But one day, scientists warn, a similar celestial display over a highly populated region of Earth could be a harbinger of death and devastation for millions on the ground, if the meteor happens to be an asteroid as small as 99 feet in diameter.
``It would destroy a city just like a nuclear bomb would, but for a lack of radiation,'' said Jim Scotti, a planetary scientist who hunts asteroids and comets for the University of Arizona's Lunar and Planetary Laboratory near Tucson.
``On the other hand, there's the potential for civilization ending, because larger objects -- a kilometer (0.62 mile) or so in diameter -- are big enough to change climate on a global scale. And objects larger still, say 10 or 20 kilometers (6.2-12.4 miles) in diameter, could cause mass extinction.''
The colorful fireball witnessed by thousands of people from southeastern Ontario to northern Virginia on July 23 is believed to have been caused by a stony chunk of asteroid no larger than a suitcase, which exploded in the atmosphere 10 to 20 miles above the Earth's surface.
But the void of space that envelopes the solar system near Earth is a speedway for much larger objects such as asteroids and comets, some the size of Manhattan, which have periodically slammed into the planet, causing cataclysmic changes from the extinction of the dinosaurs to the birth of the moon.
The largest and most destructive objects are not likely to pose a problem any time soon. But every century or so, Earth is struck by smaller apartment-building-size asteroids capable of devastating a geographic region.
On June 30, 1908, an object 165 feet wide is believed to have exploded 6.2 miles over the Tunguska region of Siberia with the force of a 10-megaton hydrogen bomb. Four hundred square miles of forest were flattened, untold numbers of reindeer were roasted and a man standing 60 miles away was knocked unconscious.
Every 100 Years
``You'd expect those to come down every 100 years or so,'' said Don Yeomans, director of the National Aeronautics and Space Administration's Near-Earth Object program.
And the 100th anniversary of the Tunguska strike is only seven years away.
``We're due, if the statistics are right,'' warned Terence Dickinson, the Canadian science writer who authored the popular astronomy book, ``NightWatch: A Practical Guide to Viewing the Universe'' (Firefly Books).
``It's not that the odds are increasing over time. It's that we now know what the odds are, and it's likely that in the foreseeable historical future, this is going to happen.''
Asteroids, remnants of the early solar system that failed to coalesce into planets 4.6 billion years ago, were discovered in 1801 by Italian astronomer Giuseppe Piazzi. Nearly all originate from the asteroid belt between Mars and Jupiter, where over a million stone, iron or nickel giants hang in orbit awaiting the day when a random collision or a shift in Jupiter's gravitational field sets one on a deadly trajectory.
Not until the latter half of the 20th Century did science come to realize that large asteroids posed more than just a potential danger for Earth, having actually struck the planet at regular intervals over billions of years.
A scientific milestone was reached in 1990 with the discovery that Mexico's Chicxulub crater near the Yucatan Peninsula was caused by a 9.3-mile-wide object that hit 65 million years ago, possibly wiping out two-thirds of animal species, including the dinosaurs.
The threat of a similar strike, dramatized by the 1998 Hollywood thrillers ``Deep Impact'' and ``Armageddon,'' has scientists mounting a constant vigil for objects that might be headed for a close encounter of the worst kind with Earth.
The hunt has grown dramatically since 1998, when NASA said it would spend over $1 billion in 10 years on asteroid and comet research, including the tracking of near-Earth objects.
Old-style telescopes have also been replaced by more sensitive digital imaging equipment and coupled with advanced robotic tracking systems and more powerful computer analysis.
Rising Number Identified
As a result, the number of asteroids that have been identified and reliably charted now stands at about 26,000, up from 10,000 in early 1999. ``And it took 198 years to get the first 10,000. That's how dramatically things are moving,'' said Brian Marsden, director of the Minor Planet Center of the International Astronomical Union, the international repository of data on asteroids, comets and meteors.
Scientists have identified and charted about 1,413 near-Earth asteroids, so called because their trajectories have the potential to intersect with Earth's orbit. Five hundred of those are 0.62 mile in diameter or larger.
A key unit of measure for asteroid hunters is the Astronomical Unit (AU), defined as the average distance between the Earth and the Sun, or 93 million miles.
The closest near-miss on record occurred December 9, 1994, when an object the size of a large house and called 1994-XM1 passed within 0.0007 AU, or 65,000 miles of Earth, well within the moon's orbit of 238,000 miles.
The next close-call for Earth is expected August 7, 2027, when a 0.62-mile-size object called 1999-AN10 passes just outside the lunar orbit at 0.00265 AU, or 245,000 miles.
Still uncertain is what authorities can do if scientists find an asteroid or comet on a collision course with Earth. ``I don't think we'd give Bruce Willis a call,'' joked Yeomans, referring to a role the actor played in the movie ``Armageddon.''
Scientists say the biggest threats should be known decades in advance, theoretically giving the world's leading nations enough time to organize a safe, effective response plan.
Some scientists favor space vehicles capable of pushing an Earth-bound object off course. Others suggest some objects could be shattered into dust by a well-aimed intercontinental ballistic missile.
But for the moment, NASA is working to gain a clearer understanding of the mass and weight of different objects, whether they be comets, stoney asteroids, chunks of solid iron, or space-going piles of rubble.
``There have been some studies trying to identify which characteristics of an asteroid or a comet we would need to know to deal with it effectively. We have to know its structure, its density, its size, its mass,'' Yeomans said.
NASA's NEAR Shoemaker mission to the asteroid Eros last February yielded a wealth of knowledge about the biggest space rocks. On July 4, 2005, the spacecraft Deep Impact is scheduled to blast a football-field-size crater into the comet Tempel-1 so that scientists can get an idea of what comets are made of.
``So, the first goal is to find 'em, track 'em and characterize each of the several classes,'' Yeomans said.
``The thinking is that, more than likely, we'll have several decades to deal with such a thing. And this might be a more rational approach than having nuclear arsenals at the ready, because in that case, the cure might be worse than the disease.''
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