The countdown continued today for Friday's sixth flight of the space shuttle Challenger, an eight-day mission with a record crew of seven people, including two American women and the first Canadian in space.

Liftoff was scheduled for 7:03 a.m. EDT. While the weather at Kennedy Space Center here was not expected to be a problem, rain and poor visibility were anticipated at emergency backup sites in Spain and West Germany.

The 13th shuttle mission will be commanded by Navy Capt. Robert L. Crippen, 47, making his fourth space flight. Other crew members are Sally K. Ride, 33, making her second shuttle flight; Dr. Kathryn D. Sullivan, 32, a geologist and oceanographer who will be the first American woman to walk in space; Lt. Cmdr. David C. Leestma, 35; Cmdr. Jon A. McBride, 41, the pilot; Canadian Navy Cmdr. Marc Garneau, 35, whose main task will be the study of the effects of acid rain on Canadian lakes; Dr. Paul D. Scully-Power, 40, an Australian-born Navy oceanographer who will photo-map large circular ocean eddies whose existence was documented from space less than two years ago.

The shuttle flight is being billed as the "first that will be totally scientific in nature and totally Earth-oriented in its science," according to Gary Graybeal of the Payloads Integration Office at Houston's Johnson Space Center. "Even the spacewalk planned for the fifth day in flight is designed to help us study the Earth."

Challenger's ascent, on an initial northeasterly trajectory, could be visible to millions of people in the southeastern United States. The spacecraft's orbital track will be inclined 57 degrees to the equator, which means it will fly above all portions of Earth between 57 degrees north latitude and 57 degrees south latitude, or from northern Canada to the tip of South America.

"I've never been at those latitudes before," said Crippen. "I'm all pumped up for this one."

In addition to crowding seven people into the shuttle cabin, about a dozen experiments are on the agenda. The main one will occur the first day in space, when Ride is to deploy the 5,000-pound Earth Radiation Budget Satellite, which will be rocketed to an altitude 380 miles above Earth, giving it the widest possible look at every part of the globe except for the poles.

The satellite is the first of three that will measure the energy Earth receives from the sun, the energy it discharges into space and the energy differences between the tropics and higher latitudes. These energy differences cause circulation of the atmosphere and oceans that determines Earth's weather and climate.

The satellite will also measure atmospheric pollutants such as volcanic dust and carbon dioxide, which will help scientists determine future changes in Earth's climate. Some scientists have forecast a new Ice Age triggered by pollutants trapping the sun's energy in the upper atmosphere.

An oversized airborne mapping camera aboard Challenger is to photograph more than 60 percent of Earth's surface during the mission, the first time such a camera has been flown in space. Challenger also will carry the Shuttle Imaging Radar system, an antenna that produces photograph-like black-and-white images of Earth's terrain at the same time it measures terrain depth and height.

A three-hour space walk by Leestma and Sullivan is planned for Tuesday. The two astronauts are to enter the shuttle's cargo bay and transfer 550 pounds of toxic hydrazine fuel from one tank to another, the first experiment of its kind in space.

The refueling is a rehearsal for a future attempt to refuel the orbiting Landsat 4 satellite and restore it to service.

A launch Friday would be exactly 30 days after the landing of the shuttle Discovery -- the fastest "turnaround" to date in the shuttle program. Challenger is scheduled to land in Florida at 12:25 p.m. EDT on Oct. 13.

Meanwhile, the National Aeronautics and Space Administration announced today that Discovery will return to orbit Nov. 7 and is scheduled to launch two communication satellites and rescue two others stranded in useless orbits.