Science - Astronomy

Scanning the skies

Hi-tech equipment used by US scientists have spotted five times more asteroids and comets zipping across Earth's orbit than other instruments, writes ROBERT COOKE ...

EXTRAORDINARY new light-sensing ability linked to ultra-fast computing power is helping scientists search the sky for things that may be coming at us.

The new system, being tested on a telescope in New Mexico, is already spotting five times more asteroids and comets than other instruments. And it's identifying some much-feared NEOs--near-Earth objects--that zip across Earth's orbit in space.

Developed by scientists at the Massachusetts Institute Of Technology/Lincoln Laboratory, the system began watching the sky in March 1998. By the end of the year it had added 15,209 large objects to the list of known asteroids and comets.

Included were 131 rocks that come close to Earth's orbit and 14 comets. Large means more than 90m in diameter, capable of doing serious damage.

"It's working quite well," said Grant Stokes, the principal investigator for the research project. "We're still learning how best to employ this very capable system. We're still tweaking and tuning our patterns and techniques."

The ultimate goal is to create a catalogue of all the space objects that are big enough, and come near enough, to do cataclysmic damage if they hit Earth.

So far, Stokes added, none of the near-Earth objects that have been spotted pose any threats in the foreseeable future.

Nonetheless, almost 20 of them have been identified as "hazardous" by the Smithsonian Institution's Minor Planet Center.

This means they have some potential, at an unknown time far in the future, to hit this planet.

By building the new sky-search system, the team in Massachusetts "has sort of revolutionised the search for near-Earth objects, by a factor of five," said astronomer Brian Marsden, head of the International Astronomical Union's Central Telegram Bureau, in Cambridge, Massachusetts. "They're beating everyone hand over fist because they're covering so much sky."

In one month, March last year, for example, the new system made 90% of the world's asteroid observations--151,035 objects of various sizes, most of them too small to join the list of large asteroids. And the system has continued performing at that level since then.

It is Marsden and his colleagues who are charged with keeping track of what's out there, how big the objects are, where they're going and how close they might come someday. The Minor Planet Center keeps updating a list that contains thousands and thousands of large space objects, including some in the so-called asteroid belt between Mars and Jupiter.

Intense interest in spotting potentially dangerous space objects was spurred by the dramatic impacts of comet Shoemaker-Levy 9, which sent about 20 large chunks of ice-bound dust smashing into Jupiter in July 1994. The event led to numerous proposals to build space-watch systems and spurred scientists to begin thinking about how to intervene if something dangerous is detected.

Although no large, formalised system is yet in place, astronomers at the University of Arizona have one system constantly scanning the sky. Another system in Hawaii, run by scientists at the Jet Propulsion Laboratory in Pasadena, California, is used occasionally. And thousands of amateur astronomers watch with smaller telescopes at night.

The greatly improved performance of the Lincoln Laboratory's system was achieved by exploiting the latest CCD--charge-coupled device--technology. The electronic detector was hooked up to a 40-inch (15.7cm) telescope in Socorro, New Mexico. It can spot objects as faint as 22nd magnitude, many, many times fainter than can be seen with the naked eye.

The system also has a fairly wide field of view, surveying a two-degree-wide patch of sky. What it looks for are objects that are moving across the sky, compared to the steady background of stars.

Its electronic system was especially designed to collect images very quickly, send them to the computer, and begin the next observation.

The detectors, the charge-coupled devices, are especially useful because they detect light almost photon-by-photon. Each light pulse is converted into an electronic signal that can be manipulated by computers. This is far better and faster than collecting the starlight on film, processing it and then trying to read the results.

Basically, the CCD is an ultra-sensitive and extra-large television camera. It consists of a small rectangular patch built up of thousands and thousands of tiny light detectors called pixels--short for picture elements--that act as light-catching traps.

The CCD used by Stokes and his colleagues contains more than five million pixels. In operation, each pixel adds one small part to the overall picture, creating an image somewhat akin to a pointillist painting.

And because it can count incoming photons one by one, it can spot extremely faint sources of light. Such sensitivity is important because asteroids tend to be dark; some of them are almost black.

Of course the danger from impacts is not just theoretical. Many impact craters, some of them very large, are known on Earth's surface. It's also clear that some past collisions have been so large and so devastating they caused mass extinctions. One that struck 66 million years ago apparently knocked off the last of the dinosaurs.

Comets are also a problem and are apt to cause severe damage in a collision. These glowing blobs of ice and dust come sweeping in from the distant edges of the solar system, and are going much faster, in relation to the Earth, than asteroids. So cometary impacts are expected to be even more devastating than asteroid collisions.

Although large collisions are rare, averaging maybe one every million years, a study done for the US National Aeronautics and Space Administration estimated that 320,000 asteroids bigger than 90m in diameter exist, and 2,100 of these are much bigger, more than 800m in diameter.

So far, only about 10% of the asteroids that large or larger have been cataloged. According to Marsden, the US National Aeronautical and Space Administration (Nasa) announced plans for a 10-year effort to seek out 90% of the space objects larger than 800m in diameter, and which might come near.

Before the new system got up and running in New Mexico, Nasa's search was missing that goal by about 30-fold. Now, even with the new detection system, they still have a factor of six to go. What the new system demonstrates, however, is that the job can be done. So now "it's only a matter of paying for it," Marsden said.

Stokes explained that the new sky survey system is a spinoff from work his team at the Lincoln Laboratory was already doing.

"Our day job has to do with developing space surveillance technology for the US Air Force," he said. The air force is more interested in what's orbiting Earth itself, rather than what's further away.

At present, a large array of telescopes is engaged in watching spy satellites, communication satellites, rocket casings and various kinds of space debris. "They track everything in orbit that is man-made," Stokes said.--LAT-WP

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