Gravitational Escape Achievement
Make 100 birds exit atmosphere
You just need to launch 100 birds out of a planet's atmosphere and have it fly off into space. You can either launch the bird from within the atmosphere, or just make it pass through the atmosphere before returning to space. This achievement should unlock easily while progressing through the game. However, it can also be quickly boosted on Level 1-19 in less than 10 minutes by rapidly firing all three birds to the left out of the atmosphere, restarting, and then repeating the process.