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.

Of course, permanent eyes in the sky aren’t new. Satellites, after all, have been a constant presence for decades, and the best military reconnaissance satellites can likely deliver stunningly precise pictures of a target of interest. However, spy satellites can't linger over a single point on the ground. By contrast, stratospheric drones will operate at altitudes that are simultaneously high enough to enable coverage of an entire city and low enough to easily collect and convey detailed images of everything in view.

What, exactly, will these drones be able to see? A lot, as it turns out. They will record the route and speed of every vehicle on the streets. They will observe the movements of individual pedestrians. At night, they will capture the precise moments when the lights in living rooms and bedrooms are turned on and off. The data they acquire, which can be correlated with information from mobile devices and smart meters, will become an important component of the growing digital record of nearly everything we do.

And what of the legal framework for the privacy of the information collected using high-altitude drones? In 1986, the Supreme Court ruled that the police use of a private plane to view otherwise hidden marijuana plants growing in a California back yard did not constitute a violation of the Fourth Amendment because the observations were made from “public navigable airspace.” This precedent suggests that the owners of drones operated in public airspace may initially enjoy broad latitude to use them for surveillance.

Ryan Calo of the Stanford Center for Internet & Society projects that drones will lead to a tightening of privacy protections precisely because they can expose so much formerly private information. For low-altitude operation, this tightening may well occur. It is hard to imagine, for example, that paparazzi and stalkers will enjoy legal protection for the long-term operation of small, camera-equipped helicopter drones above private residences.

But high-altitude drone surveillance that indiscriminately captures the detailed life of an entire city is unlikely to be viewed by the courts as a violation of the Fourth Amendment. Privacy rights, both in the formal legal sense and in terms of the accompanying societal expectations and tolerances, are often tied to the extent of individual targeting involved. For example, most of us do not object to the presence of video surveillance cameras in transit systems, office buildings, and retail stores. We recognize the necessity of this surveillance, and, importantly, we know that the video recording is not preferentially directed towards specific people. In practice, privacy is both an absolute and a relative concept, and we tend to be more accepting of privacy reductions that apply equally to everyone.

Today, no government body is actively and publicly promoting a plan to establish a permanent high-altitude surveillance drone presence above American cities. But because it will soon be inexpensive and easy to do so, and because the information gathered will be considered useful and valuable, it will inevitably happen—not this year, and likely not next year, but almost certainly by the end of the decade.

lllustration of Boeing's SolarEagle courtesy of Boeing