When Comets and Asteroids Strike Earth

Mining some of the near-Earth asteroids will exploit the considerable constructive potential of these cosmic bodies. However, many of the NEAs have a darker, more destructive potential as well. Namely, those NEAs that cross Earth's orbit can occasionally be attracted by the planet's gravity (or the Moon's gravity) and end up striking Earth.

The reality of this threat is demonstrated by the fact that small pieces of older, disintegrated comets and asteroids enter Earth's atmosphere all the time. Indeed, tens of thousands of these meteors do so each day. Most burn up in the upper atmosphere, where at night their rapid streaks across the sky are often erroneously called "shooting stars." Occasionally, larger meteors make it through the atmosphere intact and strike the ground (in which case they become known as meteorites). A number of such strikes have been witnessed and the objects recovered. One of the most celebrated cases occurred in November 1492 in Germany, where a crowd of people saw a falling object and later dug it up. It was a few feet across and weighed some three hundred pounds.

Such relatively small rocks falling from space usually cause little damage or injury. However, the danger increases considerably when these cosmic missiles are only marginally larger. An object the size of a large house could easily wipe out a good-sized city. And the impact of an asteroid or comet a little more than half a mile across would release about 1 million megatons of energy (several million times more than the energy released by the atomic bomb dropped on Hiroshima, Japan, in 1945). This would be enough to create a global catastrophe that would kill hundreds of millions of people and animals.

Cosmic Impacts Only in the Past?

Unfortunately, many people assume that such large-scale disasters happened long ago, before humans appeared on Earth. This is partly because of widespread publicity in recent years about the enormous mass extinction that occurred 65 million years ago when a large comet or asteroid struck the planet. Among the victims were the dinosaurs. The natural human tendency is to believe that this event was a rare fluke, a thing of the past, and that Earth is now largely safe from such global disasters. However, the truth is that such events can happen at any time. According to astronomer Gerrit L. Verschuur:

It is nearly impossible to admit that our lives might be wiped out through a whim of nature, a chance event, and that may be why it has been so difficult to allow that dramatic mass extinctions of the past happened suddenly. It is even more difficult to admit that another such catastrophe might be triggered by a random collision between the Earth and an object from space. This is the unpleasant likelihood suggested by the data. . . . We assume that mass extinctions happened long ago and that nothing similar will happen again. This scenario is false.⁵⁰

Thus, it is not a question of whether a comet, an asteroid, or a piece of one of these bodies could impact Earth. It is merely a question of when. Indeed, studies of past impact events reveal that such disasters, far from being rare, have occurred on a regular basis throughout the planet's history. It happened when Earth was forming, for example; for millions of years, planetesimals of all sizes rained down on our infant planet. Comets and asteroids struck the Moon as well. The Moon has about three hundred thousand impact craters with diameters of 0.6 of a mile or more. These craters have survived intact because the Moon has no air and water to erode them. In contrast, Earth, which is larger and has a more powerful gravitational pull than the Moon, has suffered far more asteroidal and cometary impacts. Very few impact craters are visible on Earth, however. This is because the effects of rain, wind, tides, volcanoes, and so forth have eroded and erased most of them.

Among the few large impact craters still visible on Earth's surface, the youngest and best preserved is Barringer Crater (also called Meteor Crater), near Winslow, Arizona. The crater is about three-quarters of a mile wide and some six hundred feet deep. The event that caused it to form occurred roughly fifty thousand years ago. (Fortunately, no people were killed in the blast because early humans had not yet migrated to North America.) University of California geochemist Alan E. Rubin provides this overview of the immense explosion:

