For aviation noobs (like me) wondering how low the balloon would have to be in order for a helicopter to be an option for retrieval:

"The maximum altitude which can be reached during forward flight typically depends on the ability of the engine to breathe the thinner air rather than the rotor's ability to provide lift. Turbine-engine helicopters can reach around 25,000 feet (7,620 meters). But the maximum height at which a helicopter can hover is much lower - a high performance helicopter can hover at 10,400 feet (3,170 meters)."

Source: https://www.virginexperiencedays.co.uk/experience-blog/how-h...