We used a latex weather balloon, and filled it with helium until it expanded to six feet in diameter. The balloon can stretch to an incredible size before breaking. We estimated that it would be 30 feet across if it got up to an altitude of 100,000 feet, which it did. The low pressure at that height causes the balloon to expand and, eventually, pop. Everybody who handled the balloon had to wear gloves, since oils from our hands could weaken the latex and make it pop early. The helium was fairly pricey. There’s a shortage, which is driving up the price.