X-37B space plane is nearing orbital record
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The US Air Force's mysterious space plane is on track to break the mission duration record of 674 days.

The solar-powered space plane, which looks a lot like a miniature version of NASA's space shuttles, had been originally designed to repair satellites before NASA discontinued the project and passed it over to the US Department of Defense back in 2004.

Since then the exact nature and purpose of the X-37B have remained a total mystery. All we know is that it is capable of spending years in orbit on a single mission without having to return to Earth.

Some have speculated that it is being used to carry out covert operations over foreign nations or that it is transporting new instruments in to orbit for use with spy satellites.

Whatever the case, its latest mission, which launched back in May 2015, is looking to be the longest yet. If it remains in orbit until March 25th it will have officially broken its record of 674 days in space.

Exactly what it has been doing up there all this time however remains as unclear as ever.