Asteroid Threat : Defending Our Planet from Deadly Near-earth Objects (9781616149147) (8 page)

The Alvarez group's discovery at Chicxulub was widely accepted as certain evidence of Earth's vulnerability to bombardment from space: that an object larger than the one that caused the long global winter that made the dinosaurs extinct and killed off so many other species could bring on Doomsday, making virtually all life on the planet extinct. It should be noted that the extinction theory was widely, but not universally, accepted. It was challenged by an international group of seven scientists with excellent credentials, led by Gerta Keller, a geoscientist at Princeton University, who maintained that the impact at Chicxulub happened about three hundred thousand years before the K-T boundary, as the dinosaur-ending Cretaceous-Tertiary time division is called in shorthand, and that it was therefore not responsible for their extinction.
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The theory that the giant reptiles and a lot of other living things were done in by the NEO that simultaneously caused the great boundary still held among the science community at large and the public, though. The K-T event killed every creature larger than a sheep that did not live near a river or in shallow water or that was not deep in a cave or underground. But many beasts survived the impact, including small dinosaurs that literally took flight and evolved into birds. Having a vague knowledge
of the mass extinction and understanding that the space agency had the technological wherewithal—at least the basics of it—to address the distinct possibility that it could happen again, or even be worse, the House of Representatives stipulated the following in its NASA Multiyear Authorization Act of 1990:

It is imperative that the detection rate of Earth-orbit-crossing asteroids must be increased substantially, and that the means to destroy or alter the orbits of asteroids when they threaten collision should be defined and agreed upon internationally. The chances of the Earth being struck by a large asteroid are extremely small, but since the consequences of such a collision are extremely large, the Committee believes it is only prudent to assess the nature of the threat and prepare to deal with it. We have the technology to detect such asteroids and to prevent their collision with the Earth.
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The committee therefore ordered NASA to conduct two workshops: one to dramatically increase the detection rate of Earth-crossers, and the other to define the systems and technologies that would be required to alter their courses or destroy them if they posed a danger to life on Earth. Since the danger was understood to be global, the committee prudently recommended international participation.
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The first workshop had twenty-four men and women from six countries—the United States, Russia, France, Finland, India, and Australia—who amounted to the NEO first team. Individuals included David Morrison, Clark Chapman, Donald Yeomans, Gene Shoemaker, Tom Gehrels of the University of Arizona, Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge (which had long kept track of NEOs), Louis Friedman of the Planetary Society, and Duncan Steel, an Australian astronomer whose
Rogue Asteroids and Doomsday Comets
was an easily understood book written for a lay audience. They, together with Alexander Basilevsky, the director of the Laboratory for Comparative Planetology at the
Russian Academy of Sciences' Vernadsky Institute in Moscow, who was not at the meeting, were a cadre of the world's leading experts on the objects, large and small, that swarm around it. The idea was to inform as many people as possible about the remote but real danger that lurks out there.

The group produced an encyclopedic report in 1992 called the Spaceguard Survey that described the situation, including the potential troublemakers in substantial detail, and was a crucial first step in the defense of the planet. As everyone in the community knew, logic required that the NEOs needed to be defined and described before the threat they posed could be calculated and articulated so an effective defense could be mounted. “There are two broad categories of objects with orbits that bring them close to the Earth: comets and asteroids. Asteroids and comets are distinguished by astronomers on the basis of their telescopic appearance,” the introduction explained. “If the object is star-like in appearance, it is called an asteroid. If it has a visible atmosphere or tail, it is a comet. This distinction reflects in part a difference in composition: asteroids are generally rocky or metallic objects without atmospheres, whereas comets are composed in part of volatiles (like water ice) that evaporate when heated to produce a tenuous and transient atmosphere [or “tail”]…. For our purposes, the distinction between a comet and an asteroid is not very important. What matters is whether the object's orbit brings it close to the Earth—close enough for a potential collision.”
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Earth-crossing asteroids (ECAs)are those that cross Earth's path as it circles the Sun the way torpedoes pass in front of a ship—they are the ones that are most likely to collide with Earth, and they are therefore the most closely watched.

