The U.S. Air Force’s second X-37B Orbital Test Vehicle (OTV-2) has landed after 469 days in space, more than double the time clocked by the OTV-1.

As with the first OTV flight, the Air Force remains secretive about the mission, saying only that the Boeing-built X-37B conducted “on-orbit experiments.” In a short statement, it says the vehicle provides “return capability” that allows the Air Force to test new technologies without the same risk faced by other programs.

The 29-ft.-long, small-winged vehicle touched down at Vandenberg AFB, Calif., at 5:48 a.m. PDT on June 16, marking the end of a 15-month mission that began with its launch on an Atlas 5 from Cape Canaveral on March 5, 2011. The extended flight of OTV-2 also demonstrated upgrades and adaptations to the 15-ft.-wingspan spacecraft, which was originally designed to stay aloft for 270 days.

Prior to its launch, the Air Force said the mission would “build upon the OTV-1 on-orbit demonstration, validate and replicate initial testing and fine-tune the technical parameters of the vehicle tests.” However, all other pertinent details of the testing, and what would be performed on-orbit, remain classified.

Further details of the OTV-2 flight are expected to emerge in the coming days. The first flight, OTV-1, was initially oriented with an orbital inclination of 40 deg. and a 250 X 260-mi. (403 km X 420 km) orbit. This was later raised to 268 X 276 mi. around five months before landing.

Paul Rusnock, Boeing vice president of government space systems, says “with OTV-1, we proved that unmanned space vehicles can be sent into orbit and safely recovered. With OTV-2, we tested the vehicle design even further by extending the 220-day mission duration of the first vehicle, and testing additional capabilities.”

Referencing the upcoming third flight of the reconditioned OTV-1, Rusnock says Boeing “looks forward to the second launch of OTV-1 later this year and the opportunity to demonstrate that the X-37B is an affordable space vehicle that can be repeatedly reused.”

The X-37 was originally built for a NASA human spaceflight technologies program under an agreement with Boeing in 1999. The deal covered development of the small, shuttle-like vehicle as a flying testbed for 40 key airframe, propulsion and operation technologies. The OTV is believed to be the only X-vehicle capable of conducting on-orbit operations and collecting test data in the Mach 25 reentry flight region. The X-37 gradually entered into the classified arena when it transitioned to the Defense Advanced Research Projects Agency in 2004 for use in its Approach & Landing Test Vehicle program.

Unlike the original X-37 designed for NASA, which was 27.5 ft. long and had the capability to stay in space for up to 21 days, the stretched X-37B has demonstrated 10 times the duration thanks to additional propellant and thermal protection capacity.