The robotic X-37B landed at 0533 ET after spending 908 days in orbit -- more than four months longer than the previous mission. 

It's the sixth Orbital Test Vehicle mission (OTV-6) with top-secret payloads that military researchers were testing in low-Earth orbit.

Even though most of the payloads are classified, some have been made public, such as the US Naval Research Laboratory's Photovoltaic Radio-frequency Antenna Module, a small device that converts solar power into radio frequency microwave energy. 

Space.com expanded more on the non-classified experiments and technologies being tested:

"Technologies being tested in the X-37B 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, re-entry and landing."

Task & Purpose has speculated some of the mysterious payloads could be "testing surveillance systems to experiments on putting satellites in lower orbits."

After the space plane landed, Jim Chilton, senior vice president at Boeing Space and Launch, wrote:

"With the service module added, this was the most we've ever carried to orbit on the X-37B, and we're proud to have been able to prove out this new and flexible capability for the government and its industry partners."