SCIENCE / MEDICINE : As Space Station Prepares to Go Up, Fears Grow About It Coming Down : NASA: The agency says failure is ‘highly unlikely.’ But engineers are studying the possibility of an ‘unplanned re-entry.’

When things go NASA’s way, what goes up doesn’t necessarily come back down.

With scores of satellites uneventfully orbiting Earth, the space agency’s track record is hard to assail. Yet when things go wrong, the failures can be spectacular.

During the 77-ton Skylab’s fiery plunge in 1979, there was massive hand-wringing around the world. Luckily, the debris fell harmlessly into the Indian Ocean and Australia’s outback.

But what if something three times the size of Skylab--or roughly the size of the U.S. Capitol building--were to experience, in NASA’s words, an “accidental inadvertent re-entry”?

That the much-touted space station, scheduled to be completed within 10 years, could fall from the sky, damaging property and taking lives, has largely been obscured by the public fascination with its futuristic design and unquestioned central role in U.S. space exploration.

Now two recent reports have served to spotlight that scenario, one that NASA says is utterly remote.

But one prominent space scientist says that simple arithmetic--based on NASA’s own numbers--shows that there is a strong likelihood, not just a possibility, that the 500-foot, 290-ton space station could run out of the fuel it needs to maintain orbit, thus falling back to Earth.

NASA, in a recently released draft environmental impact statement on the space station, said specifically that the chances of the facility ever falling are “highly unlikely.”

It nevertheless candidly acknowledged that there could be two types of “unplanned re-entries”--controlled and uncontrolled.

In a controlled re-entry, NASA said, the space station could be steered on a course of descent that would minimize any “resulting footprint”--in other words, away from population centers.

But an out-of-control space station “could produce a debris footprint at a random location,” NASA conceded. Its environmental impact statement left the rest to the imagination.

The $30-billion project is considered the steppingstone to the exploration of the moon, Mars, the rest of the solar system and beyond. Its construction, not yet under way, is expected to take about five years. It will be 1995 at the earliest before the first components are launched.

The Freedom--the largest spacecraft ever assembled--is being designed to spend no less than 30 years in orbit 150 to 270 miles above Earth at 18,000 m.p.h., circumnavigating it every 90 minutes.

But the current designs are to be thoroughly re-examined because of emerging concerns among some NASA engineers, expressed in a recent interim report, that the space station’s intricate components may require extensive repair and maintenance in outer space long before astronauts have it fully assembled.

The engineers said the space station could be only 60% to 70% complete before space-walking astronauts may have to turn from assembling it to performing inspections, maintenance and repairs.

“With any long, strung-out process of assembly, you’re running a risk of components breaking down,” said Charles R. Price, a NASA engineer the Johnson Space Center in Houston.

“One of the most difficult problems is to establish failure rate of components; that’s predicting the future,” he added.

The potential flaws, involving solar power panels, various electronic devices and assorted structural components, also could require astronauts to spend dangerously large amounts of time working outside the space station, thus exposing themselves to excessive radiation and debris.

NASA’s environmental impact statement identified several possible causes for the space station or some of its components to plunge to Earth:

* Failure of multiple and major on-board systems.

* Collision with natural debris or with the space shuttle, which will ferry construction materials, astronauts and propellant fuel to the station.

* Lack of sufficient propellant to stay in orbit.

Each scenario is “highly unlikely,” NASA said.

But aside from any structural damage or mechanical failures, the most likely cause of a space station plunge would be that it will run out of propellant, causing it to fall out of orbit, said John Pike, associate director for space policy at the Washington-based Federation of American Scientists.

The space station is supposed to have sufficient fuel to maintain orbit for 180 days, although current plans call for refueling every 73 days or so.

But given such a large number of shuttle flights during the 30-year lifespan of the space station, Pike reasoned, another catastrophe like the 1986 Challenger explosion is inevitable--a calculation based on NASA’s own odds of such a recurrence.

Another Challenger-like disaster, in turn, would lead to a prolonged grounding of all shuttles, thus making the refueling of the space station impossible, Pike said. He noted that on each of four occasions when the United States or the Soviet Union lost astronauts, manned spacecraft were grounded for more than a year.

After the Challenger tragedy, the Department of Transportation asked NASA to assess the odds of a recurrence, and the space agency came up with a worst-case scenario of one explosion in every 78 launches and a best-case scenario of one in 200.

Based on those projections, Pike said, “that would put it (the next accident) in the early next century.”

It would take 28 shuttle flights just to lift into orbit all the components of the space station.

“Simple math is going to tell you that--rather than extremely unlikely, as they describe it--one would be surprisingly lucky if it didn’t happen,” Pike said.

But Mark Hess, a NASA space station spokesman, warned against automatically assuming that the shuttle fleet would be grounded for extended time periods after another accident.

“That doesn’t necessarily follow,” Hess said, adding that measures such as repositioning the space station or rearranging its solar panels could extend its orbital life by up to 200 more days.

In any case, added Doyle McDonald, a NASA spacecraft engineer who directed the environmental impact statement, any precise calculations should be deferred until the space station’s final design and configuration are known.