Introduction to When Worlds and Comets Collide
The greatest collision between a planet and another celestial object ever witnessed by scientists happened in July 1994. For six days, Jupiter was bombarded by pieces of a disintegrated comet named Comet Shoemaker-Levy 9. One after another, more than 20 comet fragments slammed into Jupiter's dense atmosphere at speeds of about 60 kilometers (37 miles) a second, creating enormous fireballs easily visible through telescopes on Earth.
The impacts thrilled planetary scientists and amazed the world. But they also served as a reminder that the solar system can be a dangerous place. Earth, too, has been struck many times in the past by huge objects hurtling down from space. Scientists think that a mammoth collision with a large comet or asteroid about 65 million years ago may have led to the demise of the dinosaurs. And in 1908, more than 2,000 square kilometers (770 square miles) of forest near the Tunguska River in central Siberia were leveled by an immense atmospheric blast caused by an object—probably a meteor—less than 100 meters (330 feet) in diameter.
Could A Comet Collide With Earth?
Scientists say that what has happened to Earth in the past will happen again in the future. The only question is when.
In fact, astronomers recently thought they had identified a large comet that could pose a danger to Earth a little more than a century from now. In 1992, an international organization of astronomers said the comet, named Swift-Tuttle, would come close to the Earth in August 2126. Scientists estimated the odds of a collision at 1 in 10,000. Fortunately, further calculations showed that there was no danger of an impact after all. But there are many other bodies orbiting out in space. It is possible—not likely, but possible—that Earth will be hit sometime in the next 100 years by some large object that astronomers haven't yet discovered.
Such threats seemed too remote for nonscientists to worry about until the Jupiter collisions suggested to some government leaders that catastrophic strikes from space should perhaps be taken a bit more seriously. On July 20, 1994, the day the first chunks of Comet Shoemaker-Levy plowed into Jupiter, the Science Committee of the United States House of Representatives instructed the National Aeronautics and Space Administration (NASA) to begin tracking any comets, asteroids, or meteoroids—pieces of asteroids—that might someday hit the Earth. NASA officials also named a six-member panel to study the possibility of developing an early-warning system for objects on a collision course with our planet. Meanwhile, some scientists debated how such bodies might best be destroyed or deflected into a safe orbit.
Although asteroids and large meteoroids are just as likely as comets to strike the Earth, a comet would be apt to cause more damage than a rocky body the same size, because comets, on average, travel at greater speeds. But a comet would at least give us fair warning of its coming. Even with the unaided eye, a large comet can be seen from many millions of kilometers away, its misty tail glowing in the night sky. With their telescopes, astronomers would probably be able to predict a collision with a comet at least a year in advance.
From Evil Omens to Dirty Snowballs
People have always marveled at comets, and for most of human history they have feared them as well. Because comets appeared unpredictably in the skies, many cultures regarded these celestial visitors as omens of disaster—plagues or the overthrow of kingdoms, not the kind of devastation we now know they can bring.
In times past, no one knew what comets were. The ancient Greek philosopher Aristotle believed they were gaseous objects in Earth's atmosphere, and for centuries nobody could prove him wrong. In the late 1500's, however, the Danish astronomer Tycho Brahe made detailed observations of several comets and showed that their motions placed them well beyond the atmosphere of the Earth.
More than a century later, in the early 1700's, the British scientist Edmond Halley concluded that a comet seen in 1682 was the same one astronomers had observed in 1531 and 1607. Halley predicted the comet would return to the skies again in 1758. When the comet reappeared on schedule, it was named for Halley. In later years, astronomers determined that Halley's Comet, which returns at intervals of about 77 years, may have been the comet recorded in various historical records and works of art dating back as far as 240 B.C.
Modern observations of comets have revealed that they are essentially large, dirty snowballs. A comet consists primarily of water ice and various amounts of frozen gases, including carbon dioxide, methane, and ammonia, mixed with rocky material and dust. Most of the mass of a comet is contained in a solid core called the nucleus, typically 1 to 10 kilometers (0.6 to 6 miles) in diameter.
Huge Reservoirs of Comets
Astronomers think that comets were among the first objects to form in the solar system, created from the same cloud of gas and dust that gave birth to the sun and planets some 4.6 billion years ago. The outer portions of the cloud condensed into a crowded disk of icy bodies. Over the eons, gravitational interactions between these icy objects caused most of them to take up different orbits, eventually dispersing them into a huge spherical region. This vast reservoir of comet nuclei is called the Oort Cloud, named after the Dutch astronomer Jan Oort, who first deduced its existence in 1950. Astronomers think the Oort Cloud extends about halfway to the nearest star, a distance of trillions of kilometers. The objects in the Oort Cloud are so distant that not even our best telescopes can see them.
