In the videos it's mentioned that Helium gas is used to inflate the balloons. I'm curious as to why that choice was made over Hydrogen.
I was kind of interested so I played around with some calculations. One of the #AskAway
videos (they're really awesome! I'm really glad you guys are making them!) mentioned the balloon had a surface area of 500m^2, which (assuming relative sphericality) means a volume of 1051m^3. Air at 60,000ft is 72mbar and -70F and assuming the gas inside the balloons is subject to similar parameters (i.e. it's not a superpressure balloon or something), that'd be 17kg of Helium versus only 9kg of Hydrogen, with buoyant lifting force of -105kg and -113kg, at market prices of $268 and $42, respectively.
Hydrogen is a sixth of the price, 8% more buoyant and half the weight, and doesn't contribute to the depletion of a relatively scarce natural resource. I don't imagine the flammability is an issue given the relatively short (100 day) lifetime and unmanned nature and integrated contingency strategies (parachute, etc). The only disadvantage I can think of is that Hydrogen is more prone to diffusing through the membrane (and I can't find the equations for the rate of this).