Some time this year, probably in June or July, the Skylab Orbital Workshop will break up in the upper atmosphere and shower the earth beneath it with fragments of metal debris.Scientists at the National Aeronautics and Space Administration estimate that the heat and friction of re-entry will consume about three-quarters of the 157,000-pound spacecraft, and will tear the rest into about 500 pieces, some weighing a few ounces, others as heavy as 4,000 pounds. When the Skylab debris comes, it will be, in one sense, a gift from the sun. The huge space station, which cost $2.6 billion to put in orbit and took eight years to plan, is falling because of events 93 million miles away, on the surface of the sun-or, more precisely, because a team of NASA scientists guessed wrong in 1974 how many sunspots there would be in 1979.

In that sense, then, the fall of Skylab is an act of God. But the ignominious end of the Orbital Workshop-which was home to three teams of astronauts for a total of 171 days in 1973 and 1974-also represents the fruit of a seemingly unrelated series of decisions and miscalculations made in Congress, the White House, NASA headquarters on Independence Avenue, and the Marshall Space Flight Center in Huntsville, Ala.

And it is also a neat symbol of the malaise that has hit the once-proud civilian space program in the decade after its triumphal landing on the moon. For Skylab is almost literally falling into the gap between the flush years of the Apollo program and the cool cost-efficiency of the coming era of the Space Shuttle.

The original Skylab mission, despite a shaky start, was a tremendous success. Skylab's three crews remained aboard longer than planned, and used their instruments to gather new knowledge about subjects ranging from the nature of the sun's corona to the infestation of hemlock looper in Essex County, Massachusetts. Equally important, from the agency's point of view, they were the source of much new information-not all encouraging-about the ability of the body to withstand long periods of weighless space travel.

NASA loyalists like to hark back to the program's active phase; but now the public imagination has been captured by the image of a 118-foot space station-the size of a three-room house-plunging to earth, with the ominous possibility of damage, injuries and death. NASA officials testily refer to this as the "Chicken Little" aspect, and insists that the picture is overdrawn-that the risk of any damage from falling debris is relatively small.

In fact, when they planned Skylab, they knew that the workshop would probably fall out of orbit just this way, and they decided the risk was acceptable. "The conclusion was that the risk was of the same order of magnitude as that from other orbital debris," said George M. Low, who was deputy head of the agency at the time. The agency considered outfitting the space station with a retrorocket system which would bring it down in the ocean-and rejected the idea because it could endanger astronauts on board.

If there is a mistake, or a policy failure, in the history of Skylab, that was it. NASA officials don't see it as one. For them, the last days of Skylab are a melancholy drama of declining budgets and disappointed hopes, missed chances and merciless schedules. Their efforts over the past year or so to save Skylab are a technical suspense story in some ways as absorbing as the spirit for the moon or the unmanned missions to the outer planets.

But there is a difference. NASA failed to rescue Skylab. For an agency used to success, that change is difficult to adjust to. It brings a note of frustration into the normally cool technical diction of the agency's space scientists and technicians. "In the military," mused one NASA philosopher, "command is the key word. At NASA, it's control-mission control, launch control, ground control.We're used to having control and the whole mentality here is geared that way. Here, quite frankly, we don't have control. It's an agonizing process."

The key to understanding the process, though, is the sun. The wisps of the upper atmosphere produce "drag" as a satellite travels in its orbit; the drag brings the object lower until it finally falls out of orbit. During the first four years of an 11-year sunspot cycle, the increased solar activity causes the atmosphere to expand, as if yearning toward the sun; then, during the seven-year decline, it slowly contracts.

To predict orbital lifetime of a satellite, NASA must thus predict what the "Zurich sunspot number" will be for months and years ahead.

Unfortunately, no one understands why one 11-year cycle is more intense than another. "On the whole," says NASA head Robert Frosch with a resigned sigh, "I would rather look at the entrails of chickens than try to predict solar activity."

