China’s secretive spaceplane, which has spent more than 150 days circling Earth, has released an unidentified object into orbit, according to reports.

Little information is available about the experimental spacecraft, except that it’s designed for reuse. Even less is known about the technology introduced into Earth’s orbit.

The object, cataloged as 59884 by the U.S. Space Force, remains a mystery. Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, was among the first to observe the ejection on May 24, The Sun reported.

In a statement on X a few days later, Mr. McDowell speculated that the object might be a subsatellite. He suggested it could be a piece of hardware discarded at the end of a mission.

Chinese authorities released a statement shortly after the event, stating: “During this period, reusable technology verification and space science experiments will be carried out as planned to provide technical support for the peaceful use of space.”

Earlier this month, Tobias Ellwood, former British defense official, expressed concerns to The Sun about China’s ambitions to advance its satellite-based radio navigation system, which could pose challenges for the U.K. and other Western nations.

China has maintained a high level of secrecy around the spaceplane, known as CSSHQ. Insiders have leaked its only available images. Experts believe it’s China’s attempt to develop a counterpart to America’s X-37B spaceplane.

• Staff can be reached at 202-636-3000.