" But, when in outer space, what the hell do you even use as Sensors? If you are engaging in battle, you may need to locate and identify something at thousands, perhaps tens of thousands of kilometers. As far as I know we still use RADAR and various inferred sensors on ships and airplanes, and those have, what, a range of 20 or 40 miles?
So I ask, if you were going into space, what could you use for immense distances?
There are basically two ways to detect anything at range:
- Passively wait to detect emissions from the object.
- Bounce something off the object and wait for a detectable interaction.
At much closer ranges, you can do a lot more tricks, like creating a forcefield which interferes with the object and then detecting changes in the forcefield. But that's irrelevant here, where you're stuck with the two basic principles.
Electromagnetically, the passive sensor is an optical scope or telescope, which every starship should actually have, but which you never see anyone mention in a sci-fi movie because it doesn't sound futuristic enough. The receiver in the telescope would determine which kinds of frequencies you can detect. The active sensor is radar: bounce an EM pulse off a target and wait for the reflection.
Telescopes obviously work over long ranges, but most sci-fi fans ignore their limitations and think they can detect objects at arbitrary distance, with arbitrary resolution. That's utter bullshit. Radar can also work over long ranges, but the problem becomes sensitivity: as the distance increases, you need to increase your power or decrease the width of your scanning beam in order to get useful reflections. If you use a very focused beam, you can get longer range but it also takes much longer to sweep across a useful section of sky.