Photos: Delta 4 awaits blastoff

Sierra Nevada and United Launch Alliance have announced the most powerful version of the Atlas 5 rocket, with five strap-on boosters and a twin-engine upper stage, will send the first two Dream Chaser cargo missions to the International Space Station from Cape Canaveral in 2020 and 2021, a schedule that still must be confirmed by NASA.

United Launch Alliance teams rolled the Mobile Service Tower away from a Delta 4-Heavy rocket at Cape Canaveral’s Complex 37B launch pad Wednesday in preparation for liftoff on a national security mission.

The 32-story-tall mobile service tower at the Delta 4’s launch pad at Vandenberg Air Force Base in California was retracted away from the rocket Thursday in advance of a planned liftoff with a top secret National Reconnaissance Office payload.