Cool game. I love that every boss has different style of movement and attacking. Music and graphics are also good, but in my opinion they doesn't fit to each other (but they are good).
You could also add some benefits (e.g. money for more upgrades) for destroying more asteroids, because I can stay in one position and shooting all the time without losing too much health. Part of the game when you are fighting with boss is cool.
In overall, really nice entry, well done!