18/06/2012 11:19 BST | Updated 18/08/2012 10:12 BST

X37-B: Mystery US 'Spy Plane' To Return From Space After Mission Ends

A mysterious American space plane, codenamed X-37B, has returned from space after 15 months in orbit.

The 'mini space shuttle', which is also known as the Orbital Test Vehicle-2, was unmanned and made an automatic landing from space.

The American military has never said what the plane was intended to do, or what its ultimate mission objectives may be.

The plane launched in 2010 atop an Atlas V rocket from Cape Canaveral, and landed at the Vandenberg Air Force Base, north of Los Angeles after more than 400 days in space - much longer than was originally intended.

The mission was called a "spectacular success" by General William Shelton, commander of Air Force Space Command.

"Although I can't talk about mission specifics, suffice it to say this mission has been a spectacular success," he said in May.

The plane has caused an inevitable tide of speculation and conspiracy theories about its mission to erupt. Some have said it could be monitoring Chinese progress in space, or else being used to target secret military satellites.

The US military will not say what the plane is for, but a third mission is now on the cards to launch in the autumn.

The next version of the craft - the X37-C - is said to be about twice as big as the previous craft, with space to carry astronauts if needed.

X37-B Spy Plane