CHAPTER 7: HISTORIC IMPACTS

We know that, statistically, large impacts like the one that terminated the Cretaceous era occur at intervals of tens of millions of years. That is the typical interval between mass extinctions in the fossil record as well as the expected average interval between impacts by objects larger than 5 km in diameter. Most of the larger terrestrial craters discussed in Chapter 2 are also tens of millions of years old. But it is also clear to astronomers who study comets and asteroids that there are a great many more small objects than large ones, so the frequency of smaller impacts must be much higher. In Chapter 2 we showed the standard curve for estimating the typical intervals between impacts of various sizes. According to that graph, the Earth should experience an impact with the energy of the Hiroshima bomb (15 kilotons) every few months, and every few centuries we should expect to be hit by an impact with the energy of the largest nuclear weapons (15 megatons). Do these predictions make sense?

If the Earth is regularly subject to impacts the size of nuclear bombs, we should know about it. There must be historical records of such impacts, or current evidence of massive explosions in the high atmosphere for objects that break up before reaching the surface. If no such evidence exists, then the entire impact hypothesis would be in question (and our publisher would, correctly, have refused to release this book). The subject of this chapter is the contemporary and historical evidence for the continuing cosmic bombardment of our planet.

We begin with the records of recorded history and their prehistorical antecedents in mythology and oral tradition. We must approach this subject with caution, however. The ancient records are easily misinterpreted, and some investigators have gone astray when trying to base scientific conclusions on such sources. Many religious documents, in particular, were written and preserved for their religions message and not as accurate representations of historical events.

The most notorious effort in this century to extract scientific fact from ancient manuscripts is that of the Russian-American psychologist Immanuel Velikovsky. In his 1950 book Worlds in Collision, Velikovsky argued that the Earth has experienced many catastrophes in historic times caused by interplanetary collisions. Based on an extensive but uncritical reading of the ancient records, Velikovsky concluded that catastrophic myths (such as the plagues of Egypt and parting of the Red Sea in the Jewish tradition) were synchronized in widely separated cultures, thus proving that the events were global in scale. He attributed these global catastrophes to several hypothetical events: the ejection of Venus from Jupiter about five millennia ago, its subsequent near-collisions with the Earth in the second millennium BCE, and several close encounters among Venus, Mars, and Earth in the first millennium BCE. He concluded that these violent events had stopped the rotation of the Earth and reversed its direction of spin, had altered its orbit about the Sun, and had melted the surface of the Moon, among other things -- all effects that he justified from his interpretation of myth.

Velikovsky's ideas were unanimously rejected by scientists but received wide attention from the public and news media. The scientific community was accused of trying to suppress Velikovsky's ideas, but if so they were unsuccessful, since Worlds in Collision became a best seller. Only after the direct exploration of the planets by space probes contradicted Velikovsky's predictions did interest in his ideas subside, although there remains a small cult of true believers who still defend Velikovsky and even publish a journal devoted to carrying on his work. For nearly half a century, however, scientists have been suspicious of anything that smacks of "Velikovskyis­m".

While Velikovsky's idea of planets bouncing about the solar system in historic times is absurd, we cannot be sure that there were no cosmic catastrophes in the history of human civilization. We now know that catastrophes can be produced by the impact of relatively small asteroids and comets. Thus it may well be that information on long-ago impacts or meteorite falls can be extracted from the mythological and historical record.

Among the longest and most continuous such records are those kept by the Chinese imperial courts, which date back nearly three thousand years. Taken at face value, some of these documents appear to describe large meteorite falls, of a scale sufficient to destroy a village or even inflict major casualties on an army. In several cases whole armies were struck down by falling stones, perhaps like the Biblical account of the destruction of the Assyrian army of Sennacherib, so eloquently described in the poem by Byron which begins "The Assyrian came down like the wolf on the fold / His cohorts all gleaming with silver and gold". We know that stones (meteorites) do fall from the sky even today, and that they frequently fall in showers that result when an incoming projectile breaks up under the stress of atmospheric deceleration. The main reason to be skeptical of the Chinese reports is their uniqueness. There are no other records of similar meteorite falls in other cultures. We must ask why were so many ancient Chinese armies hit by falling stones, while nothing like that happened to larger and more recent armies, such as those deployed across Europe during the Napoleonic Wars or the World Wars of the 20th century? Is there something about ancient Chinese armies that attracted meteorites? Or are we perhaps misinterpreting the records? We do not know.