But somebody has to, and in 1974, when the last team of astronauts left Skylab, it was the space sciences laboratory at Marshall Space Flight Center. "We don't claim any great certainty," Charles Lundquist, head of the laboratory, said recently. "It's a bit like the court astrologer looking at the pattern of eclipses and trying to guess when the next eclipse will be."

Solar physicists must look at what has gone before-the 250-odd years for which we have sunspot records. Galileo was the first Western astronomer to record the existence of the spots; but shorly after his discovery, they vanished entirely for a period of 75 years-the so-called "Maunder minimum," or "Little Ice Age."

When the spots reappeared in 1715, the new generation of astonomers were at first unable to decide exactly what they were. A debate still goes on in the solar science community about whether the observations of the first eight cycles after the "minimum" are worth using or not. The National Oceanic and Atmospheric Administration, for example, prides itself on using a "short, clean data base"; NOAA has been far more accurate in its predictions for the current cycle than has NASA.

But it's easy to sympathize with NASA's solar specialists. In 1974 they were trying to forecast what the next sunspot cycle would be like while still paddling in the shallows of the last one. In the spring of 1974, they compared the declining cycle with previous ones, and came up with a projection for a relatively mild cycle, with a maximum sunspot number somewhere between 40 and 135-probably around 75. That meant that Skylab would stay up until about 1983.

As it turned out, they were wrong. The sunspot number in February of this year was 138-and the cycle is not yet at maximum. But nobody could know that for another two years. Skylab, meanwhile, was peacefully orbiting the earth; 1983 was comfortably beyond the completion date for the Space Shuttle system, which would be capable of revisiting Skylab and either pushing it into a higher orbit (called "reboost" in NASA language) or bringing it safely down in the ocean ("controlled deorbit").

At the time, no one at NASA made any connection between Skylab and a series of minor misfortunes that befell the Shuttle project in the early '70s. The first Shuttle orbiter was originally scheduled to fly in March of 1978. But troubles developed, one by one.In July 1971, NASA let a contract to Rocketdyne, a California corporation, to develop the shuttle's rocket engine. This was before the formal start of the Shuttle program; NASA wanted a head start on engine development, the trickiest part of a new space vehicle. But a competing contractor, Pratt & Whitney, protested to the General Accounting Office that NASA had not given it a fair chance to bid. In a 107-page opinion, Comptroller General Elmer Staats rejected Pratt & Whitney's claim; but the nine-month GAO investigation meant that the head start was completely lost.

In January 1973, with a massive re-election victory behind him and the economy showing signs of decay, President Richard Nixon decided to take a firm line against government spending. NASA's once-sacred budget had already suffered severe cuts during Nixon's first term; the Shuttle was all that remained of an impressive "wish list" for the '70s. Now Nixon went after that.

James Fletcher, then head of NASA, recalls the ensuing budget battle as "one of the most traumatic experiences of my life." An Office of Management and Budget official, who asked not to be named, saw it as merely an attempt to impose economical procedures on a new program which had not yet gotten underway. "There was a feeling that the program was just getting geared up," he said. "It didn't have to meet any particular end date."

The result was an $85 million cut in the Shuttle budget for fiscal 1974, and another $89 million cut in the 1975 budget. NASA had to push its target date for the first orbit of the Shuttle back 15 months-from March 1978 to June 1979.

Those 15 unimportant months looked more important in 1977, when the new sunspot cycle got underway. By October-with the sunspot number at 43.2-the scientists at Marshall knew their earlier prediction had been too low. The new forecast called for a maximum of 96-which would bring Skylab down in April 1980. The need for a solution had become a little more urgent.

Frosch had succeeded Fletcher as head of NASA in June. "When I looked at it in the summer of '77, we still had the evaluation that the risk was small," he said. "But facing the more imminent possiblity, we felt the responsibility to do something about it if something could be done economically."

