Not bad.
The graphics are really nice. You pulled off the 50s sci-fi feel very well. Sound would have really helped a lot though. With the great visuals, it just feels out-of-place not to have sounds that match the theme.
The gameplay was pretty standard. Straight out of the tutorials type stuff. It could have been improved greatly with the addition of something as simple as power-ups for the weapon or to repair damage (if these were in there and I just didn't play long enough to see them, then nevermind.)
Excellent work for your first flash, by the way. It's refreshing to see the work of someone with the patience to wait until they've got something solid before putting it on NG. Good job.