04/12/2013 08:58 GMT | Updated 03/02/2014 05:59 GMT

US Air Force's X37-B Secret 'Space Shuttle' Nears Year In Space

A mysterious space craft built and launched by the US Air Force has now spent roughly a year in orbit around Earth - though no one knows what it's doing there.

The X37-B spy plane is an unmanned drone similar in outward appearance to the Space Shuttle, though much smaller and without the capacity to carry astronauts.

The 11,000-pound craft is only 29 feet long, 15 wide and about a quarter of the size of the now-retired Space Shuttle.

It has previously spent two long periods in orbit - the first mission (Orbital Test Vehicle 1) lasted for 225 days, and the second (OTV2) lasted 469 days.

The latest mission, OTV3, seems set to carry on that trend, having now spent just under 358 days in space since its launch atop an Atlas V rocket from Cape Canaveral on 11 December, 2012.

Astronomers across the globe have kept track of the plane throughout its mission, noting that it has fired its thrusters several times during the flight. But what exactly it's doing, and what its payload is meant to accomplish, is classified. has an entertaining report on the speculation.

Eventually it is assumed the X37-B will return to Earth via an automatic, guided landing to either the Vandenberg Air Force Base in California or NASA's Kennedy Space Centre.