MirrorMoon's core mechanic involves traveling to planets across the universe and discovering all of their deep, dark (and sometimes bright) secrets, using moons as mirrored maps of each world's terrain. Using equipment found on the planets, players are able to slide moons in and out of the suns' rays, rotate them and manipulate gravity, among other neat tricks.
But that's the single-player portion. MirrorMoon can't be truly completed without multiplayer, which allows players to find planets and share their discoveries with others. The first person to find a planet gets to name its Star System whatever she wants, and that name is branded to the star for all other players, for as long as the game is online. There are 1,000 Star Systems in each Galaxy, so that could be a very long time indeed.
We got to play MirrorMoon at GDC Europe and we talked about that experience on the final, games-focused episode of the Gamescom Super Joystiq Podcast Special.