What Happens to a Lost Balloon?
The toy balloon sold at fairs that tugs on the end of a string is filled with a gas that is much lighter than the air around it. If you let the balloon go, it will rise and float away.
A lost balloon doesn’t keep on going forever. It will rise until the outside air is nearly as thin and light as the gas in the balloon. At this point the balloon will stop rising because the buoyant force within it equals the weight of the air around it.
As the balloon rose, the gas expanded and swelled the balloon. The balloon will come down when it bursts or when the gas leaks out.
Depending on how heavy the balloon is and how quickly it’s losing helium the balloon will keep on going upwards until either it gets heavy enough it can’t get any higher or the difference in pressure inside it and outside it is enough that it will explode.
I think if you just let go of a helium balloon it will just escape and will go up a few miles. But I couldn’t tell you exactly how many.