The crater was produced by the impact of a 50-meter [164-foot]-diameter metallic iron projectile that liberated an amount of energy equivalent to 20–60 megatons of TNT [several thousand times larger than the Hiroshima explosion]. The explosion excavated a depression 150 meters [492 feet] below the surrounding plains. . . . More than 100 million tons of rock were thrown from the crater. . . . At the instant of impact, a shock wave was produced that raced through both the target and the projectile. . . . A powerful air blast was caused by the shock wave, scouring the landscape with wind speeds exceeding 1,000 kilometers [620 miles] per hour. Trees and grasses were uprooted, and the Ice Age animals within a few kilometers of the crater were killed either by the air blast itself or by being pelted with branches, stones, and sand.⁵¹

The K-T Event

In human terms, this disaster seems big and frightening. But it pales in comparison to the explosion and loss of animal life in the impact event that occurred some 65 million years before. The comet or asteroid that collided with Earth at that time was about six miles across and traveling somewhere between twelve and forty-two miles per second. It struck the shallow ocean near the eastern coast of Mexico with the almost incomprehensible force of 100 million megatons (some 5 billion times more powerful than the Hiroshima explosion).

Within a second of the impact, an immense fireball formed. It created a powerful atmospheric shock wave that expanded outward in all directions. Every tree was leveled and every living thing was killed up to a distance of at least a thousand miles. Meanwhile, the impact carved out a crater about ten miles deep and more than a hundred miles in diameter and set off giant earthquakes that raised the ground in waves hundreds of feet high. Also, giant sea waves pounded the coastlines of the planet, crushing or drowning all animals in their path.

Later effects of the catastrophe were even worse. The explosion threw millions of tons of ash and dust into the atmosphere, blocking sunlight for many months. This made Earth's surface very dark and cold and destroyed most of the surface plants, causing most of the planet's food chain to collapse. Roughly 70 percent of all the animal and plant species on Earth, including all of the dinosaurs, died.

This global calamity, which scientists call the K-T event, was important not only for its effects on Earth and the animal kingdom in general but for the subsequent course of evolution, especially the rise of human beings as the dominant life-form on the planet. It is almost certain that the spread and ultimate success of large mammals, including humans, would not have occurred (or at least not in the same manner) if the dinosaurs had survived. If the dinosaurs "had not been wiped out," Verschuur points out,

mammals would not have arisen to dominate the world in their stead. After the dinosaurs were ushered off the terrestrial [earthly] stage, the scene was set to allow mammals to diversify until, 65 million years later, one of their kind, Homo sapiens, rose to prominence. Our species recently evolved to become conscious and clever enough to invent agriculture, technology, and science, and we have used our newly developed mental skills to uncover the secrets of nature that carry the clues to our origins, and to our future. To put this another way, if the comet that triggered the K-T event had arrived twenty minutes earlier or later, it would have missed the planet and we would not be here now, talking, reading, or writing about any of this.⁵²

City Destroyers

Disasters on the huge scale of the K-T event happen perhaps once every 50 to 100 million years. But smaller cometary and asteroidal impacts capable of destroying one or several cities occur much more frequently. In fact, one such disaster happened as recently as 1908 when an object about the same size as the one that created Barringer Crater exploded over Tunguska, a remote area of eastern Russia. Forests were completely flattened for a distance of almost twenty miles from ground zero. But few people lived in the region, so the loss of human life was very small. Still, even those who lived far from the impact site were startled and in some cases injured by the effects of the blast. One eyewitness who had been about sixty-six miles away from the impact later reported:

Suddenly . . . the sky was split in two and above the forest the whole northern part of the sky appeared to be covered with fire. . . . I felt great heat, as if my shirt had caught fire. . . . There was a . . . mighty crash. . . . I was thrown onto the ground about [twenty feet] from the porch. . . . A hot wind, as from a cannon, blew past the huts from the north. . . . Many panes in the windows [were] blown out.⁵³

Another reason that the death toll from the Tunguska impact was small was that the object struck land rather than sea. If the same object, or one larger, had struck the ocean, it would have generated huge sea waves, or tsunamis. The fireball of a large asteroid or comet striking the ocean would blast through the water and in only a few seconds carve out a crater many miles wide and thousands of feet deep in the seabed. John Lewis explains how tsunamis would be generated by such a strike:

