The Discovery astronauts are scheduled to blast skyward tonight carrying a satellite designed to diagnose what ails the atmosphere's ozone layer, which shields Earth from the sun's harmful ultraviolet rays.
Forecasters said yesterday there was a 90 percent chance that the weather would remain clear enough to launch the shuttle, its five-person crew and the 14,388-pound Upper Atmosphere Research Satellite as planned at 6:57 p.m. from the Kennedy Space Center.
From its orbit 351 miles up, the insect-like satellite will sweep the sky with nine atmospheric instruments built to study how the complex chemistry and winds at altitudes ranging from six to 50 miles interact with solar rays -- and in particular, to what extent man-made chemicals are causing the planet's irreparable ozone layer to fray like the elbows on an old coat.
A robust ozone layer is crucial to human health. It filters out most of the sun's intense ultraviolet radiation, which in large doses can cause skin cancer.
"I think there is in general an urgency to make these measurements, to understand what's really happening in the upper atmosphere, to understand the destruction of the ozone so we can come up with a policy of better controlling these chemicals," said John L. Donley, deputy project manager at NASA's Goddard Space Flight Center in Greenbelt, which will control satellite operations.
A class of industrial gases called chlorofluorocarbons, or CFCs, came to be widely regarded as a threat to atmospheric ozone in the 1970s. That led the United States to ban CFCs in aerosol cans in 1978. But CFCs continued to be used as refrigerants and in the manufacture of packaging materials.
Concern about the gases accelerated in the 1980s when a ragged hole was first detected in the ozone layer above the South Pole. Since then, scientists have found evidence of a smaller hole over the North Pole and a slight depletion of the layer above warmer latitudes. This spring, the Environmental Protection Agency reported that ozone was being destroyed twice as fast as expected in the atmosphere over the United States.
World leaders recently agreed to eliminate the use of most CFCs by the year 2000. But environmental groups say that if the $710 million satellite finds graphic new evidence of ozone destruction, governments will come under increasing pressure to speed up the implementation of the CFC ban.
"The more accurate measurements the scientific community has globally, the better we can understand the process," said Liz Cook, who directs the ozone campaign for Friends of the Earth. "Ozone depletion is one of the most life-threatening problems society faces today. If there were no ozone layer, life as we know it would not exist."
Mr. Donley said that about 100 scientists from the United States, Canada, France and Great Britain are expected to begin studying the data supplied by Goddard in about a month. In the interim, Goddard engineers will turn on, test and prepare the satellite's nine instruments.
Mr. Donley said scientists are anxious to scrutinize early data from an instrument called the Microwave Limb Sounder, which will produce a global map of the distribution of chlorine monoxide, a gas thought to play a key role in the cycle of ozone destruction.
When CFCs drift high into the atmosphere, sunlight breaks them up and releases free chlorine atoms. Those atoms break down ozone molecules, which consist of three bound oxygen atoms, into one molecule of ordinary oxygen gas and one molecule of chlorine monoxide.
Later, the unstable chlorine monoxide molecule breaks up, freeing the chlorine atom to attack again. In this way, a single chlorine atom can destroy tens of thousands of ozone molecules.
Ozone destruction reaches its height toward the end of the winter, when the sun warms chlorine gathered on the edges of ice crystals. So the launch of the Upper Atmosphere Research Satellite, or UARS, was timed to cover two winters during its initial 20-month operating period. Mr. Donley said there are already plans to extend the mission of the UARS, which can be retrieved by a shuttle if necessary and returned to Earth, for another three years.
UARS would be the first of several spacecrafts launched as part of the National Aeronautics and Space Administration's long-awaited "Mission to Planet Earth," a global project to study the planet's environment.
While environmentalists welcome the UARS mission, Ms. Cook notes that the rocket fuel used to deploy the satellite may contribute to ozone destruction.