I really like the concept and the idea of the gravitational field is super cool. Also, plotting those trajectories must've been a headache :D
I just realised that this game might simply be not for me - way to difficult and way to stressful with it being real time (and also having to change the size of asteroids mid-flight), global timer and timers for particular planets. I think this is intentional. It's clear that you were aiming at a more hectic gameplay ;)