The U.S. Air Force's secretive X-37B, also known as the Orbital Test Vehicle (OTV-6), is a robotic spacecraft circling Earth for more than 500 days, according to Space.com.
X-37B's real mission in low Earth Orbit (LEO) is classified. But a 2017 Air Force press release detailed the plane as a "host platform for experimental payloads."
However, a few unclassified experiments have been named in the latest mission. The U.S. Naval Research Laboratory (NRL) examines if it can transform solar power into radiofrequency microwave energy. The experiment is called Photovoltaic Radio-frequency Antenna Module (PRAM). Another experiment is being conducted by the U.S. Air Force Academy and sponsored by the Air Force Research Laboratory to test a FalconSat-8, a small satellite, in orbit. That's as much as we know, and for the other experiments, well, they remain classified.
Here are the most recent space missions of the plane via Space.com:
OTV-1: launched on April 22, 2010 and landed on December 3, 2010, spending over 224 days on orbit.
OTV-2: launched on March 5, 2011 and landed on June 16, 2012, spending over 468 days on orbit.
OTV-3: launched on December 11, 2012 and landed on October 17, 2014, spending over 674 days on-orbit.
OTV-4: launched on May 20, 2015 and landed on May 7, 2015, spending nearly 718 days on-orbit.
OTV-5: launched on September 7, 2017 and landed on October 27, 2019, spending nearly 780 days on-orbit.
OTV-1, OTV-2, and OTV-3 missions landed at Vandenberg Air Force Base, California, while the OTV-4 and OTV-5 missions landed at Kennedy Space Center, Florida.
The X-37B program is managed by the U.S. Space Force unit called Delta 9, established and activated July 2020. There has been no disclosure of how long the space plane will remain in orbit.
In 2019, amateur space enthusiasts captured the X-37B orbiting Earth on camera.
As for the exact mission, we'll never know what the space plane is doing in low Earth orbit.