There are also descriptions in some ancient documents that might correspond to major impact explosions rather than meteorite falls. The Biblical account of the destruction of Sodom and Gomorrah can be interpreted in these terms, but since the sites of Sodom and Gomorrah have never been identified, there is no way to acquire confirming physical evidence. Some historians have associated the destruction of Atlantis in Greek myth with impacts, but the Atlantis story seems more closely allied to volcanic events, which were familiar to the inhabitants of the Mediterranean in the second millennium BCE.

A much broader look at the legends of the past has been undertaken recently by British astronomer Victor Clube and his collaborators. Clube asks how the idea arose in so many cultures during the second millennium BCE that the gods lived in the sky, and that events in the sky influenced human lives? This was the period in which astrology arose in both Eastern and Western Asia, a religion based on the notion that our lives on Earth are influenced by the positions of the stars and planets. Today this is obviously an absurd idea, but apparently astrologer-priests searched the sky for omens and portents in many cultures of the first and second millennium BCE. What was going on then?

Clube answers these questions with a sweeping hypothesis that our ancestors lived in a world where destructive impacts were much more common than today, and where events in the sky really did have profound influence on the terrestrial realm. He suggests that the breakup of a giant comet in the inner solar system created thousands of large interplanetary projectiles in addition to streams of meteorites and cosmic dust, and that the collision of some of these projectiles and dust streams with the Earth had important influences on climate, history and culture.

Unfortunately for Clube, there is little specific historical or physical data to support his theory. Interest in the sky was not universal in ancient cultures; for example, the Egyptian civilization, probably the most advanced at the time, apparently had limited interest in the sky beyond worship of the Sun. Further, the time period in which rather reliable record keeping began in China and the Mediterranean, about 2500 years ago, seems to coincide with the end of the era of cosmic catastrophes that Clube postulates. These more reliable records do not support the idea of constant danger from celestial events, which is based primarily on earlier myths. According to Clube, we "remember" this earlier catastrophic era through the cultural-religious legacy of astrology, heavenly angels (which he says were originally comets), and planets that were identified as gods like Zeus and Aries. But the fact that the catastrophes apparently stopped just when record-keeping improved is suspicious.

Clube and his collaborators (primarily British astronomers Bill Napier, Mark Bailey, and Duncan Steel) have been widely quoted in newspaper stories and television documentaries, in part because they predict an especially intense period of impacts about a thousand years in the future. Remnants of the giant comet that they believe broke up some hundred centuries ago can still be identified with a dust stream that intersects the Earth's orbit in June and November to produce the Taurid meteor showers. They propose that denser concentrations of dust plus many large objects are imbedded in this dust stream, and that the "Taurid complex" threatens the very survival of our civilization a few centuries hence when the Earth will encounter this material. If they are correct, we are currently experiencing a lull between the intense bombardment of three millennia ago and another period of chaos soon to come. The idea of an extensive Taurid complex is not accepted by most astronomers, however, and the pre-historical interpretations are vague, leaving us with little hard evidence for impacts in ancient history.

Although the ancient historical records are frustratingly ambiguous, our situation becomes much clearer when we turn to the evidence from the 20th century. The first unequivocal example of a destructive cosmic impact dates from 1908. Although it happened in a sparsely populated wilderness, this event was witnessed directly, recorded by scientific instruments worldwide, and subsequently investigated by a number of scientific expeditions.

The scene on that morning of June 30, 1908, is set in the taiga forests of central Siberia, in an outlying part of the Empire of Tsar Nicholas II. The nearest populated areas were along the newly completed Trans-Siberian Railway, which passed through the cities of Krasnoyarsk and Irkutsk, about 1000 km to the south and south-east of the impact site. The site itself was located along the Podkamennia (Stony) Tunguska River, a tributary of the mighty Yenesei River. The impact area is generally known by its shortened form as Tunguska.

