Photos: Atlas 5 rolls out for launch

NASA cited the complexity of Sierra Nevada’s Dream Chaser space plane and an uncertainty of when the proposed crew transport craft would be ready to fly astronauts to the International Space Station as the primary reasons the agency picked Boeing and SpaceX for lucrative contracts to develop commercial space taxis.

United Launch Alliance is set to retire part of its rocket family Thursday with the final flight of the Delta 4 rocket’s “single stick” configuration, a mission set for liftoff from Cape Canaveral carrying a new GPS navigation satellite into orbit for the U.S. Air Force.

This photo gallery shows the classified National Reconnaissance Office payload, already encapsulated in the 14-foot-diameter nose cone, being lifted atop the United Launch Alliance Atlas-Centaur rocket at Vandenberg Air Force Base for liftoff Oct. 8.