US space plane touches down

Range safety officers were on hand to track its descent over the Pacific and activate a destruct mechanism if the landing needed to be aborted.

Vandenberg released infrared camera video of the craft rolling to a stop, recovery crews approaching in vehicles and working around it in protective suits, much the way space shuttles are handled upon landing.

The US air force emphasised that the primary purpose of the flight was to test the craft itself but classified its actual activities in orbit, leading to speculation about whether it carried some type of spying system in its small payload bay.

Programme manager Lieutenant-Colonel Troy Giese said in a statement that all objectives were completed and the landing culminated a successful mission.

The air force immediately announced that a second X-37B, which had only been revealed last April, is scheduled to be launched next spring.

The first X-37B, also known as the Orbital Test Vehicle, was carried into space atop an Atlas 5 rocket launched from Cape Canaveral in Florida on 22 April this year.

The flight followed the project's long and expensive journey from Nasa to the Pentagon's research and development arm, and then on from there to the secretive Air Force Rapid Capabilities Office.

Hundreds of millions of dollars have been spent on the X-37 programme, but the current total has not been released.