On May 7, the Kennedy Space Center in Cape Canaveral, Florida got a loud surprise.

One of the Air Force’s two secret X-37B space planes announced its re-entry to Earth with a loud sonic boom.

The plane is part of the Air Force’s X-37 program.

Not much is known about this program except that the Air Force has launched four missions since 2010.

Moreover, the plane that returned from orbit on May 7 stayed in space longer than any of the previous three missions.

What are those space planes doing up in space?