When I sat down to have my breakfast beside my plough, I heard sudden bangs, as if from gun-fire. My horse fell to its knees. From the north side above the forest a flame shot up.… Then I saw that the fir forest had been bent over by the wind and I thought of a hurricane. I seized hold of my plough with both hands, so that it would not be carried away. The wind was so strong that it carried off some of the soil from the surface of the ground, and then the hurricane drove a wall of water up the Angara. I saw it all quite clearly, because my land was on a hillside.
The roar frightened the horses to such an extent that some galloped off in panic, dragging the ploughs in different directions, and others collapsed.
The carpenters, after the first and second crashes, had crossed themselves in stupefaction, and when the third crash resounded they fell backwards from the building onto the chips of wood. Some of them were so stunned and utterly terrified that I had to calm them down and reassure them. We all abandoned work and went into the village. There, whole crowds of local inhabitants were gathered in the streets in terror, talking about this phenomenon.
I was in the fields … and had only just got one horse harnessed to the harrow and begun to attach another when suddenly I heard what sounded like a single loud shot to the right. I immediately turned round and saw an elongated flaming object flying through the sky. The front part was much broader than the tail end and its color was like fire in the day-time. It was many times bigger than the sun but much dimmer, so that it was possible to look at it with the naked eye. Behind the flames trailed what looked like dust. It was wreathed in little puffs, and blue streamers were left behind from the flames.… As soon as the flame had disappeared, bangs louder than shots from a gun were heard, the ground could be felt to tremble, and the window panes in the cabin were shattered.
… I was washing wool on the bank of the River Kan. Suddenly a noise like the fluttering of the wings of a frightened bird was heard … and a kind of swell came up the river. After this came a single sharp bang so loud that one of the workmen … fell into the water.
This remarkable occurrence is called the Tunguska Event. Some scientists have suggested that it was caused by a piece of hurtling antimatter, annihilated on contact with the ordinary matter of the Earth, disappearing in a flash of gamma rays. But the absence of radioactivity at the impact site gives no support to this explanation. Others postulate that a mini black hole passed through the Earth in Siberia and out the other side. But the records of atmospheric shock waves show no hint of an object booming out of the North Atlantic later that day. Perhaps it was a spaceship of some unimaginably advanced extraterrestrial civilization in desperate mechanical trouble, crashing in a remote region of an obscure planet. But at the site of the impact there is no trace of such a ship. Each of these ideas has been proposed, some of them more or less seriously. Not one of them is strongly supported by the evidence. The key point of the Tunguska Event is that there was a tremendous explosion, a great shock wave, an enormous forest fire, and yet there is no impact crater at the site. There seems to be only one explanation consistent with all the facts: In 1908 a piece of a comet hit the Earth.
In the vast spaces between the planets there are many objects, some rocky, some metallic, some icy, some composed partly of organic molecules. They range from grains of dust to irregular blocks the size of Nicaragua or Bhutan. And sometimes, by accident, there is a planet in the way. The Tunguska Event was probably caused by an icy cometary fragment about a hundred meters across—the size of a football field—weighing a million tons, moving at about 30 kilometers per second, 70,000 miles per hour.
If such an impact occurred today it might be mistaken, especially in the panic of the moment, for a nuclear explosion. The cometary impact and fireball would simulate all effects of a one-megaton nuclear burst, including the mushroom cloud, with two exceptions: there would be no gamma radiation or radioactive fallout. Could a rare but natural event, the impact of a sizable cometary fragment, trigger a nuclear war? A strange scenario: a small comet hits the Earth, as millions of them have, and the response of our civilization is promptly to self-destruct. It might be a good idea for us to understand comets and collisions and catastrophes a little better than we do. For example, an American Vela satellite detected an intense double flash of light from the vicinity of the South Atlantic and Western Indian Ocean on September 22, 1979. Early speculation held that it was a clandestine test of a low yield (two kilotons, about a sixth the energy of the Hiroshima bomb) nuclear weapon by South Africa or Israel. The political consequences were considered serious around the world. But what if the flashes were instead caused by the impact of a small asteroid or a piece of a comet? Since airborne overflights in the vicinity of the flashes showed not a trace of unusual radioactivity in the air, this is a real possibility and underscores the dangers in an age of nuclear weapons of not monitoring impacts from space better than we do.
