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.

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.