Good concept and great start. 

On my first run, it felt a tiny bit unfair because I had no way to predict direction of incoming asteroids, and I thought I had no way to speed up or improve my odds...

On my second run, I realized I could move the little asteroid and it was slightly faster than the big one, so it was a much better experience. Had a bit of a QWOP vibe to it.

(I admit, sometimes I play a game without reading the instructions carefully.)

Space games - I have to play them :-)  You have a nice idea here, and I felt the presure to protect Mars as best as I could. The controls needed some practice, but I got it decently right at some point.

Some ideas/suggestions: Let a colony grow on Mars - starting small, growing bigger and around the planet over time. That way, you give players a chance to get used to the mechanic (only a  small fraction needs protection in the beginning, growing larger over time). The more evolved parts of the colony, the more valuable and protactable they become.

That was pretty fun!