The ultra-top-secret US Air Force X-37B spaceplane has broken the record for the longest time in space for a terrestrial aircraft, with an immense 717 days and counting.

Built by Boeing, the 29-feet long and 9.6-feet high craft has been shrouded in mystery and speculation since its first mission in 2010.

With a wingspan of nearly 15 feet, the craft is launched vertically into space while strapped to a rocket, but lands rather traditionally on a runway. The current record-breaking mission (OTV-5) was launched in September 2017 by a SpaceX Falcon 9 rocket. Like each of its predecessor missions, it has broken the previously held records for time spent in orbit.

What initially began as a NASA investigation into developing cheaper reusable space shuttles is now an ultra-secretive US military project.

“The primary objectives of the X-37B are twofold; reusable spacecraft technologies for America’s future in space and operating experiments which can be returned to, and examined, on Earth,” Air Force officials wrote of the endeavor.

The reported list of technology being tested on board the craft includes advanced guidance, navigation and control as well as advanced propulsion systems, advanced materials and autonomous orbital flight, re-entry and landing technology.

Many believe that the craft is some form of orbital laboratory for testing future reconnaissance technology, especially ones that could “disappear” unpredictably, including in low orbit.

The spaceplane orbits the Earth at an altitude of about 320km (under 200 miles), lower than the International Space Station, which necessitates more manoeuvers and fuel to maintain its position, fueling speculation that it is helping the USAF to create smaller, better-performing micro spy satellites capable of ‘disappearing’ into the ether at a moment’s notice.

Also on rt.com Space plane mystery: What is US Air Force X-37B doing in orbit? (POLL)

The US Air Force owns two 29-feet-long X-37B spacecraft, which each contain a seven-by-four-feet payload bay (roughly the size of a pick-up truck bed), which adds further credence to the micro spy satellite theory.

Former secretary of the Air Force, Heather Wilson, confirmed that the craft is capable of changing its orbit at will, preventing would-be counter spies from tracking it.

Think your friends would be interested? Share this story!