Launch a hedgehog through the Earth and into space! Reach the moon (11,000 ft), and then Mars. Collect coins for upgrades to your rockets, the rubber band launcher, assisting gear, or cool looking crap that doesn't do anything.
The longer you stay airborne, the higher your money multiplier.

Sadly, once you reach Mars the game is over, and it's remarkably easy to do so with just 4 E-Rockets and a combination of either slightly upgraded launcher with ballast. So unless you're trying for the lowest days to reach Mars as possible, I recommend start moving horizontally when you're a few thousand feet above the moon.