Video: Tower rollback prior to launch of Mars Pathfinder atop a Delta 2

United Launch Alliance engineers filled a Delta 4-Heavy rocket with super-cold liquid hydrogen and liquid oxygen propellants Friday at Cape Canaveral Air Force Station during a practice countdown before the heavy-lifter’s scheduled liftoff in June with a top secret U.S. government spy satellite.

The step-by-step stacking of United Launch Alliance’s final Delta 2 rocket was completed in June at Vandenberg Air Force Base in California for liftoff Sept. 15 with an Earth science satellite for NASA.

The next Delta 4-Heavy rocket was raised vertical Friday at Cape Canaveral, signaling the start of a comprehensive series of pre-flight tests before liftoff next year with a top secret U.S. government spy satellite.