WASHINGTON- A top secret US military space plane has landed in California after almost two years in orbit around the Earth.

The unmanned X-37B, which is also known as an Orbital Test Vehicle, was launched in December 2012 by the US Air Force. It was the space plane's

third known and longest mission. The Air Force said the X-37B robotic space plane landed at Vandenberg Air Force Base in central California on Friday, ending a classified 22-month mission that marked the third in Earth orbit for the experimental program. The X-37B, a 9-metre winged craft that resembles a miniature NASA space shuttle, touched down at 9:24am (local time).

The craft was carried into orbit for its latest mission aboard an unmanned Atlas 5 rocket from Cape Canaveral Air Force Station in Florida on December

11, 2012. The spacecraft conducted unspecified experiments while in orbit, the Air Force said.

The space plane first flew in April 2010 and returned after eight months. A second vehicle blasted off in March 2011 and stayed in orbit for 15 months.

The Air Force said the orbiters, built by Boeing, "perform risk reduction, experimentation and concept-of-operations development for reusable space

vehicle technologies," although details of the missions are secret the total program costs and budget line are classified. Last week, the Air Force and NASA finalized a lease agreement to relocate the space plane program from California to Florida's Kennedy Space Center.

The Secure World Foundation, a non-profit group promoting the peacefulexploration of space, said the secrecy surrounding the orbital activitiesand payloads of the X-37B were almost certainly due to the presence ofnational intelligence-related hardware being tested or evaluated.The air force is preparing to launch the fourth X-37B mission from CapeCanaveral in 2015.