Google has also made major improvements to its stratospheric craft. One of the most significant was developing a way to accurately pilot balloons across thousands of miles without any form of propulsion. The stratosphere, which typically is used only by weather balloons and spy planes, is safely above clouds, storms, and commercial flights. But it has strong winds, sometimes exceeding 300 kilometers per hour. Providing reliable wireless service means being able to guarantee that there will always be a balloon within 40 kilometers.

Google solved that aviation problem by turning it into a computer problem. Winds blow in different directions and at different speeds in different layers of the stratosphere. Loon balloons exploit that by changing altitude. As a smaller balloon inside the main one inflates or deflates, they can rise or fall to seek out the winds that will send them where Google wants them to go. It’s all directed by software in a Google data center that incorporates wind forecasts from the U.S. National Oceanic and Atmospheric Administration into a simulation of stratospheric airflow. “The idea is to find a way through the maze of the winds,” says Johan Mathe, a software engineer working on Loon’s navigation system. A fleet of balloons can be coördinated that way to ensure there is always one over any particular area.