The national space agency for the first time pinpoints Wednesday as the most likely day Skylab will fall to Earth and predicts most of its debris will crash into the ocean.

"We are now 95 percent sure that Skylab will enter in this time frame," Richard Smith, Skylab task force director for the National Aeronautics and Space Administration, said today.

NASA, in honing its predictions on when and where Skylab will fall from orbit, narrowed its earliest and latest forecasts to a single day for the first time.

Today's prediction was the fourth in five days. NASA officials said it appeared that the 26 tons of Skylab debris would miss most continental landfalls as it scatters down a 4,000-mile path, but some debris could fall on Australia.

The latest prediction would put Skylab in the atmosphere at 12:21 p.m. EDT over the south Atlantic. If so, it would begin to disintegrate into the southwest Indian Ocean and scatter most of the debris over a vast unpopulated region of that ocean.

But the heaviest of the 500 pieces could strike Australia and could come down near the large city of Perth.

While the Wednesday noontime descent appeared the most likely, the space agency hedged its bet by declaring that Skylab could fall as early as 3:21 a m EDT and as lat as 9:21 p.m. EDT on Wednesday.

Should Skylab fall one orbit later than today's prediction, its flight path would take it over the midwestern United States and the Washington, D.C., region, although the breakup probably would not occur until it was over the Atlantic.

Should Skylab fall one orbit sooner, it would cross Canada north of the Great Lakes, pass over Newfoundland and begin breaking up over Mauritania in West Africa.

The main reason the space agency is more certain that Skylab will fall Wednesday is that the 77 1/2-ton space station fell nine miles in 24 hours to an altitude of 118 miles. The agency predicted it would fall 10 miles more by Tuesday morning.

Once below 100 miles, Skylab will begin to drop like a stone, but before it falls to 75 miles the agency plans to order the space station to begin tumbling end over end.

"The tumble is the simplest of maneuvers," Smith said. "All you do is crank up the jets in one direction and that causes it to spin up."

If Skylab is tumbling at 75 miles altitude, NASA says it can then predict more accurately where the breakup will begin and where the broken pieces that don't burn up in the atmosphere might fall. Knowing where Skylab is most likely to fall, the space agency can issue more accurate warnings to ships or to countries.

There is still a good chance that Skylab will come down over land. If it does, the most likely points are the United States, a small portion of Canada and a large swath of Africa from Mauritania in the west to Angola and South Africa in the south.

"We would attempt the delay maneuver only if we overfly Europe and Asia at descent time," Smith said. "These are the areas where world population is the densest."

NAS insists that, wherever Skylab falls, its debris will fall along a path no more than 100 miles wide. It also insists that no more than three pieces will fall in the same 100 square miles and that the chance of any person begin hit by the pieces is 152 to 1.

Two computer engineers in Washington, D.C., calling themselves Chicken Little Ssociates dispute this claim. Sam Greenlaw says that the space agency has ignored Earth's rotation rate in predicting the fallout pattern and that in fact the debris path might be 500 or 600 miles wide. NASA denies it, saying it has taken Earth's rotation into account.