A comet is made mostly of ice—water (H2O) ice, with a little methane (CH4) ice, and some ammonia (NH3) ice. Striking the Earth’s atmosphere, a modest cometary fragment would produce a great radiant fireball and a mighty blast wave, which would burn trees, level forests and be heard around the world. But it might not make much of a crater in the ground. The ices would all be melted during entry. There would be few recognizable pieces of the comet left—perhaps only a smattering of small grains from the non-icy parts of the cometary nucleus. Recently, the Soviet scientist E. Sobotovich has identified a large number of tiny diamonds strewn over the Tunguska site. Such diamonds are already known to exist in meteorites that have survived impact, and that may originate ultimately from comets.
On many a clear night, if you look patiently up at the sky, you will see a solitary meteor blazing briefly overhead. On some nights you can see a shower of meteors, always on the same few days of every year—a natural fireworks display, an entertainment in the heavens. These meteors are made by tiny grains, smaller than a mustard seed. They are less shooting stars than falling fluff. Momentarily brilliant as they enter the Earth’s atmosphere, they are heated and destroyed by friction at a height of about 100 kilometers. Meteors are the remnants of comets.* Old comets, heated by repeated passages near the Sun, break up, evaporate and disintegrate. The debris spreads to fill the full cometary orbit. Where that orbit intersects the orbit of the Earth, there is a swarm of meteors waiting for us. Some part of the swarm is always at the same position in the Earth’s orbit, so the meteor shower is always observed on the same day of every year. June 30, 1908 was the day of the Beta Taurid meteor shower, connected with the orbit of Comet Encke. The Tunguska Event seems to have been caused by a chunk of Comet Encke, a piece substantially larger than the tiny fragments that cause those glittering, harmless meteor showers.
Comets have always evoked fear and awe and superstition. Their occasional apparitions disturbingly challenged the notion of an unalterable and divinely ordered Cosmos. It seemed inconceivable that a spectacular streak of milk-white flame, rising and setting with the stars night after night, was not there for a reason, did not hold some portent for human affairs. So the idea arose that comets were harbingers of disaster, auguries of divine wrath—that they foretold the deaths of princes, the fall of kingdoms. The Babylonians thought that comets were celestial beards. The Greeks thought of flowing hair, the Arabs of flaming swords. In Ptolemy’s time comets were elaborately classified as “beams,” “trumpets,” “jars” and so on, according to their shapes. Ptolemy thought that comets bring wars, hot weather and “disturbed conditions.” Some medieval depictions of comets resemble unidentified flying crucifixes. A Lutheran “Superintendent” or Bishop of Magdeburg named Andreas Celichius published in 1578 a “Theological Reminder of the New Comet,” which offered the inspired view that a comet is “the thick smoke of human sins, rising every day, every hour, every moment, full of stench and horror before the face of God, and becoming gradually so thick as to form a comet, with curled and plaited tresses, which at last is kindled by the hot and
fiery anger of the Supreme Heavenly Judge.” But others countered that if comets were the smoke of sin, the skies would be continually ablaze with them.
The most ancient record of an apparition of Halley’s (or any other) Comet appears in the Chinese Book of Prince Huai Nan, attendant to the march of King Wu against Zhou of Yin. The year was 1057 B.C. The approach to Earth of Halley’s Comet in the year 66 is the probable explanation of the account by Josephus of a sword that hung over Jerusalem for a whole year. In 1066 the Normans witnessed another return of Halley’s Comet. Since it must, they thought, presage the fall of some kingdom, the comet encouraged, in some sense precipitated, the invasion of England by William the Conqueror. The comet was duly noted in a newspaper of the time, the Bayeux Tapestry. In 1301, Giotto, one of the founders of modern realistic painting, witnessed another apparition of Comet Halley and inserted it into a nativity scene. The Great Comet of 1466—yet another return of Halley’s Comet—panicked Christian Europe; the Christians feared that God, who sends comets, might be on the side of the Turks, who had just captured Constantinople.
