You must put the game version on title screen so when you update the explorer you can see if you are playing the new version or of the cache <img src="smileys/smiley2.gif" border="0" align="middle" />
I see the power-ups, but that appears randomly. If you spend so much time to destroy the asteroids, you can fill all health without problems.
Some ideas <img src="smileys/smiley2.gif" border="0" align="middle" />
When you kill all enemies just at start of stage and then kill the asteroids can be a little boring. Maybe could be good idea generate some enemies on amount of time. Of this form, the player need complete the level in a time and add a challenge to the stages, and the enemies spawn every time and add difficult and variety to the gameplay.
A comet could appear at some point in few moments, crossing the screen and making damage if the player touch it. maybe can hit too on asteroids and enemies.
Good work, continue on it! <img src="smileys/smiley20.gif" border="0" align="middle" />