An Air Force X-37B robotic space plane is shown after it landed at Vandenberg Air Force Base in California. The fourth mission of the program is now underway and just passed 600 days in Earth orbit.
The U.S. Air Force's mysterious X-37B space plane has now spent 600 days in Earth orbit on the vessel's latest mission, and is nearing a program record for longest time spent in space.
The robotic X-37B lifted off atop a United Launch Alliance Atlas V rocket from Florida's Cape Canaveral Air Force Station on May 20, 2015, kicking off the program's fourth space mission (which is known as Orbital Test Vehicle-4, or OTV-4).
If the uncrewed spacecraft spends 74 more days aloft, it will break the duration record set during OTV-3, which touched down in October 2014. [The X-37B's Fourth Mystery Mission in Photos]
But it's unclear how long OTV-4 will last, or just what the X-37B is doing as it circles Earth; most details about the space plane's missions and payloads are classified.