To mark our 150th year, we’re revisiting the Popular Science stories (both hits and misses) that helped define scientific progress, understanding, and innovation—with an added hint of modern context. Explore the entire From the Archives series and check out all our anniversary coverage here.

Thirty years ago, the noxious clouds of chlorofluorocarbons that had been gathering in Earth’s stratosphere for half a century would chew a seasonal hole in the protective ozone layer over Antarctica twice the diameter of Pluto. While the Antarctic feature was extreme, it underscored a disaster unfolding across Earth’s atmosphere. With less ozone in the stratosphere to shield flora and fauna from the sun’s ultraviolet rays, crops would suffer and skin cancer would soar. 

By the time Popular Science ran a feature in July 1992 describing the urgent efforts of scientists across the globe to understand the dynamics of ozone destruction, our outlook was dire. “Earth’s ozone shield seems to be failing,” wrote Popular Science Senior Editor Steven Ashley, “and researchers need to find out why—fast.” According to Ashley NASA had pulled out all the stops, building a robotic data-gathering drone to ply Earth’s polar vortex—the upper reaches of the atmosphere over Antarctica. The craft, called Perseus, used GPS and a programmed route to sniff out ozone. 

 In 1987, every country on Earth (a first and only) ratified a treaty to reverse the damage. The Montreal Protocol established guidelines to rapidly phase out a list of 100 manufactured chemicals called ozone depleting substances or ODS. Since Popular Science’s feature ran in 1992, ODS emissions have been reduced by 98 percent. And while the Antarctic ozone hole fluctuates in size and severity year to year, driven by myriad factors including seasonal temps and moisture, an improving trend has been consistent. Experts forecast full recovery by 2070. Besides representing a rare environmental success story, there’s a lesson in ozone: Amazing things are possible—even on a planetary scale—when everyone gets on board.

Unfortunately, such unity has proved elusive for greenhouse gasses. Since 1992, world leaders have taken three swings at treaties to reduce the substances, the latest being the Paris Climate Agreement. None have achieved unanimity, although the Paris Agreement is close now that the US has rejoined.

“Ozone Drone” (Steven Ashley, July 1992)

The rupture of Earth’s ozone shield has become a global concern. But how can scientists gain the high-altitude data they need to find solutions? This unmanned power glider might be the answer.

Eighty thousand feet above Antarctica’s vast frozen expanse, a lone aircraft will cruise the stratosphere on long, tapered wings. The unmanned powered glider, called Perseus, is expected in 1994 to fly higher than any previous prop plane to find out what’s gone wrong with Earth’s stratospheric ozone shield. It will be programmed to search the cold, thin air over Antarctica for ozone-killing chemicals and bring back crucial air samples that have eluded atmospheric scientists for years.

The plane’s 14.4-foot, variable-pitch propeller—so long that it is unable to spin until Perseus is aloft—will require the robot craft to be hawed into the air from its base at Antarctica’s McMurdo Station by a winch-wound cable. Once airborne, its engine will be engaged and the cable detached.

Perseus will then spiral upward toward the center of the ozone hole at about 40 knots, reaching a speed of 200 knots at altitude. Although a technician will pilot the plane remotely via line-of-sight radio controls when it’s near the ground, Perseus will largely pilot itself. Its on-board flight computer will carry preprogrammed navigation commands based on data beamed from Global Positioning Satellites.

Ultimately, it is intended that sensors mounted in the craft’s nose will respond if the high-flying probe enters a wispy, pinkish assemblage of tiny ice crystals, a suspected hotbed of ozone destruction researchers call a polar stratospheric cloud. The computer on-board will direct the craft’s air-sampling apparatus to engage. When its sensors no longer detect the ice, Perseus will reverse course and continue to fly a zig-zag pattern in order to map the boundaries of the noxious cloud.

Total flight duration will be about six hours, with an hour for air sampling. Perseus can carry only enough fuel for the climb, so it will glide silently, after the engine halts, to a landing at its base on the ice shelf.

