Without warning, the space agency early this morning abruptly changed its prediction and said Skylab would enter the atmosphere over the Pacific Ocean instead of the Atlantic and would begin to break up over southern Canada.

The breakup will take the 26 tons of debris that Skylab will scatter from a point in southern Canada north of the Great Lakes, across the most densely populated part of that country, beyond Nova Scotia and to a point in the Atlantic off the west coast of Africa about 300 miles north of the Equator.

The heaviest and most dangerous pieces are expected to fall in the North Atlantic, out of reach of land.

It was the eighth revision in six days by the National Aeronautics and Space Administration. Just six hours earlier. NASA had said that Skylab would reenter the earth's atmosphere off the west coast of Africa and begin to break up off Cape Town, South Africa, and scatter its debris over the Atlantic and Indian oceans in a path falling about 1,000 miles short of Australia.

As of 1:30 a.m. (EDT) today, NASA had no explanation for the dramatic change in its prediction.

NASA public affairs officers said that any explanation would have to come from Deputy Associate Administrator Robert A. Frosch gets briefed first, and then the press."

By no means does the dramatic change in forecast means for certain that Skylab will break up over southern Canada and the North Atlantic. NASA has said repeatedly that its predictions of Skylab's entry and breakup are subject to constant change and constant error.

However, in the earlier forecast Tuesday night, NASA had narrowed its reentry "window" sharply to between 7:50 a.m. and 4:14 p.m. (EDT), with the most likely time of reentry at 12:02 p.m. The forecast earlier this morning put the earliest reentry time at 8:48 a.m. and the latest at 1:48 p.m. (EDT), shrinking the window to five hours and moving the likely reentry time up to 44 minutes.

This moved the reentry point almost 8,000 miles to the west of the earlier one, and from the Atlantic to the Pacific.

Even this latest prediction could change.

The fact that NASA moved its forecast could mean that the upper atmosphere is expanding due to increased solar activity, which puts more drag on the space station, bringing it closer to earth at a faster rate.

NASA has repeatedly said that the reentry time for Skylab depends to a great extent on the expansion and contraction of the upper atmosphere, which at this time of the 11-year solar cycle depends on sunspot activity. An increase in sunspot activity heats up the upper atmosphere and expands it. A decrease does the opposite.

The new prediction has Skylab beginning its reentry northwest of Hawaii, moving on a path likely to bring it across Puget Sound in Washington, then across a very narrow path in southern Canada, passing north of the Great Lakes region, probably north of Toronto and Montreal, and exiting over the North Atlantic around Nova Scotia. Only the lighter pieces are expected to fall on land, NASA said.

Skylab then would complete its journey in the Atlantic just west of Africa about 300 miles north of the Equator.

Between Monday and Tuesday, Skylab fell 14 miles, to an altitude of 104 miles. It was down to 99 miles early this morning, and flight controllers at the Johnson Space Center in Houston planned to order the firing of thruster jets on Skylab at about 92 miles altitude to make the space station tumble end over end. NASA hopes this will delay the reentry point, thus missing all of Canada and landing entirely in the ocean.

Two-thirds of Skylab's metal hulk is expected to burn up in the atmosphere. The rest will fall along a long, narrow path in 500 pieces ranging in weight from three to 5,100 pounds.

The lightest pieces will fall first, since they will not have the forward velocity to carry them far. The heaviest ones will fall last, coming to earth about 3,000 miles farther down track than lightest pieces.

Ten pieces will weigh more than 1,000 pounds, meaning they will hit the earth at speeds up to 260 mph. The lightest of these is a telecope spar weighing 1,100 pounds. The heaviest are a lead-lined film vault weighing 3,900 pounds and a 20-foot-long airlock shroud weighing, 5,100 pounds.

The space agency insists that all the debris will scatter along a path no wider than 100 miles. It also insists that, even though the heat of reentry will force temperatures of 2,300 degrees Farenheit on some of the metal parts, they will have cooled to air temperatures by the time they reach earth. CAPTION: Pictures 1 and 2, Waiting for SKYLAB - It was the biggest media event in town, and news people of all breeds were camped out in the NASA press room yesterday to follow the saga of the falling space station. At left, Gary Axelson of WJLA-TV Channel 7 uses his calculator to determine from latest available data where Skylab might fall. At right, Mary Tilotson of Independent News Service chats with her husband, Warren Corbett of WRC-TV 4. Photos by Linda Wheeler - The Washington Post