You have three places to land, with increasingly greater gravitational pull:
- Moon - 1/6 of the Earth's gravity
- Earth - (it's own gravity)
- Jupiter - 2.54 times the Earth's gravity

To safely land, you need to land with a velocity less than 1 (an arbitrary number, no units given).
Anything greater, and you crash.

The Moon is fairly easy to land on, although it's also easy to give yourself too much thrust and end up flyihg to the top of the screen.

The Earth is a littel more difficult, and Jupiter is nearly impossible until you get a feel for the controls.

You keep the remaining fuel after each landing (successful or not).
When you're out of fuel, you can't fly anymore.