Also in 1950, a Dutch-born American astronomer, Gerard Kuiper (pronounced KOY per), theorized that much of the original disk of comet nuclei remains as a smaller and more densely packed reservoir of comets within the vast Oort Cloud. This region, Kuiper speculated, is still disk-shaped and begins just beyond the outermost planets. The region became known as the Kuiper Belt, but for years there was no evidence that it actually exists. Observations in the 1990's, however—including images made in 1995 by the Hubble Space Telescope—have erased most doubts about the Kuiper Belt. Several dozen icy objects orbiting beyond the planets have now been discovered.
Astronomers are certain that the number of comets visible from Earth is just a minuscule fraction of the comets in the Kuiper Belt and Oort Cloud. They think those two regions contain several trillion comet nuclei. We see only those relatively few comets whose orbits have been altered, perhaps by the gravitational influence of a nearby star or even of another comet, sending them into the inner solar system.
When A Comet Nears the Sun
Most of the time, when a comet is orbiting far from the sun, it consists of nothing but the nucleus. But when a comet moves toward the inner parts of the solar system, it starts to assume its more familiar appearance. When the comet nears the orbit of Jupiter, the sun's heat causes the frozen gases in the outer layers of the nucleus to start evaporating, producing a large cloud of thin gas and dust called a coma. The coma, which surrounds and blocks astronomers' view of the nucleus, can be more than 100,000 kilometers (60,000 miles) across.
When the comet gets about as close to the sun as the Earth is, the pressure of sunlight and of the solar wind—a stream of fast-moving particles moving outward from the sun—pushes the gas and dust away from the coma. This effect creates a long tail that may stretch for 100 million kilometers (60 million miles), always pointing away from the sun.
After a comet circles around the sun and begins to head toward the outer reaches of the solar system, its tail shrinks and its coma dissipates. Soon, the nucleus is all that remains until its next trip into the inner solar system.
Each time the comet returns, evaporation causes its mass to decrease by about 1 percent, and eventually there will be nothing left of it. How long it takes a comet to evaporate away to nothing depends on its size and its orbit. Some comets, such as Halley's Comet, are known as short-period comets because they make return trips in less than 200 years. Astronomers think short-period comets come from the Kuiper Belt. Long-period comets, on the other hand, have orbital periods that take them out of the inner solar system for centuries, and some will not return again for thousands or even millions of years. These comets probably come from the Oort Cloud. Most long-period comets have lost only a small amount of their mass, so they tend to be larger and brighter than short-period comets.
A Rare Spectacle On Jupiter
Astronomers believe that the Kuiper Belt was the original home of the comet that smashed into Jupiter. The comet was discovered in March 1993 by American astronomers Eugene Shoemaker, Carolyn Shoemaker, and David Levy at the Mount Palomar Observatory in California. The astronomers were looking for new comets by comparing photographs of regions of the sky taken on different nights. If they noticed that a tiny, faint object had changed position against the background of stars, they would identify it as a comet or an asteroid. The object's speed would reveal which of the two it is, since comets move faster than asteroids.
Studying photographic images they had made of a portion of the night sky, Levy and the Shoemakers noted what appeared to be a “squashed comet.” Further observations confirmed that the object was indeed a comet and that it looked squashed because it was broken into fragments. The pieces of the comet were hurtling through space one after another, lined up like pearls on a string.
By studying the path of Shoemaker-Levy 9, astronomers determined that the comet was in orbit around Jupiter and had probably been orbiting the giant planet for 60 to 100 years. But what had broken it into pieces? Physicists and planetary scientists theorized that when the comet passed close by Jupiter in July 1992, the planet's tremendous gravity had pulled the icy nucleus apart. Further calculations revealed that the comet fragments would smash into the far side of Jupiter in July 1994.
When the collisions came, they were spectacular. The largest comet fragments, which may have been more than 3 kilometers (2 miles) in diameter, created particularly stupendous displays, igniting huge atmospheric blasts, some of them thousands of kilometers across.
Because all the impacts occurred on Jupiter's far side, just over the planet's rim as seen from Earth, no telescope—not even the Hubble Space Telescope—could see the collisions as they occurred. Only the Galileo spacecraft, on its way to a rendezvous with Jupiter in December 1995, was able to photograph some of the comet strikes directly. Observers on Earth could see the impact sites about 30 minutes after each collision occurred, as Jupiter's rapid rotation (once every 10 hours) carried them into view. But several fiery plumes of gas—the tops of the largest explosions—were visible at the time of impact as they rose above the planet's rim.