Such a flight cannot come too soon for scientists studying ozone depletion. Earth’s ozone shield seems to be failing and researchers need to find out why—fast. Last October, NASA’s Nimbus-7 satellite measured the lowest Concentration of ozone over Antarctica in 13 years. This huge ozone hole has so far been restricted to the Southern Hemisphere, but NASA aircraft recently found an abundance of ozone-hole precursor chemicals high in the arctic air, raising the specter of a northern ozone hole. Perhaps even more alarming is the discovery of thinning ozone levels over the northern mid-latitudes, including populated areas of Canada and New England, Britain, France, and Scandinavia. (This past year’s conditions were unusually warm, say scientists, so no northern ozone hole materialized.)

Since 1988, pilots in NASNs ER-2 reconnaissance aircraft—converted U-2 spy planes—have climbed 13 miles above the remote and desolate polar regions to gather air samples for scientists. These missions are anything but routine. If one of the single-engine airplanes were to encounter trouble during these arduous eight-hour, 1,500-mile nights, the solo pilot would almost surely die.

So far, the returns have been worth the risks, however, for the high-flying collectors have provided scientists with the evidence they needed to implicate man-made chlorine compounds called chlorofluorocarbons (CFCs) in ozone’s destruction and call for their ban. Nevertheless, researchers’ ability to further model and predict changes in the ozone layer are currently limited by a dearth of crucial air samples from the heart of the hole, which lies at altitudes beyond any piloted plane’s ceiling, says Jim Anderson, atmospheric chemist at Harvard University. Anderson, also mission scientist for NASA’s six-month-long Airborne Arctic Stratospheric Experiment-2, says that current atmospheric models (used to guide the government’s environmental policy decisions) lack information on chemistry and movement at altitudes near 15 miles, or 82,000 feet-a crucial area in the formation and destruction of ozone. “Satellites are good for broad-brush maps of simple measurements,” Anderson says, “but to understand the ozone-depletion mechanism you need both-satellites for the climatological view and direct measurements by air vehicles to understand the mechanism.”

Giant helium-filled research balloons have been used for decades to haul instruments to extreme altitudes, but these unwieldy craft are subject to the vagaries of the weather, leading to launch delays and occasional lost payloads. And the only available airplane that can fly high enough is Lockheed’s SR-71 Blackbird, but the black aircraft’s supersonic speed would make sampling impossible. Perseus, then, would seem to be poised to provide many answers.

Massachusetts Institute of Technology-trained aeronautical engineer John Langford, president of Aurora Flight Sciences Corp. in Manassas, Va., is working to craft Perseus to offer extreme altitude capability, pilotless operation, and the ability to carry scientific instruments aloft at relatively low cost. The nucleus of the Aurora staff are veterans of the MIT Daedalus Project, which developed the lightweight, human-powered aircraft that was pedaled 69 miles between the Greek isles Crete and Santorin [“88- pound Pedal Plane,” Feb. ‘87]. The development of Perseus owes a lot to its seemingly simple forerunner.

Daedalus’ high-efficiency wings, designed by Mark Drela, associate professor of aeronautics and astronautics at MIT, kept the flimsy-looking composite craft airborne despite being driven only by its human engine. Langford and Drela knew that its long, thin wing shape would work in the thin air and extreme altitudes relevant to ozone sampling. “It was obvious that much of the airfoil and structures technology would be applicable to high· flying aircraft,” Drela recalls.

The need for a low-cost, high-altitude, unmanned platform for in situ atmospheric research was established a few years ago by a panel of experts from NASA, the National Oceanic and Atmospheric Administration, and the National Science Foundation. Besides ozone chemistry, the panel wanted a vehicle that could help determine the role of clouds in global warming, investigate a stratosphere/troposphere mixing phenomena for a new Department of Energy study on climatic change, find the causes of severe storms, and assess the impact of future supersonic airliner exhaust emissions [“The Next SST,” Feb. ‘91].

