I've started to hack the game out in Processing. Although there are some nice games engines for Flash, I decided that it might be more valuable to have an example of a Processing games engine for other aspiring developers.
I've made some progress with the physics engine: gravity and collision-handling. There's a known bug-by-design, but otherwise it's mostly working to spec.
Physics prototypeAnother thing I've been doing is trying to work out how the game will play. The current concept is to directly control the astronaut's arms with the mouse; essentially, once you're within arm's reach of an asteroid, you can click-and-drag to propel yourself. I've also got more ideas for gameplay, but I'll have to see how much time I have--I've kept this intentionally minimal in scope so far.