An X-37B space plane recently touched down in California after spending two years in orbit on a secret mission. The return of the vehicle to Earth is driving speculation on the Internet over the nature of the classified mission.

The Boeing Orbital Test Vehicle (OTV) lifted off on Dec. 11, 2012, landing 675 days later at Vandenberg Air Force Base on Oct. 17. The landing was completely automated. Touchdown occurred at 12:24 EDT.

"With a program total of 1,367 days on orbit over three missions, these agile and powerful small space vehicles have completed more days on orbit than all 135 Space Shuttle missions combined, which total 1,334 days," Ken Torok, director of experimental systems at Boeing, said.

The first test mission of the OTV took place over 224 days, from April 22, 2010 to Dec. 3, 2010. The second test flight launched on March 5, 2011 on a 468-day flight that concluded on June 16, 2012.

The Air Force is tight-lipped about experiments performed aboard the X-37.

"The X-37B program is demonstrating a reliable, reusable unmanned space test platform for the Air Force. Its objectives include space experimentation, risk reduction and concept-of-operations development for reusable space vehicle technologies," Boeing officials reported the landing.

Theories on the Internet run a gamut, from the plausible to the improbable and beyond. Some people have proposed the spacecraft was packed with spy gear, while others say it may have been designed to trail the Tiangong-1 space lab built by China, in orbit around the Earth since 2011.

Surveillance, the most likely of any of the scenarios, could have been carried out from the spacecraft, using high-tech equipment. This would allow controllers to move to a place of interest faster than they could using traditional satellites. However, the cost in fuel for these maneuvers could prove prohibitive.

These same concerns over fuel use would likely eliminate any notion that the vehicles could have been used as a bomber. Some speculated the craft would be able to carry heavy inert cylinders, called "rods from god" that would explode on impact with the force of a tactical nuclear weapon.

Satellites operated by other nations could be the target of the X-37, possibly being able to capture some orbiting vehicles. However, the OTV was tracked by both professional and amateur observers, and no one reported a close approach with another satellite.

Tiangong-1, on the other hand, orbits at right angles to the flight of the X-37, and the two pass each other far too quickly for useful observation.

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