Earthgrazers and Fireballs: The Strange Side of The Leonid Meteor Shower

But while the 2001 Leonids will likely be remembered for the sheer volume of shooting stars, there are some strange characters to look for as the shower's source ekes above the eastern horizon late Saturday night and early Sunday morning.

A handful of meteors will first zoom across the horizon for long stretches of time. Earthgrazers, they're called. And if you're real lucky, you might spot some fireballs -- larger meteors that explode upon impact with Earth's atmosphere, generating spectacular blazes of light (not to mention fear of alien spacecraft and calls to local law enforcement offices).

Leonid meteors will take their time arriving Saturday night. Wherever you are on Earth, you're viewing location has to rotate into the stream of space dust that causes the Leonids. The shooting stars will appear to emanate from a point in the sky known as the radiant, which for the Leonids happens to be in the constellation Leo (hence the name).

No knowledge of this is needed to find an earthgrazer. Just go out and look to the East. The timing depends on where you live. Figure mid-evening for high northern latitudes, such as Canada; late evening hours for mid-northern latitudes, as in most of the United States; and after midnight for equatorial regions and the Southern Hemisphere.

"When the radiant lies near the horizon the Leonid meteors cannot penetrate far into the Earth's atmosphere," explains Robert Lunsford of the American Meteor Society. "At this time they are only able to skim the upper atmosphere."

These earthgrazers, as scientists call them, often last several seconds and can span a great distance of the sky, Lunsford said.

Later, as Earth continues rotating, the Leonid radiant moves higher into the sky, along with its host constellation and all the stars. Meteors will strike the atmosphere at a more direct angle, Lunsford explains, creating shorter paths. But the paths will still span much of the sky, so you don't need to face East. In fact, the best views will be everywhere but directly East.

Most Leonid meteors are created by sand-sized grains of dust that vaporize about 60 miles up due to the friction caused by Earth's atmosphere. But Tempel-Tuttle, the comet that has left all this Leonid raw material in space, also deposits a few larger chunks of itself each time it swings around the Sun (which it does every 33 years).

A comet fragment the size of a marble can generate a glorious fireball of light as it burns up. Instead of slicing through the atmosphere like a small bit of dust, such a pebble sometimes goes splat upon meeting up with a certain density of air.

"The Leonids can have fireballs, but they're not especially noted for them," said Bill Cooke, a meteor researcher at NASA (news - web sites)'s Marshall Space Flight Center. Cooke said the number of fireballs each year depends in part on which streams of cometary debris Earth plows through.

In 1998, observers noted several fireballs when the planet moved through a stream that comet Tempel-Tuttle had deposited in the 14th Century. The Sun's radiation had blown much of that ancient dust into a widely dispersed region of space, so the 1998 Leonids did not produce a great number of shooting stars.

But the larger material -- fireball material -- was still relatively concentrated. In fact, Cooke said, scientists are learning that gravity acts on these larger fragments, causing them to be huddled more closely together over time. They call the process "gravitational focusing."

People in Asia will see shooting stars caused by material that has been waiting to be swallowed up by Earth since 1633, so there should be some fireballs there, Cooke said. The North American peak will be caused by material left by Tempel-Tuttle in the 1700s, however, and should provide fewer fireballs, but probably still some.

Cooke is quick to point out that the Leonids can surprise, however. There could be fewer meteors overall. Or there could be more fireballs. Meteor forecasting is a young profession. And, for now at least, meteor showers are still somewhat strange -- even to the scientists.

(Okay so I'm a worry wart, I'm a catastrophist.)

A cosmic trail with destruction in its wake

by Nick Nuttall
Copyright 1990 Times Newspapers Limited The Times, May 24, 1990, Thursday

Over the next few weeks the Taurid stream, a procession of vast cosmic rubble and dust that snakes around the Sun and out towards Jupiter, will swing through Earth's orbit for the first of its bi-annual crossings.

Within the stream are probably thousands of bodies including asteroids, mountain-and island-sized boulders, smaller meteoroids, Encke's Comet and assorted fragments of celestial refuse.

The exact number, size and location of objects, however, remains a mystery and according to Dr. Mark Bailey, research Fellow in astronomy at Manchester University, it is likely that for every object which is confirmed, there are nine others that have so far eluded detection.

All that is certain is that the rubble, believed by some astronomers to have been formed by a collision in the asteroid belt of a defunct comet which was captured by the solar system up to 30,000 thousand years ago, will bisect Earth's orbit in late June and again in November.

According to astronomers such as Dr. Victor Clube, of Oxford University's Department of Astrophysics, the coming and goings of the Taurid stream should be a source of concern to politicians, planners and anyone who cherishes life on Earth.

A ''catastrophist'', Dr. Clube is one of many astronomers who are convinced that within this celestial procession lie the seeds of mass destruction an Armageddon of biblical proportions. ''The matter requires urgent attention. It is crucial that everyone is woken up to the danger,'' Dr. Clube says.

The chilling scenario envisaged is of Earth and one of the 46,000mph objects in the Taurid stream colliding during one of the orbital crossings.

