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:

Rather than a sphere, Loon used a lobed structure (like a pumpkin) with a tendon network. This shape allows pressure-induced stress to distribute along the seams, not the film. The film itself was a 0.076 mm thick co-extruded polyethylene with a specific UV-resistant additive. The seams were reinforced with load tapes.

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.

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.

Project Loon was born from a counterintuitive question: What if the cell tower floated?

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

The "Gravity Balloon" (a nickname derived from its buoyancy-based altitude control) was not a balloon in the party sense, but a operating in the stratosphere—a realm colder, drier, and more violent than most aircraft ever encounter. 2. The Physics of Floating Against Gravity To understand Loon, one must first understand the stratosphere (10 km to 50 km altitude). Below 10 km, weather dominates: wind shear, turbulence, precipitation. Above 20 km, the atmosphere is stable, with predictable zonal (east-west) wind bands. However, at 20 km, air density is only 7% of sea level.

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.