Balloon: Google Gravity

[ V = \frac{m_{air}}{\rho_{strat}} \approx \frac{30 \text{ kg}}{0.088 \text{ kg/m}^3} \approx 340 \text{ m}^3 ]

That’s a sphere ~8.7 meters in diameter—roughly a tennis court’s width. The final Loon balloons used a pumpkin-shaped lenticular envelope to reduce drag and manage stress. Traditional weather balloons are zero-pressure : they have an open duct at the bottom. As gas expands (daytime heating, rising altitude), excess vents out. At night, the balloon contracts and descends. This is fine for a 2-hour radiosonde flight but disastrous for a 100-day mission. google gravity balloon

The optimization problem: maximize the number of user-hours connected given constraints on battery (solar recharge rate), wind prediction error, and balloon longevity. This became a partially observable Markov decision process (POMDP) with >10^6 state variables. As gas expands (daytime heating, rising altitude), excess

Loon’s envelope used helium. To lift a 15 kg payload (electronics + batteries) plus a 15 kg envelope, the balloon required displacing ~30 kg of air. At 20 km altitude (pressure ≈ 50 hPa), the volume needed is: The optimization problem: maximize the number of user-hours

1. Introduction: The 95% Problem In 2011, Google X (now X Development) proposed a radical solution to a persistent economic reality: while satellites offered global coverage but were expensive and high-latency, and cell towers offered high bandwidth but were geographically limited, nearly 95% of the world’s population lived within range of a cellular signal—yet only half were connected. The problem wasn't coverage; it was economic viability in rural and remote regions.

Loon required —a fully sealed, rigid envelope that maintains internal pressure higher than the external atmosphere at all times. The challenge: as the sun heats the balloon, internal pressure rises, stressing the polyethylene film.