A chapter of the survey was devoted to the hazard of impacts, and specifically to the relationship between the size of the impactor and the damage it would likely cause, from local destruction to the threshold size for a global catastrophe.
Asteroids of stony or metallic composition that are larger than 100–150 meters can make it to the ground intact and blast out relatively small craters, while those that are larger than 150 meters can make craters that are three kilometers in diameter. Worse, the larger-than-150-meter asteroids' zone of destruction would extend well beyond the impact area, damaging or flattening buildings with the shock wave from their explosions. The good news is that they come along only about once every five thousand years. And the planet is very likely out of harm's way for at least another five generations. “No object that is now known has an orbit that will lead to a collision with our planet during the next century, and the vast majority of the newly discovered asteroids and comets will also be found to pose no near-term danger. Even if an ECA has an orbit that might lead it to an impact, it will typically make hundreds of moderately near passes before there is any danger, providing ample time for response. However, the lead time will be much less for a new comet approaching the Earth on a long-period orbit,” the report explains. The authors claim that no object that is now known, that is being tracked, is in an orbit that will lead to a collision with Earth during the next century. That would undoubtedly draw sneers from the residents of Chelyabinsk and probably an expression of astonishment from Michelle Knapp.

And it would very likely dumbfound the people of the Eastern Mediterranean. What happened there on June 6, 2002, was a real attention-getter. There was an explosion by an undetected asteroid between Libya and Crete that had the power of a small atomic bomb. It caused no damage and, since it occurred over the sea, no fragments were recovered for study (or sale). But Gen. Simon P. Worden, the deputy director of operations at US Space Command (which scrupulously tracks manmade objects such as spacecraft circling Earth, as well as some objects that are made elsewhere), noted that had the explosion occurred over or near the Indian subcontinent a few
hours earlier it could have started a nuclear war between India and Pakistan, both of which were having a military standoff with tensions running even higher than usual. The blast might have been interpreted by either side as the prelude to an attack by the other, and either could have answered with an immediate, reflexive counterattack.

“A few weeks ago the world almost saw a nuclear war,” Worden said in a speech on July 10. He continued,

Pakistan and India were at full alert and poised for a large-scale war—which both sides appeared willing to escalate into nuclear war. The situation was defused—for now! Most of the world knew about this situation and watched and worried. But few know of an event over the Mediterranean in early June of this year that could have had a serious bearing on the outcome. U.S. early warning satellites detected a flash that indicated an energy release comparable to the Hiroshima burst. We see about 30 such bursts per year, but this one was one of the largest we've ever seen. The event was caused by the impact of a small asteroid—probably about 5–10 meters in diameter on the earth's atmosphere…. The event of this June caused little or no notice as far as we can tell. But had it occurred at the same latitude, but a few hours earlier, the result on human affairs might have been much worse. Imagine that the bright flash accompanied by a damaging shock wave had occurred over Delhi, India or Islamabad, Pakistan…. The resulting panic in the nuclear-armed and hair-trigger militaries there could have been the spark that would have ignited the nuclear horror we've avoided for over a half century. This situation alone should be sufficient to get the world to take notice of the threat of asteroid impact.
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Worden was and remains mindful of the NEO threat and is therefore a strong proponent of robotic and crewed missions to them to increase our knowledge.

