If you're playing vanilla/Warlords, you don't launch it, whoever completes all of the parts for their spaceship first wins, unless the game has already been won by some other means—then nothing happens.
In BTS, a "Launch" button pops up on the spaceship screen when you have the minimum number of completed components—when launched, the game carries on for 10-20 more turns with a "___ Turns To Victory!" notice in the upper right corner. The game doesn't decide a winner until someone's interstellar spaceship gets to its destination in one piece. Launching a minimal ship carries a pretty high risk of a failed mission meaning you have to start over. A fully completed ship always successfully arrives.