IT has circled the Earth for a year at 27,000km/h on a top secret mission.

Now the US Air Force's unmanned space plane, the experimental Boeing X37-B, will land in June - ending its mission in orbit.

At launch, the craft was accompanied by staff in biohazard suits, leading to speculation there were radioactive components on board, The Daily Mail reports.

The plane resembles a mini space shuttle and is the second to fly in space.

It was meant to land in March, but the mission of the X-37B orbital test vehicle was extended for unknown reasons.

The first one landed in December at Vandenberg Air Force Base in California after more than seven months in orbit.

The 8.8m, solar-powered craft had an original mission of 270 days.

Secretive U.S. space mission launches An experimental miniature space plane blasts off from Cape Canaveral Air Force Station on a secret mission that could last nine months.

Originally published as FLYING, WHY? Plane on mission to ... er ... we'll get back to you on that one