You're protecting a planet from wave after wave of enemy ships. Under your command is a space ship which can move back and forth, and shoot rapid-fire guns at great distances in any direction. Just move the crosshairs where you want to fire, and blast away.

A scanner above will show you incoming enemies, and allow you to head them off before they get above your planet. But don't move too far in one direction, because they attack from both sides.

The ships will use different attacks against you. Some will crash into the planet, some will bomb it, others will blast it with lasers. Just one well-aimed attack can take out nearly your whole planet. And they only get faster.

Lose three planets, and it's game over.

A trackball. Orbitron is a great, near-classic arcade game. There's plenty of action, and while the joystick works pretty well, it's just not fast enough to keep up after a few attack waves. A trackball would have been perfect for this game, and could have helped establish it as a true classic.