would cause a planetary disaster; at least a billion people would be killed, and modern civilization would be destroyed.31
In December 1997 an Earth-crossing asteroid with a diameter of almost two kilometers was discovered by astronomers in the United States. Classified as asteroid 1997 XF11, its course was studied closely over the next three months. Then in March 1998 Harvard University astronomer Brian Marsden announced the results of these calculations: There was, he warned, a possibility of a collision in 2028. Headlines on 12 and 13 March of 1998 were dominated by this announcement and astronomers around the world attempted to refine Marsdens calculations. Some concluded that the asteroid would pass closer to Earth than the Moon, perhaps as close as 40,000 kilometers. Others argued that the distance might be more than a million kilometers. Marsdens conclusion was that “the chances of impact are very small, but not impossible.” Jack Hills, an asteroid specialist at Los Alamos National Laboratory in the United States commented: “It scares me. It really does. An object this big hitting the Earth has the potential of killing many, many people.”32
In 1968 the asteroid Icarus, two kilometers in diameter, missed Earth by 6 million kilometers—“an uncomfortably small distance in the scale of the solar system,” as the Massachussetts Institute of Technology commented at the time.33
In 1991 asteroid BA passed just 170,000 kilometers from Earth, less than half the distance to the Moon. It has a diameter of 9 meters (about the size of a double-decker bus), sufficient “to destroy a small town.”34
On 16 March 1994, Duncan Steel gave the following briefing to the Australian media:
About six hours ago Earth had a near-record observed near-miss by an asteroid. The miss distance was about 180,000 kilometers, which is less than half the distance to the Moon. The object is only about 10–20 meters in size. Its name at this stage is 1994 ESI. It was discovered by the Spacewatch team (University of Arizona) at Kitt Peak National Observatory, near Tucson, Arizona. If it had hit Earth it would have done so at a speed of 19 kilometers per second (44,000 miles per hour). Unless it is made of solid nickel-iron (as are many meteorites) it would have exploded in the atmosphere at a height of 5–10 kilometers. The total energy released would be equivalent to a nuclear explosion of energy about 200 kilotons (around 20 times the Hiroshima bomb).35
Destructive airbursts caused by asteroids are in fact routinely recorded by the infrared scanners of U.S. military satellites—the recently declassified data for 1975 to 1992 indicates 136 atmospheric explosions with yields of a kiloton or more.36 One particularly spectacular burst, with a yield estimated at 5 kilotons, was observed over Indonesia in 1978.37 Even more spectacular was a 500-kiloton airburst between South Africa and Antarctica on 3 August 1963.38 On 9 April 1984, the captain of a Japanese cargo plane reported a brilliant airburst approximately 650 kilometers east of Tokyo. “The blast formed a mushroom cloud rising from 4,267 to 18,288 meters in only 2 minutes.”39
FIREBALLS AND COMETS
On 19 February 1913 a small asteroid entered the earths atmosphere as a fiery apparition over Saskatchewan, traveling east at a speed estimated at around 10 kilometers per second. It was observed at an altitude of 50 kilometers over Winnipeg and Toronto and over several cities of the northeastern United States. It passed over New York and into the Atlantic. Two minutes later it was observed again over Bermuda.40 Thereafter all trace of it was lost. It probably fell into the ocean.
