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.

Each mission has been highly secretive, leading to public speculation that the planes could be used for spying activity or testing space weapons.

The Air Force has provided some hints about its program. In a press rele