"Comets are like cats. They have tails, and they do precisely what they want."
-- David Levy
"The history of cosmology is not the easy story of the rejection of absurd ideas
in favour of what (perhaps after a little thought) is seen to be patently true, but
the heroic saga of the hard-won rejection of the patently true in favour of the
-- Michael Hoskin
Ever since childhood I've been interested in the interaction of the history of astronomy with archaeology and mythology. I remain intrigued by the possibility that small solar-system bodies like comets, asteroids, and meteoroids have played a significant role in shaping the destiny of proto-historic and historic peoples all over all the world. In my youth in the 1960s I devoured the works of authors such as Hans Bellamy, Immanuel Velikovsky, Comyns Beaumont, and Ignatius Donnelly, each of whom suggested that mythology encoded catastrophic astronomical events resulting from decaying moons, wildly careening planets, or asteroidal and cometary impacts.
As I grew older and learned more about astronomy and physics, I realized the untenability of the physical scenarios embraced by these authors. Yet I could not shake the idea that the mythology, religion, and ritual of different people all over the world stemmed at least in part from astronomical causes, including catastrophic impacts.
The evidence for such impact events throughout the solar system is undeniable. We need only look to the surface of our Moon to see a myriad of craters left by impacting objects. But the Moon is not alone. During the past three decades, pictures sent back from space probes demonstrate that nearly every solid body in our Solar System exhibits craters. Planets like Mercury, Venus, and Mars; the moons of the giant outer planets Jupiter, Saturn, Uranus, and Neptune; and even asteroids like Toutatis and Vesta -- all show the scars left by impacts. Geologically active bodies like Jupiter's moon Io show few craters, while geologically inactive bodies like the Earth's moon are covered with craters.
The Earth also bears impact scars. At present scientists have identified nearly two hundred terrestrial impact craters. Three to four more craters are located each year. Most of these have only been discovered since 1950. Fewer visible craters appears on the Earth's surface as compared with most other solar system bodies because the Earth is so active geologically.
The heaviest bombardment of the Earth and Moon occurred early in the history of the solar system, prior to 3.2 billion years ago. (I always use billion in the sense of thousand million.) The impact flux has greatly decreased since that time, but asteroids and comets still strike the Earth on a regular basis. By counting the craters on the Moon, whose geologically inactive surface retains a complete record of bombardments over the past few billion years, we can determine the size and frequency of impacts over the long term. We can also use estimates of the total population of near-Earth asteroids and comets ("near-Earth objects" or "NEOs" for short) derived from search and rediscovery programs for near-Earth objects. What these tell us is that there are, on average, about a thousand objects one kilometer or larger in size that cross the Earth's orbit. As of August 2000 about half of these objects have been discovered.
The impact of a one kilometer object onto the Earth would have global effects. There may be more than five hundred thousand objects one hundred meters or larger in Earth crossing orbits. The impact of an object one hundred meters in size is sufficient to level a city. There may be an astonishing one hundred million objects greater than ten meters in size. A ten meter object striking the Earth at about 20 km/sec produces a one hundred kiloton blast. This is five times the yield of the atomic bombs dropped on Nagasaki and Hiroshima which were about 20 kilotons each. On average, the Earth is struck about once a year by an object releasing energy equivalent to a one hundred kiloton explosion. A one megaton impactor strikes once every ten to twenty years. In general, the energy from the smaller impactors is expended high in the atmosphere so no ground damage results.
Asteroid fragments generally strike the Earth at speeds between twelve and thirty six km/sec. Comets may strike at twice this speed, up to seventy two km/sec.
Of the seven hundred to one thousand near-Earth objects greater than one kilometer in diameter, about one third will eventually strike the Earth. A kilometer sized asteroid impacts with an energy equivalent to a million megatons of TNT. An impact of this size occurs about once every hundred thousand years or so on average. An all-out nuclear exchange with the current world arsenal would deliver about twenty thousand megatons. The impact of a kilometer sized object would deliver fifty times the maximum possible destructive power of a global nuclear war.
