Here's my rating:
The gameplay is an interesting and innovative idea, and it's fun to play! Good job on that! The only problem is that collisions turn off at certain points in the game, which completely defeats the purpose. My rating: 4 stars
Good use of the theme! I'm glad you didn't stick to the outer space trope and instead thought of ways to use physical space to make a game. Great! My rating: 5 stars
The art is very simple, but it looks pretty good! It's consistent and clean, although it could use some polishing (and the circles might be better with antialiasing). My rating: 4 stars
The music is really good and perfectly suited to the genre of game. The sound effects work with it too, although they don't exactly stand out. You could also stand to add a few more here and there, such as a little sound on any collision with the walls. My rating: 4 stars
Overview: The game is quite well executed, with a sorta well rounded approach to the game's general feel rather than a focus on just art, just mechanics, or just sound. My overall rating: 4.25 stars (rounded down to 4).