The mysterious X37-B plane is headed back to space tomorrow for a third mission, though we're still no closer to knowing exactly what the United States Air Force is using the aircraft for. Yet again the cargo being carried inside the unmanned Orbital Test Vehicle 3 (OTV-3) is heavily classified. The Air Force is touting one aspect of the mission however, and that's reusability. The X37-B plane being prepared for Tuesday's liftoff was the same one used in the inaugural 224-day OTV-1 flight in 2010. Officials say repurposing the aircraft in this manner demonstrates "a reliable, reusable, unmanned space test platform for the United States Air Force."

OTV-2 returned in June after an extensive 469-day mission — 200 days longer than originally planned. As the Pentagon has refused to offer any insight into these missions, critics maintain the Orbital Test Vehicle is being used for reconnaissance purposes. One report even accused the X37-B of spying on China, though that theory has met resistance of its own.