The U.S. Air Force’s mysterious space plane is back in orbit for the sixth in a series of a secretive missions. The crewless X-37B blasted off on an Atlas V rocket on Sunday and will remain in orbit for an unspecified amount of time; the previous X-37B mission was two years long. Space.com reports that the shuttle-style plane is carrying a small satellite and two NASA experiments designed to study the effects of radiation on seeds—but its overall mission remains a secret.