LOS ANGELES—The U.S. Air Force X-37B spaceplane landed at Vandenberg AFB, California, on Oct. 17, completing a record-setting 675 days in orbit.

The end of the third Orbital Test Vehicle (OTV-3) mission comes as the Air Force prepares its second X-37B for a return to space. The Air Force operates two of the Boeing-built vehicles and last flew the second on a mission that concluded at Vandenberg in June 2012 after 469 days in space.

The winged X-37B, which at 29-ft. in length, resembles a scaled-down space shuttle, was first flown into orbit on a United Launch Alliance Atlas V launch vehicle in April 2010. The purpose of the first OTV-1 mission, like all subsequent tests, remains unknown. The Air Force denies speculation that the vehicles are designed to test advanced space-based intelligence, surveillance and reconnaissance systems or actively engage, capture or disable other craft. Instead it says the X-37B is designed as a flying technology testbed.

The Air Force says the suite of technologies under evaluation 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, re-entry and landing systems.

While the precise purpose of the long-duration flights remains something of a mystery, the orbital path of the vehicles has been tracked by ground observers from the first mission. For OTV-3, which was launched on a ULA Atlas V in December 2012, the orbital height was reportedly lower than the previous missions at around 218 mi., while its path ranged from 43.5 deg. north to 43.5 deg. south.

OTV-3 may also be the last X-37B to land at Vandenberg. The Air Force commented in July that the X-37B’s yet-to-be confirmed fourth mission could land in Florida at Kennedy Space Center’s Shuttle Landing Facility. Boeing has begun work to modify a former space shuttle processing facility at the site for X-37B refurbishment and will complete the work at the end of this year.