“The key point was that the vehicle be available in the 1993-’94 time frame,” recalls Jennifer Baer-Riedhart, project manager of the resulting Small High-Altitude Science Aircraft program at NASA’s Ames-Dryden Flight Research Facility in Edwards, Calif. Aurora, already well on its way to developing such a craft, was awarded a $2.25 million, two-year NASA contract to deliver two Perseus planes.

To keep costs down, Langford notes that the strategy has been to modify off-the-shelf components and existing designs, rather than developing custom technology.

The result is a lightweight 1,320 pound), “unmanned version of a sailplane,” Langford says, with a 59- foot wingspan and low-drag aerodynamic design. The wings, propeller, tail surfaces, and tail boom are molded from resin-impregnated Kevlar aramid cloth, Nomex honeycomb cores, and graphite cloth.

“Perseus’ composite structure is like that of a sport glider pushed to extremes,” says Siegfried Zerweckh, who has worked as leader of Aurora’s aerostructures group. “The fact that the plane is unmanned and that its structures don’t have to perform forever like those of a commercial aircraft [that is, without an inspection following each flight] means that we can push the materials to the limit.

“We use sandwich construction for stiffness in almost every part, including the wings, tail surfaces, and tailboom,” Zerweckh continues. The three-piece, 30-foot wings, for example, have only four ribs supporting them in the span-wise direction, 80 the structural sandwich panels must be largely self-supporting. A 19.7-foot· long wing panel, for instance, weighs in at 170 pounds. The result is a relatively light structure.

An on-board flight control/navigation computer, a fly-by-wire electronic control system, and an unusual closed-cycle propulsion system complete much of the plane’s bulk. NASA thought Perseus’s propulsion system was important enough to the success of the project to fund it in a separate, half-million-dollar effort.

In keeping with Aurora’s penchant for classical monikers, the propulsion system for Perseus was dubbed Arion. It is an unusual closed-cyc1e system that includes a liquid-cooled, 65-horsepower rotary Norton, a two-speed reduction gearbox with provisions for clutching and locking the propeller, a stiff carbon-fiber drive shaft, the large, variable-pitch propeller, storage tanks for gasoline and liquid oxygen, and a large condenser to cool the exhaust.

Much of this is the work of Martin Waide, former chief engineer for Aurora, who has been an engineer for Group Lotus in Britain and various American manufacturers of military remotely piloted vehicles.

A closed-cycle combustion engine system, which was chosen for Perseus because it was cheapest and fastest to develop, derives from work done for torpedoes and submarines. Instead of compressing external air in a heavy, expensive turbocharger to maintain power, the engine exhaust is fed back into the intake along with fuel and oxygen. Senior propulsion engineer Stephen Hendrickson reports that the entire engine complement was ground tested in May—successfully.

Burning the fuel-air mixture produces exhaust temperatures of nearly 2,000° Fahrenheit, which ordinarily would be dumped overboard. But because Perseus’s exhaust will be recycled, large radiators above the wing must carry off its heat. The Aurora team is developing large stainless steel and aluminum fin-and-tube-type heat exchangers that will work at low atmospheric pressure, where heat transfer is slow.

This past November, the prototype Perseus A reached nowhere near its extreme altitude goals in its maiden flights over the El Mirage dry lake bed in California’s Mojave Desert, limited as it was to a 3,000-foot safety ceiling. But the three short test flights provided data that will pave the way for high-flying missions two years hence, when Perseus A will be airlifted in pieces to McMurdo Station. There, a ground crew of seven will quickly assemble and prepare the aircraft for launch.

Harvard’s Anderson designed the lightweight, nose-mounted instrument package that Perseus will carry. His 110-pound air sampling/analysis system employs an optical ultraviolet-absorption technique to measure ozone concentration and a more sophisticated photon-scattering apparatus that measures the levels of ozone-destroying precursor compounds in parts per trillion. In March, NASA balloon specialists completed a series of difficult test flights during which the miniaturized sensor package and its’ electronics survived -80°C temperatures when they were lofted from the western coast of Greenland.

