[vkasdfsf]

Giant comets would be GREAT exploratory starships, you just rehuse frozen water for reorient the thing, somehow attach an habitat ontop / inside the comet, then you take off right to the inner solar system, or outer solar system regions.

You could send a relatively small probe to the Oort cloud, look briefly for a comet falling to Sun's gravity well, a plug it with hardware, then the thing would approach Earth as stealthy as some giant alien starship can get, you look at it, just seing some, another giant comet just doing its usual business around the system. No fuss.