U.S. Air Force's X-37B space plane in orbit more than 620 days

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.

Credit: 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 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.

US Military's Secretive Robot Space Plane: X-37B Mission Passes 5-Month Mark

NASA's Marshall Space Flight Center image shows on-orbit functions for the reusable X-37 space plane, now under the wing of the U.S. Air Force. 

CREDIT: NASA/MSFC

The U.S. Air Force's robotic X-37B space plane has quietly passed the five-month mark on its latest secret mission in Earth orbit.

The unmanned X-37B spacecraft launched into space atop an Atlas 5 rocket from Florida’s Cape Canaveral Air Force Station on Dec. 11, 2012, kicking off a mission whose objectives and payloads are classified.

The winged craft is known as Orbital Test Vehicle-3 (OTV-3), since it is conducting the third mission of the Air Force's X-37B program.

What OTV-3 is up to on its confidential cruise remains unknown.

However, a network of vigilant skywatchers is monitoring the mission as it progresses.

"It’s certainly important not to forget about these programs," said Ted Molczan of Toronto, a leader in the worldwide community of satellite trackers.

"Careful observation over a long time may provide the clues to finally solve the mystery."

Molczan said that, even then, he suspects that any breakthrough in knowledge regarding X-37B’s orbital missions will more likely result from leaks by insiders to journalists.

"Hobbyist observations can provide corroboration and some interesting, even useful details, but seldom are sufficient to expose the big picture…especially with new programs," he reported. "With X-37B, we can only watch and wait."