A spacecraft capable of travelling at 20% of lightspeed would have to be generations away. Voyager I is about 11 billion miles out, and that's still in the Kuiper belt. But a manned mission to another system? Just being able to provision a spacecraft for that voyage would be an incredible feat. And would it be possible to put a crew in some form of stasis/hibernation for the journey? I would also think that navigating through interstellar space to a nearby system would be an unprecedented challenge, too.