If Skylab comes down when the space agency tentatively predicted it would, its 26 tons of debris will fall along a 4,000-mile path that starts over the state of Washington and ends up in the Atlantic Ocean off South America.

Right now, very inaccurate prediction is that Skylab will reenter the atmosphere at 3 p.m. EDT on July 11," the National Aeronautics and Space Administration's depauty associate administrator, Richard G. Smith yesterday at a news conference.

"But I want to emphasize that the accuracy of that prediction falls within plus or minus 20 percent of the time still to go. By that, I mean it could just as easily come down 24 hours ahead of time of 24 hours after," he said

Should Skylab fall at 3 p.m. Wednesday, as predicted, it will begin to break up over Washington, Idaho and Wyoming and then follow a long, Curving path that takes it over Nebraska, Missouri, Tennessee, Alabama, the Florida Panhandle, Cuba and Venezuela before the path ends in the south Atlantic.

Its largest and heaviest pieces would fall at the end of the breakup -- over the Atlantic off the east coast of South America.

"The heaviest pieces will sustain the forward velocity for the longest time," Smith explained. "That means they'll tend to fall at the end of this 4,000-mile footprint we've been talking about."

Smith said Skylab will break into about 500 pieces, most of them weighting less than 10 pounds. These light pieces will almost float down in the last 10 miles of their desent.

NASA figures 38 pieces will weight at least 250 pounds, 10 will weight at least 1,000 pounds, one will weight 3,900 pounds and the heaviest piece will weight 5,100 pounds. The two big pieces are the lead safe, built to protect the Skylab film from cosmic radiation, and the airlock shroud. These will strike the earth at speds of more than 260 miles an hour.

The 77 1/2-ton Skylab was circling the earth at an altitude of 135 miles yesterday a falling at the rate of three miles a day. Its desent rate has been increasing each day. By the time it gets down to 120 miles it will begin to lose the speed that keeps in orbit and will plunge downward.

At an altitude of 65 miles, the space station's solar panels will be torn off, and its telescope mount will break loose. Minutes later the entire space station will begin to come apart.

If it appears that Skylab is about to break up over a heavily populated part of Earth, Smith said, the space agency may attempt a maneuver to delay its reentry. Skylab is now flying around the Earth in a side ways position, which puts the most atmospheric drag on it and pulls it down more rapidly. But the sideways position also keeps it from tumbling out of control if ground controllers are to attempt to delay its reentry.

Smith said that controllers might reposition Skylab so that it is not the drag and keeping the space station aloft as much as six more hours. This could shift the break-up pattern as much as 3,000 or 4,000 miles to the west. But NASA would try this only if it would place the debris over lesspopulated regions.

Smith emphasized that Skylab's reentry can be predicted with only minimal accuracy. He said the breakup date of Wednesday could be off by two days. Controllers could be off by as many as three orbits in predicting which orbit will be Skylab's last, he said.

The North American Air Defense Command tracked the upper stage of a Soviet Rocket that reentered the atmosphere several months ago and predicted it would reenter on a certain orbit. But the stage hit the upper region of the atmosphere, bounced back into orbit and came down two orbits later.

Smith was asked if the Soviet Union was being notified of Skylab's progress so that when it reentered the Soviets would not mistake it for a ballistic missile.

"The Russians know its orbit as well as we do," Smith said. "Skylab is not a new object in space."