For nearly two years, the Air Force's X-37B autonomous space plane has been circling around the Earth doing who knows what. Now it's finally coming back to Earth, carrying all its secrets with it.

The X-37B has been the subject of intense speculation ever since it was launched on its very first mission in April 2010 with very little explanation from the Air Force. A second plane followed in March 2012, In December 2012, that first plane was launched back into space, where it has been presumably spying on Earth ever since. It'll land at an Air Force base in California on Tuesday, as long as the weather holds up.

The Daily Beast reports that the X-37B most likely carries a bevy of radars, cameras, and other sensors that you might find on your regular spy satellite. But a plane that can go to space and back has a leg-up over a dumb old satellite:

The idea is that the X-37B carries "specialized" sensors packages that can be reconfigured as needed for each mission when the aircraft returns to Earth. That ability to reconfigure the robotic spacecraft makes the X-37B cheaper and more flexible than a satellite—which goes up once with one package of sensors and is eventually discarded. Satellites can often cost billions of dollars and cannot reconfigured or reused, unlike the X-37B.

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That seems like a perfectly reasonable explanation, but that hasn't stopped the fevered speculation over what the X-37B was really doing up in space. Rumors abound that the X-37B is supposed to snatch foreign satellites or strike our enemies as an orbital bomber. Whatever it is, the Air Force is still keeping mum. [The Daily Beast, U.S. Air Force]

Top photo: The X-37B being launched from Cape Canaveral, Florida, in December 2012. Credit: U.S. Air Force