At the end of October 1977, Frosch ordered a crash program to produce a device that could ride the Shuttle into orbit, link up with Skylab, and move it into a higher orbit.

Known as the teleoperator retrieval system, or TRS, the device was designed to emerge from the Shuttle's wide cargo bay and fly across a half-mile of space to Skylab. An astronaut would fly TRS by remote control, using a television signal relayed from its nose.

A TRS unit had been a dream of NASA's since the inception of the Shuttle. But the agency had not planned on building it until 1982 or 1983. So the emergency TRS which was designed to save Skylab became a kind of orbital gooney bird, concocted of equipment left over from other programs. The central unit of TRS would be small-4 feet by 4 feet by 5 feet; four detachable booster tanks would push the space station about 75 nautical miles higher. TRS was designed with that much power because the new sunspot projection showed it would be enough to put Skylab into a stable orbit.

By February 1978, however, the sunspot number was 93.6.

In March, a team of NASA scientists flew to the Bermuda space tracking station to try to communicate with Skylab. No one had been aboard the orbital workshop since February of 1974. This part of the story is atypical: things went better than expected. The Bermuda team found that Skylab had hardly degenerated at all during the four-year hiatus. NASA began to suspect that if they could reach the workshop in time, it might play a valuable role during the Shuttle era.

The Bermuda crew was directed by Herman Thomason, a quiet control systems engineer from Marshall who speaks of Skylab today with paternal pride and regret. The group at first had a simple mission: to see if they could talk to Skylab, and, if so, to try to put it into a tumble, or rolling motion, which would reduce the drag of the upper atmosphere and keep it up longer.

But when they turned on the telemetry control system, they received a confusing jumble of data, followed by silence, followed by a blank signal, and then silence again. In fact, Skylab had already fallen into a complex tumble. Its solar power panels were providing electricity to the telemetry system only when they were pointing at the sun.

The intervals of signal and silence were the one concrete piece of information Skylab could provide. Thomason and his crew used this opening to devise ways to control a vehicle which was not designed to be controlled from the ground. Their problem might be compared to the story about the boy who tries to fish a quarter out of a sewer grate with a wad of gum on a string-but, in this case, the boy was trying not to retrieve the coin, but to flip it over in the dark, and then to guess from the feel of the string whether the quarter was heads or tails.

The first job was to restore electrical power to the discharged batteries. These could be charged by the sun; but they were designed to shut themselves off whenever their charge was low. A fraction of a second after NASA turned them on, they would dutifully shut themselves off. Thomason's crew began sending frantic, repeated orders to turn on the batteries whenever their instruments showed the panels were pointing at the sun-"bootlegging" power in.

By the end of May they had 13 of the original 18 batteries working. Because of a ground equipment shortage, they could communicate with Skylab only through the Bermuda station-sometimes only for 10 minutes a day. This cost them dearly when they tried to adjust Skylab's attitude. Once again, they used the sun as a point of reference, instructing the onboard computer to fire the attitude control, or TAC, rockets and stabilize the station when the panels were pointing at the sun. It worked-at first.

"We were feeling pretty good that day," Thomason recalls. But they did not know that an instrument designed to handle computer commands had developed a defect that caused it to misread certain numbers. The next day, when they tried to turn on the standby computer, the device mistakenly shut off the two giant gyroscopes that keep Skylab steady. After Bermuda had lost contact, the space station developed a continous wild wobble; the computer, under orders to steady up on the sun, vainly fired the TAC rockets, wasting irreplaceable fuel.

But by the end of July, Thomason and his team had Skylab under firm control, and since then they have been able to do virtuoso tricks with it, like yo-yo aces switching rapidly from "around the world" to "walking the dog" and back again. Thomason affectionately gives the credit to Skylab itself. "Every time somebody would call at 2 o'clock and say, 'Something is wrong,' you'd bleed a little," he said. "But it's been a forgiving vehicle.We've made some stupid mistakes, and it's helped us get back."