In 1972 another fireball was observed in the United States, this time rising up steeply to escape from the earths atmosphere in which it had only temporarily become enchained. The astronomers L. G. Jacchia and John Lewis calculate:
It approached with a relative speed of 10.1 kilometers per second and was accelerated to 15 kilometers per second by Earth’s gravity as it fell toward the top of the atmosphere. Its point of closest contact to Earth was at an altitude of about 58 kilometers over southern Montana…. The body had a diameter of 15 to 80 meters and a mass of at least several thousand metric tons and possibly as high as a million metric tons. It passed within 6,430 kilometers of the center of the earth. If it had passed only 6,410 kilometers from the center of the earth it would have exploded or impacted somewhere in the populated strip of land stretching from Provo, Utah, through Salt Lake City, Ogden, Pocatello, and Idaho Falls. The explosive power would have probably been [equivalent to about] 20 kilotons of TNT.41
On 1 February 1994 a bolide entered the earths atmosphere over the Pacific Micronesian islands, crossed the equator traveling in a southeasterly direction and eventually exploded northwest of Fiji, 120 kilometers above the island of Tokelau. It was calculated to have traveled at 72,000 kilometers per hour.42 The explosion was blindingly bright and may have had a yield equivalent to 11 kilotons of TNT.43
Larger and much faster objects have also approached close to Earth. On 27 October 1890 observers at Cape Town, South Africa, witnessed the apparition of an immense comet, with a tail as wide as a full moon, that stretched across half the sky. During the 47 minutes that it was visible (from 7:45 P.M. until 8:32 P.M.) it traversed about 100 degrees of arc. “Supposing this was a typical small comet,” observes John Lewis, “traveling at about 40 kilometers per second relative to the earth, then its observed angular rate of two degrees per minute implies that the comet must have passed within 80,000 kilometers of Earth, about a fifth of the distance of the Moon.”44
Another fast-moving comet, which streaked across the sky at the rate of 7 degrees a minute, was detected in March 1992 by astronomers at the European Southern Observatory.45 Its nucleus appeared to be about 350 meters in diameter:46
Again taking the most probable flyby speed as 40 kilometers per second, typical for long-period comets, this comet must have flown by at a distance of about 20,000 kilometers. Remembering that the diameter of the earth is about 13,000 kilometers, this is very close indeed.47
MERCURY
The more we learn about the vast arsenal of projectiles flying around in space, the easier it becomes to understand how neighboring Mars—which may once have offered a congenial home to life—could have been reduced to a tortured and barren hell-world. Indeed, what has happened to Mars is actually the norm among the inner planets. It is Earths continued survival as a functioning ecosystem that seems hard to explain.
Mercury, the innermost planet, is brutally pockmarked with craters and, like Mars, appears to have been stripped of huge segments of its crust:
Something smashed into Mercury with such violence that its outer layers were torn away and, lost to space, fell into the Sun.48
Another characteristic that Mercury shares with Mars—and also with Earth—is the phenomenon of massive craters in one hemisphere being matched by reactive disruption at the antipodal point in the opposite hemisphere. As we have seen, the Martian crater Hellas, which has a diameter of almost 2,000 kilometers, has been connected to a bizarre feature known as the Tharsis Bulge, which is nearly antipodal to it. On Earth the 200-kilometer Chixculub crater in Mexico, the epicenter of the K/T Boundary Event, has been connected to the volcanic scabs of the Deccan Traps in India. In the case of Mercury, NASA photographs show a gigantic crater, 1,300 kilometers in diameter, which has been named the Caloris Basin. Exactly on the opposite side of the planet is an extensive area of “chaotic terrain” where there are no impact craters but where the ground appears to have been smashed to bits by gigantic pile-drivers and then shaken up into a new and extraordinary configuration. Duncan Steel offers this explanation:
When Caloris was formed, huge seismic waves were focused through the interior of Mercury, meeting at the antipodal point and breaking up the smooth terrain that previously existed there.49
VENUS
If in our imaginations we look down on the solar system from above, that is, from the north, we will see that all the planets are orbiting the Sun in a counterclockwise direction. The majority of them also rotate counterclockwise about their own axes. The notable exception is Venus, the second planet out from the Sun, which
rotates in the direction opposite to its revolution.50
Astronomers regard the retrograde rotation of Venus as “quite remarkable.”51 The generally accepted explanation is that at some point in its history it “was struck so hard”—probably by a titanic asteroid or comet—that its rotation was momentarily halted and that it then “began to spin in the opposite direction.”52 The cataclysm is thought to have taken place billions of years ago, during the early stages of the formation of the solar system, but there is also evidence of a much more recent giant impact:
The entire surface of Venus was wiped clean…. Geologists describe this event as having “resurfaced” the planet with lava from its interior as great blocks of the surface cracked and subsided.53
EARTH
Earth is the third planet out from the Sun—a glowing sphere of light and consciousness soaring in dark space, a kind of magic, a kind of miracle. Some see it as a living being. Plato described it as a “blessed god …”54
a single spherical universe in circular motion, alone but because of its excellence needing no company other than itself, and satisfied to be its own acquaintance and friend.55
It is also, with our as yet extremely rudimentary knowledge of our cosmic environment, the only place in which we can be absolutely certain that life exists. The balance of probability is that there is life, perhaps much more intelligent than ourselves, on other planets orbiting other suns. But we just can’t be sure. For all we know cosmic smashups like those that ruined Mercury, reversed the rotation of Venus, and flayed the planet Mars may be commonplace not only within the solar system but in the universe as a whole.