The incoming object was first seen by trappers and hunters near the northern end of Lake Baikal, as a brilliant white fireball traveling in a north-west direction at high speed. Shortly after the fireball disappeared over the horizon, distant observers witnessed an explosion and "flame" and "smoke" near the apparent point of impact. At the settlement of Varanova 60 km south of the impact point, a flash of intense, burning heat was followed by an earthquake and blast that shattered windows, shook goods off the shelves in the general store, and knocked one man from his chair, sending him sprawling on the ground. A native hunter about 20 km west of "ground zero" was thrown against a tree with such violence that he was pierced by the branches and died from his wounds. While there are no other records of direct human casualties, it was later reported that several small herds of reindeer were killed and that nomad shelters near the impact were destroyed by the heat and explosion.

The sound of the Tunguska explosion was heard in Krasnoyarsk and Irkutsk, and the earthquake was recorded at the Irkutsk Magnetic and Meteorological Observatory. The attenuated atmospheric blast wave itself was picked up on a variety of sensitive instruments called microbarographs, which had recently been installed in Germany, England, and Russia. The Potsdam microbarograph station near Berlin recorded both the direct wave traveling from Tunguska and a second wave that arrived from the opposite direction nearly 24 hours later after circumnavig­ating the world. Most of these records went unnoticed at the time and were not located until the 1930s, after the Tunguska event was more widely recognized. From the measurements of these pressure waves, scientists estimated that the energy released into the atmosphere was at least 10 megatons.

One of the phenomena associated with the Tunguska event was the bright night skies seen over Europe following the impact. At several locations in Russia, it was reported that a newspaper could be read at night without artificial light. These strange white nights were widely reported at the time, and the effect seems to have persisted in diminished form for more than a month after the impact. The exact cause is still unknown, but it may be related to dust injected into the stratosphere and carried rapidly westward by motions in the upper atmosphere.

The Tunguska impact took place in a remote wilderness area, and it was not until many years later that the first scientific expedition was dispatched. In 1927, an expedition of the USSR Academy of Sciences, led by L.A. Kulik, reached the site of flattened forest known previously only from rumors. They discovered that the trees were uprooted over a huge area, all pointing away from the presumed location of the explosion. There was no obvious crater, but Kulik suspected that a large marshy depression 7-10 km across might be the remains of an impact structure. Forced by bad weather and lack of supplies to retreat after a cursory inspection, Kulik returned with larger expeditions in 1928 and 1929-30. Since this permafrost country becomes marshy and mosquito-infested in summer, most of the expeditions were made in cold weather; contemporary pictures show a heavily bearded Kulik wrapped in fur-lined skins, while his baggage train of loaded reindeer stretches across the snow-covered land. You had to be tough to undertake scientific expeditions in Siberia then!

Kulik's primary objective was to recover the presumed meteorite, much like the objective of Daniel Barringer, digging at the same time at Meteor Crater in Arizona. The Soviet scientists excavated in the central marshy depression and sank several boreholes in other locations, but found only peat and permafrost. No meteorite was recovered, nor any convincing evidence of craters. Kulik concluded that the explosion must have taken place before the meteorite hit. Later estimates from the distribution of downed trees suggested that the height of the explosion was about 8 km, and the energy released was about 15 megatons. (Published estimates of the force of the Tunguska blast range from 5 to 50 megatons, but we will stick to the standard value of 15 megatons, about the same as that of the Meteor Crater blast 50,000 years earlier in Arizona).

From its energy, the mass of the Tunguska impactor can be estimated at hundreds of thousands of tons, but the fate of this meteoric material has remained a mystery. Presumably it was distributed over a wide area by the airburst, and if an expedition had reached the site immediately a great deal of this fine meteoric dust could have been collected. After 19 years of rain and weathering, however, the 1927 expedition had no hope of recovering the material. It was not until 1994 that the first microscopic fragments of the Tunguska projectile were extracted from resin in trees that were exposed to the explosion.

The basic facts about the Tunguska event are rather simple: an object entered the atmosphere at high speed and descended at an angle of about 45 degrees, radiating intense heat and ending its path with an explosion at 8 km altitude equivalent to a 15-megaton nuclear weapon. The explosion knocked down trees but left no trace that could be recovered 19 years later. What could have caused this unprecedented event?

