高度是18600米，直徑不到30米，裏麵不全是氦氣。

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"

這個圖說明裏麵一半是氦氣，另一半是個氣囊，加減球外空氣而控製高度。