THE US Air Force's top secret X-37B space plane will return to Earth sometime next month after spending over a year in orbit.

The X-37B, also known as the Orbital Test Vehicle 2, or OTV-2, was launched in March 2011 atop an Atlas V rocket from Cape Canaveral, Florida.

It is due to land at California's Vandenberg Air Force Base sometime in early- to mid-June, with the exact date dependent on technical and weather conditions, the military said.

"The men and women of Team Vandenberg are ready to execute safe landing operations anytime and at a moment's notice," Colonel Nina Armagno, 30th Space Wing commander, said in a statement.

The military originally said the 8.8-meter-long, 2.4-meter-wide craft, which is shaped like a small space shuttle, was only supposed to spend up to 270 days in space.

The Boeing-build OTV-2, funded through the Pentagon's budget, was performing classified experiments for the Air Force's Rapid Capabilities Office.

Its landing will mark the third automated runway landing for a space plane - the first was achieved by the former Soviet Union's Buran space shuttle, and the second was completed by the OTV-2's sister vehicle, the OTV-1, which was launched in April 2010 and landed later that year.

Originally published as US's secret space plane to return to Earth