The latest secretive mission of the United States Air Force's X-37B space plane has cruised beyond 500 days in Earth orbit since its launch last year. 

The U.S. military launched the robotic X-37B space plane on May 20, 2015, marking the fourth flight for the Air Force program. A United Launch Alliance Atlas V rocket lofted the spacecraft from Florida's Cape Canaveral Air Force Station to kick off the OTV-4 mission (short for Orbital Test Vehicle-4).

Exactly what the winged space plane's duties are while it's in orbit continues to remain a tight-lipped affair. Similarly, how long the vehicle will remain in orbit has not been detailed. [The X-37B's Fourth Mystery Mission in Photos]

The first OTV mission launched in April 22, 2010, and concluded on Dec. 3, 2010, after 224 days in orbit. The second OTV mission — which used a different vehicle than the first — began March 5, 2011, and concluded on June 16, 2012, after 468 days on orbit. The subsequent OTV-3 mission reused the X-37B that flew on the first mission, and chalked up nearly 675 days in orbit.

So far, the U.S. military has not stated where the OTV-4 mission's craft will ultimately land once it's current flight ends. In the past, all three X-37B flights ended at Vandenberg Air Force Base in California, gliding to a runway landing on autopilot.

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