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A Boeing-built unmanned space shuttle that has been in orbit for a year on a secret U.S. Air Force mission is expected to land this week.

The experimental Boeing X-37B Orbital Test Vehicle was launched in March 2011. The Air Force has not revealed the nature of the mission.

A spokesman for Vandenberg Air Force Base, Calif., told ABC News the spacecraft s first landing availability was Wednesday, but weather or technical conditions could delay that. later reported that the landing could happen Friday.

During its first flight in 2010, the X-37B stayed in orbit 225 days.

According to Boeing, the X-37B is designed to operate in low-earth orbit, 110 to 500 miles above the Earth at 17,500 miles per hour. At 29-feet long and with a wingspan of 14 feet, 11 inches, the X-37B is approximately one-fourth the size of one of the retired space shuttles.

The X-37B was built in Southern California.

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