- #36
cepheid
Staff Emeritus
Science Advisor
Gold Member
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- 38
In response to the OP, the balloon topped out at just under 128,000 ft, because this was the float altitude of the balloon, meaning the altitude at which the pressure of helium inside is equal to the ambient pressure outside. At this point, any excess helium vents out the bottom, and there is no more lift. Also, these are stratospheric balloons, they don't go up into the thermosphere, whoever said that.