The water displaced from the explosion cavity is partly ejected in a broad, open cone at many times the speed of sound. The seabed is cracked by the blast wave, melted and scoured by the one-hundred-thousand-degree fireball. Hundreds of cubic kilometers of water are vaporized, blasting an immense column of steam back out to space. . . . When the surface of the fireball coasts to a stop in the water, the ocean surface collapses back into the cavity . . . from all sides, converging on the center of the crater. As the wave crest approaches the center of the crater, fast-moving waves converging from all directions pile into each other, rushing headlong into a monstrous surge that shoots up a towering pillar of water higher than the highest mountains on Earth. The sea sloshes back and forth in the blast region, pumping the surrounding ocean and generating circular wave fronts which, like the ripples from a pebble tossed into a puddle, spread out in all directions.⁵⁴

As such waves approach land, they can grow to towering heights. The waves generated from a one-thousand-megaton strike (about twenty times larger than the Tunguska explosion), for instance, would be well over five hundred feet high, as tall as a fifty-story building. These could easily destroy one or more cities.

Warnings of Cosmic Perils

The destruction of a city, or perhaps dozens of cities, by the effects of a cosmic impact would naturally be appalling. But at least most of the human race would survive. However, if a comet or asteroid the size of the one that caused the K-T event, or one even larger, were to strike, humanity might not be so fortunate. A warning of the possible perils that await Earth in the depths of space came in 1994 when twenty-one pieces of the recently fragmented Comet Shoemaker-Levy 9 plowed into Jupiter. At just over a mile across, each piece blasted a hole the size of Earth in Jupiter's upper atmosphere. If these twenty-one cometary fragments had struck Earth instead, all life on our planet would have been annihilated.

It is impossible to say for sure how likely it is that such an object will hit Earth in the near future. However, if recent strikes and near misses are any indication, events that could kill millions of people could potentially occur at least once, and perhaps as many as three times, per century. In 1890, people in Capetown, South Africa, witnessed a comet as it passed rapidly by Earth at a distance of fifty thousand miles, only one-fifth the distance to the Moon. The Tunguska event occurred in 1908. And in 1992, a comet missed our planet by only twelve thousand miles. Some experts estimate that the impact of this object would have generated an explosion of fifteen thousand megatons. If it had struck land, it would have annihilated most life in an area the size of the state of Texas; if it had plunged into the sea, it would have generated tsunamis thousands of feet high, killing even larger numbers of people.

Looking into the future, astronomers have calculated that an even bigger object—Asteroid 1950 DA—is likely to strike Earth on March 16, 2880. This cosmic body is about three-quarters of a mile in diameter. And depending on how and where it hits, it could conceivably blast humanity back into the Stone Age. "This extraordinary object," remarks science writer David L. Chandler,

will remain a major focus of small-body [i.e., asteroid and comet] research for a long time. . . . The process of devising a plan, and carrying out the first rescue of our world from a devastating blow from above, could be a project that prods human consciousness for much of the millennium to come.⁵⁵

Trying to Prevent Doomsday

Astronomers and other scientists agree that the threat of cosmic bombardment is real and that steps need to be taken to meet that threat. The eight-hundred-year waiting period for Asteroid 1950 DA should not lull humanity into a false sense of security, they say. Thousands of undiscovered NEAs exist, new comets enter the inner solar system on a regular basis, and one or more of these objects could suddenly emerge as a planet killer at any time. "The only way to beat the odds," asserts David Levy, codiscoverer of Comet Shoemaker-Levy 9, "is to locate physically every possible asteroid or comet that could pose a threat to the Earth."⁵⁶

Some small-scale attempts to find and calculate the orbits of potentially dangerous asteroids and comets were launched in the 1970s and 1980s in southern California. One was the Planet-Crossing Asteroid Survey (PCAS); another was the Palomar Asteroid and Comet Survey (PACS). Both were conceived and guided by the late Eugene Shoemaker, the other codiscoverer of the comet that struck Jupiter. These programs found large numbers of asteroids and comets before shutting down in 1994.

