Errr.. If it really was an spaceship, what makes you think this would be its top speed? I mean, I'm not saying that it was indeed an spaceship. But if it was, then I think it is safe to assume that it would be able to accelerate and decelerate at will.
Seems we have plenty time to track that theory It is only 1.5x faster than the Voyager probe. We should be able to catch up with it with more modern tech.