Jupiter consists primarily of gases, so the impacts caused no long-term damage. The only noticeable effects were a few dark blotches in the atmosphere that persisted for several months and could still be seen as stretched-out atmospheric streaks in 1995.
Big Collisons Are Rare
If our own planet had been the target of Comet Shoemaker-Levy 9, the outcome would have been much different. Earth, too, would have survived the bombardment, but scientists say that the collisions would have caused widespread devastation and greatly damaged the planet's complex web of life.
The extent of the destruction that would result from a comet striking the Earth at a typical speed of about 50 kilometers (30 miles) a second would depend on the size of the comet. A strike by a comet 10 kilometers (6 miles) or so in diameter would be truly catastrophic, equivalent to a billion 1-megaton hydrogen bombs exploding all at once. Besides the immediate effects of the impact, trillions of tons of pulverized debris would be thrown into the atmosphere, where it would blot out the sun for months. Much of the planet's vegetation would die, and with it much of the human and animal life that depends on it. Civilization itself might well be destroyed, and whatever scattered remnants of humanity survived—if any did—would sink into a prolonged dark age.
That is the magnitude of the collision many scientists believe occurred 65 million years ago, when the last of the dinosaurs and many other species of animals vanished forever from the Earth. Geologists think they may have identified the ancient crater produced by that event off the coast of Mexico's Yucatan Peninsula.
Fortunately, such collisions are rare. The geological record of impact craters on Earth shows that a 10-kilometer-wide object strikes our planet, on average, only once every 100 million years.
Smaller Comet Strikes Are More Common
Strikes by smaller bodies are more frequent, simply because small objects in the solar system are much more numerous than large ones. The best-preserved impact crater made within the last few tens of thousands of years—Meteor Crater in Arizona—was created by a relatively small object. About 50,000 years ago, an iron meteorite about 30 meters (100 feet) in diameter smashed into the Arizona desert to produce the crater, a depression more than 1.2 kilometers (0.75 mile) across and about 175 meters (575 feet) deep. Despite the meteorite's fairly small size, it struck with the explosive force of 20 million tons of TNT.
The blast that gouged out Meteor Crater was comparable to the Tunguska explosion of 1908. Scientists estimate that a comet or meteor collision of that size occurs once every 200 to 300 years, on the average. One reason we don't have more impact craters on Earth is that many objects rapidly disintegrate in the atmosphere, just as happened in Siberia. Only very large or very hard objects make it all the way to the ground. Moreover, it is all but certain that the great majority of strikes have occurred in the ocean, because more than 70 percent of the planet is covered with water. But the scarcity of impact craters on land is due largely to erosion. As centuries pass, wind and water slowly obliterate all traces of most craters.
Contemplating Catastrophe
Although a Tunguska-sized explosion over a populated area would be a calamity, the destruction would be limited to a relatively small area. Many scientists and government leaders are more concerned about the possibility of a collision with a larger body, one with a diameter of 1 kilometer (0.6 mile) or more. An object that size hits the Earth at least once every million years or so, on the average.
At a speed of 50 kilometers a second, a comet that size would hit the Earth with a kinetic energy (energy of motion) equivalent to 1 million 1-megaton hydrogen bombs, or 1 trillion tons of TNT. Experts say the destruction and loss of life resulting from such a collision would surpass anything humanity has ever experienced from a single event.
Flashing through Earth's atmosphere in about two seconds, the comet would smash into the ground with incredible force and explode in an immense fireball. The shock wave from the blast would level virtually everything for a radius of more than 100 kilometers. Within much of that area, the heat of the fireball would reduce the debris to ashes and shapeless blobs of melted stone and metal. Beyond the ring of total destruction, damage from the shock wave and heat would be severe to moderate for another 1,000 kilometers (600 miles).
The collision would produce a crater at least 20 kilometers (12 miles) across and several kilometers deep. The force of the impact would hurl molten material long distances, igniting forest fires, and eject an immense volume of dust and vaporized rock into the atmosphere. The dust and gas would spread around the planet and obscure the sun.
The atmospheric darkening would be much less severe than would result from a truly enormous impact such as the one that occurred 65 million years ago, but it would still be significant. The atmospheric effects from a 1-kilometer-comet strike might cause widespread crop failures and starvation. So even if we had adequate warning of the collision and evacuated the areas most apt to be devastated, these secondary effects could still cause great loss of life.
If A Comet Plunged Into the Ocean
But what about the more likely possibility that a comet would hit in the ocean? Unfortunately, that too would be a disaster. Although a comet strike far out at sea might spare cities from being flattened or burned, the collision would still do plenty of damage.
