A robotic U.S. Air Force space plane is set to land in California as soon as this week after a secretive 22-month flight hundreds of miles above Earth.

Artist's concept of an X-37B spacecraft in orbit. Credit: Boeing Built by Boeing Co., the X-37B space plane will touch down on a runway at Vandenberg Air Force Base, Calif., the Air Force said Friday, but officials did not say when the spacecraft was scheduled to land.

The return will end the mysterious X-37B space plane program's third mission, which passed 671 days in orbit Monday. Its activities in space have been kept secret by the Air Force, but some analysts speculate the winged spaceship could test next-generation surveillance, communications and intelligence-gathering instruments, deploy small satellites, or demonstrate new materials for use in future military programs.

The program's cost is also kept under wraps by the Pentagon.

Resembling a miniature space shuttle, the 29-foot-long X-37B space plane takes off on top of a conventional launch vehicle, deploys solar panels to generate electricity in orbit, then returns to Earth like a glider for an automated landing on a runway.

"Preparations for the third landing of the X-37B, the Air Force's unmanned, reusable space plane, are underway at Vandenberg Air Force Base," the Air Force said Friday in a press release. "The exact landing date and time will depend on technical and weather considerations."

The spaceship's return was expected to come as soon as Tuesday.

The Air Force had warned pilots to stay away from Vandenberg's three-mile-long runway from 8 a.m. to 5 p.m. local time (1500-0000 GMT; 11 a.m.-8 p.m. EDT) Tuesday. But the airspace closure was rescheduled for the same time Thursday during a follow-up notice to pilots.

"Team Vandenberg stands ready to implement safe landing operations for the X-37B Orbital Test Vehicle, the third time for this unique mission," said Col. Keith Balts, commander of the Air Force's 30th Space Wing.

The X-37B space plane launched on the Orbital Test Vehicle 3, or OTV 3, mission from Cape Canaveral on Dec. 11, 2012, aboard a United Launch Alliance Atlas 5 rocket.

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X-37B space plane scheduled to return to Earth soon