WASHINGTON - A pilotless space plane developed by the US Air Force has landed safely back on Earth after spending 469 days in orbit, officials said. The robotic X-37B, a sort of miniature space shuttle weighing just five tonnes and measuring some 29 feet long, touched down Saturday at Vandenberg Air Force Base in western California, the Air Force said in a statement. The reusable plane had launched from Cape Canaveral in southern Florida in March 2011 carrying "secret" equipment, feeding media speculation about its true mission. "The vehicle was designed for a mission duration of about 270 days," said X-37B program manager."We knew from post-flight assessments from the first mission that OTV-1 could have stayed in orbit longer. So one of the goals of this mission was to see how much farther we could push the on-orbit duration."A similar craft carried out a mission in 2010, staying seven months in space before also landing at the Vandenberg base some 160 miles (260 kilometers) northwest of Los Angeles.The space plane, which has a wing span of about 15 feet (4.5 meters) and operates at speeds 25 faster than the speed of sound, is a quarter of the size of the US space shuttle. The shuttle program was grounded in July 2011.Results from the X-37B will "aid in the design and development of NASA's Orbital Space Plane, designed to provide a crew rescue and crew transport capability to and from the International Space Station," NASA said in fact sheet.McIntyre said that following the retirement of the space shuttle fleet, the X-37B program "brings a singular capability to space technology development.""The return capability allows the Air Force to test new technologies without the same risk commitment faced by other programs," he added.Boeing, which is developing two of the vehicles, congratulated the Air Force on the craft's successful landing.Paul Rusnock, Boeing vice president of government space systems, said this second craft allowed them to test "the vehicle design even further by extending the 220-day mission duration of the first vehicle, and testing additional capabilities."We look 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."Among the new technologies being tested are the X-37B's advanced thermal protection and solar power systems, along with environmental modeling and range safety technologies, according to McIntyre."Each mission helps us continue to advance the state-of-the-art in these areas," he said.