Imagine the responsibility, therefore, if we are the only life. Imagine the responsibility if our spark of consciousness is the only consciousness that has survived in the entire universe. Imagine the responsibility if some avoidable threat is looming which through complacency we do nothing about.
JUPITER
What is already clear is that Earth is at present the only planet in the solar system that is inhabited by intelligent beings. This may not have been true 10,000 or 20,000 or 50,000 years ago—who knows?—but today all our neighbors are dead and show signs of having suffered massive bombardments of cosmic debris.
Mercury is dead. Venus is dead. The Moon is dead. Mars is dead. And although Earth still lives, with us upon it, there is no evidence that the bombardments have stopped just because we are here. On the contrary, as recently as 1994 humanity was offered spectacular proof that objects of world-killing size do still collide with planets. That was the year in which a swarm of massive fragments from the disintegrating comet Shoemaker-Levy 9 hit Jupiter, an event taken by many astronomers as a timely reminder that Earth, too, could suffer such a fate—and theoretically at any time. As David Levy, the co-discoverer of the comet, observed:
It was as if Nature had called over the phone and said “I’m going to drop 21 comets on Jupiter at 134,000 miles an hour…. All I want you to do is watch.56
The impacts were watched—with great interest and attention. Dozens of observatories and the Hubble space telescope, as well as the NASA probe Galileo, focused their attention and cameras almost exclusively on Jupiter during the month of July 1994 when the collisions took place, and ominous photographs of all the major impacts were broadcast as headline news to billions of people around the world.
Mercury … Venus … the Earth-Moon system … Mars …
Jupiter is the fifth planet out from the Sun; its orbit lies about 500 million kilometers beyond that of Mars. With a diameter of nearly 144,000 kilometers, it is the giant of the solar system—one-tenth of the size of the Sun itself, ten times larger than Earth and 20 times larger than Mars. Its surface is not thought to be solid, but fluid and gaseous, “composed mainly of hydrogen and helium in near-solar proportions.”57 Nevertheless its mass is 318 times greater than that of Earth and, indeed, greater than the combined mass of all the other planets in the solar system.58
The ability of such a leviathan to shoulder aside or destroy objects approaching it from space, and to absorb the impacts of those that penetrate its atmosphere, seems virtually limitless. And yet Jupiter was horrifically battered and bruised by its high-speed encounter with the 21 fragments of comet Shoemaker-Levy 9.
COSMIC TRACER
Caroline Shoemaker, the late Eugene Shoemaker, and David Levy discovered their eponymous comet on 24 March 1993. It initially showed up as a fast-moving smudge on grainy photographic plates. Big observatories then turned their telescopes on the object, and Jim Scotti of the University of Arizona’s Lunar and Planetary Laboratory, using the 90-centimeter Spacewatch telescope, was the first to confirm that S-L 9 was not in fact one object but “a string of 21 fragments.”59 Early photographs showed images that were beautiful but scary—like tracer bullets arching across the night sky—and astronomers began to calculate how large the individual fragments might be, where they had come from, and where they were going.
