MOUNTAIN VIEW, CALIF., JULY 28 -- Space agency scientists announced today an international probe of the mysterious ozone hole over the Antarctic to see whether man-made chemicals, nature or both are destroying the gas that is vital to life on Earth.

A specially equipped ER2 plane, capable of flying to the edge of space, and a modified DC8 airliner taking off from the southern tip of Chile will be part of the $10 million effort from Aug. 17 to Sept. 29.

"I believe this is probably the most singularly important earth science project in a decade," Robert Watson, program scientist for the National Aeronautics and Space Administration, said at the agency's Ames Research Center.

A few years ago, he said, scientists were "worried that a 5 percent change {in ozone} over the next century could be adverse. Instead of 5 percent, we're looking at 50 percent in only 10 years . . . . This was absolutely unexpected; it's caught our attention."

NASA will be joined in the effort by at least three other federal science agencies; the Chemical Manufacturers Association; scientists from Harvard, the University of Denver, the University of Washington, and the governments of Argentina, Chile, France, New Zealand and Britain.

Other countries also are cooperating by allowing aircraft flights over their territories. In all, more than 160 scientists, pilots and others will be involved in the Airborne Antarctic Ozone Experiment.

The ER2, a modified version of the U2 spy plane, and a specially equipped DC8 airliner will make at least 10 passes each through the ozone hole, flying from Punta Arenas, Chile, across the South Pole to New Zealand.

The ER2 will fly in the region of maximum ozone depletion, up to 65,000 feet, and carry instruments to gather information on winds, pressure and temperatures. Other instruments will take samples of chemicals. The DC8 will fly up to 40,000 feet and carry instruments to measure ozone and other chemicals. Other data will be recorded by ground and satellite equipment.

The timing of the flights coincides with the onset of the hole, or thinning of the ozone in the atmosphere over the Antarctic, which begins at the end of winter in the southern hemisphere and lasts through spring.

Ozone, a form of oxygen produced at the upper reaches of the atmosphere by a photochemical reaction with the sun's ultraviolet radiation, is essential to life because it limits the amount of ultraviolet rays penetrating to the Earth's surface.

"If the ozone decreases, the amount of ultraviolet radiation will increase and could have adverse effects on human health, such as melanoma, skin cancer, suppression of the immune response system and the terrestrial and aquatic ecosystems," Watson said.

The key questions the probe will address are whether man-made fluorocarbons, such as those emitted by chemicals in refrigerators, air conditioners and aerosol sprays, are causing the ozone depletion, and whether the hole will spread and affect larger portions of the world.

Adrian Tuck, project scientist for the Commerce Department's National Oceanic and Atmospheric Administration, said he believes the probe will show that "both chemistry and {natural} dynamics are going to be found to play a role" in ozone depletion.

He noted that although most scientists thought the chemical changes were occurring only in the upper atmosphere, recent tests surprisingly showed they are also happening at lower levels.

An international meeting of policy-makers in Montreal in September will look at findings from an experiment earlier this year in Australia and at other data to set guidelines for the use of fluorocarbons, which have a lifetime of up to 100 years in the atmosphere.

Watson said new regulations are "almost inevitable. The question is the timing of the regulations and the magnitude. Will there be a freeze in the worldwide production of fluorocarbons, a 20 percent cutback or a 50 percent cutback?"

Such drastic cutbacks could have significant health and economic implications. Of the more than 1 million tons of fluorocarbons Watson said are produced each year, all of which drift into the atmosphere, 30 percent are made in the United States, 40 percent in Europe, 25 percent in Japan and about 5 percent in the Soviet Union, he said.

However, with the development of China and its emphasis on refrigeration, the world may see a vast increase in fluorocarbons in coming years, he said.

Although virtually none of the chemicals that could affect the ozone layers are released in the area near the Antarctic, Watson explained that fluorocarbons released anywhere in the world are inevitably swept around by winds and other weather factors. Thus, he said, it is possible that chemicals released in North America or Europe have been gathering in the atmosphere over both poles.