Autonomous and Top Secret

Already on its fourth orbital mission, the United States Air Force's X-37B remains the world's most mysterious aircraft. Also called the X-37 Orbital Test Vehicle (OTV), the X-37B is expected to return from what could be its longest-ever clandestine mission in space — potentially beating its third mission's 674-day record.

That is, if it gets back to Earth after March 25. Of course, there isn't much we could say about it, as details about the X-37B's missions are highly classified. However, USAF spokeswoman, Captain AnnMarie Annicelli, did divulge a few crumbs of information in an interview with

"The landing date will be determined based on the completion of the program's on-orbit demonstrations and objectives for this mission," Annicelli said.

Developed by Boeing, the X-37B is an unmanned, autonomous spacecraft designed for prolonged orbital missions. The X-37B is a product of combined projects by NASA and DARPA with Boeing dating back to 1999, when the aerospace company was asked to develop a reusable orbital vehicle. Currently, the Air Force has two of these in service.

Long-Lasting Missions

The first X-37B mission, or OTV-1, lasted for 224 days (from April 22 to Dec. 3, 2010). OTV-2 managed to stay in space for more than a year (468 days), from March 5, 2011 until June 16, 2012. On its third, even longer orbital mission, the X-37B was launched on Dec. 11, 2012, and landed on Oct. 17, 2014, completing more than 670 days in space. Its current mission began May 20, 2015.

While we don't know much about the nature of these missions, Boeing's description of the X-37B gives us an idea. This drone spacecraft has been used "to explore reusable space vehicle technologies in support of long-term space objectives."

The USAF's 2015 X-37B fact sheet adds, "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, advanced propulsion systems, and autonomous orbital flight, re-entry, and landing."

This is especially relevant, as missions to Mars (and back to the Moon) are in the works.

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