Love it! Nice and simple idea and I think you've hit a pretty good spot in terms of difficulty.
My main feedback would be to maybe limit it so the aliens can't spawn super close to the edge of the screen, as being hit by an asteroid that spawned half a millisecond previously just isn't fun.
Also a couple of times it seems that upon collecting an alien an asteroid spawns already part way onto the screen, presumably as a side effect of the way you're increasing their spawn rate. Similarly to above an asteroid killing you before you've even had a chance to react to it appearing should probably be avoided