The Air Force's Top-Secret Space Plane Has Returned To Earth

The U.S. Air Force's unmanned space plane has returned to Earth, completing a top-secret mission that lasted nearly two years.

The X-37B spacecraft landed safely at NASA's Kennedy Space Center Sunday morning after spending 718 days in orbit. 

It was launched into space by an Atlas 5 rocket back in May 2015.

It's still unclear exactly what the plane was doing up there. 

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The Air Force will only say it "performs risk reduction, experimentation and concept of operations development for reusable space vehicle technologies."

But, as CNN points out, many wonder if it was up to something a little more mysterious, like spy activities or testing a new weapon.

This was the fourth and longest trip for the space plane so far. The Air Force says it plans to launch it back into orbit again later this year.