Asteroid Threat : Defending Our Planet from Deadly Near-earth Objects (9781616149147) (5 page)

Science is wholly dependent on sharing discoveries, so, unlike Thomas Jefferson Jackson See, those who investigated the disappearance of the dinosaurs tended to be generous to each other and highly professional in describing their monumental collective accomplishment.

The first important clue about this planet's turbulent history, and therefore about the state of perpetual violence in which it exists, turned up in Mexico in 1952. Geologists working for Petróleos Mexicanos, the Mexican national petroleum cartel, were searching for oil off the Yucatan Peninsula when they found pieces of hard, dense, crystalline rock instead of the porous, relatively soft, sedimentary rock in which oil is found. Crystalline rock is called that because it contains crystals. And crystals—as in glass—are made when minerals are subjected to enormous pressure and heat. An analysis of the rock samples also showed that they contained a composition similar to andesite, a fine-grained rock that is found in volcanic rock.
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They did not turn up any oil, and the reason quickly became clear, or at least the geologists thought it did. Volcanic rock does not contain oil. It was therefore decided that a long-dormant volcano was under the water just off the Yucatan coast, beside the village of Chicxulub.

But Petróleos Mexicanos, ever thinking about the lucrative market for crude oil, would not abandon hope that it was there somewhere. It therefore sent two geophysicists, Antonio Camargo Zanoguera, who came from the area, and an American named Glen Penfield, to press the search in the
late 1970s. They, too, could not find oil. What they did find, however, revolutionized humanity's understanding of its history and, to a considerable extent, its place in the Solar System and the universe beyond. Zanoguera and Penfield discovered the clear outline of an almost-perfect semicircle that measured 180 kilometers across. The perfect geometry intrigued them, so they took a closer look, collecting rock and sediment samples as they went. Since volcanoes do not contain oil, the cartel called off the search. But Zanoguera and Penfield continued to explore the area and soon found the outline of another partial arc on the Yucatan Peninsula itself; on land. The two semicircles came together almost perfectly to form a 180-kilometer-wide circle whose center was near Chicxulub. That circle could have been caused only by an enormous natural “event” of some kind, and there were only two possibilities: a volcanic eruption or an impact. But, except for the material that closely approximated andesite, there was no evidence that the circle had been caused by a volcano. That, in addition to the crystalline rocks, finally started them thinking about the possibility that the great circle had been caused by an asteroid or a comet, which they calculated would have had to have been roughly ten kilometers wide and moving at blistering speed. To use Shoemaker's analogy, it would have been like a bullet fired into a pumpkin.

Walter Alvarez, a geologist at the University of California at Berkeley, became intimately involved in proving that the devastation at Yucatan was the work of an impactor. He has described what he believes would have happened when a very large impactor that he called “Doom” suddenly appeared:

Doom was coming out of the sky, in the form of an enormous comet or asteroid—we are still not sure which it was. Probably ten kilometers across, traveling tens of kilometers a second, its energy of motion had the destructive capability of a hundred million hydrogen bombs. If an asteroid, it was an inert, crater-scarred rock, dark and sinister, invisible until the last moment before it struck. If a comet,
it was a ball of dirty ice, spewing out gases boiled off by the heat of the Sun, and it announced impending doom with a shimmering head and a brilliant tail splashed across half the sky, illuminating the night, and finally visible even in the daytime as Armageddon approached.
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Alvarez is the son of the late Luis Alvarez, a Nobel laureate in physics. He, his father, and two colleagues who were chemists, Frank Asaro and Helen V. Michel, decided to investigate the impact theory in 1980, soon after Zanoguera and Penfield went public with it. Alvarez and his colleagues knew when they began that the element iridium is relatively scarce on the surface of the Earth because it is very dense and much of it, therefore, sank when the planet was forming. But there is a lot of iridium in asteroids and in some meteorites. They found that more of it is concentrated in the sediment at Yucatan than is scattered elsewhere on the planet. In addition, they, like their oil-prospecting predecessors, turned up “shocked quartz granules,” which is to say minute amounts of primitive glass. That, again, was clear evidence that the area had once been incredibly hot and under enormous pressure.

