A mysterious ‘space drone’ will touch down after orbiting the Earth for nearly two years.

The U.S. Air Force's “top secret” X-37B space drone, otherwise called the Orbital Test Vehicle, has been in flight since December 2012 on a mission almost no one knows anything about.

As with many mysteries, the interests of the internet have come up with thousands of ideas for missions X-37B could have been undertaking; mostly theories related to spying.

The 8.8 metre shuttle is packed with kerosene-based fuel and a large hydrogen peroxide tank. It has room labelled for “experiments”, but it is too small to carry any human passengers.

X-37B has been in space for over 670 days, far more than its two previous flights of just 225 and 469 days.

Insiders say two hangars at the Vandenberg Air Force Base in California have been cleared for its arrival, with Air Force Colonel Keith Baits, 30th Space Wing Commander, quoted as saying: “Team Vandenberg stands ready to implement safe landing operations for the X-37B Orbital Test Vehicle, the third time for this unique mission”.

The space drone has flown three missions to date, three that anyone has noticed at least.

Each time it carries a mystery payload and performs long-duration flights in low Earth orbit.

The craft travels into orbit on a traditional rocket, but can land on wheels like a terrestrial plane.