Object of Alien Ambush!:

The object of the game is clear as many sectors of aliens as possible. Each sector consists of two screens of aliens. After a sector is cleared you are given the opportunity to refuel your ship. You must navigate through the asteroid field surrounding the mother ship. Then you can refuel.

If you run out of fuel you will lose a ship. If you crash in the asteroid field surrounding the mother ship you will lose the chance to refuel and lose a ship.