Systems are pumping liquid hydrogen into the Centaur upper stage as today’s countdown proceeds toward a liftoff at 5:42 p.m. EST. The Centaur, pictured above during stacking of the Atlas V at Space Launch Complex 41, burns liquid hydrogen fuel combined with liquid oxygen during the ascent into space. The Centaur’s liquid oxygen tank is filled to flight level. Since the propellants are cryogenic, a portion boils off during the countdown and is vented out of the tanks. Therefore, the system continues to trickle in propellant until just before liftoff to make up for the amount that boiled off.