The US Air Force's unmanned X-37B space plane recently passed its 500-day mark on its fifth flight as part of the mysterious Orbital Test Vehicle (OTV) program.

All previous OTV flights have established new records for the program, with OTV-4 staying in orbit for a whopping 718 days. The fourth installment of the program was launched in May 2015 and landed at NASA's Kennedy Space Center in Florida in May 2017. At that point in time, the OTV program marked 2,085 days in space.

The current mission, known as OTV-5, began on September 7, 2017, when the spacecraft was launched from the Florida space center's Launch Complex 39A atop a SpaceX Falcon 9 rocket.

Although details of the program's mission are scarce and locked down on a need-to-know-basis, Space.com did report that the plane was likely being used for intelligence-gathering purposes due to its ties to the Schriever Air Force Base, which known for space-based demonstrations and collecting intelligence on space objects. In partnership with the Air Force Rapid Capabilities Office, the Colorado base's 3rd Space Experimentation Squadron oversees all operations carried out by X-37B.

Air Force officials previously revealed that the craft is carrying in its payload an Advanced Structurally-Embedded Thermal Spreader, which was developed by the US Air Force Research Laboratory to test experimental electronics and oscillating heat pipes.

In August 2018, nearly a year after OTV-5 blasted off from Florida, the X-37B plane was photographed by Netherlands-based satellite tracker Marco Langbroek. Flying over the Dutch city of Leiden, the aircraft was estimated to be cruising at an altitude between 193 and 202 miles.

— Dr Marco Langbroek (@Marco_Langbroek) August 20, 2018

​Langbroek had previously caught sight of X-37B in April 2018 as it appeared to travel past the so-called "heavenly twin" stars Castor and Pollux, which are located within the Gemini constellation.

While it's unclear when OTV-5 will end its flight and return to Earth, it has been speculated by space enthusiasts that its successor, OTV-6, will likely take flight sometime in 2019 aboard a United Alliance Atlas V rocket, according to Space.com, which noted that the launch may take place at Florida's Cape Canaveral Air Force Station.