Very Cool, it did take me an entire life to figure out you could buy upgrades from the planet but that's just because I am prone to push the play button instead or reading :P The only words of criticism I can come up with is that a would prefer to choose where my ship would shoot rather than the closest enemy but your version forces the place to play more aggressively thus heightening the danger and fun.(which is probably much better then having the player be able to shoot where ever they want). Also the player continues to shoot when they are in a safe zone, but that's not really much of an issue
A fun game with some neat mechanics, Great Work :)