But if Skylab was forgiving, other things were not. In July, the sunspot number was 70.4; by September it had shot up to 138.2

Both the Space Shuttle and the TRS, meanwhile, were running into problems. To withstand the strain of repeated take-offs, the Shuttle engine must be highly durable; engine research had been delayed for nine months while Pratt & Whitney and Rocketdyne slugged it out before the GAO. In late 1977 and early 1978, problems developed in the engine fuel valves.

The orbiter must also be covered with 30,000 silica tiles, and many of them had turned out to be defective.

On August 30, NASA concluded that the orbiter could not fly until September 1979.

TRS, meanwhile, was being whipsawed by inflation. Component prices were running higher than expected; computer programs were also proving more complicated and expensive than anticipated. TRS could probably still have been built on or near its schedule-but costs would have risen $10 to $15 million. Congress had required NASA to make an interim report before spending more than $10 million for TRS. That figure was approaching.

The sunspot number was 122.7 in early December. Skylab was falling faster than had been predicted only six months earlier. The Shuttle was behind schedule and might slip further. Even if they got the money to cover the TRS cost overrun, that program might run into schedule delays as well. And NASA had become convinced, in the words of Robert Aller, head of the combined TRS-Skylab rescue program, that "Skylab's going to burp too heavily one of these days."

The orbital workshop had been built with three gyros; one had failed during the origninal mission. Now another was having problems. Thomason was working on a plan to control Skylab with only one gyro; but no one was certain that was possible. If the malfunctioning gyro seized up suddenly, Skylab would spin out of control, and there was not enough fuel left in the TAC rockets for course corrections.

But the main problem was simply that, even if everything went as planned, the TRS did not have enough power to push Skylab into a stable orbit. The sun had beaten NASA again. Because of the higher sunspot activity, TRS would intercept Skylab at an altitude of about 150 nautical miles, and push it up to 225 nautical miles-25 miles too low. Skylab would soon begin falling again.

Frosch recommended to the White House that the Skylab effort be dropped. "The philosophy was: Why spend the money to do something that you can't do anyway, just in a valiant effort to save it?" he said later. On Dec. 18 last year, Yardley and Aller announced that Skylab would be allowed to fall. "Skylab has been a good friend," Yardly said. "But we do have to consider the realities of life and have to conclude now that it is not worth it any longer."

Skylab will be the largest orbiting body ever to re-enter the atmosphere. No one denies that there is some danger of injury or property damage, and the "Chicken Little" aspect has grabbed the public's attention. NASA's words of calm, couched in the soothing language of comparative probability, have a hard time catching up with the grim fantasy of disaster.

NASA's studies show that Skylab will break up into about 500 pieces. Most will weigh less than 20 pounds, but a few will be quite large-there is a 5,000-pound air-lock shroud, a 4,000-pound lead film vault and a set of six oxygen tanks which total 3,000 pounds. But no one site will be subjected to a hail of deadly debris, NASA says. Instead, the fragments will spread out along a huge "footprint," 100 miles wide and 4,000 miles long. Within that area, the densest concentration of debris will be one piece per 50 square miles.

The debris is likely to miss people and houses, NASA says, simply because human beings-and the structures they inhabit-occupy a relatively small portion of the earth's surface. "The odds of hitting anyone are just awfully remote," said Arnold Frutkin of NASA's external affairs division."If you drop a safe from a tall building in New York City at high noon on a main street, you could have a problem. But you'd have no certainty of hitting anyone."

NASA also points out that a number of other large rockets and satellites have re-entered without mishap-for example, a 90,000-pound Saturn rocket and a 22,000-pound Pegasus satellite. In addition, between 1,200 and 7,500 meteorites-some weighing more than 200 pounds-fall from space every year. In the past 200 years, there have been seven recorded injuries-and no deaths-from meteorites.

