This achievement requires that you hit a cumulative total of 10 UFOs across all replays of all levels. UFOs are scattered across many of the worlds and when you hit them they will rotate and send you flying. You will hit way more than 10 while completing the game, but it you want to get this done early, the easiest level you can earn this on is Sweet Home 5. There is a single UFO above and to your left. Launch the Rabbit in the direction with full jetpack power and try for the UFO. Repeat until you get 10.