“It is very difficult to appreciate the impact that was about to occur, because such an extreme event is far beyond our range of experience—for which we can be most grateful!” Walter Alvarez has written.
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One can write down the measures of what happened—an object about 10 km in diameter slammed into the Earth at a velocity of perhaps 30 km/sec. But these measures only acquire meaning when we try to visualize them, or make analogies to help our understanding. How can we imagine a comet 10 km in diameter? Its cross section about matches the city of San Francisco. If it could be placed gently on the surface of the Earth it would stand higher than Mount Everest, which only reaches about 9 km above sea level. Its volume would be comparable to the volume of all the buildings in the entire United States. It was a big rock, or a big ice ball, but not of a scale beyond our comprehension.
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What turned it into a cataclysmic weapon was its velocity. The estimated impact velocity of 30 km/sec is 1,000 times faster than the speed of a car on the highway and 150 times faster than a jet airliner.
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Walter, his father, Asaro, and Michel wrote a paper on what they discovered for the journal
Science
, which published their findings in its June 6, 1980, issue.
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That paper played a crucial role in shaping the United States' planetary-defense space program that its authors could not possibly have foreseen.

When that monster struck, it sent so much debris into the atmosphere that the sky was darkened for months, blocking life-giving sunlight and therefore not only finishing off the dinosaurs but also many other species of animals and vegetation. It was, in effect, what would be called a “nuclear winter” during the so-called Cold War that occurred sixty-five million years later, when other dinosaurs, now grossly shrunken but still walking on two legs, reappeared on both sides of what they called an “iron curtain” and threatened to obliterate most of the world with nuclear weapons. The extinction of the great reptiles and more than half of the other species of the time is generally relegated to a time so ancient, so lost in the mist of history, that it is irrelevant. But that is wrong. The huge asteroids and comets are still out there, and the possibility of this planet suffering another catastrophic hit is not only real but also probable. In the words of Robert F. Arentz, a corporate executive who is highly knowledgeable on the subject, “It's not a matter of if; it's a matter of when.”
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What happened over the Podkamennaya Tunguska River basin in the Central Siberian uplands just after dawn on June 30, 1908, made his point. Eyewitnesses reported that a very intense blue-white streak suddenly appeared in the sky, followed by the sound of a tremendous, thunderous explosion. The blast was so powerful that, sixty kilometers away, it knocked people off their feet. “I was sitting on the porch of the house at the
trading station, looking north,” a man who lived close by later reported. “Suddenly in the sky north…the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire. I felt a great heat, as if my shirt had caught fire…. At that moment there was a bang in the sky, and a mighty crash…. I was thrown twenty feet from the porch and lost consciousness for a moment…. The crash was followed by a noise like stones falling from the sky, or guns firing. The earth trembled…. At the moment when the sky opened, a hot wind, as if from a cannon, blew past the huts from the north. It damaged the onion plants. Later, we found that many panes in the windows had been blown out and the iron hasp in the barn door had been broken.”
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And this from another survivor: “The ground shook and incredibly prolonged roaring was heard. Everything round about was shrouded in smoke and fog from burning, falling trees. Eventually, the noise died away and the wind dropped, but the forest went on burning. Many reindeer rushed away and were lost.”
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Nineteen years later, in 1927, the first group of scientists reached the desolate area and were stunned by what they found. Most of the trees within thirty kilometers of where the midair explosion had occurred were down and heavily charred. And the fact that all of them were in a circle that pointed away from the center, plus the absence of a crater, left little doubt as to what had happened. Soviet scientists concluded that a stony meteorite measuring about one hundred meters in diameter had exploded as it penetrated the thickening atmosphere.
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Scientists have disagreed for years over whether the explosive near miss over Tunguska was a comet or a meteoroid. There was no disagreement within the community, however, about its having been part of a large population of near-Earth objects (NEOs), some of which are potentially very dangerous.

David Morrison is the director of the Carl Sagan Center for
the Study of Life in the Universe and a senior scientist at NASA's Ames Research Center in California. He has been researching NEOs for most of his professional life and is an acknowledged expert on the subject. In 2000, he began sending out via the Internet frequent bulletins that were loaded with NEO developments. They were and still are simply called
NEO News
, and they live up to their name.

“We have a long-standing research program to understand more about comets and asteroids, including those that come close to the Earth,” Morrison has explained. “While the research program is motivated primarily by a desire to understand the scientific aspects of these small bodies, it is also designed in part to provide the information that will be essential for planning a defense program against hazardous impacts.” Like his colleagues in NASA and elsewhere, Morrison is not an alarmist and thinks about the situation objectively. “I can tell you with confidence that for the ten percent of the big ones that have been discovered, there is no danger. But I can tell you nothing about the ninety percent that we have not discovered. So yes, we understand the nature of the risk, but we have not taken any concrete efforts to protect ourselves or even to look to see if there's anything headed our way.”
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Morrison continues, “Cosmic impacts are highly infrequent, and the largest (mass extinction level) events have characteristic time-scales of tens of millions of years. Even the smaller localized events have low probability relative to other more familiar natural hazards such as earthquakes, tsunami waves, and severe storms. Until astronomers began to survey for potential impactors, the risk was perceived as random, and little, if any, warning could be expected. From the perspective of an elected official, the chances of having to deal with such a catastrophe within a term of office are extremely low, whether we are discussing local or global events. Yet the potential exists for an impact catastrophe at any time, in any country, with little or no warning.”
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“For some people, meteorites are trophies, to be cherished and displayed,” Neil deGrasse Tyson wrote in his autobiography,
The Sky Is Not the Limit
.

For me, they are also harbingers of doom and disaster. Consider that the slowest speed a large asteroid can impact Earth is about six or seven miles per second. Imagine getting hit by my overpriced objet d'art moving that fast. You would be squashed like a bug. Imagine one the size of a beach ball. It would obliterate a four-bedroom home. Imagine one a few miles across. It would alter Earth's ecosystem and render extinct the majority of Earth's land species. That's what meteorites mean to me, and it's what they should mean to you because the chances that both of our tombstones will read “killed by asteroid” are about the same for “killed in an airplane crash.”

About two dozen people have been killed by falling asteroids in the past four hundred years, but thousands have died in crashes during the relatively brief history of passenger air travel. The impact record shows that by the end of 10 million years, when the sum of all airplane crashes has killed a billion people (assuming a conservative death-by-airplane rate of a hundred per year), an asteroid is likely to have hit Earth with enough energy to kill a billion people. What confuses the interpretation of your chances of death is that while airplanes kill people a few at a time, our asteroid might not kill anybody for millions of years. But when it hits it will take out hundreds of millions of people instantaneously and many more hundreds of millions in the wake of global climatic upheaval.
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The likelihood of that happening, or at least of an asteroid colliding with Earth irrespective of the death and destruction it could cause, was put on a scale in 1999 by Richard P. Binzel, a professor of earth, atmospheric, and planetary sciences at MIT. He called it the Torino Impact Hazard Scale in honor of the Torino Observatory in Turin, Italy, which had done advanced research of asteroids for two decades. Binzel first described the scale at the United Nations International Conference on Near-Earth Objects that was held in April 1995. It takes into account the NEO's size, speed, and direction and assigns it a number from zero (which means that there is virtually no
chance of impact or damage) to ten (which means that there will almost certainly be a catastrophic collision). The number is calculated by the astronomers who track the asteroid and is then announced to the scientific community and to the public. The idea was to come up with a way of categorizing threats that is consistent, and it worked.