The spacecraft which blasted off in December 2012 for its third space mission touched down at California's Vandenberg Air Force Base Friday, Xinhua reported.

"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 US Air Force said in a statement.

The programme 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 metres long and 4.5 metres wide.

The aim of the spacecraft is to "demonstrate a reliable, reusable, unmanned space test platform for the US Air Force," the statement added.

 

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