WASHINGTON - A burst of high altitude cloud activity over Antarctica in January 2003 was caused by the exhaust plume of the space shuttle Columbia during its final flight, reports a team of scientists who studied satellite and ground-based data from three different experiments. The data call into question the role these clouds may play in monitoring global climate change.
"Our analysis shows that the Columbia's exhaust plume approached the South Pole three days after launch," said Michael H. Stevens, a scientist at the Naval Research Laboratory and lead author of a paper scheduled to be published in Geophysical Research Letters on 6 July. "The lower temperatures and high concentrations of water vapor over Antarctica caused a significant increase in polar mesospheric cloud activity."
Polar mesospheric clouds are the highest on Earth, forming at an altitude of about 84 kilometers [52 miles]. They normally form when temperatures fall below minus 125 degrees Celsius [minus 193 degrees Fahrenheit].
"Because the brightness, occurrence, and range of the clouds have been increasing, some scientists have suggested that they are indicators of global climate change," said Xinzhao Chu, a research scientist at the University of Illinois at Urbana-Champaign and a co-author of the paper. "That role needs to be reconsidered, however, because of the potential influence of water vapor in shuttle plumes."
On 16 January 2003, Columbia lifted from Kennedy Space Center in Florida on its final flight before the loss of the crew and orbiter 16 days later. As with previous shuttle flights, Columbia released about 400 tonnes [tons] of water, the primary product of its liquid hydrogen and liquid oxygen fuel, while flying nearly horizontally at an altitude of 110 kilometers [68 miles]. The resulting plume was about three kilometers [two miles] in diameter and about 1,000 kilometers [650 miles] long.
"The plume was detected and tracked by the Global Ultraviolet Imager [GUVI] on NASA's Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics satellite," Stevens said. "The GUVI images reveal rapid movement of the shuttle plume toward the South Pole."
At the Rothera Research Station in Antarctica, Chu was measuring upper altitude iron densities and polar mesospheric clouds, using a special lidar system designed at the University of Illinois and operated in collaboration with the British Antarctic Survey. Three days after the launch, the lidar detected iron in the atmosphere at altitudes much higher than usual.
"In addition to a persistent layer of iron near an altitude of 90 kilometers [56 miles], produced from ablating meteoroids entering Earth's atmosphere, three anomalous iron features were found at altitudes between [ 100 and 110 kilometers] 64 and 71 miles," Chu said. Too high to be caused by meteoroids, these iron features originated in the shuttle plume, the researchers report, and had been produced by the normal ablation [shedding of particles] of main engine components during launch.
"Within the next two weeks we measured almost all of the polar mesospheric clouds we were to see that season," Chu said. "Only four hours of cloud observations were recorded before mid-January. From January 19 to 26, however, 18 hours of cloud observations were recorded." The increase in polar mesospheric clouds was also observed with the Solar Backscatter Ultraviolet instrument on the NOAA-16 satellite.
Additional evidence that the shuttle plume was responsible for the burst of cloud activity can be found in the mesopause temperature, inferred from the iron observations near an altitude of 90 kilometers [56 miles], the researchers report. At Rothera, the mesopause temperature was minus 120 degrees Celsius [minus 184 degrees Fahrenheit], which is too warm for polar mesospheric clouds to form under typical water vapor concentrations. By dumping so much water vapor into the mesosphere, the shuttle raised the concentration enough to allow the clouds to form.
"Our data will force scientists to rethink the role of polar mesospheric clouds in monitoring global climate change," Stevens said. "Any interpretation of recent trends in cloud activity must consider the potential influence of the space shuttle program."
Co-authors of the paper with Stevens and Chu are Robert R. Meier of George Mason University, Matthew T. DeLand at Science Systems and Applications Inc., and John M.C. Plane at the University of East Anglia.
The National Science Foundation, NASA, and the Office of Naval Research supported this work.
Home | Site Map | Search | About | Contact
Copyright © 2005, Brian Webb. All rights reserved.