By Steve Gorman
LOS ANGELES (Reuters) - The U.S. military landed its X-37B robotic space plane at Vandenberg Air Force Base in central California on Friday, ending a classified 22-month mission that marked the third in Earth orbit for the experimental program, the Air Force said.
The X-37B, a 29-foot-long (9-meter) winged craft that resembles a miniature NASA space shuttle, touched down at 9:24 a.m. local time at the coastal air base and launch facility near Lompoc, California, 160 miles (258 km) northwest of Los Angeles.
Also known as the Orbital Test Vehicle, the X-37B was carried into orbit for its latest mission aboard an unmanned Atlas 5 rocket from Cape Canaveral Air Force Station in Florida on Dec. 11, 2012.
The spacecraft conducted unspecified experiments for 674 days while in orbit, marking the lengthiest mission to date for the secretive program, which is managed by the Air Force Rapid Capabilities Office, the Air Force said.
The space plane first flew in April 2010 and returned after eight months. A second vehicle blasted off in March 2011 and stayed in orbit for 15 months. Those first two missions also ended at Vandenberg.
The Air Force says the orbiters, built by Boeing, "perform risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies," although details of the missions are secret.
Total program costs and budget line are likewise classified.
Last week, the Air Force and NASA finalized a lease agreement to relocate the space plane program from California to Florida’s Kennedy Space Center, where the runway once used for shuttle landings may be used for future X-37B touchdowns.
The Secure World Foundation, a nonprofit group promoting the peaceful exploration of space, says secrecy surrounding the orbital activities and payloads of the X-37B are almost certainly due to the presence of national intelligence-related hardware being tested or evaluated.
"The landing of OTV-3 marks a hallmark event for the program," an unnamed program manager said in a statement released by the Air Force's 30th Space Wing at Vandenberg.The Air Force is preparing to launch the fourth X-37B mission from Cape Canaveral in 2015.
(Additional reporting by Irene Klotz in Cape Canaveral, Florida; Editing by Doina Chiacu and Will Dunham)