Great game!
A few ideas to make it better (hopefully). As you have 3 lives per tryout, maybe you could make the "turn" end by time rather than end of impulse as the latter almost everytime puts you in infinite distance to the target making it easier to just restart.
To sum up with that, there is the fact that there is no friction in space and you shouldn't slow down. Sometimes I tried to play from afar but I almost stop before reaching anywhere and get captured by a planet with no hope of escaping.
The last point is the fact that bigger planet should have bigger bubbles not only to be more accurate but also to make you juggle with the different attractions to get to the target. I don't know if you did it in more advanced levels, I just got to the third one so far.
Good luck with your games!