HAZARDS: IMPACTS IN OUR FUTURE?

THOUSANDS OF ASTEROIDS AND COMETS CROSS PATHS WITH EARTH. HOW DANGEROUS ARE THEY?

The great majority of asteroids orbit the Sun in well-ordered bands between Mars and Jupiter. But occasionally, a collision between two asteroids followed by a gravitational nudge from Jupiter shifts an asteroid into a new orbit that crosses paths with Earth.

Fortunately, the largest and potentially most destructive asteroids-measuring several miles across-are extremely rare. Astronomers have already spotted most of them, and so far none poses a serious threat. But many thousands of asteroids, each as wide as a sports stadium, remain undiscovered. Although the odds of one striking any particular place are exceedingly small, such an impact could ruin a large metropolitan area.

GLOBAL CATASTROPHE

IMPACT FREQUENCY: 100 MILLION OR MORE YEARS

Asteroids large enough to cause widespread extinctions collide with Earth very rarely. These asteroids measure more than 10 kilometers (six miles) across. Because they're so large, astronomers think they've spotted all of the ones that could cross Earth's orbit during the next two centuries.

All of Earth's most devastating impacts happened many millions or even billions of years ago. Researchers have found about a dozen of the colossal craters that resulted-one impact 35 million years ago gouged open the mouth of Maryland's Chesapeake Bay. But the best-known impact happened 65 million years ago.

One of Earth's largest craters, the Chicxulub Crater, formed on Mexico's Yucatán Peninsula 65 million years ago.

DID AN IMPACT DO IN THE DINOSAURS?

A thin layer of grayish clay marks the geologic boundary between the period when the dinosaurs flourished and the time when mammals rose to dominance. That clay boundary between the Cretaceous and Tertiary periods represents a mass extinction, in which 70 percent of all species died out-including the nonavian dinosaurs.

In 1980 researchers found evidence in that layer of a giant meteorite or comet impact. They found high levels of iridium-an element common in meteorites yet very rare in Earth's crust-and tiny spheres of glass that must have condensed from rock vaporized during the impact. After a decade of intense searching for this impact's crater, researchers hit pay dirt at the northern tip of Mexico's Yucatán Peninsula. A crater 185 kilometers (115 miles) wide had formed at essentially the same time the extinction took place.

In the fraction of a second after the impact, both the meteorite and the rocks beneath it turn from solid into liquid and vapor. The superheated plume of vapor shoots up above the atmosphere and spreads around the globe. Heat from the falling molten and gaseous rock ignites fires across the planet. Dust, soot and smog block out sunlight for months. This combined destruction kills off many plant species and leads to months or years of global cooling.

GIGANTIC TSUNAMI

FREQUENCY: 100,000 OR MORE YEARS

An asteroid with a diameter of one kilometer (0.6 mile) could completely devastate a land area the size of Europe and blanket Earth with dust. But because two-thirds of Earth's surface is covered by ocean, a more likely outcome would be an enormous tsunami, or "tidal wave."

Roughly 1,100 asteroids of this size have Earth-crossing orbits, and the Spaceguard detection program, supported by NASA, is scheduled to map 90 percent of them by 2008. Only one asteroid found so far has even a minimal—one in 300-chance of striking Earth, on March 16, 2880.

No one has ever witnessed a tsunami caused by a meteorite impact. But an undersea earthquake near Alaska in April 1946 generated waves as high as 17 meters (55 feet), which pounded the Hawaiian coast. In this photo a wave crushes the pier in Hilo; the man at the left was among the 165 people killed.

If an asteroid this size landed in the ocean 640 kilometers (400 miles) from New York, waves higher than the Statue of Liberty would hammer the coast from Massachusetts to North Carolina. The waves reaching Africa or Europe would be smaller—only 20 meters (65 feet) high.

REGIONAL DESTRUCTION

FREQUENCY: 2,000 YEARS

Less damaging but more numerous are the asteroids with Earth-crossing orbits that are roughly 200 meters (660 feet) across, the size of a large sports stadium. Astronomers have discovered only a minority of the estimated tens of thousands of Near-Earth asteroids this size. An organized program to detect 90 percent of them would take at least 10 years and cost several hundred million dollars.

