Thrust

Thrust is a gravity-defying cross between Asteroids and Lunar Lander. You rotate and thrust your little ship down through underground caverns to tow a heavy energy pod back out into space. Gravity and momentum fight you all the way, and limpet guns guard each pod. Shooting the nuclear reactor shuts the guns down for a while, but hit it too often and the whole planet goes critical. You also need to collect fuel as you go.