A helium-filled balloon was used instead of a hot-air balloon to lift the Red Bull Stratos capsule to 128,000 ft because helium is a much lighter gas than air. This means that a helium-filled balloon can lift heavier objects to higher altitudes compared to a hot-air balloon. In the case of the Red Bull Stratos mission, they needed to reach a very high altitude of 128,000 ft, so using a helium-filled balloon was the most efficient and effective way to achieve this goal. Additionally, helium is a more stable and reliable gas, making it a safer option for such a high-stakes mission. rephrase and shorten
A helium-filled balloon was chosen over a hot-air balloon for the Red Bull Stratos capsule because helium is lighter and more reliable, allowing it to lift heavier objects to greater altitudes, such as the record-setting 128,000 ft target.
