Love this game.
Press the arrow keys on your keyboard to move your asteroid around. You can crash into asteroids which are smaller, but anything bigger than you crushes you, and resets the level. Keep going and you'll turn into a planet and be able to eat bigger planets. At level two, it zooms out, and you become the little planet (or was it fish? Anyway...) in a big, erm, space. Pretty demotivating if you ask me, but still a great game. Enjoy!