The mysterious air force plane that landed on Sunday at NASA’s Kennedy Space Center was actually in space for 718 days. The plane is called X-37B, and its only one of the two test advanced spacecraft sent outer space. It is 29 feet long and is built with 14-foot wingspan which is four times smaller than NASA’s space shuttles.

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This spacecraft is a part of the X-37 program, and its first launch happened in 2010. The vehicle returned to Earth just after eight months but had another launch the following year. The second trip lasted for 15 months followed by its third launch which lasted for 22 months in 2012. Its last trip started in May 2015 and lasted for almost two years.

According to the 45th Space Wing commander, General Wayne Monteith, the team has been working on the project for some years and is very happy with the spacecraft’s successful landing. “I am extremely proud to see our hard work and dedication culminate in today's safe and successful landing of the X-37B," he said.

Many astronomers, including amateurs, have already spotted the spacecraft on their telescopes. The vehicle was flying lower than the altitude of International Space Station. The purpose of the plane’s launching is to program "performs risk reduction, experimentation, and concept-of-operations development for reusable space vehicle technologies," according to the Air Force.

The Air Force didn’t provide further details of the experimentation but said it is for advanced guidance, control and navigation, advanced propulsion systems and much more. Photos of the project that shows the technical aspect of the vehicle has been released. The engine of the spacecraft appears to run on nitrogen tetroxide and hydrazine.