The landing of an unmanned military space plane at Kennedy Space Center Sunday morning concluded an almost two-year mission in orbit, according to the U.S. Air Force.

The Air Force tweeted shortly after 8 a.m. that the reusable X-37B Orbital Test Vehicle had landed safely, completing its fourth classified mission.

OTV-4 launched from Cape Canaveral on a United Launch Alliance Atlas V rocket on May 20, 2015.

Air Force officials have called the X-37B program, which is managed by the Air Force Rapid Capabilities Office, "the newest and most advanced re-entry spacecraft."

The Air Force's two mini-shuttles have completed two missions each since the first launch from the Cape in 2010.

Three prior X-37B landings occurred at Vandenberg Air Force Base in California.

The sonic boom could be heard throughout Central Florida. Some Brevard County residents might have heard the booms before the space plane landed at the Shuttle Landing Facility.