The military won't say what it has been doing with its experimental miniature space shuttle, but the pilotless spaceship, known as the X-37B, has been in orbit for a year now.

The 29-foot robotic spacecraft, also known as the Orbital Test Vehicle, or OTV, was launched on March 5, 2011, on a follow-up flight to extend capabilities demonstrated by a sistership during a 244-day debut mission in 2010.

"We are very pleased with the results of ongoing X-37B experiments," Tom McIntyre, with the Air Force Rapid Capabilities Office, wrote in a statement emailed to Discovery News.

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"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," McIntyre said.

No word on when the spaceship will be landing, but the military is planning to refly its first X-37B craft this fall.

Amateur satellite watchers last spotted the spaceship on March 4 as it circled between 204 and 212 miles above the planet in an orbit inclined 42.8 degrees relative to the equator.

The spaceship's altitude hasn't changed much since launch, satellite-hunter Ted Molczan told Discovery News.

As of Monday, the spaceship was flying over the same ground track every 31 orbits, which takes slightly less than two days.

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"Ground tracks that repeat every two to four days are a common feature of U.S. imagery intelligence satellites," Molczan said. "It gives you a fairly frequent revisit of the same targets from the same vantage point."

The Air Force says it is using the experimental space plane to test technologies, but provides no details. It also wants to figure out if it is possible to quickly and easily refurbish and reuse a spaceship.

(Image: The military's first X-37B spaceplane, the Orbital Test Vehicle-1 landed in California on Dec. 3, 2010, and is being prepared for a reflight this fall. Its sistership, OTV-2, has now been in orbit for more than one year. Credit: U.S. Air Force.)