Photos: X-37B gallery archive

The mission, dubbed Cert-2 by United Launch Alliance, is the final mission required before the rocket can begin launching national security missions. Liftoff is set for the beginning of a three-hour window at 6 a.m. EDT (1000 UTC).

Climbing away from Earth on the power of its liquid hydrogen-fueled main engine and twin solid rockets, a Delta 4 booster successfully launched a new Global Positioning System satellite Wednesday to benefit U.S. military operations and civilian society as a whole.

Ground crews at Vandenberg Air Force Base in California retracted a mobile gantry away from the Delta 2 rocket Monday afternoon, revealing the 128-foot-tall (39-meter) booster on its launch pad awaiting liftoff with a new polar-orbiting weather observatory.