If an asteroid this size did hit Earth, it would be very likely to land in a relatively unpopulated area or in the ocean, so the death toll would be relatively low. If it landed in a large metropolitan area, however, it could kill millions of people.

The impact from an asteroid of this size would release 10 times the energy of the largest nuclear weapon test, with destruction equivalent to history's worst natural disasters. The heat blast and shock wave from an impact on land would devastate an area the size of greater Los Angeles. An ocean impact could generate a tsunami 10 meters (33 feet) high along many coastal areas.

LARGE MIDAIR EXPLOSION

FREQUENCY: 100 YEARS

Asteroids the size of a 10-story building, 30 meters (100 feet) across, are about the smallest that can cause significant damage. Typically, these asteroids explode several miles above Earth's surface in a large fireball. The great majority of them would blow apart harmlessly over the ocean or an unpopulated area.

Because these asteroids are so small, we have little hope of reliably detecting or tracking the estimated 200 million asteroids this size that cross Earth's orbit.

The pressure of the meteorite's impact on Earth's atmosphere causes it to explode with the force of a few megatons of TNT, more than 100 times the strength of the nuclear bombs used in Japan during World War II. Shock waves may damage objects on the ground. Some meteorite fragments might survive the explosion and rain down over the area.

SMALL FIREBALL

FREQUENCY: MANY PER YEAR

Every year U.S. military satellites detect about a dozen small asteroid explosions in the atmosphere. These asteroids, measuring several feet across, pose almost no direct threat. However, observers on the ground occasionally mistake them for nuclear or conventional weapons attacks, raising the danger of a misguided military retaliation.

Some of the larger fragments from these explosions land as meteorites. Several meteorite falls were reported during just a four-year period earlier this century, including: a shower of dozens of pebble-sized meteorites in Canada's Yukon Territory, more than 400 fragments falling in rural Lesotho, Africa, and a shower of meteorites large enough to damage six houses and three cars in a Chicago suburb.

When a meteorite crashed into Robert Donahue's living room in Wethersfield, Connecticut, it hit the floor, then bounced up to the ceiling of the adjoining room, hit a chair and came to rest on the floor. The meteorite, which fell on November 8, 1982, was the second one to hit a house in Wethersfield in 11 years.

These meteorites explode high in the atmosphere. While the explosion may be visible from the ground, it poses no direct threat. Meteorite fragments may make holes in buildings or other objects on the ground.

PROTECTING THE PLANET

"Who knows whether, when a comet shall approach this globe to destroy it, ... men will not tear rocks from their foundations by means of steam, and hurl mountains, as giants are said to have done, against the flaming mass?" - Lord Byron, 1821

In the unlikely event that astronomers discovered a civilization-ending asteroid or comet destined to impact Earth in a month, we wouldn't have enough time to stop it. But with more warning, we should be able to deflect a doomsday rock.

The most commonly suggested strategy so far is a nuclear explosion to blast the object aside. However, if the asteroid was a loosely bound collection of rubble instead of a dense solid mass, it might absorb the explosion's energy.

Other proposed schemes include attaching a nuclear-powered or solar-powered rocket to steer the asteroid out of the way, or setting up a scooping mechanism to repeatedly fling bits of rock away from the asteroid, thereby nudging it off course. Some researchers think that painting an asteroid white would cause energy from sunlight to push the rock out of the way.

WILD-CARD COMETS

FREQUENCY: UNKNOWN

Comets—clumps of ice and rock—come near Earth much less frequently than asteroids, but they travel much faster—one hundred times the speed of sound on Earth. So even a relatively small comet would impact with very high energy, and with little warning.

As a rule, comets are much harder to find and track than asteroids. Trillions of comets are hidden in a zone far beyond Pluto's orbit called the Oort cloud. Because comets are less reflective than asteroids, they can remain invisible until they're very close to Earth. As a comet nears the Sun, solar energy begins eroding its surface, forming a bright tail that always points away from the Sun.

Astronomers don't know exactly how hazardous a comet impact would be, because comets are less dense than asteroids, and because so little is known about comet composition. However, the incredibly high speeds of most comets mean that the energy of an impact would be very high. A comet could probably cause about as much devastation as an asteroid of the same size.