A widely held theory reported recently by Anderson and two colleagues spells out why tracking these precursor compounds is so vital.

It is known that unimpeded ultraviolet (UV) radiation can cause skin cancer, cataracts, disabled immune systems, as well as disruptions of natural ecosystems and agriculture.

In winter when the sun leaves the poles, the stratospheric air rapidly becomes so cold that nitric acid trihydrate (NAT) in the air freezes. These tiny nitric acid crystals seed the formation of water-ice particles, which gather into wispy, pinkish clouds (the very clouds that Perseus’ detectors will be trained on).

As soon as the ice-nitric acid particles form, fast reactions involving hydrochloric acid and chlorine nitrate occur on the ice surface, which acts as a catalyst (see The Chlorine Connection). The former is adsorbed onto the edges of the crystals, while collisions of the ice particles with the latter liberates molecular chlorine (C12). “Nobody expected that the ice surfaces would act as catalysts for the release of molecular chlorine,” Anderson says.

While the polar air masses cool, they sink. As surrounding air flows in to take the cold air’s place, the Coriolis Force—caused by the spinning Earth—steers the in-rushing air into continent-size rotating jets. These polar vortices act as semi-impermeable walls, isolating the air inside them. Despite the polar subsidence, the free molecular chlorine remains high up.

With the return of the spring sunlight, virtually all chlorine molecules split into free chlorine radicals—chlorine atoms hungry to recombine. This chlorine feeds a series of catalytic reactions that together destroy ozone.

“Free chlorine monoxide chews up ozone like Pac-Man,” Anderson notes. “At the concentrations we’ve observed-more than one part per billion by volume, we estimate that 1 percent of the ozone is lost each day.”

Later in the season, planetary-scale air waves pummel the polar vortices, breaking them up and replenishing the polar ozone. It’s thought that the arctic ozone hole has yet to form because the northern vortices are unstable due to nearby mountain ranges.

A number of scientists are aware that sampling the stratosphere is vital to finding a solution to our ozone depletion problems. Several other high-flying planes are planned. Already developed, but as yet unused, is the giant Condor pilotless aircraft, which was developed by the Boeing Co. of Seattle in a secret Defense Department project. The 20,000-pound Condor is powered by 8 pair of liquid-cooled, 175-hp Teledyne Continental engines with two-stage turbocharging and intercooling driving three-bladed, 16-foot-long props. Though the reportedly $20 million craft completed eight test flights in 1989, the government lacks the funds to operate it. In one of those flights, Boeing’s Condor set the world altitude record for propeller-driven aircraft at 67,028 feet. In another, the classified drone stayed aloft for two and a half days, flying an estimated 20,000 miles.

Other aircraft developers are taking the manned route. A German group from the Deutsche Forschungsanstalt fiir Luft and Rahrfahrt (DLR) in Oberpfaffenhafen has proposed development of a two-seat plane called Strato·2C that is to be capable of reaching 85,000 feet or flying for 10,000 miles. The composite aircraft is to be powered by twin 402-hp Teledyne Continental engines with turbochargers.

Aurora’s engineers are planning several derivative versions of the Perseus “jeep” (as NASA terms the next larger size vehicle). Fitted with an efficient turbocharged engine, Perseus B could cruise for several days at somewhat lower heights than the A model to circle above hurricanes, for instance. With a 188-foot wingspan and twin pusher-prop power plants, Theseus—a “van”-size craft —could fly a 440-pound payload at around 100,000 feet for about a month. Farther down the road, the solar-powered Odysseus “truck” could cruise the stratosphere for as long as a year with a 110·pound payload on board.

By working to extend flight duration and elevation, these propeller-driven stratospheric cruisers may well come to act nearly as “poor man’s satellites.”

From the archives: NASA dispatches drone to help rescue the ozone layer
The cover of the July 1992 special issue of Popular Science, focusing on the intersection of environment and technology.

Some text has been edited to match contemporary standards and style.