The leading astronomers of the sixteenth and seventeenth centuries were fascinated by comets, and even Newton became a little giddy over them. Kepler described comets as darting though space “as the fishes in the sea,” but being dissipated by sunlight, as the cometary tail always points away from the sun. David Hume, in many cases an uncompromising rationalist, at least toyed with the notion that comets were the reproductive cells—the eggs or sperm—of planetary systems, that planets are produced by a kind of interstellar sex. As an undergraduate, before his invention of the reflecting telescope, Newton spent many consecutive sleepless nights searching the sky for comets with his naked eye, pursuing them with such fervor that he felt ill from exhaustion. Following Tycho and Kepler, Newton concluded that the comets seen from Earth do not move within our atmosphere, as Aristotle and others had thought, but rather are more distant than the Moon, although closer than Saturn. Comets shine, as the planets do, by reflected sunlight, “and they are much mistaken who remove them almost as far as the fixed stars; for if it were so, the comets could receive no more light from our Sun than our planets do from the fixed stars.” He showed that comets, like planets, move in ellipses: “Comets are a sort of planets revolved in very eccentric orbits about the Sun.” This demystification, this prediction of regular cometary orbits, led his friend Edmund Halley in 1707 to calculate that the comets of 1531, 1607 and 1682 were apparitions at 76-year intervals of the same comet, and predicted its return in 1758. The comet duly arrived and was named for him posthumously. Comet Halley has played an interesting role in human history, and may be the target of the first space vehicle probe of a comet, during its return in 1986.
Modern planetary scientists sometimes argue that the collision of a comet with a planet might make a significant contribution to the planetary atmosphere. For example, all the water in the atmosphere of Mars today could be accounted for by a recent impact of a small comet. Newton noted that the matter in the tails of comets is dissipated in interplanetary space, lost to the comet and little by little attracted gravitationally to nearby planets. He believed that the water on the Earth is gradually being lost, “spent upon vegetation and putrefaction, and converted into dry earth.… The fluids, if they are not supplied from without, must be in a continual decrease, and quite fail at last.” Newton seems to have believed that the Earth’s oceans are of cometary origin, and that life is possible only because cometary matter falls upon our planet. In a mystical reverie, he went still further: “I suspect, moreover, that it is chiefly from the comets that spirit comes, which is indeed the smallest but the most subtle and useful part of our air, and so much required to sustain the life of all things with us.”
As early as 1868 the astronomer William Huggins found an identity between some features in the spectrum of a comet and the spectrum of natural or “olefiant” gas. Huggins had found organic matter in the comets; in subsequent years cyanogen, CN, consisting of a carbon and a nitrogen atom, the molecular fragment that makes cyanides, was identified in the tails of comets. When the Earth was about to pass through the tail of Halley’s Comet in 1910, many people panicked. They overlooked the fact that the tail of a comet is extravagantly diffuse: the actual danger from the poison in a comet’s tail is far less than the danger, even in 1910, from industrial pollution in large cities.
But that reassured almost no one. For example, headlines in the San Francisco Chronicle for May 15, 1910, include “Comet Camera as Big as a House,” “Comet Comes and Husband Reforms,” “Comet Parties Now Fad in New York.” The Los Angeles Examiner adopted a light mood: “Say! Has That Comet Cyanogened You Yet?… Entire Human Race Due for Free Gaseous Bath,” “Expect ‘High Jinks,’ ” “Many Feel Cyanogen Tang,” “Victim Climbs Trees, Tries to Phone Comet.” In 1910 there were parties, making merry before the world ended of cyanogen pollution. Entrepreneurs hawked anti-comet pills and gas masks, the latter an eerie premonition of the battlefields of World War I.
