The exact details of these experiments are kept secret. In a statement, the Air Force revealed only that the program "performs risk mitigation, experimentation and development of concepts for operations for reusable space vehicle technologies."
This is the fifth X-37B spacecraft to orbit in the last decade, each flight longer than its predecessor.
"Our team was preparing for this event and I am extremely proud to see their hard work and dedication reach their peak in today's safe and successful landing of the X-37B," said Brig. Gen. Doug Chias in a statement issued Sunday.
Each mission is highly secretive, leading to public speculation that the planes could be used for spy work or space weapons testing.
Here's what we know about the X -37B: The craft is designed to test new navigation systems, as well as methods for safely introducing the Earth's atmosphere and land.
The X-37B spacecraft is approximately 29 feet long and 9.5 feet (2.9 meters) high, with a wingspan of about 4.5 feet (4.5 meters).
The planes also have a unique design. They launch powerful rockets into orbit and then break away to accomplish their mission. When they return to Earth, they touch the runway horizontally, like a commercial plane or a space shuttle coming for a landing.
The plane that just landed was launched into space in September 2017 on a SpaceX Falcon 9. In its Sunday press release, the Air Force announced that it was preparing to launch its sixth X-37B mission in 2020