LOS ANGELES, Oct. 17 (Xinhua) -- After spending about 22 months orbiting Earth on a mysterious mission, the U.S. Air Force's X-37B unmanned, reusable space plane finally returned to the ground on Friday.
The secretive spacecraft, which blasted off in December 2012 for its third space mission, touched down at California's Vandenberg Air Force Base at 9:24 a.m. local time (1624 GMT), the base announced.
"The mission is our longest to date and we're pleased with the incremental progress we've seen in our testing of the reusable space plane," the Air Force told media.
The program is being run by the Air Force's Rapid Capabilities Office, and there are two space planes in the fleet, both built in tight secrecy by Boeing.
The X-37B space planes look much like NASA's retired space shuttles, only much smaller. Each X-37B spacecraft is about 8.8 meters long and 4.5 meters wide.
Although media reports are abuzz with speculations about the true nature of the program, the Air Force said it is to "demonstrate a reliable, reusable, unmanned space test platform for the United States Air Force."
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