Some confusion about comets continues to our own time. In 1957, I was a graduate student at the University of Chicago’s Yerkes Observatory. Alone in the observatory late one night, I heard the telephone ring persistently. When I answered, a voice, betraying a well-advanced state of inebriation, said, “Lemme talk to a shtrominer.” “Can I help you?” “Well, see, we’re havin’ this garden party out here in Wilmette, and there’s somethin’ in the sky. The funny part is, though, if you look straight at it, it goes away. But if you don’t look at it, there it is.” The most sensitive part of the retina is not at the center of the field of view. You can see faint stars and other objects by averting your vision slightly. I knew that, barely visible in the sky at this time, was a newly discovered comet called Arend-Roland. So I told him that he was probably looking at a comet. There was a long pause, followed by the query: “Wash’ a comet?” “A comet,” I replied, “is a snowball one mile across.” There was a longer pause, after which the caller requested, “Lemme talk to a real shtrominer.” When Halley’s Comet reappears in 1986, I wonder what political leaders will fear the apparition, what other silliness will then be upon us.
While the planets move in elliptical orbits around the Sun, their orbits are not very elliptical. At first glance they are, by and large, indistinguishable from circles. It is the comets—especially the long-period comets—that have dramatically elliptical orbits. The planets are the old-timers in the inner solar system; the comets are the newcomers. Why are the planetary orbits nearly circular and neatly separated one from the other? Because if planets had very elliptical orbits, so that their paths intersected, sooner or later there would be a collision. In the early history of the solar system, there were probably many planets in the process of formation. Those with elliptical crossing orbits tended to collide and destroy themselves. Those with circular orbits tended to grow and survive. The orbits of the present planets are the orbits of the survivors of this collisional natural selection, the stable middle age of a solar system dominated by early catastrophic impacts.
In the outermost solar system, in the gloom far beyond the planets, there is a vast spherical cloud of a trillion cometary nuclei, orbiting the Sun no faster than a racing car at the Indianapolis 500.* A fairly typical comet would look like a giant tumbling snowball about 1 kilometer across. Most never penetrate the border marked by the orbit of Pluto. But occasionally a passing star makes a gravitational flurry and commotion in the cometary cloud, and a group of comets finds itself in highly elliptical orbits, plunging toward the Sun. After its path is further changed by gravitational encounters with Jupiter or Saturn, it tends to find itself, once every century or so, careering toward the inner solar system. Somewhere between the orbits of Jupiter and Mars it would begin heating and evaporating. Matter blown outwards from the Sun’s atmosphere, the solar wind, carries fragments of dust and ice back behind the comet, making an incipient tail. If Jupiter were a meter across, our comet would be smaller than a speck of
dust, but when fully developed, its tail would be as great as the distances between the worlds. When within sight of the Earth on each of its orbits, it would stimulate outpourings of superstitious fervor among the Earthlings. But eventually they would understand that it lived not in their atmosphere, but out among the planets. They would calculate its orbit. And perhaps one day soon they would launch a small space vehicle devoted to exploring this visitor from the realm of the stars.
Sooner or later comets will collide with planets. The Earth and its companion the Moon must be bombarded by comets and small asteroids, debris left over from the formation of the solar system. Since there are more small objects than large ones, there should be more impacts by small objects than by large ones. An impact of a small cometary fragment with the Earth, as at Tunguska, should occur about once every thousand years. But an impact with a large comet, such as Halley’s Comet, whose nucleus is perhaps twenty kilometers across, should occur only about once every billion years.
When a small, icy object collides with a planet or a moon, it may not produce a very major scar. But if the impacting object is larger or made primarily of rock, there is an explosion on impact that carves out a hemispherical bowl called an impact crater. And if no process rubs out or fills in the crater, it may last for billions of years. Almost no erosion occurs on the Moon and when we examine its surface, we find it covered with impact craters, many more than can be accounted for by the rather sparse population of cometary and asteroidal debris that now fills the inner solar system. The lunar surface offers eloquent testimony of a previous age of the destruction of worlds, now billions of years gone.