The U.S. Air Force's second X-37B space plane marked one year in orbit Monday, continuing its clandestine mission more than 200 miles above Earth.

Artist's concept of an X-37B spacecraft in orbit. Credit: Boeing Co.
 
The robotic spacecraft's purpose is secret, but Air Force officials acknowledge the vehicle is performing well one year after it blasted off on a United Launch Alliance Atlas 5 rocket on March 5, 2011.

"We are very pleased with the results of the on-going X-37B experiments," said Lt. Col. Tom McIntyre, X-37B program director in the Air Force's Rapid Capabilities Office. "The X-37B program is setting the standard for a reusable space plane and, on this one-year orbital milestone, has returned great value on the experimental investment."

The X-37B space plane, also known as the Orbital Test Vehicle, features an unpressurized cargo bay about the size of the bed of a pickup truck. The Air Force has not disclosed its contents, only saying the mission is experimenting with the spacecraft, which is flying on its second mission.

The solar-powered vehicle is designed to return cargo intact inside its payload bay.

Another space plane orbited Earth for 224 days in 2010 before gliding to an unpowered landing at Vandenberg Air Force Base, Calif.

A third flight, reusing the first vehicle, is in the Air Force's plans.

"Upon completion of all objectives we look forward to bringing the mission to a safe, successful conclusion," McIntyre said in a statement. "The next program phase, the third X-37B mission, has been tentatively planned for the fall of 2012."

The landing date has not been set, according to Maj. Tracy Bunko, an Air Force spokesperson at the Pentagon.

"The mission is still on track and the vehicle is performing well," Bunko said. "As stated before, we are extending the mission as circumstances allow in order to get maximum value out of the mission. We don't know when we'll land, but we regularly evaluate that based on test objectives."

File photo of the launch of the second X-37B mission on March 5, 2011. Credit: Pat Corkery/United Launch Alliance
 
The Air Force decided to continue the space plane's mission beyond its 270-day design life, which ran out in late November. Before its launch in March, officials hinted the second X-37B flight would test the spacecraft's design, pushing it farther than engineers did the first space plane.

But other than demonstrating the craft's ability to loiter in space and return to Earth, the mission's other objectives remain shrouded in secrecy.

The space plane could ferry into orbit materials science payloads, experimental reconnaissance sensors, innovative communications instruments, or a variety of other potential cargo.

The X-37B stretches 29 feet long and has a wing span of 14 feet. It can weigh up to 11,000 pounds fueled for launch, according to an Air Force fact sheet.

Built by Boeing Co., the X-37 space plane started off as a NASA project. The Defense Department took over in 2004, and responsibility for the X-37 ended up with the Air Force in late 2006. Boeing built two space-worthy X-37 vehicles.

Once its mission is over, the X-37B will fire a powerful engine to fall from orbit. After weathering the brutal heat of re-entry, the stubby-winged spacecraft will automatically approach a 15,000-foot runway at Vandenberg with the help of GPS navigation, line up with the landing strip, deploy a landing gear and touch down at more than 200 mph.

Independent satellite trackers say the space plane is circling Earth between 205 miles and 212 miles high, with an orbital inclination of about 42.8 degrees.

"Its ground track repeats almost exactly every 31 revolutions, which takes just under two days," said Ted Molczan, a respected amateur satellite observer based in Canada.

Molczan and other satellite tracking hobbyists compare notes online to keep tabs on spacecraft in orbit.

"Ground tracks that repeat every two to four days are a common feature of U.S. imagery intelligence satellites, but that is insufficient information to confidently assess the mission of OTV 2-1."

The X-37B's orbit tracks above the part of Earth between 42.8 degrees north and 42.8 degrees south latitude.