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An unmanned Air Force space plane steered itself to a landing early Saturday at a California military base, capping a 15-month clandestine mission. The spacecraft, which was launched from Cape Canaveral Air Force Station in Florida in March 2011, conducted in-orbit experiments during the mission, officials said. It was the second such autonomous landing at the Vandenberg Air Force Base, 130 miles northwest of Los Angeles. In 2010, an identical unmanned spacecraft returned to Earth after seven months and 91 million miles in orbit. The latest homecoming was set in motion when the stubby-winged robotic X-37B fired its engine to slip out of orbit, then pierced through the atmosphere and glided down the runway like an airplane. "With the retirement of the Space Shuttle fleet, the X-37B OTV program brings a singular capability to space technology development," said Lt. Col. Tom McIntyre, the X-37B's program manager. "The return capability allows the Air Force to test new technologies without the same risk commitment faced by other programs. We're proud of the entire team's successful efforts to bring this mission to an outstanding conclusion." With the second X-37B on the ground, the Air Force planned to launch the first one again in the fall. An exact date has not been set. The twin X-37B vehicles are part of a military program testing robotically controlled reusable spacecraft technologies. Though the Air Force has emphasized the goal is to test the space plane itself, there's a classified payload on board — a detail that has led to much speculation about the mission's ultimate purpose. Some amateur trackers think the craft carried an experimental spy satellite sensor judging by its low orbit and inclination, suggesting reconnaissance or intelligence gathering rather than communications. Harvard astrophysicist Jonathan McDowell, who runs Jonathan's Space Report, which tracks the world's space launches and satellites, said it's possible it was testing some form of new imaging.