After its plunge through the atmosphere, the comet would plow through several kilometers of seawater in a fraction of a second, breaking apart from the force of the impact. Still moving at immense speed, the comet would burrow into the sea floor and explode, creating a crater more than 10 kilometers in diameter and spewing material in all directions. Vast amounts of steam and vaporized rock would be thrown upward before the parted water could rush back to cover the hole in the sea floor.
The worst effect of an ocean strike might be the resulting tidal wave. The comet's sudden displacement of a huge volume of water, together with the titanic blast on the sea floor, would create a tidal wave a kilometer or more in height that would surge outward at almost 1,000 kilometers an hour. Many low-lying coastal cities would be submerged.
Watching For Dangerous Comets
Because a collision with a comet would have such terrible consequences, experts say it's worth considering how such a disaster might be prevented. Some astronomers have proposed the construction of a network of telescopes dedicated to searching the solar system for all near-Earth objects (NEO's), objects whose paths cross Earth's orbit and which are large enough—1 kilometer or more in diameter—to cause large-scale damage. Such a system would most likely enable scientists to identify all these threats, which astronomers think may number 2,000 or more. At present, only about 100 are known.
NASA's six-person panel was expected to recommend the development of just such a system, probably to be called Spaceguard. NASA already has a much more limited program, the Spacewatch survey, which uses a telescope at the Kitt Peak National Observatory near Tucson, Ariz., to watch for NEO's. Spacewatch observations are detecting about 30 new NEO's a year, ranging in size from about 6 meters (20 feet) to 6 kilometers (3.7 miles) in diameter. A similar project is being carried out at Mount Palomar.
In 1995, NASA and the Air Force were also funding the development of improved electronic detectors, known as charge-coupled devices (CCD's), to increase the light-gathering ability of telescopes. The more sensitive CCD's would give small instruments the resolving power of considerably larger telescopes, making it possible to use many existing telescopes to search for NEO's. Once astronomers identify an NEO, they can chart its orbit and predict its future motions.
It may thus be possible to find and catalog every potentially dangerous asteroid, large meteoroid, and known comet. But a new short-period comet making its first trip around the sun would pose an unforeseen danger. Likewise, a long-term comet returning from a million-year circuit through the Oort Cloud might sneak up on us by surprise. Experts say we must be particularly on the lookout for these previously unknown comets.
Unfortunately, detecting a comet on a collision course with Earth will undoubtedly be easier than preventing the impact. So far, scientists and engineers have proposed several schemes, but all involve considerable risk.
Bombing A Comet
One solution is to send nuclear-armed rockets into space to blow up the comet or nudge it into a new orbit. Such a mission would require extremely accurate calculations. Astronomers would have to be absolutely certain that the object was sure to strike the Earth—otherwise, the nuclear explosion might change what would have been a near-miss into a direct hit. And spaceflight engineers would have to give the rockets just the right trajectory and explode the warheads at precisely the right time to get the desired effect.
Attempting to destroy the comet outright could be a chancy proposition, however. If we were to simply blow it into large pieces, those chunks might rain down over a large region of the Earth. Calculations indicate that such an outcome could be worse than a single large collision. To be successful, engineers would have to make sure that the comet was completely pulverized. Simply altering the object's orbit might be a safer bet. That could be done by detonating warheads close enough to the object to affect its motion but not break it apart.
The farther away a comet could be intercepted, the easier it would be to push it into a safe orbit. Only a small change in the object's path would make a large difference over a distance of several billion kilometers, just as moving a rifle barrel a couple of millimeters can be the difference between hitting a far-off target dead center or missing it completely. Thus, early detection of a comet on a collision course with Earth would give engineers a tremendous advantage in diverting it.
Intercepting A Comet or Asteroid
With enough warning, it might even be possible to avoid explosive devices altogether. Some scientists have proposed sending astronauts to an approaching comet or asteroid to mount powerful rocket engines on it or even fit it with a huge “solar sail.” The latter would be a giant reflector made of metallic foil, which would capture the pressure of sunlight the way a sloop's sails catch the wind, slowly easing the object into a new orbit. But such schemes assume a warning time measured in years. If we discovered a comet just months away from hitting the Earth, nuclear-tipped rockets might be the only feasible solution.
The Odds Are With Us
Thankfully, astronomers say we probably have plenty of time to weigh our options. Even though comets and asteroids have crashed into our planet many times in the past, the long intervals between the largest impacts make it likely that the next big one won't arrive for thousands more years.
Still, the experts caution, we would do well to keep a close watch on the skies, especially for the smaller objects that arrive more frequently. But on the rare occasion that a major comet makes its majestic way around the sun, the odds will be good that we can simply sit back and enjoy the show.