Story highlights The unmanned X-37B looks like a small space shuttle

Space plane has been in orbit for more than 22 months

Air Force says mission of X-37B is to demonstrate technologies

Speculation abounds about secret weapons, spying

(CNN) The U.S. Air Force's mysterious unmanned space plane, the X-37B, is about to come back to Earth after nearly two years in orbit on a mission the military won't tell us much about.

The X-37B is expected to land at Vandenberg Air Force Base in California, the Air Force said.

The base did not give an exact time for the landing, but a notice to aviators and mariners on the Federal Aviation Administration's website said airspace around the Southern California base would be closed from 8 a.m. to 5 p.m. PT Tuesday.

Photos: Air Force X-37B space plane Photos: Air Force X-37B space plane Air Force X-37B space plane – The Air Force launched its third mission of its Orbital Test Vehicle, the X-37B, on December. 11, 2012, from Cape Canaveral, Florida. Hide Caption 1 of 6 Photos: Air Force X-37B space plane Air Force X-37B space plane – The X-37B is lifted into orbit by an Atlas V rocket. Hide Caption 2 of 6 Photos: Air Force X-37B space plane Air Force X-37B space plane – The last mission of the X-37B landed at Vandenberg Air Force Base, California, on June 16, 2012. It spent 469 days in space during its mission. Hide Caption 3 of 6 Photos: Air Force X-37B space plane Air Force X-37B space plane – The Air Force's first X-37B mission landed at Vandenberg in the early morning of December 3, 2010. It spent more than 220 days in orbit during its maiden voyage. Hide Caption 4 of 6 Photos: Air Force X-37B space plane Air Force X-37B space plane – Technicians in self-contained atmospheric protective ensemble suits conduct initial checks on the X-37B after its landing on December 3, 2010. Hide Caption 5 of 6 Photos: Air Force X-37B space plane Air Force X-37B space plane – The X-37B Orbital Test Vehicle sits on the runway at Vandenberg Air Force Base on December 3, 2010, during post-landing operations. Hide Caption 6 of 6

"Team Vandenberg stands ready to implement safe landing operations for the X-37B Orbital Test Vehicle, the third time for this unique mission," Col. Keith Baits, commander of the 30th Space Wing, said in a statement.

The X-37B, which looks like a small space shuttle, lifted off from Cape Canaveral in Florida on December 11, 2012. At the time, the Air Force said its mission would last about nine months.

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