Hong Kong (CNN Business) The US Air Force's X-37B space plane landed Sunday back on Earth after spending 780 days in orbit, the longest mission in the mysterious military test program's history.

The uncrewed plane, which looks like a small space shuttle , conducted in-orbit experiments that could then be brought back to Earth for examination, according to the Air Force

The exact details of these experiments are a closely guarded secret. In a statement, the Air Force only revealed that the program "performs risk reduction, experimentation and concept of operations development for reusable space vehicle technologies."

This is the fifth X-37B space plane to be launched into orbit over the past decade, with each flight longer than its predecessor.

"Our team has been preparing for this event, and I am extremely proud to see their hard work and dedication culminate in today's safe and successful landing of the X-37B," said Brig. Gen. Doug Schiess in the statement released Sunday.

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