roselady,
When you build the last component, it will take you to the Spaceship screen. You will not be able to exit that screen without Launching.
IF any other Victory Condition has been satisfied, i.e. the game is 'officially over', you will not be able to view the Spaceship screen or Launch.
This frustrated me in a previous game. I got to the 2050 year end of the game and was 1 or 2 turns from either Spaceship or UN or something. Once the game is officially over, you can't do anything that would construe a 'Victory Condition' (except possibly continue until you've wiped-out everyone else.)
Steven