The U.S. military's secret space plane is preparing to return from its second mission after an incredible 453 days in orbit (as of today). The robotic X-37B, which resembles a miniature space shuttle, is due to land at Vandenberg Air Force Base in California sometime in early to mid-June, depending on weather and other technical factors.

"The men and women of Team Vandenberg are ready to execute safe landing operations anytime and at a moment’s notice,” Col. Nina Armagno, commander of the Air Force's 30th Space Wing, said in a statement.

The X-37B Orbital Test Vehicle 2, or OTV-2, launched March 5, 2011. That means it's been in orbit for more than twice the duration of the orbit-hopping inaugural mission of its predecessor, OTV-1. It's also been up there much longer than anyone anticipated – the mission was originally planned for 270 days, only a few weeks longer than the 225-day mission of the first flight. Why so long? Some have speculated that it might be spying on the Chinese space station Tiangong. Or maybe the Air Force is just trying to squeeze out all possible value from an experimental spacecraft in a time of relative budget austerity.

We still don't know what the 29-foot-long X-37B has been doing up there. In the wake of the first X-37B mission, and with its pickup-bed-sized payload bay, analysts theorized it could be a commando transport, a bomber or an orbital spy. It could launch, repair or reposition U.S. satellites in low orbit. It could carry sensors. It could even sneak up and disable or steal enemy satellites.

It could also just be a way to test technology in space, and do so cheaply, as the Air Force maintains. Though, a reusable – albeit unmanned – spacecraft shaped like an airplane can also be more expensive to launch and more difficult to maneuver than disposable spacecraft. The ambiguity surrounding the X-37B even sparked a minor space race last year as Russia and China threatened to build similar vehicles.

"We are very pleased with the results of ongoing X-37B experiments. The X-37B program is setting the standard for a reusable space plane and, on this one-year orbital milestone, has returned great value on the experimental investment," Lt. Col. Tom McIntyre of the Air Force Rapid Capabilities Office told Space.com in March, as the plane hit its one-year mark in orbit.

Whatever the X-37B's fate may be, the Air Force still plans to put it back into space for another mysterious mission. OTV-1 is slated to blast back into orbit in October.