(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.

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