Of all the things that can bring down a rocket, bubbles seem the most unlikely. They are a threat to rockets that use very cold liquid fuels, like the upper stage of NASA’s new Ares 1 rocket, which will carry astronauts to the space station and join up in orbit with spacecraft carried by the larger Ares 5 rocket for missions to the moon and beyond. Bubbles can form when frigid liquid hydrogen and oxygen fuel are pumped into a rocket engine. The problem arises when they pop.