By Leonard David, SPACE.com's Space Insider Columnist

An artist's illustration of the U.S. Air Force's X-37B space plane in orbit. The solar-powered winged spacecraft has spent more than 620 days in orbit as part of the military's secret OTV-3 mission, which launched in December 2012.NASA Marshall Space Flight Center

The U.S. Air Force's classified X-37B space plane is prepared for its first spaceflight, OTV-1, in April 2010. The same space plane launched on the third X-37B mission, OTV-2, on Dec. 11, 2012. As of Aug. 29, 2014, the mission has reached 627 dNASA

General William Shelton (retired), former commander of the Air Force Space Command, has been a leader in rallying support for future of U.S. military space capabilities, including the X-37B robotic space plane.Credit: U.S. Air Force/Staff Sgt. Christopher Boitz

The U.S. Air Force's mysterious unmanned space plane has winged beyond 600 days in orbit on a classified military mission that seems to have no end.

The X-37B space plane is carrying out the Orbital Test Vehicle-3 (OTV-3) mission, a long-duration cruise that marks the third flight for the unpiloted Air Force spaceflight program.

The Air Force launched the miniature space shuttle into orbit on Dec. 11, 2012 using an expendable Atlas 5 rocket. By the end of Friday (Aug. 29), the space plane had spent 627 days in orbit. That's one year, eight months, 19 days and counting, to be exact.

"The Air Force continues to push the envelope of the solar-powered X-37B capabilities," said Joan Johnson-Freese, a professor of National Security Affairs at the U.S. Naval War College in Newport, Rhode Island. [Amateur Skywatcher Spots X-37B Space Plane (Video)]

The reusable X-37B looks like a mini version of NASA's now-retired space shuttle. This space plane is 29 feet long and 9.5 feet tall, and has a wingspan of nearly 15 feet.

The X-37B's payload bay is the size of a pickup truck bed. In contrast, NASA's space shuttle payload bay could fit two X-37B space planes comfortably inside. At liftoff, the X-37B space plane weighs 11,000 lbs.

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Space plane's epic orbit