Tuesday 3 August 2010

Space junk. Where can we put it?

Since the first satellite, Sputnik 1, was launched in 1957, thousands of space probes, satellites and telescopes have been sent into space. Just as we have created rubbish mountains on Earth, we've also accumulated a blanket of junk around the Earth.

This debris silently zooms around the globe at speeds of up to 25,000 miles per hour with altitudes ranging from hundreds to thousands of miles. NASA has frequently replaced windows on the space shuttle that have been damaged by objects as small as a flake of paint. There may be over a million pieces of space junk currently orbiting the Earth; however, all but 9,000 of these are smaller than a tennis ball. About 70,000 objects about the size of a postage stamp have been detected between 850 and 1,000 km above the Earth. They are probably frozen bits of nuclear reactor coolant that are leaking from old satellites but even they can cause severe damage if it strikes a rocket or satellite at speeds of 25,000 miles per hour.

A one-milimetre sized metal chip could do as much damage as a .22-caliber long rifle bullet. Bits this size doesn’t generally pose a large threat to spacecraft, but can erode more sensitive surfaces and disrupt missions.

A pea-sized ball moving this fast is as dangerous as a 400-lb safe travelling at 60 miles per hour Debris this large may penetrate a spacecraft. If this happens through a critical component, such as the flight computer or propellant tank, this could be fatal not to mention crushing an astronaught in its direct path.

A metal sphere the size of a tennis ball is as lethal as 25 sticks of dynamite. This debris will penetrate and seriously damage a spacecraft.

Some research has suggested that the collision risk from small particles is minimal. One study showed that a craft that had been in space for more than five years was struck by particles more than 30,000 times, with no ill effect.

Ten years ago, it was estimated that as much as 193,000kg (425,000 pounds) of material re-entered the Earth's atmosphere. However, unless the material was extremely large, the material would vaporize before it reached the surface of earth. My concern however is that some of it may not completely vaporize and strike an airliner in flight. So far, that hasn’t happened as of yet.

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.

The number of objects currently in orbit had surpassed a critical mass; or, in scientific terms, the critical spatial density, the point at which a chain reaction becomes inevitable.

Early in the year 2007, after a half-century of growth, the 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.

China's test on January 11th of that year of an anti-satellite rocket that shattered one of their old satellites into hundreds of large fragments means the chain reaction started sooner. The cascade of debris 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.

Sometime perhaps in this decade, some piece of whirling debris will start the cascade, experts say. A significant piece of debris will run into an old rocket body, and that will create more debris. Many of China’s satellites are now facing a heightened risk of destruction.

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 their 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. 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.

China 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.

The paths of the speeding Chinese debris, following the laws of physics and of celestial mechanics, expanded in many directions, including upward and downward. 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, perhaps centuries from now, the costs would outweigh the benefits. It gets more and more expensive. Sooner or later, it will get too expensive to do any form of business in space.

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