shoot at the nearby asteroids enough and they'll explode, dropping 4 little pellets, each containing 4 energy. The further out you go, the tougher the asteroids are, but the more pellets they drop, and the more energy each pellet contains, following perfect squares. Get out far enough and a single asteroid will drop 10 pellets, each having 10 energy. It's sort of required if you wanna speed through the different upgrades