The US Air Force’s secretive X-37B space plane returned to Earth yesterday, after spending 718 days in orbit, but we still don't know what it was used for.

The unmanned reusable vehicle – called an Orbital Test Vehicle (OTV) – was launched back on May 20, 2015 on its fourth mission, OTV-4. This was its longest mission to date, with the previous record being 674 days.

It touched down at NASA’s Kennedy Space Center Shuttle Landing Facility in Florida, once used for Space Shuttle landings. The total number of days in orbit for the OTV program now stands at an impressive 2,085 days. The return to Earth on this occasion was also quite dramatic, as the vehicle caused a sonic boom that was heard by residents of Florida.

The Boeing-built vehicle measures 2.9 meters (9.5 feet) in height and is 8.9 meters (29.2 feet) long, with a wingspan of 4.5 meters (14.8 feet). It weighs about 4,990 kilograms (11,000 pounds) and is powered by a combination of solar cells and lithium-ion batteries.

"The landing of OTV-4 marks another success for the X-37B program and the nation," said Lt. Col. Ron Fehlen, X-37B program manager, in a statement.

"This mission once again set an on-orbit endurance record and marks the vehicle's first landing in the state of Florida. We are incredibly pleased with the performance of the space vehicle and are excited about the data gathered to support the scientific and space communities.”

But the exact purpose of this vehicle remains a mystery. The Air Force are quite open on the launch and landing, with the vehicle being taken to space by an Atlas V rocket. But what it does in orbit is unknown, although we do know thanks to amateur astronomers that it stays in a relatively low orbit around 350 kilometers (220 miles) high.

^{The vehicle on the runway yesterday. US Air Force}

Speculation for the X-37B’s purpose ranges from surveillance to launching bombs from space. The US Air Force, for its part, says the goal of the program is simply to test reusable spacecraft technologies, and also new ways to perform experiments in orbit.

This likely isn't the end for the program, though. The X-37B could fly again, while Boeing is also building a successor called the X-37C that’s about 165 to 180 percent bigger.