Media playback is unsupported on your device Media caption The pictures released give little away about the plane's purpose

An unmanned US plane on a top-secret, two-year mission to space has returned to Earth and landed in California.

The aircraft, resembling a miniature space shuttle and known as the Orbital Test Vehicle or X-37B, spent 674 days in orbit around the planet.

It was the unmanned plane's third space flight, but its mission has been shrouded in mystery.

A theory that it was taking a look at China's space lab has been downplayed by experts.

Air Force officials have only told US media the aircraft performs "risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies".

X-37B - American military spaceplane Mission: Described as a re-usable testbed for new sensors and other space technologies

Length: 9m (29ft); Wingspan: 4.5m (14ft); Height: 3m (9.5ft); Mass: 5t (11,000lb)

Origins: Started as a Nasa project in 1999 before being handed to the military in 2006

Cost: The budget line for the X-37B programme continues to be classified information

The X-37B programme, started in 1999 and is currently run by the Air Force's Rapid Capabilities Office.

The first plane flew in April 2010 and returned after eight months. The second launched in March 2011 and remained in space for 15 months.

The current aircraft - built by Boeing - uses solar panels for power in orbit, measures over 29ft (9m) long, has a wingspan of nearly 15ft and a weight of 11,000lbs (4,989 kg).

It looks like a mini space shuttle and can glide back down through the atmosphere to land on a runway, just like Nasa's re-usable manned spaceplane used to do before its retirement.

Image copyright AP Image caption An infrared view of the spaceplane was released in 2012

Image copyright AP Image caption A fourth X-37B mission is currently scheduled for 2015

A fourth X-37B mission is said to be planned for launch from Cape Canaveral, Florida, in 2015.

Europe is to test its own automated space plane technology in the coming weeks.

The IXV vehicle will be launched into space atop a rocket from French Guiana and then make its way back through the atmosphere to splash down in the ocean.

BBC News science correspondent Jonathan Amos

The reality is that no-one really knows what this vehicle does. The only credible explanation I have seen is that it is testing technologies that could find their way on to future satellite missions.

If you consider how expensive a satellite mission is - several hundred million dollars - you'd like to be sure that any innovations are going to work straight out of the box.

By flying early prototypes on the X-37B, you can test these technologies so that when you put them on future satellite missions, you can be sure they will deliver.

Nasa recently agreed to give over work space formerly used to service the shuttles at the Kennedy Space Center to the X-37B programme, which tells us this is a long-term project for the Air Force. Whatever it is they are doing up there, they deem it to be high value.