Snapshot's advice is extremely helpful. I must add a tip for day 2, however. Focus on altitude; altitude is the key. When you're running low on fuel, hold either the left or right arrow to build up as much speed as you can. This lets you run for quite a while, bouncing off of platforms and not losing much altitude. I managed Mars on day 3 easily, with only $1100 from day 1, and I didn't hit the moon until day 3.
Yeah, the only thing that really needs improving is the gravity. Make it stronger. Also, possibly, keep it from rolling so much when the ground is being rotated. Have it move with the ground, not trying to hold its original position.