Prizes are the only aliens that cannot be destroyed. Instead, when a prize appears, you have a few seconds to travel down the corridor, claim the Prize, and then get back to the center again before an indestructable Ghost Ship reaches you. If you do not claim a Prize in time, it turns into a Supersonic Cannon Ball!

As added incentive to claim prizes and then get back as quickly as possible, you are not permitted to fire in any corridor where a Prize is present, nor can you fire when you are anywhere but in the center alleyway.