Has anyone done a story about an alien starwisp blowing through the solar system at .2c, and humanity figuring out where it came from and sending someone off to take a look? Sounds vaguely familiar -- I know there've been stories about various other signs of alien life prompting an expedition -- but I'm not thinking of any with exactly this scenario; specifically, an alien vehicle moving through the solar system at such high speed that we don't have time to react to it while it's here. (If I'm calculating right, at .2c it would cross the entire solar system in about 60 hours; assuming we saw it and recognized it for what it was, we still wouldn't have time to do anything about it.)

...Hey, what's the fastest speed (relative to Earth, say) of any not-artificially-propelled piece of matter we know of? If something came through at .2c, would we be able to assume it was of artificial origin?

Good question — I suppose some comet in a hyperbolic orbit would be the fastest thing we’ve seen. I’ll try to remember to check my The New Solar System when I get home. (Thanks, Dr. Lisa!)

Offhand, I can’t think of any stories that use that scenario. Conceptually, though, is it that different from any other scenario where we find evidence of alien intelligence at a distance?

Anyway, I think we’d have to have an unreasonably high level of technology to detect an actual starwisp.

This may be a little too far away, but it is local (cosmically speaking):

MICRO-QUASAR WITHIN OUR GALAXY !

The announcement is also almost ten years old, so maybe there's something closer to report by now.