Mysterious American ‘space plane’ has been in orbit for 700 days - and no one knows why

The U.S. Airforce's X-37B Orbital Test Vehicle mission 4 after landing at NASA's Kennedy Space Center Shuttle Landing Facility in Cape Canaveral, Florida, U.S., May 7, 2017.  U.S. Air Force/Handout via REUTERS  ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY.     TPX IMAGES OF THE DAY
The U.S. Airforce's X-37B Orbital Test Vehicle (Reuters)

A mysterious robotic ‘space plane’ has stayed in orbit for nearly two years - and few people know what it’s really up to.

The Boeing X-37B craft - also known as Orbital Test Vehicle 4 - has now been in orbit for 700 days.

It launched on its latest mission on September 7, 2017, Space.com reports, and is now just a few days short of its all-time record for remaining in space.

Observers have speculated that the Space Shuttle-esque vehicle might be designed to destroy satellites - or work as a ‘movable’ satellite itself.

The X-37B Orbital Test Vehicle mission 3 space plane is shown after landing at Vandenberg Air Force Base, California October 17, 2014 in this handout photograph provided by Vandenberg Air Force Base. The United States military landed the robotic space plane in central California on Friday, ending a classified 22-month mission that marked the third in Earth orbit for the experimental program, the Air Force said. REUTERS/Boeing/Vandenberg Air Force Base/Handout via Reuters (UNITED STATES - Tags: SCIENCE TECHNOLOGY MILITARY) ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY, CONTENT, LOCATION OR DATE OF THIS IMAGE. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS PICTURE IS DISTRIBUTED EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS
The X-37B Orbital Test Vehicle (Getty)

When it landed from a previous mission, it was greeted by ground crew in biohazard suits - sparking gossip that there was something radioactive on board.

The aircraft has a wingspan of less than 15ft - and is taken into space on a rocket, but glides back to Earth like a space shuttle.

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NASA has said that there is a materials experiment on board - but its main payload remains a mystery.

During the aircraft’s previous missions, amateur astronomers were able to detect the orbital pattern of the first X-37B which included flyovers of North Korea, Iraq, Iran, Pakistan and Afghanistan, heightening the suspicion that the vehicle was being used for surveillance.

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