Fortunately, though, some other ambitious programs began in the 1990s. One, the Spacewatch Camera, which operates at the observatory on Kitt Peak in Arizona, finds hundreds of new asteroids each year, about thirty of them NEAs. Another program, dubbed Near-Earth Asteroid Tracking (NEAT), is centered on the summit of Mount Haleakala on Maui, Hawaii. NEAT found four NEAs in its first day of operation alone and usually discovers up to fifty per month. Still another promising program is the Spaceguard Survey, an international program funded mainly by NASA. Its truly ambitious goal is to find 90 percent of all NEAs measuring 0.6 of a mile across or larger by 2009.

Searching for Planet Killers

In this excerpt from an article in the February 2002 issue of Astronomy, NASA astrobiologist David Morrison summarizes the Spaceguard Survey and its goals.

The Spaceguard Survey is an international search for potentially threatening asteroids. It's named after the similar . . . search proposed by author Arthur C. Clarke in [his novel] Rendezvous with Rama. The survey uses a half-dozen specialized optical telescopes. The most successful of these efforts is operated by Grant Stokes and his colleagues at the MIT Lincoln Laboratory. Their twin telescopes, located in New Mexico, scan the sky every clear night except around the full moon, when faint asteroids cannot be detected. . . . Once a new asteroid is found, astronomers need to compute its orbit. The additional positions necessary are mostly obtained by a few dedicated amateur astronomers (many of them in Japan and Italy). The amateurs take measurements over successive weeks. Astronomers then calculate the position of each asteroid forward in time to ensure that it poses no danger of hitting Earth. NASA's goal is to find 90 percent of the near-Earth asteroids larger than 1 kilometer in diameter by 2009.

If one of these programs discovers that an object like Asteroid 1950 DA will strike Earth in the near future, what can be done to avert catastrophe? In a number of popular movies dealing with this subject, including Armageddon and Deep Impact, scientists plant nuclear bombs on the object in an attempt to blow it up. However, in most situations such an approach may cause more harm than good. Shattering the intruder will turn it into a mass of fragments, most of which will remain on a collision course with Earth. And hundreds or thousands of smaller impacts spread out over the planet's surface might be almost as destructive as one large impact.

Most scientists agree that a more prudent approach would be to push a threatening asteroid or comet into a different orbit, one that takes it away from Earth. One of the first steps in such an endeavor, says Levy,

involves launching a reconnaissance spacecraft to study the asteroid, [to] determine whether it is made of soft stone or solid iron and plant a transponder [tracking device] on its surface. Signals from that device would then allow us to track the asteroid accurately as it soars through space. . . . The next step, moving the asteroid, is most efficiently done at . . . the place in its path closest to the sun. At that moment . . . a nuclear warhead exploding near [not on or in] the asteroid would change the path by a small amount. . . . A few years later . . . a second shot would add to the change. By now, the asteroid should be in a new orbit that would miss the Earth. Doomsday [would] thus [be] averted.⁵⁷

There is no way to know for sure if doomsday will ever be prevented in this manner. Perhaps humanity will someday succeed in mounting a foolproof planetary defense system that will keep it safe from bombardment from space. On the other hand, maybe global governments will not take the threat seriously enough until it is too late, and humanity will become extinct. Either way, science's fairly recent discovery of what comets and asteroids are and what they can do to the planet forces people to confront a sobering realization: Human civilization is a fragile entity existing always at the mercy of frightening cosmic forces. As Verschuur puts it, "If there is one thing the study of . . . comets and asteroids has given me, it is a profounder sense of the nature of life on Earth, our place in space."⁵⁸