For the second time this year, Australians have been warned to duck to avoid being clobbered by American-owned space debris.

Australia's minister for science, Sen. James Webster, issued the warning today about the possible landing on Australian soil this weekend of the 10-ton NASA spacecraft Pegasus 2. He reminded Australians that much of the Skylab space vehicle landed on Western and central Australia when it came down in July.

"The probability of anyone being injured by Pegasus debris is extremely small," Webster said. "There is less risk than there was for Skylab. However, if it appears that Pegasus might fall in their area, people would be safer indoors than outside."

NASA's target hour for the craft's reentry in the atmosphere is 6:14 p.m. EST Saturday but the agency said it could come as much as 14 hours either side of that. NASA could not predict where it will land. Webster, who is an elected senator from the southern state of Victoria as well as a member of the Cabinet, ordered the reopening of an emergency alert room for Pegasus in the Australian capital.

The only other time the sophisticated communications center was used was in July when Australian officials realized that Skylab could hit Australian territory.Their fears were justified.

In a spectacular space smashup, the remains of Skylab roared across the southwest coast in the middle of the night of July 12 and thudded down over 3,000 miles of deserted Australian outback.

A handful of Australians in tiny towns such as Kalgoorlie, Balladonia, Rawlinna and Ferramunjup caught a glimpse of the fantastic fireworks show caused by the bits of Skylab slowing as they sped to earth through the atmosphere. The NASA craft started hitting land near a 3,000-mile-long fence built 60 years ago to keep rabbits, imported by the British, from crossing to the arable lands of southwestern Australia.

But no damage was done to the fence and, although several pieces of Skylab have been retrieved since, there have been no reports of damage to any Australian property. No chances are being taken on Pegasus 2, however.

Apart from the alert room in Canberra, officials from the Ministry of Science are keeping radio stations, newspapers, television stations and local police informed of the latest predictions from NASA.

Not as much of Australia is "at risk" as when Skylab approached the earth in its final erratic orbits. Pesasus 2, which was launched 14 years ago to detect micrometeroids, travels between 31.8 degrees north and south. Two Australian states -- Victoria and Tasmania -- are completely outside its orbit -- as are the three major cities of Canberra, Sydney and Melbourne.

However, Perth, Brisbane and Darwin, all state capitals, are within the orbit and are theoretically under threat. Webster put the risk to Australia as "one in forty" on the grounds that Australia comprised about one-fortieth of the area traversed by the Pesasus orbits.

Seventy-five percent of the course is over water but Pegasus 2 does fly over parts of Florida, Texas, Louisiana, Mississippi and Alabama as well as much of Africa and South America.