Several astronomers suggested in the 1930s that the Tunguska explosion was caused by the collision of a small comet with the Earth. At the time, comets were generally thought to be loose agglomerations of icy rubble like a flying gravel pile. It seemed reasonable to suppose that such an insubstantial body might break apart during its fiery plunge through the atmosphere. The ice and other volatile materials would be vaporized by the heat of entry, and the shattered fragments of dust or rock dispersed. Although not supported by any detailed calculations, this seemed like a reasonable scenario, and it is still to be found in most astronomy texts or other accounts of the Tunguska events.

However, there is something about Tunguska that inspires less prosaic explanations. The absence of recovered fragments augmented by the remoteness of the site has endowed the Tunguska event with an aura of mystery. In Russia even today, there are many amateur and professional scientists who disbelieve the official stories and speculate on various exotic causes of the event. It has even been noted that a disproportionate number of scientists in Soviet mental asylums seemed preoccupied with the Tunguska event.

One bizarre explanation for Tunguska surfaced after the 1947 "discovery" of unidentified flying objects by the American news media. Some people suggested that the Tunguska event was due to a crashed UFO that lost control and was destroyed upon entering the atmosphere. In some variants of this idea, the explosion was caused by the detonation of the nuclear power source of the alien spacecraft. As recently as 1994, we heard a member of the Russian Academy of Sciences present a paper in which he argued that the recovery of the forest at Tunguska showed biological anomalies similar to those of the ecology near former nuclear test sites, darkly hinting that a nuclear explosion of some sort was implicated.

Another Tunguska theory, although perhaps not a serious one, was that the impacting object was a mini-black hole. In the 1970s British physicist Steven Hawking had postulated the existence of such black holes not much larger than single atoms but containing substantial mass and energy. If such exotic things exist (and there is no evidence they do), then perhaps the impact of such an object travelling at nearly the speed of light could precipitate an explosion while leaving no physical trace of its passage.

What appears to be the correct explanation was proposed in 1992 by NASA scientists Chris Chyba and Kevin Zahnle, together with Paul Thomas of the University of Wisconsin. Chyba, who later was selected as a Presidential Fellow and honored by Time magazine as one of the 50 most promising young Americans for 1994, had investigated in his doctoral thesis the effects on the Earth of its early bombardment by comets, seeking to determine how much water and organic material comets might have brought to our planet. Zahnle, for his part, was working on mathematical models for the breakup of large meteorites. They were bothered by one part of the comet impact theory: if Tunguska was due to a comet impact, then why do we not see evidence elsewhere on Earth of recent impacts from the much more common stony projectiles? Why weren't there lots of small craters due to impacts by such mini-asteroids?

Chyba, Thomas, and Zahnle calculated what would happen if projectiles of different composition and strength, but each with 15 megatons of energy, entered the atmosphere. They developed a quantitative model for the effects of atmospheric drag on a projectile, showing how it would fragment and disperse from the stress of hypersonic entry. Depending on its strength, any object will begin to break up when the air resistance is high enough; smaller objects then disperse explosively, while larger ones hang together until they penetrate deeper or strike the surface. These calculations showed that 15-megaton comets (loosely bound volatiles and dust) come apart very high in the atmosphere, disintegrating at altitudes near 50 km. At the other extreme, 15-megaton iron objects are only slightly slowed by the atmosphere and crash, whole, into the ground. Stony materials, however, disintegrate at altitudes of 8-10 km, producing airbursts very similar to that observed at Tunguska. They concluded, therefore, that the Tunguska impactor was a common stony asteroid with a mass of half a million tons and a diameter of about 60 meters -- about the size of a 20-story building. Another team of scientists from Los Alamos National Laboratory reached the same conclusion and published their similar calculations at the same time.

The new asteroidal theory for Tunguska was also consistent with other evidence. It predicted that comets with 15 megatons of energy would burn up at such high altitudes that there would be no measurable blast wave at the ground. The more common stony projectiles would, like Tunguska, penetrate to the lower atmosphere where the explosions could do great harm, but they would make no craters. At 15 megatons, only the very rare irons would form craters. In fact, all the small craters that have been studied were formed by iron projectiles rather than stony objects. An example is Meteor Crater in Arizona, which was caused by an object with the same energy -- 15 megatons -- as Tunguska, but one made of iron rather than stone. In order to reach the surface and make a crater, a stony projectile must to be more than 100 m in diameter (energy greater than 100 megatons).

