Balloons go to 12 miles all the time, they stay at whatever altitude they are neutrally buoyant at. The OP says hot air balloons, but if you ignore that requirement you can get a non propulsed balloon to 30 miles up. It doesn’t just pop, it does leak slowly bc no bag is perfect.
Balloons go to 12 miles all the time, they stay at whatever altitude they are neutrally buoyant at. The OP says hot air balloons, but if you ignore that requirement you can get a non propulsed balloon to 30 miles up. It doesn’t just pop, it does leak slowly bc no bag is perfect.
https://www.isas.jaxa.jp/j/topics/topics/2013/0920.shtml