Image copyright Other Image caption The X-37B was launched into orbit seven months ago on a classified mission

A prototype spaceplane built for the US military has returned to Earth after seven months in orbit.

The unpiloted X-37B touched down at Vandenberg Air Force base in California at 0116 PST (0916 GMT).

The project has been shrouded in secrecy, prompting widespread speculation about the craft's purpose.

The Air Force has not said whether it carried anything in its cargo bay, but insists the primary purpose of the mission was to test the craft itself.

Officials have said the X-37B could be used to carry out experiments in orbit.

X-37B Mission: Testing reusable space vehicle

Testing reusable space vehicle Length: 8.9m (29ft 3ins)

8.9m (29ft 3ins) Wingspan: 4.5m (14ft 11ins)

4.5m (14ft 11ins) Height: 2.9m (9ft 6ins)

2.9m (9ft 6ins) Launch Weight: 11,000lbs (4,990kg)

11,000lbs (4,990kg) Power: Gallium arsenide solar cells with lithium-ion batteries Source: US Air Force

The robotic X-37B was launched atop an Atlas 5 rocket from Florida's Cape Canaveral Air Force Station on 22 April, with a maximum mission duration of 270 days.

"We are very pleased that the programme completed all the on-orbit objectives for the first mission," the project's programme manager Lt Col Troy Giese said in a statement.

Jeremy Eggers, an Air Force spokesman based at Vandenberg said the craft is expected to return to space in Spring 2011.

At 8.9m (29ft 3ins) long and with a 4.5m (14ft 11ins) wingspan, the reusable spaceplane is about one-quarter the size of the space shuttle, with a large engine mounted at the rear of the ship for changing orbit.

While the space shuttle uses a fuel-cell power-system, the military vehicle is powered by a solar array and lithium-ion batteries.

The Boeing-built spacecraft returned to Earth on "auto-pilot"; the successful return marks the first autonomous re-entry and landing in the recorded history of the US space programme.

Because the X-37B (also known as the Orbital Test Vehicle, OTV-1) started life as a Nasa programme, the Air Force is in a position to talk openly about the craft's design, but its precise purpose remains classified.

The secrecy surrounding the project has prompted much speculation about uses to which the craft might be put.

Image copyright bbc

In April, Gary Payton, the Air Force's deputy undersecretary for space programmes, sought to allay worries about the X-37B and the potential weaponisation of space.

"I don't know how this could be called weaponisation of space. It's just an updated version of the space shuttle type of activities in space," he said.

"We, the Air Force, have a suite of military missions in space and this new vehicle could potentially help us do those missions better."

But some countries could be unsettled by speculation the craft might be capable of inspecting foreign military satellites.

According to amateur satellite watchers, who have been tracking the experimental vehicle since its launch, the craft changed its orbital path around six times.

Some of those skywatchers have also claimed that characteristics of the X-37B's orbit are shared with spy satellites that carry out imaging reconnaissance, as well as scientific remote sensing spacecraft.

Earlier this year, Ted Molczan, a key member in the amateur network, told BBC News the X-37B was "repeating its tracks". This pattern allows some satellites to pass over the same area of ground every few days and so re-visit a target of interest, he said.

Paul.Rincon-INTERNET@bbc.co.uk