A self-replicant probe with solar sail for propulsion, to explore space. This is a probe that gets to a solar system, and starts mining resources. Then, it builds facilities to assemble more copies of itself. The probe explores the local system, sending data back to Earth. New probes moves to other star systems, and this process keep going, always reporting data back to us.

If I'm not mistaken this was considered by a famous scientist, but I can't seem to recall his name. Though the idea is not mine, it is damn too good not to bring it into focus here.

It should be beyond difficult to turn this idea into reality. The AI alone to coordinate such operations would be eons beyond anything we have.

I find it puzzling that we've never came across an alien probe before. It brings some doubts about the feasibility of the idea, and to the existence of extraterrestrial intelligence itself.