Tunguska is the prototypical contemporary impact, the only major destructive impact explosion witnessed in modern history. According to the standard frequency curve for impacts (see Chapter 2), we can expect a 15-megaton impact on Earth about once every 300 years, which translates to an average interval of nearly a millennium between events of this size on land. We are thus fortunate to have witnessed such an impact in the present century, and it is no surprise that Tunguska remains unique in history. Smaller impacts occur more frequently, however, and we should also see some evidence of these.

Probably the most dramatic impact event since Tunguska took place on February 12, 1947. Once again the target was Siberia. (The Russians often point out that they have a special interest in impacts, since their country seems to be a preferred target.) This strike occurred in the Sikote-Aline mountains of western Siberia, about 400 km from the port of Vladivostok, and it is referred to as the Sikote-Aline meteorite fall.

The projectile entered the atmosphere in broad daylight and was witnessed by many people, who reported that it was brighter than the Sun. The fireball left a dark column of dust or smoke that remained visible for many minutes after its passage. After the disappearance of the fireball, loud detonations were heard, followed by rumblings and roarings like distant thunder. Russian scientists reached the site of the impact a few days later, finding a total of 122 small craters distributed over an area about 1 km wide and 3 km long. The four largest craters were each about 20 m across, or the size of a 60-foot house lot. The craters were irregular in shape and did not resemble explosion pits, and the surrounding forest was largely untouched.

The Russian scientists recovered 12 tons of iron meteorites of all sizes, including one individual object weighing nearly 2 tons. From the appearance of the recovered fragments, they concluded that many of the meteorites had broken apart upon striking the ground and had not been disrupted in flight. They inferred that the original mass of meteoritic metal was close to 100 tons at entry, which would correspond to an energy of about 10 kilotons.

From all the evidence, we can conclude that Sikote-Aline was an iron meteorite that had only begun to break up at the time it hit the surface. Being so small, no bigger than a truck in size, the incoming body had been slowed by atmospheric friction so that it hit the ground at relatively low speed. Since nearly all of the 10 kilotons of its original energy had been dissipated in the atmosphere, there was no explosion, and the damage was largely confined to the direct points of impact of the various fragments.

A stony asteroid of comparable energy entered the atmosphere over Revelstoke National Park in Canada in 1965. As we might expect, the incoming projectile broke up in the atmosphere, spreading small fragments over a wide area. Since it was winter and fresh snow had recently fallen, scientists recovered many of these small stony fragments from the ground. From microbarograph measurements, the energy of the Revelstoke object was measured to be 12 kilotons, but only a tiny fraction of the material from the projectile was ever recovered.

In addition to these and other recorded meteorite falls, there was one impactor that, like the proverbial fish, "got away". On August 10, 1972, tourists in the Teton-Yellowstone National Parks were startled to see a bright point of light moving across the daytime sky from south to north trailing a luminous column like the contrail of a jet. Hundreds of people photographed this strange, silent intruder, and one beautiful series of 8-mm movies was also obtained. The object was tracked for hundreds of miles as it headed across the border into Canada, and it was also observed by a U.S. Defense Department satellite in space. Apparently, however, it never fell to the ground, but passed out of the atmosphere having lost only a little of its energy to atmospheric friction. Various estimates suggest that the diameter of the projectile was about 10 m (energy several hundred kilotons), and that it never got lower than about 60 km altitude. Had it come lower, it would presumably have broken up in the atmosphere and might have produced a meteorite fall like Sikote-Aline or Revelstoke.

Upper atmosphere explosions of kiloton or greater yield should be visible from space, and the standard frequency curve for impacts predicts many of these. As long ago as 1972, there were surveillance satellites in space capable of detecting an intruder like the Teton fireball. Since then these capabilities have steadily increased. During the 1991 Gulf War it was reported that surveillance satellites could detect the firing of individual scud missiles from distances of more than 1000 km. Surely these systems must also detect the atmospheric entry of fireballs, and such data can be used to check our theories about the frequency of such events and the way these objects break up in the atmosphere. However, it has not proved easy for scientists to gain access to this information.

