The falling Skylab space station will be visible in the evening sky this week, possibly for the last time before it breaks up in the atmosphere and drops to Earth.

At 9:39 Eastern Daylight Time tonight, the 85-ton Skylab can be seen in the Washington area in the Northwest part of the sky, moving east. It will be visible for a little over four minutes at about 27 degrees above the horizon. It will resemble a medium-bright star, though moving perceptibly across the sky.

On Thursday, Skylab will show up at 9:34 p.m., again moving east from the northwest but higher above the horizon and visible about half a minute longer.

The best viewing time will be on Friday night starting a little after 9:28, when the space station will fly almost directly overhead and be visible for four minutes and 44 seconds. It will be moving from the northwest to the southeast and will appear brighter than the two previous nights.

On Saturday, Sunday and Monday night, Skylab will be dimmer and visible for successively shorter viewing times until by Tuesday it will be so close to the horizon that it will drop out of sight of Washington viewers. Its course across the sky will aslo change, moving south out of the western sky.

The National Aeronautics and Space Administration has said that Skylab should fall to Earth sometime between July 7 and 25, with a 50 percent chance of its dropping on July 16.

When it falls, the space agency said, most of the space station will burn up from high-speed friction with the atmosphere. About 500 pieces will fall all the way to Earth, most of them weighing less than 10 pounds and dropping along a stretch of Earth 1,200 miles long.

An estimated 38 pieces weighing more than 250 pounds each will fall in a region 600 miles long. Included in this fallout are 10 pieces of more than 1,000 pounds, one weighing 3,900 pounds and another 5,000 pounds.

The space agency insists the two heaviest pieces, the airlock shroud and a lead-lined film vault, will drop about 500 miles apart. The reason it gives is that the airlock is on the outside of the space station and the film vault on the inside, 15 feet from the airlock.

These two pieces will strike the Earth at 260 miles an hour, NASA says, basing its estimate on Air Force studies of other re-entering satellites and burned-out rockets.