Is radar better than visible light in deep space?
On the earths surface, radar is pretty sweet. It apparently penetrates the atmosphere well and bounces back and can be detected. I think this has to do with the wavelengths absorbed by our atmosphere.
But what about space? For detecting things at a distance (for example - rock on a collision course) does radar offer any advantage over visible light?
I am trying to justify an interstellar ship tricked out in klieg lights, looking for asteroids like Londoners looking for bombers during the blitz.
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1 answer
The big problem with using radar in outer space is simply range. The received flux of a radar signal falls off as $1/r^4$ instead of the $1/r^2$dependence we're used to getting for signals emitted by a source far away. The $1/r^4$ arises from the fact that the signal has to travel from the transmitter to the object (a factor of $1/r^2$) and then back to the receiver (another factor of $1/r^2$).
This means that the signal gets extremely weak extremely quickly. We can use radar to detect spacecraft-sized objects at perhaps 10-20 times the distance to the Moon, but beyond that, we'd need transmitters much more powerful than any equipment we currently have. For bodies the size of comets or asteroids, the problem is slightly lessened because they have substantially larger cross-sections than spacecraft, but we still have to deal with the $1/r^4$ problem.
(We of course detect these bodies in the optical by looking at reflected light - this time from the Sun - but the source of that light is extremely powerful, so range is less of a problem. The Sun is obviously much stronger than any radar source we humans could produce!)
Another problem you have is that you would need to get reasonably lucky to detect an unknown object via radar. Space is, well, big, so angular sizes are small, and it would be quite easy to simply miss an object that's very far away. It's much easier to detect something if you have a good idea of where it is. Perhaps radar could be useful for measuring the trajectories of extremely dark, low-albedo asteroids which would reflect very little visible light (although perhaps they would also be poor reflectors of radio waves, too). Performing an all-sky survey, on the other hand, might be a job for optical telescopes.
On the third hand, in interstellar space, far from any light source, it might be easier to find an asteroid with radar because you would be able to generate your own signal. At the same time, of course, any such objects would be exceedingly rare, so the odds of finding one by any method are low.
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