These long-term averages do not tell us whether the impact flux is uniform over time. It may exhibit large peaks and valleys with impacts clustered near each other in time. Many astronomers believe that there is long-term fifteen or thirty million year cycle to impacts as the result of galactic tides affecting comets in the Oort cloud believed to surround the solar system like a halo. A few astronomers like Victor Clube, Bill Napier, Duncan Steel, and D. J. Asher believe the impact flux varies on an even shorter cycle. They suggest that a giant comet (over fifty kilometers in diameter) arrives in the inner solar system every hundred thousand years on average. As the giant comet disintegrates, its debris may threaten the Earth with a greatly enhanced impact flux for a few years or decades every millenium or two. This notion of coherent catastrophism is highly controversial.
What happens when a comet or asteroid strikes the Earth? That depends upon the size of the impactor, its composition, its incoming speed, and whether it strikes the land or the sea. A sufficiently large impact can have global consequences by degrading climate world-wide, possibly even resulting in mass extinctions. In 1980 the father and son team of Louis and Walter Alvarez, along with their colleagues Frank Asaro and Helen V. Michel, proposed a giant impact model to explain the extinction of the dinosaurs. The dinosaurs, along with many other species of plants and animals, all died out at the Cretaceous-Tertiary boundary about sixty-five million years ago.
While the idea that a large impact might have effected the dinosaurs's demise wasn't new, the Alvarezes and their team offered something no one previously had: solid physical evidence for a large impact at the right time. Since 1980 the evidence has mounted in favor of the impact theory. The impact occurred on the Yucatán platform with the port city of Progreso, Mexico near the bullseye. The 200 kilometer (or larger) crater lies buried beneath 1,100 meters of limestone deposited since the time of the impact. This buried crater is now known as the Chicxulub structure. The impactor that formed the crater was probably about ten kilometers in diameter. Such "doomsday" objects strike the Earth only once every hundred million years on average.
Since 1980 scientists have looked for evidence that other extinction events may also be linked to impact events. Some scientists believe that all mass extinctions may have been caused by impacts. Other are more cautious and suggest that mass extinction may result from a variety of other causes besides large impacts.
An interesting counterpoint to the idea that cosmic impacts cause extinctions is that cometary impacts may have brought water and the building blocks of life -- or even life itself -- to Earth. Recent suggestions that meteorites, possibly of Martian origin, include exotic biotic materials has refocussed attention on the possible extraterrestrial origins of life.
The Earth is still threatened by cosmic impacts. Our best line of defense would be to mount a thorough search for as many near-Earth objects as possible. A few researchers are actively engaged in this search for near-Earth objects. Unfortunately, the total number worldwide is only about two dozen -- something to ponder considering the magnitude of the potential threat posed by cosmic impacts. At least public awareness of the problem has been raised over the past few years by a number of recent books, article, and television programs discussing the impact threat.
While many view asteroids and comets as a source of danger, others look to them as an untapped source of natural materials. The idea of mining asteroids has a long history in science fiction. Now organizations such as PERMANENT seek to turn science fiction to science fact by using asteroids as building blocks for cities and factories in space within the next few decades.
Cosmic impacts are not the only catastrophic occurrences to affect the Earth. Terrestrial catastrophes such as volcanos and earthquakes offer a more common source of danger. Both can unleash powerful tsunamis ("tidal waves") which can devastate coasts thousands of miles away from the center of the earthquake or volcanic eruption. In addition, powerful volcanos can inject enough ash into the upper atmosphere to form dust veils, just as large impacts can. These dust veils can lower temperatures and result in dimmer days, leading to crop failure, starvation, and forced population migration. Extensive, prolonged vulcanism may also play a part in mass extinctions.
If you find other sites with information about catastrophism, pro or con, please send me email telling me about them.
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Last modified by pib on July 17, 2010.