Watching them fly off up into the sky, there seems no limit to how high a helium balloon can go. In reality, there are two major constraints: the strength of the balloon material, and Archimedes’s principle. As a balloon ascends, the pressure of the surrounding air drops while the helium inside expands. Toy balloons burst at around 10km, while professional meteorological balloons reach heights of 30km.
The ultimate limit is set by Archimedes’s Principle, which says balloons will stop rising once their density matches the surrounding air. So there’s no chance of balloons entering the vacuum of space. However, in 2002, a helium balloon built by Dr Takamasa Yamagami and colleagues at the Institute of Space and Astronautical Science climbed to 53km – half way to the official edge of space.