For decades, space experts have worried that a speeding bit of orbital debris might one day smash a large spacecraft into hundreds of pieces and start a chain reaction, a slow cascade of collisions that would expand for centuries, spreading chaos through the heavens.

In the last decade or so, as scientists came to agree that the number of objects in orbit had surpassed a critical mass -- or, in their terms, the critical spatial density, the point at which a chain reaction becomes inevitable -- they grew more anxious.

Early this year, after a half-century of growth, the federal list of detectable objects (4 inches wide or larger) reached 10,000, including dead satellites, spent rocket stages, a camera, a hand tool and junkyards of whirling debris left over from chance explosions and destructive tests.

Now, experts say, China's test Jan. 11 of an anti-satellite rocket that shattered an old satellite into hundreds of large fragments means the chain reaction will most likely start sooner. If their predictions are right, the cascade could put billions of dollars' worth of advanced satellites at risk and eventually threaten to limit humanity's reach for the stars.

Federal and private experts say early estimates of 800 pieces of detectable debris from the shattering of the satellite will grow to nearly 1,000 as observations continue by tracking radars and space cameras. At either number, it is the worst such episode in space history.

Today, next year or next decade, some piece of whirling debris will start the cascade, experts say.

"It's inevitable," said Nicholas Johnson, chief scientist for orbital debris at NASA. "A significant piece of debris will run into an old rocket body, and that will create more debris. It's a bad situation."

Geoffrey Forden, an arms expert at the Massachusetts Institute of Technology who is analyzing the Chinese satellite debris, said China perhaps failed to realize the magnitude of the test's indirect hazards.

Forden suggested that Chinese engineers might have understood the risks but failed to communicate them. In China, he said, "the decision process is still so opaque that maybe they didn't know who to talk to. Maybe you have a disconnect between the engineers and the people who think about policy."

China, experts note, has 39 satellites of its own -- many of them now facing a heightened risk of destruction.

Politically, the situation is delicate. In recent years China has played a growing international role in fighting the proliferation of space junk. In 2002, for instance, it joined with other space-faring nations to suggest voluntary guidelines for debris control.

In April, Beijing is to play host to the annual meeting of the advocacy group, known as the Inter-Agency Space Debris Coordination Committee. Donald Kessler, a former head of the orbital debris program at NASA and a pioneer analyst of the space threat, said Chinese officials at the forum would probably feel "some embarrassment."

Kessler said Western analysts agreed that China's new satellite fragments would speed the chain reaction's onset. "If the Chinese didn't do the test, it would still happen," he said. "It just wouldn't happen as quickly."

Last week in Beijing, a foreign ministry spokeswoman failed to respond directly to a debris question. Asked if the satellite's remains would threaten other spacecraft, she asserted that China's policy was to keep space free of weapons.

"We are ready to strengthen international cooperation in this regard," the spokeswoman, Jiang Yu, told reporters.

Cascade warnings began as early as 1978. Kessler and his NASA colleague, Burton Cour-Palais, wrote in the Journal of Geophysical Research that speeding junk that formed more junk would produce "an exponential increase in the number of objects with time, creating a belt of debris around the Earth."

During the Cold War, Moscow and Washington generally ignored the danger and, from 1968 to 1986, conducted more than 20 tests of anti-satellite arms that created clouds of jagged scraps. Often, they did so at low altitudes from which the resulting debris soon plunged earthward. Still, the number of objects grew as more nations launched rockets and satellites into orbit.

In 1995, as the count passed 8,000, the National Academy of Sciences warned in a thick report that some crowded orbits appeared to have already reached the "critical density" needed to sustain a chain reaction.

A year later, apprehension rose as the fuel tank of an abandoned American rocket engine exploded, breaking the craft into 713 detectable fragments -- until now, the record.

Amid such developments, space experts identified the first collisions that threatened to start a chain reaction, putting analysts increasingly on edge.

On Jan. 17, 2005, for instance, a piece of speeding debris from an exploded Chinese rocket collided with a derelict American rocket body that had been shot into space 31 years earlier. Warily, investigators searched though orbital neighborhoods but found to their relief that the crackup had produced only four pieces of detectable debris.

A year later, Johnson, the chief scientist for NASA's orbital debris program, and his colleague J.C. Liou, published an article in the journal Science that detailed the growing threat. They said orbits were now so cluttered that the chain reaction was sure to start even if space-faring nations refrained from launching any more spacecraft.

"The environment is unstable," they wrote, "and collisions will become the most dominant debris-generating mechanism." It was in this atmosphere of rising tension that China last month fired a rocket into space that shattered an old weather satellite -- its first successful test of an anti-satellite weapon.

David Wright, a senior scientist at the Union of Concerned Scientists, a private group in Cambridge, Mass., calculated that the old satellite had broken into 1,000 fragments 4 inches wide or larger, and millions of smaller ones.

Federal sky-watchers who catalog objects in the Earth orbit work slowly and deliberately. As of Monday, they publicly listed 647 detectable pieces of the satellite but were said to be tracking hundreds more.

The breakup was dangerous because the satellite's orbit was relatively high, some 530 miles up. That means the debris will remain in space for tens, thousands or even millions of years.

Kessler, the former NASA official, now a private consultant in Asheville, N.C., said China might have chosen a relatively high target to avoid directly threatening the International Space Station and its astronaut crew, which orbit at a height of about 220 miles.

"Maybe the choice was to endanger the station in the short term or to cause a long-term problem," he said. "Maybe that forced them to raise the orbit."

Even so, the paths of the speeding Chinese debris, following the laws of physics and of celestial mechanics, expanded in many directions, including upward and downward. As of last week, outliers from the central cloud stretched from roughly 100 miles to more than 2,000 miles above Earth. A solution to the cascade threat exists but is costly. In his Science paper and in recent interviews, Johnson of NASA argued that the only sure answer was environmental remediation, including the removal of existing large objects from orbit.

Robots might install rocket engines to send dead spacecraft careering back into the atmosphere, or ground-based lasers might be used to zap debris.

The bad news, Johnson said in his paper, is that "for the near term, no single remediation technique appears to be both technically feasible and economically viable."

If nothing is done, a kind of orbital crisis might ensue that is known as the Kessler Syndrome, after Kessler. A staple of science fiction, it holds that the space around Earth becomes so riddled with junk that launchings are almost impossible. Vehicles that entered space would quickly be destroyed.

In an interview, Kessler called the worst-case scenario an exaggeration. "It's been overdone," he said of the syndrome. Still, he warned of an economic barrier to space exploration that could arise. To fight debris, he said, designers would have to give spacecraft more and more shielding, struggling to protect the craft from destruction and making them heavier and more costly in the process.

At some point, he said, perhaps centuries from now, the costs would outweigh the benefits.

"It gets more and more expensive," he said. "Sooner or later, it gets too expensive to do business in space."