Early Sunday, a sonic booming could be heard across central Florida as an unmanned, reusable space plane landed at NASA’s Kennedy Space Center. Shortly after the unannounced landing, the Air Force tweeted that an X-37B Orbital Test Vehicle had returned from orbit.

The space plane had been circling Earth for over 700 days. It was launched by an Alliance Atlas V rocket on May 20, 2015. The X-37B’s orbital mission was entirely classified and we still don’t know exactly what it was doing up there all this time.

Government officials wear protective suits as they greet the just-landed plane. 

What we do know, according to an Air Force statement, is that the purpose of the X-37B is to demonstrate reusable spacecraft technologies:

“Technologies being tested in the program include advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, advanced propulsion systems, advanced materials and autonomous orbital flight, reentry and landing.”

The X-37B was also created to have the capability to operate experiments “which can be returned to, and examined, on Earth.”

The X-37B Orbital Test Vehicle mission 4 after landing at NASA 's Kennedy Space Center in Florida.

At 29 feet long, the X-37B Orbital Test Vehicle is a sturdy, delta-winged vehicle. It’s equipped with a 4-foot-by-7-foot payload bay, solar power boom, and it’s designed to land itself autonomously.

This is the fourth successful X-37B mission for the Air Force, which first launched the vehicles in 2010. The OTV program has now clocked 2,085 cumulative days in low Earth orbit. According to CBS News, this is the second mission for this specific X-37B craft.

The X-37B Orbital Test Vehicle at Vandenberg Air Force Base, California, in June 2009.

The Air Force plans to launch a fifth mission from Cape Canaveral Air Force Station, Florida, later in 2017.

The Air Force also released this background about the Orbital Test Vehicle initiative:

The Air Force Rapid Capabilities Office is leading the Department of Defense’s Orbital Test Vehicle initiative, by direction of the Under Secretary of Defense for Acquisition, Technology and Logistics and the Secretary of the Air Force. The Air Force OTV effort uses extensive contractor and government investments in the X-37 program by the Air Force, NASA and the Defense Advanced Research Projects Agency to continue full-scale development and on-orbit testing of a long-duration, reusable space vehicle.

NASA’s original X-37 program began in 1999 and ran until September 2004 when NASA transferred the program to DARPA. NASA envisioned building two vehicles, an Approach and Landing Test Vehicle, or ALTV, and an Orbital Vehicle. The ALTV validated flight dynamics and extended the flight envelope beyond the low speed/low altitude tests conducted by NASA from 1998 through 2001 on the X-40A, a sub-scale version of the X-37 developed by Air Force Research Labs. DARPA completed the ALTV portion of the X-37 program in September 2006 by successfully executing a series of captive carry and free flight tests. NASA’s X-37 Orbital Vehicle was never built, but its design was the starting point for the Air Force’s X-37B Orbital Test Vehicle program.

The Air Force has successfully flown four X-37B missions, OTV-1 through OTV-4, beginning with its first launch on April 22, 2010 from Cape Canaveral Air Force Station, Fla. OTV-1 through OTV-3 all landed successfully at Vandenberg AFB, Calif., and the most recent mission, OTV-4, successfully landed at Kennedy Space Center, Fla. on May 7, 2017, after 718 days in orbit. The first four OTV missions have spent a total of 2,085 days on orbit, successfully checking out the X-37B’s reusable flight, reentry and landing technologies as well as operating experiments to benefit the national space community.

Photos via Secretary of the Air Force Public Affairs, U.S. Air Force