高度是18600米，直径不到30米，里面不全是氦气。

https://en.wikipedia.org/wiki/2023_China_balloon_incident

"The balloon was flying at an altitude of 60,000 feet (18,000 m) over Billings at the time"

"The balloon itself was about 90 feet (27 m) in diameter"

这个图说明里面一半是氦气，另一半是个气囊，加减球外空气而控制高度。