In fact, NASA says that the total risk from Skylab is about the same as one year's worth of meteorites. The most recent risk study concludes that there is one chance in 152 that there will be a single casualty from Skylab. That is not the same as no chance at all, however. So, on White House orders, NASA has begun to develop a contingency plan for a possible Skylab disaster.

The only situation in which the government took comparable precautions was the re-entry of the Soviet Cosmos 954 satellite over Canada in January 1978. That satellite, which injured no one, carried a small nuclear reactor, making it in many ways more dangerous than Skylab. However, according to a senior Defense Department official who worked on the Cosmos task force, the probability of injury from Cosmos was less than that of Skylab.Cosmos was "a very small probability of a very bad outcome," the source explained, while Skylab is "a lukewarm probability of a lukewarm outcome." Even if there are no casualties, Skylab could do serious damage-there is one chance in seven that a piece of debris will fall on a city of more than 100,000 and one chance in 40 that it will weigh more than 250 pounds.

As the end approaches, NASA will set up a Skylab coordinating center. The North American Air Defense Command will track Skylab by radar as it falls, and, in the last day or so, will begin to give estimates of where it will re-enter.

As the re-entry predictions become firmer, NASA-working in conjunction with the White House-will alert Civil Defense authorities-or foreign officials, if the projected footprint falls abroad; but there will probably be no public announcement until after Skylab falls. No one can be certain the predictions are right. With Cosmos, NORAD's final predictions jumped during the last six hours from Hawaii to North Africa and then back to Canada, because Cosmos "skipped out" of the atmosphere, like a rock skipping across the surface of a pond. The same thing may happen with Skylab.

"Those calculations are absolutely no good for operating purposes," says NASA's Frutkin. "You can't go evacuate a town on the basis of the first footprint. We could cause riots that could really hurt people far more than the stuff itself."

Once Skylab drops off NORAD's screen, NASA will try to calculate where the footprint has fallen. If it is over land, there will begin the agonizing process of waiting for damage reports and trying to verify them. In this country, verification-and emergency medical help if needed-will be handled by the Federal Preparedness Agency. If Skylab hits in a foreign country, the Pentagon's Military Airlift Command will have medical units, airplanes and military hospitals on alert around the world.

There will be contingency plans for mass casualties, according to a Pentagon source. "This isn't as alarming as the Cosmos," he said. "But this one is still alarming. Just because it's unlikely doesn't mean it can't happen. You're got to look at all the what-ifs."

NASA is currently planning for a "random deorbit", in which they have no control over Skylab. But a team of specialists, lead by William Goldsby, is considering using the gyros and attitude rockets to pick the moment of re-entry. Some of them believe that NASA will be able to pick which orbit to bring the space station in on.

The problem is that they might not be able to control anything else. Skylab's orbit describes a wandering path across the earth's surface, covering every spot between 50 degrees North latitude and 50 degrees South-a band that includes 90 percent of the earth's population, and most world capitals (Moscow is an exception). NASA officials are somewhat worried about the consequences of choosing one orbit over another, if something goes wrong. "You're making a funny kind of choice," says Frosch. "I'm not even sure we know how to make it."

A large body of thought holds that the choice may not be possible-that Skylab will begin an uncontrollable tumble. But Marshall Kaplan, a Penn State aeronautical engineer, has a NASA contract to work on a plan he thinks may even be able to bring Skylab down in a stretch of ocean. Others at NASA privately consider him too optimistic. The agency must decide early this month whether they have a reasonable chance of making the situation better by controlling Skylab during re-entry. If so, it will probably re-enter in late June or early July. NASA will hold it up that long because during that period energy from the sun will fall on the solar power panels strongly enough to allow full power inside the workshop.

If the decision is against controlling Skylab, they will just let it come down when and where it will. Depending on the sun, that may come as early as May or as late as September. In either case, by New Year's Day 1980, Skylab will be a memory for most of us. If we are very unlucky, it will be a memory of sorrow and disaster, like a plane crash or an earthquake. But the odds are that it will be remembered by the public if at all, as something like the Comet Kohoutek of 1973-a scary cosmic pheonomenon which did not live up to its advance billing.

