Shoot. This is one well put-together game. So let me share my thoughts.
The game definitely has a solid premise. It definitely takes skill to sharpshoot those meteors with your default cannon. The shop offers some nifty looking power-ups too. The graphical style, general mood, and overall gameplay definitely come together to make a cohesive, engaging foundation for this game.
My biggest beef at this point is that I didn't necessarily quit playing per se because it was so hard, but because the action is way too repetitive. I shot meteors, more meteors, even more meteors... yeah, that's about it. Gameplay gets stale, especially given its a more moderate than fast pace at which you play, so after two plays I wasn't much inclined to... ya' know, shoot more of the same meteors.
Past that, it's some of detail stuff. What in the world is with the intro theme suddenly dropping into that 8-bit/electronica funk-like theme? Sweet snot, it was a total character and mood breaker for me. It was just weird. Also, stuff in the shop can be very pricy, and (as I've learned from one of my past games) working towards expensive albeit shiny looking equipment is not always enough in itself to motivate a player to keep playing.
Anyway, I think you have a very solid premise here that with a few tweaks could be a lot better. Maybe you could add some different meteor types (like ones that fragment) or maybe even space junk, and then maybe have different correlating shop items the user should buy. You could say something like "We have detected incoming (insert thing here). You should purchase the (insert weapon power-up here) to properly handle them." That's one idea, anyway.
Ultimately, I think it needs a little more flow and direction. In my experience, trying to do a "little bit better" or shooting for higher scores/stages without any other goals (unless you are an exceptionally solid arcade-style game like Galaga, but even then that switches it up some) can be weak motivation to keep playing.