Air Force's X-37B robotic space plane wings past 500 days in Earth orbit

That enigmatic U.S. military X-37B robotic space drone has now chalked up more than 500 days circling the Earth.

The Orbital Test Vehicle (OTV-6) is also called USSF-7 for the U.S. Space Force and launched May 17, 2020, on an Atlas V 501 booster.

OTV-6 is the first to use a service module to host experiments. The service module is an attachment to the aft of the vehicle that allows additional experimental payload capability to be carried to orbit.

Related: The US Space Force's secretive X-37B space plane: 10 surprising facts

Primary agenda: classified

While the Boeing-built robotic space plane's on-orbit primary agenda is classified, some of its onboard experiments were identified pre-launch.

One experiment onboard the space plane is from the U.S. Naval Research Laboratory (NRL), an investigation into transforming solar power into radio frequency microwave energy. The experiment itself is called the Photovoltaic Radio-frequency Antenna Module, PRAM for short.

Along with toting NRL's PRAM into Earth orbit, the X-37B also deployed the FalconSat-8, a small satellite developed by the U.S. Air Force Academy and sponsored by the Air Force Research Laboratory to conduct several experiments on orbit.

In addition, two NASA experiments are also onboard the space plane to study the effects of the space environment on a materials sample plate and seeds used to grow food.

Previous flights

OTV-1: launched on April 22, 2010 and landed on Dec. 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 Dec. 11, 2012 and landed on Oct. 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 Sept. 7, 2017 and landed on Oct. 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.

There is no word on when and where OTV-6 will return to Earth.

According to a Boeing fact sheet, "the X-37B is one of the world's newest and most advanced re-entry spacecraft, designed to operate in low-earth orbit, 150 to 500 miles above the Earth. The vehicle is the first since the space shuttle with the ability to return experiments to Earth for further inspection and analysis. This United States Air Force unmanned space vehicle explores reusable vehicle technologies that support long-term space objectives."

Delta 9

The X-37B program is flown under the wing of a U.S. Space Force unit called Delta 9, established and activated July 24, 2020.

"Delta 9 Detachment 1 oversees operations of the X-37B Orbital Test Vehicle, an experimental program designed to demonstrate technologies for a reliable, reusable, unmanned space test platform for the U.S. Space Force," according to a fact sheet issued by Schriever Air Force Base in Colorado.

"The mission of Delta 9 is to prepare, present, and project assigned and attached forces for the purpose of conducting protect and defend operations and providing national decision authorities with response options to deter and, when necessary, defeat orbital threats," the fact sheet explains. "Additionally, Delta 9 supports Space Domain Awareness by conducting space-based battlespace characterization operations and also conducts on-orbit experimentation and technology demonstrations for the U.S. Space Force."

Leonard David is author of the book "Moon Rush: The New Space Race," published by National Geographic in May 2019. A longtime writer for Space.com, David has been reporting on the space industry for more than five decades. Follow us on Twitter @Spacedotcom and on Facebook. This version of the story was published on Space.com.