The U.S. surveillance satellites are operated by the Air Force Space Command from locations in Colorado and California. The main complex for these activities is buried deep within Cheyenne Mountain outside Colorado Springs, Colorado, with a secondary control center at Onizuka Air Force Base in Sunnyvale, California. These are among the most sensitive and secure elements of the national defense system. Onizuka Air Force Base, known locally as the Blue Cube, is in a 9-story windowless building constructed to withstand earthquakes and natural disasters and protected from terrorist attacks by a wall and ditch that resemble the defense works of a medieval castle. Cheyenne Mountain is, as the name implies, constructed in caves dug out of the mountain and supposedly safe even against a direct nuclear hit.

The nerve center for this intelligence operation is at Falcon Air Force Base, located in the prairies east of Colorado Springs. Here, again, the level of security is daunting to the visitor. The complex is surrounded by double chain-link and razor-wire fences, and the space between the fences is swept by microwaves to detect the motion of any intruder. Heavily armed soldiers guard the gates, which are specially constructed so that no vehicle could crash through, and the staff are screened by the most advanced electronic identification systems, including one that reads the pattern of veins in the eye's retina for positive identification of each individual who enters. Behind these secure boundaries, the data from a constellation of surveillance systems are scanned for evidence of nuclear attack against the United States or its allies.

Both visible-light and infrared sensors continually scan the Earth from space, and these systems "see" all of the large meteors or fireballs that enter the atmosphere. However, the software that screens and analyzes these data has been constructed to remove signals that are not relevant to the defense mission of the orbital surveillance systems. From the military perspective, natural events such as meteors are "noise" that should be filtered out. Therefore, no systematic records have been kept of fireball data. Nevertheless, individual operators over the years have noted bright fireballs, and gradually a fair amount of information has accumulated on these events.

The first public discussion of these results did not come about until 1993, after the collapse of the Soviet Union, and the first technical papers were not published until 1994. Between 1975 and 1992, it is reported that 136 atmospheric impacts were recorded worldwide by either the nuclear burst monitoring system (visible light) or a scanning infrared sensor, both operated by the U.S. Department of Defense. Since 1992, the rate of reporting for such detections has increased. The largest event so far took place on February 1, 1994, when a cosmic projectile entered the atmosphere over the Western Pacific, within the territory of the island nation of Micronesia. The object, with an energy later estimated at about 100 kilotons (more than 5 times larger than the Hiroshima bomb), began to break up at an altitude of 35 km, and several fragments were tracked by 6 different optical and infrared sensor systems in space. The largest of these fragments penetrated to 21 km before exploding. Two eyewitnesses were later located from the island of Palau, and these fishermen reported that the final explosion was "brighter than the Sun" but that no sound or concussions were noticed. Chyba and Zahnle applied their model for the Tunguska event to this fireball and concluded that its behavior was consistent with a stony object -- basically a miniature Tunguska, consisting of a house-sized boulder only about 10 m in diameter rather than 60 m like Tunguska.

The space surveillance records are incomplete, and it is estimated that the majority of the impacts might have gone unreported by the Air Force. In 1996 a second independent record of atmospheric explosions was located in previously classified documents, this one based or arrays of low-frequency sound detectors that had been placed around the globe to "hear" nuclear test explosions. As expected, these sonic records include more than twice as many "events" as the satellite data, and they are interpreted to indicate at least one atmospheric explosion per month of yield 10 kilotons or greater.

Putting all of this information together, we can formulate a consistent picture of the current impact environment of the Earth. The most common impacts are by stony objects, with airbursts like Tunguska expected somewhere on the land area of the planet about once per millennium. The fact that history records only one Tunguska-scale event is consistent with the time and area of the Earth covered by historical records. Smaller impacting bodies (except for the rare metallic projectiles) do not penetrate to the lower atmosphere but explode harmlessly at high altitudes, and these events are being detected by surveillance satellites even though most go unnoticed by ground-based observers. Based on the data that have been released, the Earth experiences a Hiroshima-scale high-altitude atmospheric explosion several times per year, consistent with predictions by the astronomers. There is no evidence that we live in "special" times of cosmic onslaught, and the current impact rate appears to be similar to the long-term average derived from observations of the lunar craters.
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