US Air Force X-37B space plane finally set to land after secret two year mission?

One of the most mysterious craft ever to be flown by the US military has been in orbit for almost two years.

The X-37B space plane, an experimental program run by the Air Force, launched atop a United Launch Alliance Atlas V rocket on May 20, 2015.

Now, experts claim it may be about to finally return to Earth - almost two years later.

'The historic Shuttle Landing Facility at the Florida spaceport is preparing to once again host an end of mission landing as the Air Force's X-37B mini spaceplane prepares to return from a near two year mission on orbit,' nasaspaceflight.com said.

However, the Air Force claimed the movement was simply a test exercise, meaning the mission continues.

'The X-37 is still on-orbit. 'The program is conducting a regularly scheduled exercise this week,' said Capt. Annmarie Annicelli, media operations officer at the Pentagon's Air Force Press Desk. nasaspaceflight.com says even if there is a test, the landing equiment has been left in place for a possible landing. While some details of its payload have been revealed - we still do not know exactly what's on board.

Theories have ranged from it being a space bomber, to a clandestine probe on a mission to 'take out' spy satellites. United Launch Alliance's 206ft Atlas V rocket launched the space plane from Launch Complex 41 at Cape Canaveral last May. The mystery vehicle, essentially a technology test bed, is designed to orbit the Earth and then land like one of Nasa's old shuttles.

According to X-37B manufacturer Boeing, the space plane operates in low-earth orbit, between 110 (177km) and 500 miles (800km) above earth. ....... The U.S. Air Force's unmanned X-37B space plane has flown three previous secret missions to date.

Each time it has carried a mystery payload on long-duration flights in Earth orbit.

The spacecraft looks similar to Nasa's space shuttle but is much smaller. The X-37B is about 29ft (8.8m) long and 9.5ft (2.9 m) tall.

It has a wingspan of just less than 15ft (4.6 m). At launch, it weighs 11,000lbs (4,990kg).

The craft is taken into orbit on a rocket but lands like the space shuttle by gliding down to Earth.

While it's main mission payload is a mystery, Nasa has revealed it has a materials experiment aboard.

The Planetary Society is tagging along with a solar-sail demo.

Ten CubeSat nanosatellites are also taking a piggyback ride into orbit.

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