It quickly became apparent that the 21 nuclei in the S-L 9 string had all originally been part of a single, much more massive comet, probably between 10 and 20 kilometers in diameter.60 The largest fragment was estimated at 4.2 kilometers in diameter and others at 3 kilometers and 2 kilometers in diameter.61 As astronomers plotted their course and calculated their orbit backward it was discovered that “these nuclei had made a very close passage by Jupiter in July 1992.”62
Further investigations showed what must have happened: the original comet had approached too close to Jupiter, falling to an altitude of just 20,000 kilometers above its surface on 7 July 1992 and breaching the planet’s Roche limit. David Levy describes the effects this way:
Like a giant hand reaching up and pulling the comet apart, Jupiter’s gravity pulled on the closest part harder than it pulled on the most distant. As the comet started to stretch out like a noodle, with a shudder it simply became unglued.63
Only narrowly managing to avoid collision at that time, it seems that S-L 9 was torn out of its own long-distance orbit through the solar system by this encounter and forced instead into a perilously close-orbit around Jupiter.64 By mid-May 1993 astronomers had calculated that this orbit would bring the 21 fragments into an even closer encounter sometime in July 1994.65 Further calculations revealed that this next encounter would be so close that a collision was inevitable:
Although the comet fell apart in 1992, its pieces survived the graze with Jupiter, but only to buy a little time. The ancient comet would have one orbit left, a last chance to swing away from Jupiter, look back, and return again to crash into the planet.66
COMETS REALLY DO HIT PLANETS
Traveling at a speed of 60 kilometers a second, fragment A—one of the smallest—hit Jupiter on 16 July 1994 creating a gigantic plume of fire. A few hours later, fragment B, surmised to be a “loosely held-together group of dust and boulders,”67 produced a faint plume that lasted for 17 minutes.68 Two impacts separated by an interval of an hour were associated with fragment C, closely followed by a “short-lived fireball” associated with fragment D.69 The first large fragment was E. It hit at 11:17 Eastern Daylight Time, sending up a plume of material “more than 30 times the brightness of Europa” (one of Jupiter’s moons).70 As the initial atmospheric turbulence subsided it became clear that the fragment had opened up three huge scars in Jupiters swirling surface—including one bright spot with a diameter of more than 15,000 kilometers.71
Fragment F produced an even bigger impact scar with a diameter of 26,000 kilometers. Then, recounts David Levy, “the gates of hell opened as the central mass of fragment G blew up, leaving a mighty fireball soaring some 3,000 kilometers above the clouds.”72 The fireball rose at 17 kilometers per second and was fueled by superheated gas—twice as hot as the surface of the Sun.73
The impact ring created on Jupiter’s surface by fragment G was an equally turbulent feature. It expanded outward at the rate of 4 kilometers per second and soon reached a diameter of 33,000 kilometers74—just 7,000 kilometers less t
han the equatorial circumference of Earth. Within another hour it had grown into a spot so big that it could have swallowed Earth, and so bright that it outshone Jupiter’s own radiance and temporarily “blinded” telescopes.75
“I began to think about what all this meant,” remembers Gerrit Verschuur:
Given that fragment G was supposed to have been 4.2 kilometers across, and given that it was traveling at 60 kilometers per second, its impact energy would have been about 100 million megatons of TNT, something like the K/T impactor that wiped out the dinosaurs. And there it had happened on Jupiter in 1994! What now were the odds on it happening here? The impact produced the equivalent of 5 million Hiroshima-sized explosions going off simultaneously. Incredible! It wasn’t so long ago, back in 1991 at the First International Symposium on Near-Earth Asteroids in San Juan Capistrano, California, that I had heard it predicted that we would never see objects of this size slam into planets in our lifetimes.76
Gene Shoemaker was asked what he thought was the most important lesson learned from S-L 9. “Comets really do hit planets,” he replied.77
In an interview with the BBC in London, Caroline Shoemaker was asked to describe what would happen if a fragment like G were ever to hit the earth. Her reply was brief and to the point: “We would die.”78
20
Apocalypse Now
BY the time all 21 fragments of comet S-L 9 had buried themselves in the massive body of Jupiter, many people who had previously taken little interest in the sky began to look heavenward with feelings of vague anxiety. It took no more than common sense to realize that what had happened to Jupiter could just as easily have happened to Earth—and probably would one day. An old idea of using nuclear missiles to divert potentially dangerous comets or asteroids was revived and there was talk of adapting “Star Wars” technology to defend the earth. It was of course not an accident that only two days after the armageddon-like impact of fragment G, the House of Representatives wrote a clause into the NASA Authorization Bill (quoted in the last chapter) instructing the agency to “identify and catalogue the orbital characteristics of all comets and asteroids that are greater than 1 kilometer in diameter and are in an orbit around the Sun that crosses the orbit of the earth.”