I really enjoyed this. It was like Pilotwings...in space. And since that game was one of my favorite games when it was out, I gave a really high rating. Here are some small considerations:
1: Music was great. I loved the very intro music, but it wasn't obvious if I could artificially advance from that screen (was playing the Web build). It would have been nice to be able to skip it once I was done reading it, rather than wait for it, but it wasn't a huge deal, just a Nice to Have.
2: It was somewhat difficult to tell at times when I was going to hit some debris. I might recommend a proximity alert of some kind. This could be a sound, or it could be (probably preferably) visual.
3: I liked how the different levels were setup, but the stratosphere at least felt dramatically harder than the troposphere. I had lost no HP in the latter, while I died pretty close to instantly in the former. I think those rotating things could be a bit slower maybe? Or, failing that, it might make sense to include a small, personal booster that recharges that you can use, as well as the ring boosts - something like "maneuvering jets". Thinking about this more, there has to be a reason you're flying at a near-constant speed toward the moon from the earth - not so much the Final Cause, but rather the Efficient Cause: was the player jettisoned from a near-earth station? Were they rocketed up somehow? Perhaps something could be done with that, in order to give the player just a little bit more control over the gameplay.
4: I'm wondering if it would make any sense to include an oxygen indicator and have that be a third aspect of the game. It would make it harder, both for the player and for you the designer (to find the best places to put oxygen refillers), but to combat that you could have there be mini-stations between levels which would both give the player a minute to pause, which is nice, and be an auto-refill, and you would only run out of air if for instance you didn't get any rings at all. Perhaps for the first couple levels, you don't have to get rings to survive, but after that, you have to get more rings or risk running out of air before the end. I dunno, I definitely like your simplicity, and maybe this is just over-complicating it.