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 The second X-37B Orbital Test Vehicle marked its one-year anniversary in space Monday, March 5, as Air Force officials continue to remain mum about any landing plans.

Once its mission is complete, the unmanned, reusable mini-space shuttle is scheduled to touch down on Vandenberg Air Force Base’s 3-mile-long runway. 

The vehicle was launched aboard an Atlas 5 rocket from Cape Canaveral Air Force Station, Fla., on March 5, 2011.

Its purpose in space remains top secret, with Air Force officials saying only that it’s conducting experiments.

“We are very pleased with the results of ongoing X-37B experiments,” Lt. Col. Tom McIntyre, Air Force Rapid Capabilities Office, said this week. “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. Upon completion of all objectives we look forward to bringing the mission to a safe, successful conclusion.”

Air Force officials are tight-lipped about the landing date, saying nothing has been set yet.

“We regularly evaluate that based on test objectives and will make an announcement when we have a firm date,” officials said.

The first X-37B spent more than 224 days in orbit before gliding onto the Vandenberg runway in the dark early on Dec. 3, 2010. It had logged 91 million miles, according to the Air Force.

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Officials said the vehicle, which has a 15-foot wingspan and length of 29 feet, was designed to orbit Earth for 270 days.

In an X-37B fact sheet, officials said the vehicle’s mission is twofold: proving reusable spacecraft technologies as well as operating experiments that can be returned to, and examined, on Earth.

Canadian amateur satellite observer Ted Molzcan, who along with other space hobbyists has been keeping an eye on the vehicle’s location in orbit and sharing his findings online, said X-37B’s orbit appears similar to U.S. imagery intelligence satellites.

A third X-37B mission has been tentatively planned for fall 2012, Air Force officials said. 

The X-37B, built by Boeing, began a decade ago as a NASA project and then was taken over by the Defense Advanced Research Projects Agency. When the those agencies abandoned the program, the Air Force stepped up through its Responsive Capabilities Office.