By then, the United Nations had long since been concerned about the threat. It held a near-Earth-objects conference at UN headquarters and at the Explorers Club in New York in April 1995, at which many of the astronomers, earth and planetary scientists, and those specializing in astronautics who had begun
to devote their professional lives to studying the problem made presentations that fundamentally helped to shape the international planetary-defense program. They included Yeomans, Marsden, Ostro, Chapman, Keller, Morrison, and Gehrels. The conference also included John L. Remo of QuantaMetrics, a company that specializes in selling and marketing to scientific researchers; Mark B. E. Boslough, a physicist at the Sandia National Laboratories and an expert on planetary impacts who had made many science documentaries and television appearances; Michael R. Rampino and Bruce M. Haggerty, professors in New York University's Earth and Environmental Science Program; and several other scientists from around the world. In all, they gave forty-seven presentations during the three-day session that, as custom required and would continue to require, first described the NEO “population” and then focused on ways to detect and then mitigate them.
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Given that there are far fewer really menacing asteroids and comets out there because of the relative rarity of the very big ones, there is indeed breathing time, provided something terrible does not come out of the Oort Cloud unexpectedly. The cloud is believed, in theory, to envelop this Solar System—it has not been seen as more substantial parts of the universe have—and contain billions of comets. A kilometer-size impactor would cause destruction in the megaton range (a term that inevitably suggests a comparison with the effects of nuclear weapons).

Larger attackers—those measuring one to five kilometers—would cause serious global consequences, the Spaceguard Survey reported, and specifically gouge out craters up to fifteen times the diameter of the projectile. The primary hazard that would come from such a strike—provided no people or property were hit—would be a global “veil of dust” in the stratosphere that could cause massive, worldwide crop failures that could threaten civilization. That added a dimension to the threat. Damage had always been characterized as immediate
death and destruction: cities or regions destroyed and mass fatalities. But now a more subtle element of the equation was added: starvation. The authors admitted that the threshold size of the impactor that would cause such a “global catastrophe”—its minimum size—is not accurately known. And contrary to the popular portrayal of asteroids as smooth, potato-shaped rocks like 433 Eros, they are most often jagged since they are chunks of rocks or metal—fragments—that were violently broken off of larger bodies, including planetoids, which are very large, pretentious asteroids.

As every police officer, sports coach, physician, lawyer, and businessman or businesswoman knows, competition requires knowing the opponent, and the more thoroughly, the better. That certainly applies to planetary defense—to competition with NEOs—which is why the Spaceguard Survey called for increased observation, notably by optical astronomy, radar astronomy, and physical observation with photometric equipment. In common with other asteroid and comet watchers around the world, its collective membership also called for international cooperation. “That the hazard posed by NEO's is a problem for all humankind hardly needs repeating,” they repeated. “The likelihood of a particular spot being the target of an impact is independent of its geographic position, so that we are all equally at risk.”
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The community has been well aware of the danger for decades. Congress got on board in 1998 after hearings were held that May. There, Clark Chapman, William Ailor of the Aerospace Corporation, Gregory Canavan of the Los Alamos National Laboratory, John Lewis of the University of Arizona, and Carl Pilcher, the director of NASA's science program, testified about the threat and what was being done to address it.

Pilcher told the Subcommittee on Space and Aeronautics that NASA was so concerned about what its and other telescopes picked up that it was committed to cataloging within a
decade 90 percent of all NEOs with a diameter larger than one kilometer and that the program was on track to do so. He added that the budget for completing this task had been doubled to $3 million and that that allocation—relatively meager as it was—would at a minimum be maintained. Canavan told the subcommittee that improved technology had increased the detection rate, but that long-period comets—those that cross Earth's path—remained a serious concern and could constitute as much as half of the NEO threat. He also repeated the axiom that asteroids had to be characterized (meaning adequately defined) for their threat to be reduced, and he called for cooperation between the Department of Defense and NASA, as happened in the Clementine II mission in 1994, when they combined to test spacecraft components and sensors by sending the spacecraft to observe the Moon and asteroid 1620 Geographos. Usable data came back from the Moon but not from the asteroid because of a technical malfunction. Ailor discussed the Leonid meteor shower and the danger it would pose to satellites when it started in November. It was a spectacular show and caused no apparent harm to the Earth-circlers. And Lewis sounded a distinctly upbeat note by telling the subcommittee that asteroids were economically valuable because they could be mined, and he carefully pointed out that the keys to successful exploitation for minerals were lower launch costs and carefully choosing asteroids that are the most accessible and have the richest mineral concentrations.
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