An unmanned US Air Force space plane steered itself to a landing at a California military base on Saturday, 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, 209km from Los Angeles.

In 2010, an identical unmanned spacecraft returned to Earth after seven months and 146million kilometres in orbit.

The latest homecoming was set in motion when the robotic X-37B fired its engine out of orbit, then pierced through the atmosphere as it glided down the runway.

"With the retirement of the Space Shuttle fleet, the X-37B OTV programme brings a singular capability to space technology development," said Tom McIntyre, the X-37B's programme manager.

"The return capability allows the Air Force to test new technologies without the same risk commitment faced by other programmes. We're proud of the entire team's successful efforts to bring this mission to an outstanding conclusion."

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.