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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 antisatellite 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 L. 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 E. 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 spacefaring 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 J. 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.”