If aliens were to bring a space ship to the solar system, and put it in orbit around a Jupiter moon, and then begin colonising the surface of the planet, what would we be able to see from using current day technology?
Presuming the best case scenario, i.e. we are at a close orbit with Jupiter, etc.
How big would their ship need to be to be visible to us? Are there any 'major' telescopes regularly pointed at Jupiter, or would it be more likely picked up by someone with a (big) telescope in their backyard? How long would it take us to develop / deploy a probe to investigate (and how long to get there)?
The main question here is about visibility. With today's technology, what would we see?