LOS ANGELES (AP) — 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.