A secretive, experimental Chinese spacecraft returned to Earth after 8 months. It’s still a mystery what it was up to.

The type of spacecraft China used is often called a space plane because it can operate like an airplane and fly in the Earth’s atmosphere.

Space planes aren’t a new concept. As Business Insider previously reported, this type of spacecraft can fly independently and orbit into space using a detachable rocket booster. They also have cargo bays for satellites and other objects.

The US Space Shuttle and the Soviet Buran are perhaps the most well-known space planes. More recently, the US Space Force teamed up with Elon Musk’s company to launch an X-37B space plane.

China’s new space plane is a mystery

The details surrounding the spacecraft – from its name to its purpose – are murky.

CNN said some commentators believe the spacecraft is the Shenlong, a space plane that took around 20 years to construct.

A briefing from Xinhua gave a vague description of its purpose, saying it would “carry out reusable technology verification and space science experiments as planned, providing technical support for the peaceful use of space.”

Commentators have compared the spacecraft to the US’ X-37B, which launched two weeks later than China’s space plane and is still in orbit at the time of writing.

“It’s probably no coincidence that they’re trying to match us in timing and sequence of this,” General Chance Saltzman, Space Force’s Chief of Space Operations, told reporters in December, per the Daily Mail.

“It’s no surprise that the Chinese are extremely interested in our spaceplane,” he added. “And we’re extremely interested in theirs.”

Space observers and experts have tracked multiple objects as being released from China’s spacecraft since its launch, South Morning China Post reported.

Some experts detected the release of a possible “sub-satellite” and said the spacecraft made efforts to operate around it in May, CNN and The Telegraph reported.

Juliana Suess, a research fellow for space security at London-based defense think tank the Royal United Services Institute, told CNN that the capability to move close to another object in space could be used for inspections or “potentially the clearing of orbits of space debris.”

“But they could be used for nefarious purposes, including interception of signals or the physical damaging of other spacecraft,” she added.

Nonetheless, analysts told the outlet there’s no evidence to support the theory that China’s space plane is being used as a weapon.