Dr. Clube says: ''It is analagous to a nuclear war with a megatonnage of the same order and all the effects of nuclear war with debris from the impact causing sunlight to be blocked causing a Dark Age or Ice Age.''

He has coined the phrase ''Multiple Tunguska Bombardment'' to describe the worst nightmare which, he believes, will eventually happen. Tunguska refers to a Siberian River near which, in June 1908, a 100 yard body from the Taurid stream ploughed into Earth, exploding and devastating an area 25 miles wide with the impact of a 20-megaton bomb.

Fortunately the encounter occured in an unpopulated part of the globe but if the impact had been on London it would have devastated the city, killing millions. The Tunguska event may have been only a chance occurence.

Yet, according to Dr. Clube and Dr. Bill Napier, of the Royal Observatory in Edinburgh, whose book Cosmic Winter is published next month, the history of Earth is littered with subtle evidence that cosmic debris have consistently intervened, often with catastrophic consequences.

One of the most popular theories to explain the sudden demise of the dinosaurs is that, 65 million years ago, a huge asteroid ploughed into the planet, triggering either a nuclear-style winter or huge fires.

This popular theory was given a boost only last week when scientists at the University of Arizona reported the discovery of an apparent 180-mile-wide crash site in the Caribbean of an asteroid six miles wide. They claim this could be linked with the great reptiles' extinction.

Dr. Clube ascribes other events including the Old Testament story of Noah and his Ark to a Dark Age linked with colliding heavenly bodies. He also believes that climatic changes, including fears of present global warming, may have a cosmic component.

There is sufficient evidence, he says, to indicate that collisions happen within centuries and millenniums rather than millions and billions of years, with multiple encounters more likely than sceptics claim.

Dr. Clube emphasizes that predicting when a bombardment may occur is impossible without more scientific evaluation of the Taurid stream. ''We are probably a little safer at the moment because the intersecting orbits are far away. But we are on the inward run and in 500 years we will start getting close again,'' he says.

According to Dr. Clube, the last time that the stream was closest within Earth's orbit was in the first millennium BC, from about 500BC up to 0AD, the time of Christ.

It is vital to overcome complacency about the threat from cosmic debris, he says. This complacency is relatively new, as pagan and ancient civilisations such as the Babylonians were firm believers in the threat of of cosmic destruction. Part of the blame for this complacency rests with the breakup, in 1845, of Comet Biela without any easily visible effect on Earth.

''This rather relaxed attiude to comets, which has persisted to the present day, helped turn 19th century opinion against a prevailing catastrophist view of evolution,'' Dr. Clube says.

''Indeed, the eventual disintegration of Comet Biela into dust made it no longer out of place for biologists and geologists to explain evolution in processes that were non-violent and slow-acting.''

''In short, it became fashionable to assume that the world is safe when in fact multiple Tunguska bombardments, releasing around five-thousand megatons, the equivalent of a full-scale nuclear war, may happen at intervals of about 1,500 years, producing a Dark Age,'' he says.''To suggest the planet is safe is absurd.'' The Oxford astrophysicist is not alone in his views. Similar concerns were echoed last week by the respected American Institute of Aeronautics and Astronautics (AIAA).

The institute is calling for studies aimed at defending the Earth from asteroid attack, including the possible redeployment of nuclear weapons to shatter incoming celestial bodies.

The call comes in the wake of thawing East-West relations and what is being claimed as a recent, potentially disastrous near-collision.

Last year, 1989 FC, a cosmic boulder bigger than an aircraft carrier, passed within 400,000 miles of Earth, a mere whisker in astronomical terms, before being noticed by astronomers.

''Such an object could cause a disaster of unprecedented proportions if it had struck. Although the probability is very small, its consequences in terms of the casuality rate could be enormous,'' the institute argues in a paper it released about the problem.

Apart from putting nuclear warheads on standby for intercepting and shattering asteroids, the institute is calling for studies into power units that could attach and divert the celestial boulders away from Earth.

''We have the technology needed to detect and track such an object and possibly to divert if from an impending impact. We would be derelict if we did nothing,'' the institute says.

Dr. Clube is hoping to get access to an infra-red telescope to study the Taurid stream during the November crossover.

In 1983, a satellite revealed what appeared to be dust following Comet Encke, but some scientists, including Dr. Clube, now believe that this contains the single large missing body, perhaps as large as 20 miles wide, shrouded in dust and boulders.

The best chance of detecting the defunct comet might come in 1994 when the American National Aeronautics and Space Administration (Nasa) is expected to launch the infra-red telescope, ISO.

Dr. Bailey says: ''We are learning more about these objects almost every week. We are realizing that there are quite a large number of fairly large objects, ranging in size from just a few hundred yards to six miles across, which are in Earth's collision orbit.''

Along with Dr. Bailey, Dr. Clube supports the institute's call for improved monitoring. But both British astronomers are concerned at suggestions of shattering incoming asteroids.

They believe that there is the danger that by solving one large threat, it may create scores of smaller ones. Cosmic Winter by Dr. Victor Clube and Dr. Bill Napier. Published in June by Basil Blackwell (Pounds 16.95). The Origin of Comets by Dr. Mark Bailey, Dr. Victor Clube and Dr. Bill Napier. Pergamon Press.

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