The reusable space plane designed and built by Boeing is flying its sixth mission, known as Orbital Test Vehicle-6 or OTV-6, which was initially launched from Cape Canaveral Space Force Station in Florida on May 17, 2020. It remains unclear when the top-secret mission will end. 

On Jul. 7, Boeing Space tweeted the X-37 "has set another endurance record — as it has on every mission since it first launched in 2010."

Many of OTV-6's experiments and activities are classified. But some experimental payloads have been made public, such as the U.S. Naval Research Laboratory's Photovoltaic Radio-frequency Antenna Module, a small device that converts solar power into radio frequency microwave energy. 

Space.com expands 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."