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

by William E. Burrows

  Although some members of Congress undoubtedly read about what happened to Hodges and Knapp, they didn't need those incidents to conclude that their planet was and still is under continuous attack by large and small rocks coming from space and that the dinosaurs were almost undoubtedly terminated by a really big one. Those isolated events would have gone virtually unnoticed by Congress, and certainly by the international community, since, by their nature, they were of no real consequence to the rest of the world. Except that just about everyone who was paying attention knew that some of the rocks out there were a lot bigger than what came down on Sylacauga and Peekskill. Gene Shoemaker had not yet participated in the film Asteroids: Deadly Impact for National Geographic Video, but it was generally understood that a collision with a large-enough impactor could inflict more damage than a nuclear war and even end their planet's existence. The explosion of a 450-ton meteor twelve miles above Chicora, Pennsylvania, on June 24, 1938, and others over British Columbia in March 1967, Lake Huron in September 1966, and Greenland in December 1997, made the point. As British poet and critic Samuel Johnson said, “Depend upon it, sir, when a man knows he is to be hanged in a fortnight, it concentrates his mind wonderfully.”

  When the comet Shoemaker-Levy 9 slammed into Jupiter in July 1994, it was extensively covered by the news media and concentrated the legislators’ and the rest of the world's collective mind. The images of the successive impacts and the inevitable references to what any one of them would have done to Earth drove home the need to find out what, precisely, the nature of the threat was and then come up with an effective defense, or “mitigation,” as the community calls it. Every other conceivable problem was dwarfed by the prospect of Doomsday: by the possibility, however remote, that Earth would be terminated and that existence would end. Lights out forever.

  The primal know-the-enemy impulse took concrete form when the Johns Hopkins University Applied Physics Laboratory (APL) designed a spacecraft for NASA that was packed with instruments and that would be the first to be sent to an asteroid to study it in detail as part of the new Discovery program. As was mentioned in chapter 2, the asteroid candidate of choice was 433 Eros, a smooth, pockmarked near-Earth boulder that was thirty-three kilometers long and thirteen kilometers wide—about the size of Manhattan—and that was incongruously named after the Greek god of love (as in erotic).

  The mission was at first called NEAR, for Near-Earth Asteroid Rendezvous, and then NEAR Shoemaker for the legendary godfather of planetary science. It entailed having the spacecraft land on Eros and stay on it—ride it—for a year, all the while collecting data on its prodigious bulk, composition, mineral content, internal mass, magnetic field, interaction with the solar wind and the space around it, and more. The spacecraft was launched from Cape Canaveral on February 17, 1996, flew by asteroid 253 Mathilde on June 27, 1997, and then went on to 433 Eros. After the first attempt at orbital insertion (as its handlers called it, which describes the adjustment of a spacecraft's momentum to enter into orbit around a space object) failed, it began flying ever-tighter circles around its quarry and started the first orbital study of an asteroid in February 2000. The visitor from Earth eventually landed on 433 Eros on Valentine's Day, February 12, 2001.

  NEAR Shoemaker then began sending home everything it learned from its very close encounter. Not surprisingly, the asteroid's mineral composition was typical of other asteroids, and the tough guy had itself been the target of hits—“successive impacts”—in the shooting gallery. Andrew F. Cheng, who was also at the Johns Hopkins APL and who was a member of the science team, reported that there were many impact craters, most of them relatively large, and that the asteroid was “intact but deeply fractured,” probably because it had broken off of an even larger rock.13 The mission also showed that landing people on this or other large asteroids, either for scientific research or to mine them for rare minerals, was feasible. The spacecraft stopped sending data home on February 28, 2001, and an attempt to communicate with it in December 2002 was unsuccessful. It is still riding its asteroid, however, mute in the bitter cold.

  The 433 Eros asteroid was no threat to Earth and never will be. But the mission provided the first detailed look at one of the large rocks in its element as representative of others, larger and smaller, that could be potentially hazardous or worse. The images of a scarred, potato-shaped boulder that was twice the size of Manhattan speeding through space were vivid reminders, yet again, that many dangerous objects prowl the neighborhood, far from home and near it. So the NEAR Shoemaker encounter, like the attack on Jupiter, remained lodged in the minds of the space-science community and those on Capitol Hill who were responsible for knowing about the potential danger posed by asteroids and comets that get too close.

  The idea that asteroid and comet impacts are still a hazard had only just taken hold in 1980, as David Morrison has written, which is when the Alvarez team published what it found at the Chicxulub crater. Morrison adds that asteroids are statistically by far the greater threat than comets are; the threat from comets is comparatively miniscule.14 But the threat from NEOs is potential, not immediate. The Wide-field Infrared Survey Explorer telescope, which was called “WISE” by its handlers at JPL, turned up 911 (that number again) of an estimated 981 NEOs larger than a kilometer, but none were thought to be potentially hazardous asteroids. That gestated in the scientific community and in Washington for a little more than a decade. And so, understandably, did the idea that what had come out of the sky to cause such death and destruction could come again, a point that is made to students in Astronomy 101. Those who are assigned Contemporary Astronomy by Jay M. Pasachoff, the standard text for that college-level course, learn that some Apollo asteroids—those that cross Earth's orbit—are potential impactors. “Most Apollo asteroids will probably collide with the Earth eventually,” the author explains, “because their orbits may intersect the Earth's. Luckily there are only a few dozen Apollos greater than 1 km in diameter.”15 The widely used textbook—and justifiably so—might have pointed out that even Earth-crossers a lot smaller than a kilometer can, and have been, devastating.

  With the Apollo Moon-landing program, NASA scored a monumental victory over the Soviet Union, followed by that enormously successful Solar System exploration program—the high point of which was unquestionably Voyager 2’s twelve-year grand tour of four of the outer planets and the library of knowledge it sent home—and then the Cold War ended and a highly productive “space race” was over. That left NASA with a conundrum. It became the victim of its own success. All of those accomplishments were by definition spectacularly unique, but repeating them was considered irrelevant and wasteful. The resources were there, but there was no mission. So NASA became a powerful and famously inventive agency without a major long-term goal that was inherently dramatic enough to hold the public's (and congressional appropriations committees’) interest. There is such a mission, however. It is planetary defense done in conjunction with the international community.

  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.16

  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 larg
er 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.17

  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.18

  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.”19 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.20

  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-d
efense 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.21

  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.