Last week President Obama signed a sweeping aviation bill that, among other things, will open the skies to “unmanned aircraft systems,” more commonly known as drones. Much of the discussion regarding the coming era of domestic drones has been focused on the many important questions regarding their use at low altitudes. To what extent will it be legal, for example, for drones to hover 300 feet above residential neighborhoods snapping pictures into backyards and windows? What level of human-in-the-loop control is needed to ensure safety in a crowded airspace? And how can we stop terrorists from piloting drones at treetop level towards a target?

But there is another portion of the airspace—the stratosphere—that while mostly empty today, will in the coming years will become increasingly populated by gossamer-like, solar-powered drones turning silent, lazy circles in the sky. These drones will stay aloft for years at a time, running on energy collected during the day using solar panels mounted on paper-thin wings. As their slowly turning propellers push them along at bicycle speeds, arrays of high-resolution cameras on their undersides will record the daily comings and goings of the population of entire cities.

The stratosphere lies roughly between 40,000 and 150,000 feet in altitude. Commercial airliners often ply its lower reaches, but above about 55,000 feet the traffic is limited to a few military reconnaissance planes, unmanned weather and scientific balloons, and at rare intervals, a rocket arcing upward on its way to orbit. The stratosphere is mostly empty, cold, and quiet, closer to the blackness of outer space than to the din of human commerce.

Like so much in aviation, that is about to change. The technology to turn the stratosphere into the domain of the drones is already well under development. The Zephyr, a high-altitude, solar-powered drone designed by British company QinetiQ and weighing under 120 pounds despite having a 74-foot wingspan, stayed aloft for two continuous weeks in a summer 2010 test in Arizona. In September 2010, Boeing announced that it had been awarded a contract by the Defense Advanced Research Projects Agency to develop the Solar Eagle, a craft that will eventually be able to fly above 60,000 feet for five continuous years. And many of the information technologies needed perform detailed surveillance from these platforms are already found in common consumer electronics devices...