The US Air Force's unmanned X- 37B space plane, thought to be a spy bomber against targets like a Chinese space station, has returned to Earth after a nearly two-year secret mission.

The X-37B Orbital Test Vehicle mission 3 (OTV-3) touched down at Vandenberg Air Force Base in California on Friday after conducting experiments in orbit for 674 days, the US military said.

Conspiracy theorists endlessly conjecture on what the Pentagon is doing with "the newest and most advanced re-entry spacecraft."



The Air Force's two vehicles resemble small space shuttles, and have now logged a combined 1,367 days in space, the military said.

In the latest mission, the X-37B lifted off from Cape Canaveral in Florida on December 11, 2012. At the time, the Air Force said its mission would last about nine months.

The military has spoken only in generalities about the spacecraft and its mission, CNN reported today.

"Technologies being tested in the program include advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, and autonomous orbital flight, re-entry and landing," an Air Force statement said.

Theorists speculate the spacecraft is a space bomber, a spy plane against such targets as the Chinese space station, or merely an experiment as the government states, according to a Popular Mechanics story in 2012.

The previous mission of the X-37B, which landed at Vandenberg on June 16, 2012, lasted 469 days, according to the Air Force. That mission was flown by the second of the orbiters. The latest mission is the second for the first of the orbiters, which was refurbished after it spent 224 days in orbit following an April 2010 launch.

When the latest mission launched, the Air Force said it might not be the last.

"Officials anticipate multiple missions will be required to satisfy the test programme objectives, but the exact number of missions has not been determined," a statement said.

The X-37B spacecraft is 29 feet, 3 inches long and 9 feet, 6 inches high with a wingspan of 14 feet and 11 inches. It weighs about 5.5 tonnes. It is lifted into space by United Launch Alliance's Atlas V rockets.