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

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 Sept. 1, the space plane had spent 630 days in orbit. That's one year, eight months, 22 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)]

A secretive space plane

The reusable X-37B looks like a mini version of NASA's now-retired space shuttle. This space plane is 29 feet (8.8 meters) long and 9.5 feet (2.9 m) tall, and has a wingspan of nearly 15 feet (4.6 m).

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. (4,990 kilograms).

The key to the X-37B's longevity in space rests with its ability to use solar panels to generate power., the solar panels extend the craft's longevity. [How the X-37B Space Plane Works (Infographic)]

"While far above the longevity of any other reusable spacecraft, it is far below that of most U.S. satellites, which are built to last for years, even decades," Johnson-Freese told Space.com. "That certainly confirms the broad, officially stated goal of the X-37B as a test bed vehicle."

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Secret space plane passes 600 days in orbit