Made this game for the Global Game Jam this past weekend, it's a 2D gravity simulation game with 3D graphics where you fly a little U.F.O. and attempt to gather orbs dangerously close to the surface of various celestial bodies without crashing. The levels are very hard, but all are winnable. Replayability in improving your high score by beating levels using less fuel. Some levels are more challenging simply because of the amount of fuel you start with. As a graphical touch there are particles in space that are also effected by gravity, this as well as looking nice will give you an idea of how strong and what direction the gravitational forces are around the map.
Here's a little trailer I put together: