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.”28
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.”29
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.30 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.”31
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.”32
“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.33
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.
By the start of the last decade of the twentieth century, Congress had become sufficiently concerned about the impact threat, so it mandated NASA to locate within ten years 90 percent of NEOs with diameters of a kilometer or more. Then, in 2005, Congressman George E. Brown of California, whose constituents included several aerospace companies, led a campaign that expanded the number of potentially threatening intruders the space agency had to locate and catalog to 90 percent of those that measured 140 meters or larger by 2020 (as in perfect vision). But the Obama administration did not ask for funds to complete that task, and the concern that was dramatically proclaimed in Congress did not extend to authorizing enough of an appropriation to do the necessary work. As has been ruefully noted for a very long time, the members of the House of Representatives seem to have attention spans that are limited to two-year election cycles. The space agency therefore had to try to carry out its mandate with restricted funds. It was like a modern farmer being ordered to produce a bumper crop using only a plow pulled by a mule.
But two usable crops—that is, studies—were harvested. The first, titled Asteroid and Comet Impact Hazards and conducted by the Spaceguard Survey Report, was released in 1992. The second, Report of the NEO Survey Working Group, came out three years later. The Spaceguard Survey Report owes its name to Arthur C. Clarke, who invented the term and used it as the title of chapter 1 in his Rendezvous with Rama: “SPACEGUARD.” So too does the concept behind the
report owe itself to Clarke: it made the obvious point that defense against asteroids and comets depends on understanding their nature—that is, their size, their composition, their velocity, their location, and the direction in which they are moving. The Spaceguard Survey Report ranked the potential impactors’ destructive capacity according to size, which translated to “kinetic energy.” Smaller ones—those with less kinetic energy—approach Earth more frequently than their larger counterparts but inflict little or no damage. The bigger ones come in less frequently, but when they do, they can inflict severe damage. And coastal populations are at greater risk than inland populations because of tsunamis. “Persons living in coastal regions,” the report warned, “run a risk from impact-generated tsunamis as much as two orders of magnitude greater than that from land impacts.”34
As the Spaceguard Survey Report was accepted and began to take hold within the space community, so did the effect of an important paper that was published in February 2001 that brought the planetary-defense situation around full circle. The Comet/Asteroid Impact Hazard: A Systems Approach was written by Clark R. Chapman, Daniel D. Durda, and Robert E. Gold, three leading space scientists. Chapman and Durda were in the Department of Space Studies at the prestigious Southwest Research Institute in Boulder, Colorado, and Gold was in the equally prestigious Space Engineering and Technology Branch of the Applied Physics Laboratory at Johns Hopkins University in Laurel, Maryland. They maintained that the danger of a serious impact was potentially so great that an integrated approach to the problem across the scientific, technological, and public-policy sectors was fully justified. While the emphasis at that point had been on astronomers spotting potential impactors and finding ways to deflect them, the authors contended that little or no thought had been given to connecting the astronomers with the military and civilian agencies that would be responsible for pushing approaching NEOs off course; to planning other kinds of mitigation and dealing with the repercussions of predicting an impact (mass civil panic, for example); and to the need for an informed, international effort to actively plan a mitigation strategy that would replace what they derided as “unbalanced, haphazard responses.” Mitigation in the NEO community means “to stop the threat.” “For example,” the authors contended, “we believe it is appropriate, in the United States, that the National Research Council develop a technical assessment of the impact hazard that could serve as a basis for developing a broader consensus among the public, policy officials, and government agencies about how to proceed. The dinosaurs could not evaluate and mitigate the natural forces that exterminated them, but human beings have the intelligence to do so.”35 Then, poignantly, they took note of the seed that had been planted two decades earlier:
The scientific community began to understand the implications for life on Earth of errant small bodies in the inner Solar System in 1980 when Nobel Laureate Luis Alvarez and his colleagues published an epochal paper in Science (Alvarez et al. 1980) advocating asteroid impact as the cause of the great mass extinction 65 million years ago that led to the proliferation of mammal species based on an extraordinary discovery they made at the Chicxulub impact crater underwater off Yucatan on Mexico's gulf coast.36
The National Research Council, which is part of the prestigious National Academy of Sciences, was paying attention, and so was the rest of the international space-science community. In October 2002, Chapman met with former astronauts Russell L. “Rusty” Schweickart and Ed Lu, as well as Piet Hut, an astrophysicist at Princeton's Institute for Advanced Study who specialized in planetary dynamics (specifically in preventing asteroid impacts). They gathered at the Johnson Space Center and hatched the B612 Foundation, which was named in honor of the little prince who lived on the asteroid. The foundation's purpose was to protect Earth from asteroid strikes by funding the development of ways to deflect them.
In February 2000, a far-ranging space probe called the Near-Earth Asteroid Rendezvous Shoemaker (NEAR Shoemaker) was ordered to fly in close formation with a thirty-three-kilometer-long, thirteen-kilometer-wide, potato-shaped asteroid named 433 Eros for a year, collect data as it went, and then land on it. Eros, as in erotic, is the god of sex and making love in Greek mythology. NEAR Shoemaker's controllers, who, like many in the space program, had a sublime sense of wit and irony, ordered it to mount Eros on February 12, 2001. It was therefore on Eros two days later, which was Valentine's Day.
The next step in planetary protection was a big one, and it happened at the Hyatt Regency Hotel in Garden Grove, California, from February 23 to 26, 2004, when the National Research Council held the first international Planetary Defense Conference. The conference was dedicated to “Protecting the Earth from Asteroids”37 and was precisely what Chapman, Durda, and Gold had called for three years earlier in The Comet/Asteroid Impact Hazard: A Systems Approach.
Eighty-one papers were presented at that meeting on topics such as “Order-of-Magnitude Analyses of Planetary Defense Problems,” “Orbit Determination for Long-Period Comets on Earth-Impacting Trajectories,” “The Impact Imperative—A Space Infrastructure Enabling a Multi-tiered Earth Defense,” and “The Mechanics of Moving Asteroids.” In addition to heavy participation by Americans, scientists from Russia, Italy, Ukraine, Bulgaria, Spain, South Korea, France, Germany, and India came to make presentations.
At about the same time, and unrelated to the meeting, David Morrison began sending out NEO News on the Internet, and it subtly but effectively helped to bring the NEO community together. And a galaxy of astronomers and others who were interested in the impact threat began to coalesce as a separate group.
The reach of Morrison and several other astronomers soon extended to Europe. On March 26, 1996, the Spaceguard Foundation was established in Rome with the declared intention of protecting “the Earth environment against the bombardment of objects of the solar system (comets and asteroids).”38 Its members were mostly European, of course, but included a Japanese scientist on its board of directors. And the foundation's two trustees were definite attention-getters. The first is Fred Whipple, a very well-known astronomer. The second, none other than Arthur C. Clarke, who was billed as the author of “several famous science fiction novels, including…2001: A Space Odyssey (1968); in Rendezvous with Rama (1973) he described the effects of the collision of an asteroid with the Earth and the settlement of a Spaceguard organization for the protection of the Earth against such events.”39
Then the European Space Agency weighed in. In April 2006, it announced that it planned to “slam” an impactor probe into an asteroid in a mission it wryly called Don Quijote. The mission was to involve two spacecraft: one named Sancho that would orbit the asteroid and study it for several months, and a second, Hidalgo, that would collide with it. Then Sancho was to return to the asteroid to assess the damage and report home.
The mission was originally scheduled for 2011; then it was postponed to 2015. And then, in late December 2009, Anatoly Perminov, the head of Roskosmos (the Russian space agency), declared that he was considering inviting the international community to send out a robot to meet a large asteroid named Apophis that is headed in this general direction and to nudge it off course before it makes its first pass by Earth in 2029. He was referring to 99942 Apophis. The huge rock takes its name from an evil ancient-Egyptian serpent that dwelled in eternal darkness in the center of Earth. No professional astronomer seriously believes that Apophis is going to hit this planet the first time around, but there is some concern that it, too, will be ensnared by Earth's gravity and swing into an orbit that becomes increasingly closer until it impacts.40
The possibility of Earth attracting a potential impactor came to mind on June 6, 2002, when the Eastern Mediterranean event took place. Yet another meteor exploded in the sky between Libya and Crete, without warning and with the power of a small atomic bomb. It happened during the 2001–2002 India-Pakistan confrontation, and, in the opinion of Gen. Simon P. Worden, vice director of operations for the US Space Command, had it occurred three hour
s later, it could have caused a nuclear war between the two enemies.41 Either side could have thought the explosion was caused by a ballistic missile that had been launched by the other side, blowing up prematurely, or that it was meant to knock out the other side's communication capability as a prelude to a nuclear war. Seen in that light, the name impactor takes on two distinct and dangerous meanings. They are, of course, physically dangerous. But to the extent that they can trigger war or other forms of manmade violence, they are abidingly dangerous politically as well.
“So it is said that if you know others and know yourself, you will not be imperiled in a hundred battles,” Chinese warrior-philosopher Sun Tzu wrote more than two thousand years ago in his classic work, The Art of War.1 “Others” is often translated as “enemies,” which is precisely what he meant. “Know thine enemy” is a widely used variation on the theme. And since there are an infinite number of others—enemies—in this world, the advice is universally applicable to individuals and to the multifarious relations between nations.
There are also infinite enemies out of this world, and knowing all about them—what they are, where they are, and where they are going—could make the difference between this planet's safety and survival and its either suffering a severe wound or being annihilated altogether. The early Greeks and Romans saw that rocks fell from the sky and pondered its meaning. Aristotle was convinced that they were first lifted off Earth's surface by strong winds and were then thrown back. He and Pliny the Elder are thought to have both written about an impact that occurred in 467 BCE, when a meteorite fell on Aegospotami in Thrace, on the European side of the Dardanelles. And a comet was seen the same year.2
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