Photos: Boeing space plane returning from year-long secret mission

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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.