A top secret US robot space plane landed back on Earth on Friday after a 22-month orbit, officials said, although the craft's mission remains shrouded in mystery.

The unmanned X-37B, which looks like a miniature space shuttle, glided into the Vandenberg Air Force Base in California after having launched on December 11, 2012, on a mission that military officers say is still strictly secret.

"I'm extremely proud of our team for coming together to execute this third safe and successful landing, said commander Keith Balts of the US military's 30th Space Wing, after the 9:24 am (1624 GMT) landing.

Analysts say the X-37B could be a platform for spying from space, including possibly snooping on other countries' satellites.

But officials have previously denied the project had anything to do with creating a "space weapon" that could knock down other satellites.

The Air Force says the X-37B can test technology for "reusable" spacecraft and conduct unspecified experiments that can be studied on Earth.

The latest mission was the third and the longest so far for the vehicle. An initial flight launched in 2010 lasted about eight months and a second flight had the spacecraft in orbit for more than 15 months.

The X-37B, manufactured by aerospace giant Boeing, weighs five tonnes and measures about 29 feet (8.8 meters) long, with a wing span of roughly 15 feet across.

Traveling at speeds 25 times faster than the speed of sound, the vehicle is launched into space on the back of a rocket and, once its mission is complete, returns from orbit like a plane.

But, unlike NASA's civilian shuttle, it has two stabilizers in the rear instead of one, forming a "V" shape.