Photos: Atlas 5 rocket soars from West Coast

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).

SpaceX launched its 16th mission of the year early Monday, lighting up a nighttime sky with a Falcon 9 rocket to deliver a Canadian-owned commercial communications satellite to orbit.

After receiving more than $150 million in U.S. military funding to design and develop a reusable winged spaceplane named Phantom Express, Boeing said Wednesday it is ending its work on the vehicle, effectively killing a program military officials hoped would offer regular, reduced-cost launch opportunities for small satellites.