CAPE CANAVERAL, Fla. – 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.

The #AirForce #X37B #OTV4 has returned from orbit and has landed safely @NASAKennedy. Stay tuned for more info. — U.S. Air Force (@usairforce) May 7, 2017

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.

The #X37B #OTV4 is the newest and most advanced re-entry spacecraft. Find out more about today's landing here: https://t.co/GUGgOMQiYg pic.twitter.com/HfHHVnWhYc — U.S. Air Force (@usairforce) May 7, 2017

Officials said several technologies are being tested in the program.

"The primary objectives of the X-37B are twofold; reusable spacecraft technologies for America’s future in space and operating experiments which can be returned to, and examined, on Earth," officials said.

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Air Force officials said the X-37B is the first vehicle able to return experiments to Earth for further inspection and analysis since NASA's Shuttle Orbiter. The X-37B is able to stay in space longer because if its on-orbit time of 270 days or more, according to officials.

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