http://www.nytimes.com/2007/02/06/sc...ce/06orbi.html

Orbiting Junk, Once a Nuisance, Is Now a Threat

By WILLIAM J. BROAD
Published: February 6, 2007

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 (four 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 on 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 that 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 the National Aeronautics and Space Admin-
istration. "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.

Dr. 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 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."

Mr. 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. Mr. Kessler and
his NASA colleague, Burton G. 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 antisatellite 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, Mr. 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 spacefaring 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 antisatellite weapon.

David C. 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 four inches
wide or larger, and millions of smaller ones.

Federal sky-watchers who catalogue objects in the Earth orbit
work slowly and deliberately. As of yesterday, 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.

Mr. 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 the Earth.

A solution to the cascade threat exists but is costly. In his
Science paper and in recent interviews, Mr. 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, Mr. 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 Mr. 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, Mr. 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 will 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 will outweigh the benefits.

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