A mysterious space plane operated by the US Air Force has passed its 600th day in orbit – but we’re still not any wiser on what it’s actually doing up there.

The Boeing X-37B space plane, also called the Orbital Test Vehicle (OTV), was launched on May 20, 2015, from Cape Canaveral in Florida. This is the fourth mission in the program, and the second for this vehicle. The previous longest mission in the program lasted 675 days, from December 11, 2012, to October 17, 2014.

Despite the program's spaceflights spending almost 2,000 days in orbit in total, the purpose of the space plane remains mysterious. The X-37B is a small vehicle, measuring 8.8 meters (29 feet long), about four times smaller than the Space Shuttle. Like the Shuttle, it also has a payload bay, which can use a robotic arm.

The plane launches vertically on a rocket and lands horizontally back on a runway. It’s also solar-powered, which is how it has been able to stay in orbit for so long, traveling at 28,000 kilometers per hour (17,500 miles per hour).

We do know some of the payloads these spacecraft have carried into orbit. On this launch, there is known to be a NASA materials science experiment and an ion thruster. But the overall goal of the space plane isn’t clear.

One theory is that the plane is testing thrusters in a relatively low orbit (about 320 kilometers/200 miles), with a view to place reconnaissance satellites here in the future to get higher resolution images of the ground. Or it may already be performing reconnaissance.

According to the US Air Force’s website, 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.” But many think it may serve another purpose, too.

At any rate, it’s pretty cool that there is a working space plane in orbit. Even if we may never know what it’s really up to.