Just what it was doing up there for more than 450 days is a mystery.

Space.com:

Exactly what the spacecraft, which is built by Boeing's Phantom Works division, was doing up there for so long is a secret. The details of the X-37B's mission are classified, as is its payload, with its mission overseen by the Air Force's Rapid Capabilities Office.

This secrecy has led to some speculation, especially online and abroad, that the X-37B could be a space weapon of some sort -- perhaps a sophisticated satellite-killer. Some experts also suspect that the vehicle may be an orbital spy platform.

The Air Force, however, has worked to tamp down such speculation, stressing repeatedly that the X-37B isn't doing anything nefarious hundreds of miles above the Earth's surface.

"This is a test vehicle to prove the materials and capabilities, to put experiments in space and bring them back and check out the technologies," Richard McKinney, the Air Force's deputy undersecretary for space programs, said shortly after OTV-1 landed in December 2010.

"My words to others who might read anything else into that is 'just listen to what we're telling you,'" McKinney added. "This is, pure and simple, a test vehicle so we can prove technologies and capabilities."

The X-37B looks a bit like NASA's recently retired space shuttle, but it's far smaller. The X-37B is about 29 feet (8.8 meters) long and 15 feet (4.5 m) wide, with a payload bay about the size of a pickup truck bed. Two X-37B vehicles could fit inside the payload bay of a space shuttle.

The spacecraft's orbital longevity is enabled by its solar array, which generates power after deploying from its payload bay and allows for longer spaceflights.

With China sending three astronauts - including a woman for the first time - into space, there is more urgency for the US to get back into the manned flight business. It's possible the military will turn the X-37B into a crew carrier - it's one of several options being considered. But the plane probably has other missions more important to national security that would be planned by the Pentagon.