Photos: Gallery of Delta 2 launch

Riding the rails between its assembly building and the launch pad, an Atlas 5 rocket topped with a clandestine national security satellite was rolled out this morning for Thursday’s liftoff at 4:07 a.m. EDT (0807 GMT).

A recent federal regulatory filing by SpaceX suggests the company may attempt to return a Falcon 9 rocket booster to Vandenberg Air Force Base in California for the first time later this year.

SpaceX’s first Falcon Heavy launch for the U.S. Air Force took a major step closer to liftoff with a hold-down engine firing Wednesday night at launch pad 39A at NASA’s Kennedy Space Center in Florida. Liftoff with a cluster of experimental satellites and weather monitoring craft is set for June 24.