Up in the sky! An asteroid 10 miles wide!

Rating: A new scale ranks threats posed by "near-Earth objects" from 0 to 10. A 0? No sweat. A 10? Start praying.

August 08, 1999|By Martin Merzer

THE WORLD'S leading astronomers recently adopted a new rating system likely to have a Deep Impact on many people. Similar to the Saffir-Simpson hurricane scale, it predicts the devastation likely to be wrought by a collision between Earth and an asteroid.

Category 0 or 1: "You hit the snooze button and go back to sleep," said prominent astronomer Richard Binzel, who developed the 0-10 rating system for the International Astronomical Union.

Category 10: "It will ruin your day." At a minimum.

A Category 10 collision would deliver global catastrophe, creating an immense tidal wave, carving out a 20-mile-wide crater and raising a cloud of debris and dust that could engulf the planet in frigid darkness for months.

"This new scale is very important, because we know there are a lot of Earth-crossing asteroids out there, but we need to know which are most likely to hit us," said Florentin Maurrasse, a geology professor at Florida International University and an expert on Earth-asteroid collisions.

It was a Category 10 event -- a gigantic impact near what is now Mexico's Yucatan Peninsula -- that rendered the dinosaurs extinct 65 million years ago, scientists believe.

Such catastrophes are rare, but inevitable. A Category 10 asteroid -- greater than a half-mile wide -- smacks Earth every 100,000 to 1 million years. The monster that wiped out the dinosaurs was 6.5 miles wide. The vastness of space and the Earth's atmosphere generally protect the planet from collisions with these objects. But with Earth-asteroid encounters the stuff of popular fiction, and after a few false alarms from astronomers, scientists sought a tool to help them communicate with the public.

Thus was born Binzel's scale for gauging potential impacts with asteroids and comets, collectively called near-Earth objects.

Asteroids are rocky or metallic objects, primarily debris left over from planetary formation. Found mostly in the Main Asteroid Belt between Mars and Jupiter, some have escaped, and orbit the sun on irregular paths.

Comets are bodies of ice, rock and organic compounds that also travel irregular orbits around the sun.

Comets are rare, but astronomers using new technology have identified 2,000 asteroids of Category 10 size. Also lurking in our neighborhood: more than 250,000 smaller asteroids.

"It's a cosmic shooting gallery out there," said Binzel, a professor of earth, atmospheric and planetary sciences at the Massachusetts Institute of Technology.

He hopes to avoid situations such as the one last year surrounding the XF11 asteroid and the one this year involving the AN10 asteroid. Initial reports of both suggested possible impacts with Earth. Later reports based on more sophisticated measurements ended those concerns.

The International Astronomical Union, the world's leading organization of astronomers, agrees. It announced Wednesday in Vienna that it has officially adopted Binzel's scale for use worldwide.

Called the Torino Impact Hazard Scale (for the Italian city Turin, in which it was introduced last month), the system measures two criteria -- the size of the object and the chance that it could collide with Earth.

The scale is divided into 11 categories within five general sections ranging from white (no chance of impact) to red (dinosaur city).

"As you go higher up the scale, the chances of the object hitting you increase. Also, the size of the object increases," Binzel said. "Eight, nine and 10 are certain collisions. You definitely want to stay out of the red zone."

All known asteroids have scale values of zero.

While more asteroids are regularly being identified, astronomers point out there is no increase in the number -- only in our awareness of their existence and their passage close to our planet.

"This doesn't mean that the Earth is in any greater danger," Binzel said. "Fortunately, the odds favor that newly discovered objects will miss."

On the other hand, space-borne objects do hit Earth. Sand-like grains bombard us constantly, and objects the size of a small car strike our planet several times a year.

Those objects are of relatively little concern. But one large enough to enter Category 10 -- that's a different story. "It would initiate instant global change," Binzel said. "You would see horrific destruction at the site of the impact."

But if you wake up one morning to a Category 10 alert, he said, do not surrender hope, do not immediately quit your job and insult your boss. Things could change rapidly. "When an object is discovered, we will immediately put it on this scale," he said. "But as we watch the object over the next few months and improve our understanding of its orbit, there's a good chance we will re-evaluate it. It could go from a 10 to a zero in an instant."

Martin Merzer wrote this article for the Miami Herald, in which it first appeared.

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