version1: The rocket responds to keystrokes.
version2: The rocket can wrap around the edge of the scene.
version3: Hitting the sun is a losing condition.
version4: The sun can apply gravity on the rocket.
version5: Three planets circle around the sun.
version6: The planets, too, add gravity to the three planets.
(To demonstrate that the planets themselves have gravity, the
simulation turns off sun gravity and keeps the planets from spinning.)
version7: Hitting earth is a winning condition.
version8: A satellite rotates around the earth.
version9: Rocket trails follow the rocket's path.
version10: The game responds differently to winning or losing.
version11: A fuel indicator shows how much fuel is left.