This top secret unmanned Air Force aircraft is like nothing you’ve ever seen, and it’s back in orbit. The X-37B just launched on its fifth flight on Sept. 7, lifting off from NASA’s Kennedy Space Center in Florida on a Falon 9 rocket via SpaceX. Its mission is classified, but experts think it is going to be in a low orbit around Earth using the second stage of the Falcon launch vehicle. The first stage returned to the Florida launch pad minutes after the launch.

The X-37B launched for the first time back in April 2010 after 11 years in development. It spent 718 days in orbit after that initial launch. The Air Force said at the time that the spacecraft was meant to perform risk reduction, experimentation, and development of reusable space vehicles. But many experts think that the X-37B will be used to test new military space technologies meant for spying.

One of the key purposes of the X-37B program has been to test reusable spacecraft technologies, so it’s no surprise the Air Force partnered with SpaceX, as Elon Musk’s company has been doing just that with their Falcon 9 rocket boosters. They’ve demonstrated multiple landings both on land and on a barge at sea.

“The X-37B Orbital Test Vehicle, or OTV, is an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the U.S. Air Force,” the Air Force says on its 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.”