They won't. I think the first space travels will be much more conservative in their targeting, like establishing a base nearby, with a one-way carrier-ship. The technical difficulties involved in getting the crew back, even from distances as close as Mars, are enormous - so they'll probably pick people willing to die (or AI-led robots). For trying to reach earth-like planets, several light years away, that'll probably take centuries indeed - unless they make some wormholes to warp time/space... that could be an option, who knows...