Had a lot of fun with this. I wish there was a downloadable demo as the browser was a bit glitched during gameplay. Asteroids should be a hazard to the base. Just let them hit it if the player can't get the miners and/or defenses deployed fast enough. Seeing them just bounce off is disappointing. At least have that as an option. Also, asteroids should break apart when they collide with each other breaking into smaller pieces. Oh the chaos that would ensue would be glorious! Maybe, make those features as part of a harder difficulty setting. Really great start.
Edit: Obviously the asteroids breaking apart would have to have a limit on how small the fragments could get, but still would be a lot of fun as an option.