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The Nasa ER-2 plane (foreground) flies above the Golden Gate in San Fransisco

On a typical day in the field, Kristie Boering awakens at 2 a.m. and spends the next few hours in an airplane hangar checking the carbon dioxide (CO2) measurement apparatus aboard the National Aeronautics and Space Administration (NASA) ER-2 aircraft, a modified U-2 spyplane. At daybreak, she watches the ER-2-which is essentially a rocket with glider wings-take off with her experiment. After 87 launches, the Bunting Institute science scholar says that her heart still skips a beat at the sight.

The lone pilot, outfitted in a pressurized suit, guides the plane into the stratosphere, 20 kilometers (13 miles) above Earth's surface. He flies 20 degrees north and back, maintaining an altitude of 15 to 20 kilometers. During the eight-hour trip, Boering's measurement device monitors stratospheric air four times per second, passing it by a light source that looks for CO2 molecules. The instrument then records the density of CO2 molecules in comparison to other stratospheric gases.

Boering analyzes this data to determine the environmental feasibility of supersonic aircraft. Specifically, she is investigating the impact of a fleet of 500 high-speed commercial transports (HSCTs) proposed by Boeing and McDonnell Douglas. HSCTs, flying at Mach 2.4-that is, 2.4 times the speed of sound-would reduce travel time from Los Angeles to Tokyo from 10 hours to four. A flight from New York to London would take three hours instead of seven.

Commercial travel by supersonic transport (SST) isn't new. During the 1960s and 1970s, the United States, the Soviet Union, France, and Great Britain competed to develop prototype SSTs. Several factors combined to end the U.S. program by 1971. Escalating costs and poor economic prospects for operations weakened federal support. Congress questioned the government's role in funding aerospace industry research. Environmental concerns, including sonic booms, noisy takeoffs and landings, and uncertainties about the effect of exhaust on the ozone layer, sealed the SST's fate in the United States.

Today, two types of SST exist, but only one flies. In 1968, the Soviets designed the Tupolev Tu-144, but stopped production after building the model. In 1969, the British and French developed the Concorde. British Airways and Air France built 15; 10 remain in service. The 100-passenger Concorde reaches a speed of Mach 2, meaning a three-and-a-half-hour trip from New York to London. The aircraft's heavy structure prohibits longer flights or more passengers, and its high fares-$7,606 for a round-trip special on the New York-London route-restrict consumer access. The SST has been a commercial failure.

Boeing and McDonnell Douglas envision a lighter, more affordable HSCT that would carry 300 people, at a cost of less than $1,630 per person for the New York-to-London route. Since that's only about 20 percent more than today's full fares, industry analysts anticipate considerable demand for this new generation of supersonic passenger aircraft.

Congress seems to agree. Since 1989, it has allocated nearly $500 million to fund NASA's High-Speed Research Program, which is laying the groundwork for the technology to make HSCTs viable.

Only the ozone layer stands in the way. To gain supersonic speeds, HSCTs travel in the less pressurized ("thinner") air of the stratosphere. Coincidentally, ozone production occurs in this part of the planet's atmosphere. The proposed HSCTs would deposit exhaust emissions directly into the ozone layer that protects Earth's surface from damaging ultraviolet radiation. In the past decade, the atmosphere has lost 6 percent of its ozone. Because of the complexity of stratospheric chemistry, the degree of harm that HSCT emissions might do remains uncertain.

In 1991 Boering became a research associate in atmospheric chemistry in Harvard's department of earth and planetary sciences to solve this piece of the puzzle. She and her colleagues Steven C. Wofsy, Ph.D. '71, McKay professor of atmospheric and environmental sciences, and Bruce C. Daube, project engineer, are part of the science team on NASA's Stratospheric Transport of Atmospheric Tracers mission. The trio measures CO2 to determine the direction and speed of stratospheric air flow. "Air tends to rise into the stratosphere at the tropics and fall back to Earth at the mid-latitudes, in a funnel shape," Boering explains. "But there's a lot of uncertainty about how quickly this happens. Measuring CO2 helps quantify how long HSCT emissions will stay in the stratosphere. Most of the emissions will be deposited in the mid-latitudes. They could descend and be 'cleansed' by rain, or they could stay up there and cause ozone depletion. If our numerical models predict significant ozone depletion, the HSCTs won't be built. But we need more data-up beyond where the ER-2 can fly-before we can come to any conclusions. We're constructing a balloon instrument for that purpose. If all goes well, a decision about HSCTs will be made in 1998 and planes could be flying in the next decade."

Boering's experiments dictate a strenuous travel schedule, with field sessions varying from monthly to several times a year. Since 1992, she has commuted from Cambridge to such farflung locales as San Francisco, Hawaii, Fiji, and Christchurch, New Zealand. If HSCTs were available, would she fly them? "Absolutely," Boering laughs. "Then I'd have time to answer my mail and feed my cats."

Elaine Mar

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