Around 3pm EST on May 24, China’s mysterious Shenlong spacecraft launched an unidentified flying object 372 miles (600 kilometers) above the Earth’s surface. Experts outside China don’t know exactly what the object is, but they believe it could be a small satellite or a piece of hardware discarded before the vehicle was scheduled to deorbit.

The U.S. Space Force has been tracking the spacecraft since it was launched on a Long March 2F rocket from Jiuquan Launch Center on December 14. Although it resembles the Boeing X-37B stealth spaceplane, little is known about the vehicle. It was developed by the United States capable of multi-year trips into orbit. China’s stealth spaceplane was first launched in 2020 and spent just two days in orbit. However, the second journey, which started in late 2022, took 276 days.

Shenlong (meaning “Divine Dragon”) deployed six small facilities shortly after becoming operational last year; each emitted a signal. So far, Chinese authorities have kept the plane and its activities secret, without addressing or even acknowledging the nature of the mysterious objects. Meanwhile, the US army’s X-37B space plane is also in Earth orbit. The ship was launched on a SpaceX Falcon Heavy rocket on December 28. The fact that the two missions were running simultaneously was “probably not a coincidence,” Chief of Space Operations B. Chance Salzman told Air & Space Forces magazine.

Perhaps unsurprisingly, Chinese state media did not indicate that the spaceplane was used for military purposes. “After operating in orbit for a certain period of time, the experimental spacecraft will return to the designated landing site in China,” Xinhua said, according to Gizmodo. “During this period, it will carry out reusable technology validation and space science experiments as planned and provide technical support for the peaceful use of space.”

Reusable spaceplanes have many potential commercial and scientific applications, including transporting passengers and launching satellites more efficiently. With all this said, the Space Force will continue to monitor Shenlong’s mission until it returns to Earth.