Shuttle Liftoff Set for Mission to Study Ozone : Space: Discovery will peer into Earth’s middle atmosphere and conduct scientific tests. Weather could still snag tonight’s scheduled launch.

Equipped with a four-ton laboratory outfitted to look down into the Earth’s middle atmosphere, the space shuttle Discovery is scheduled to lift off tonight to study the condition of the Earth’s ozone layer and sample 30 to 40 other gases.

Officials at Kennedy Space Center in Florida said Sunday that the countdown was moving forward without a hitch, and forecasters said the chances of unacceptable weather conditions were only about 20%.

With a crew of five astronauts aboard, the eight-day mission is due to get under way at 10:32 p.m. PDT.

A night launch was required because some of the atmospheric observations must be carried out when the shuttle flies into sunrise over the Northern Hemisphere as it circles the Earth every 90 minutes.

While recent shuttle missions have been focused increasingly on preparations for a permanently occupied space station, Discovery’s assignment is part of the National Aeronautics and Space Administration’s “Mission to Planet Earth.” The mission is a long-term effort to study the Earth’s resources and environment from orbital altitude.

The condition of the Earth’s ozone layer has been a subject of intense scientific interest since the discovery of severe ozone depletion over the Antarctic and evidence of a thinning of the layer in both the Northern and Southern hemispheres.

In the stratosphere, ozone protects the Earth from the sun’s intense ultraviolet radiation. Evidence of its depletion, raising alarm over the increased incidence of skin cancer and cataracts, has led to worldwide controls on chlorofluorocarbons, the family of chemicals linked to ozone destruction.

Observations taken by instruments in Discovery’s cargo bay will be correlated with data from satellites that continuously gather information on stratospheric ozone.

Because the instruments in the shuttle can be finely calibrated, Discovery will serve to verify the information from satellites that have been in orbit for years.

Working round the clock, mission specialists will train their instruments on a slice of the atmosphere from 6 to 85 miles above Earth.

Besides its studies of atmospheric chemistry, the Atlas-2 payload will take a series of observations of solar radiation reaching the Earth’s atmospheric envelope.

The launch will take the spacecraft on a sharply northward course across the Atlantic so that it will fly over much of the Earth’s surface during its voyage. The astronauts will pass as far north as Juneau, Alaska, and as far south as Tierra del Fuego at the tip of Argentina.

Landing is scheduled at Kennedy Space Center early April 14.

The flight will be commanded by Marine Col. Kenneth D. Cameron, 43, who is making his second shuttle flight.

Crew members are Stephen S. Oswald, 41, the shuttle pilot, plus mission specialists Michael Foale, 36, Kenneth D. Cockrell, 42, and Ellen Ochoa, 34. The only rookie aboard, Ochoa was born in Los Angeles and grew up in La Mesa, near San Diego. She received a bachelor of science degree in physics from San Diego State and a master’s and doctorate in electrical engineering from Stanford.