The secretive X-37B space plane of the U.S. Air Force has finally returned to Earth after almost two years of mystery-shrouded mission in orbit.

The 29-foot long Boeing-made reusable spacecraft, which can fly and stay in orbit unmanned, arrived at the Vandenberg Air Force Base in California on Friday, Oct. 17 at 9:24 m. local Pacific time. The vehicle was in orbit for an unprecedented 675 days for the Orbital Test Vehicle 3 (OTV-3) mission.

"The landing of OTV-3 marks a hallmark event for the program" an unnamed X-37B program manager said in a statement. "The mission is our longest to date and we're pleased with the incremental progress we've seen in our testing of the reusable space plane. The dedication and hard work by the entire team has made us extremely proud."

The OTV-3, which was the Air Force's third Orbital Test Vehicle mission, was launched in December 2012 but just with the first and second orbital missions, the military is mum on the details fueling speculations and conspiracy theories on what the space plane was particularly doing during tis covert mission.

Former Air Force space operations officer Brian Weeden, who is now technical advisor to the Secure World Foundation, a not for profit organization that promotes the peaceful exploration of space, said that the plane was probably conducting surveillance missions in orbit.

Weeden said that the secrecy of the mission suggests the plane could be doing work for the National Reconnaissance Office, the agency in charge of the country's intelligence satellites. Based on the plane's path, Weeden said that the X-37B was possibly gathering information from Middle East, Afghanistan, Africa, Southeast Asia and Latin America but not Russia.

"It was probably serving some sort of intelligence/surveillance/reconnaissance (ISR) function," Weeden said. "And the secrecy surrounding the mission being performed by the X-37B suggests the mission was being done for the NRO, perhaps to test out and evaluate new sensor technologies or techniques."

The Air Force does not provide extensive information about the vehicle's tasks but it did said that the objective of X-37B is to test technologies.

"Technologies being tested in the program include advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, and autonomous orbital flight, reentry and landing," the Air Force said in a statement.

Air Force officials said that the fourth X-37B mission is set to be launched sometime next year. 

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