The Boeing-built X-37B

The US Air Force’s unmanned space plane returned on early on Saturday to a California military base after a mysterious 15-month mission.

The unmanned spacecraft X-37B, which was launched from Cape Canaveral Air Force Station in Florida in March 2011, conducted in-orbit experiments during the mission, Air Force officials said.

“With the retirement of the Space Shuttle fleet, the X-37B Orbital Test Vehicle-2 (OTV-2) program brings a singular capability to space technology development,” CBS quoted Lt Col Tom McIntyre, the X-37B’s program manager, as saying.

“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,” McIntyre added.

The X-37B space planes are part of a military program to test robotically controlled reusable spacecraft technologies.

The latest X-37B plane was designed to stay in orbit for nine months as the Air Force wanted to test its endurance. After determining the space plane was performing well, the military decided to extend the mission.

Many sceptics think that the vehicle’s mission is defence or spy-related. There are rumours circulating that the craft has been kept in space to spy on the new Chinese space station, Tiangong.

However, analysts have pointed out that surveillance would be tricky, since the spacecraft would rush past each other at thousands of metres per second.

Other industry analysts have speculated that the Air Force is just making use of the X-37B’s amazing fuel efficiency and keeping it in space for as long as possible to show off its credentials and protect it from budget cuts.