great game! the concept is really nice and original, most entries are about the player getting stronger, but you gave it a twist. the simple art and colors works really well since it helps you realize where the danger is. the music fits very nice and gives the sensation of being in the space.
and abt the gameplay, it's very difficult to move yourself, move the shield, and shoot at the same time hahaha but after a few tries you get used to it and it is very fun! the mechanic of waiting the enemies to gather more material is very interesting, and i always like when games have upgrade systems in it ;)
the only thing i have problems with is the balancing, the waves 1 and 2 are very enjoyable, since begins with only one enemy on screen, but from wave 3, not only there are more enemies, they shoot more bullets and there are too many asteroids, maybe you could introduce this things gradually, and it doesn't help that the shop is very expensive and the upgrades don't make much difference.
but overall this is a great game! it has so much potential ^_^ good job!