For NASA it will be a memory of loss and embarrassment, of an asset that was, through a series of circumstance, converted into a liability. The Shuttle, after all, was originally conceived as a way to fly back and forth to a large orbiting space station But because of budget austerity, there is no space station in sight. The mighty Saturn rockets that put Skylab in orbit have now been scrapped. A station could be orbited by building it in modules which could be carried up one by one in the Shuttle and assembled in space. But there are no plans, and no funds, for doing that. Skylab was NASA's last shot at a long-term, large-size space station for some time to come.

The Soviet Union, meanwhile, has a Salyut 6 station in orbit, which is being used by a team of cosmonauts right now. "I have a personal disappointment that the U.S. does not have a space station," says William Schneider, who ran the original Skylab program. "My curiosity is aroused when I read of the Soviets having a continuous presence in space."

"It's a hell of a waste," says Sen. Harrison Schmitt (R-N.M.), the Apollo 17 astronaut-turned-politician. Schmitt, is convinced that industrial technologies developed in space hold the answer to America's faltering economy and world leadership; and Skylab would have been a perfect place to begin experimenting with such things as vacuum-melted steel alloys and superconductors. It could also have been useful as a long-range weightlessness laboratory, which would provide essential information on how astronauts would survive the long flights to the planets, or as a chance to study the upper atmosphere.

Serendipitous discovery is a major part of scientific advance; and a reactivated Skylab would have been a $2.6 billion dividend in a lean time for the American space effort.

That, in the end, may be the real meaning of the losing battle to save Skylab. NASA lost because they have shaved it too close, because each decision had to be justified in terms of cost-efficiency, because the agency was assigned to deal with the unknown and given no real margin for error. "The problem you're in when you're a bureaucrat," mused former NASA chief James Fletcher, "is that you have to justify everything you do."

During the Apollo program, space was the favored child of an expanding economy. If the Shuttle system works as well as expected, it may rekindle some of America's lost enthusiasm for space, making the '80s a new era of innovation and progress. But in the history of space exploration, the '70s are likely to be remembered as a pause, a stutter, a time of retrenchment-the decade when NASA gambled against the sun and lost. CAPTION: The cover photo of Skylab is courtesy of the National Aeronautics and Space Administration.; Picture 1, The Skylab Orbital Workshop still hangs uneasily suspended above the earth. This 157,000-lb cylinder will enter the atmosphere some time this year-perhaps as early as June-and break apart.; Picture 2, Guessing the sunspot number, says NASA, is as easy as reading chicken entrails; Pictures 3 and 4, The Space Shuttle orbiter Enterprise (top), made its first successful test flight in August 1977 atop a jumbo jet. NASA'S plans called for an orbital flight in 1978. They were scrapped when Richard Nixon slashed NASA's budget in 1973 and 1974, and the Shuttle is now scheduled to go into orbit late this year. NASA officials hoped that the Shuttle would be ready in time to carry a teleoperator retrieval system (artist's conception, at bottom) up to Skylab. The TRS, which was never built, was designed to dock with the workshop and push it up into a stable orbit.; Picture 5, Mission Commander Charles "Pete" Conrad (left, back to camera) and Science Pilot Joseph Kerwin (right) check out a backpack manuevering unit in the weightlessness of the first Skylab mission.; Pictures 6 and 7, NASA head Robert Frosch (top) and William Goldsby (bottom) must decide whether to try to control Skylab's descent.; Picture 8, A crew member of the second Skylab mission manuevers in space outside the huge space station. NASA's effort to save the orbital workshop centered on the agency's hopes of reusing Skylab as a "shirtsleeve environment" and large-scale laboratory when the Space Shuttle is ready to fly. Photographs courtesy of NASA