An unmanned US Air Force spacecraft with a vaguely defined military mission landed early Friday in California after a seven-month mission, officials said.

The X-37B, the US Air Force's first unmanned re-entry spacecraft, landed at Vandenberg Air Force Base at 1:16 am local time (0932 GMT), according to an Air Force statement.

The orbital spacecraft "conducted on-orbit experiments for more than 220 days during its maiden voyage," the statement said.

"It fired its orbital maneuver engine in low-earth orbit to perform an autonomous reentry before landing."

"Today's landing culminates a successful mission based on close teamwork between the 30th Space Wing, Boeing and the Air Force Rapid Capabilities Office," said Lieutenant Colonel Troy Giese, the X-37B program manager.

"We are very pleased that the program completed all the on-orbit objectives for the first mission."

The robotic space plane, lifted off from Cape Canaveral atop an Atlas V rocket in April.

Resembling a miniature space shuttle, the plane is 8.9 meters (29 feet) long and has a wing-span of 4.5 meters.

The reusable space vehicle has been years in the making and the military has offered only vague explanations as to its purpose or role in the American military's arsenal.

Officials have said the vehicle is designed to "provide an 'on-orbit laboratory' test environment to prove new technology and components."

Pentagon officials have sidestepped questions about specific military missions for the spacecraft, as well as the precise budget for its development -- estimated at hundreds of millions of dollars.

Industry analysts have speculated the Pentagon must have military capabilities in mind for the unmanned spacecraft or else would not have invested so much time and money in the effort.

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(c) 2010 AFP