In 1982, two British astronomers, S. V. M (Victor) Clube and William Napier, published a book entitled The Cosmic Serpent. Clube and Napier suggested that the outer planets occasionally divert giant comets (more than 50 kilometers in diameter) into the inner solar system into short-period orbits. Debris from the resultant disintegration of these giant comets can adversely affect the environment of the Earth. Dusting can block sunlight, resulting in globally cooler conditions. Impact events in the super-Tunguska class may result in not only heavy localized destruction but also the occasional "impact winter" or dust veil with global climatological effects.
Clube and Napier identified the progenitor of the Taurid complex as such a giant comet whose injection into a short-period (about 3.3 year) orbit occurred sometime in the last twenty to thirty thousand years. The Taurid complex currently includes the Taurid meteor atream, Comet Encke (the only known currently active comet in the Taurid complex), "asteroids" such as 2101 Adonis and 2201 Oljato, and copious amounts of dust. All ten of the numbered asteroids in the Taurid complex appear to have associated meteor showers and therefore are likely to be extinct comets masquerading as asteroids.
The effects of the disintegration of the Taurid progenitor object in an Earth-crossing orbit should appear in the geological and climatological record. Clube and Napier marshalled evidence for such effects in "The Cosmic Serpent" as well as their later book Cosmic Winter published in 1990. Clube and Napier, following in the footsteps of earlier catastrophists, also sought evidence of catastrophic events in ancient mythology and history. These authors have also written papers in standard peer-reviewed journals about the role giant comets play in constructing a tenable physical theory of coherent catastrophism.
The giant comets normally reside far beyond the planets, in a spherical cloud surrounding the Sun, called the Oort cloud. There is also evidence for a flattened disk of comets closer to the inner solar system, called the Edgeworth/Kuiper belt. What prompts members of either of these comet repositories to enter the realm of the planets? Clube and Napier suggest a galactic influence. The solar system periodically passes through the plane of the galaxy as the Sun (and the solar system with it) orbits the galactic center. Each passage may dislodge giant comets and divert them closer to the Sun. The outer planets, particularly Jupiter, may then perturb some of these giant comets into orbits which enter the inner solar system. These comets, stressed both by gravity and by heat from the sun, may fragment into a cloud of smaller objects with dynamically similar orbits.
Chiron offers a good example of a giant comet as called for by Clube and Napier's giant comet hypothesis. Chiron is somewhere between 148 and 208 kilometers in diameter. Currently Chiron's unstable "parking orbit" lies mostly between Saturn and Uranus. Chiron may end up injected into the inner solar system within a hundred thousand years, or ejected from the solar system on a similar time scale. It is also possible that Chiron has already visited the inner solar system.
The Taurid complex and the Kreutz sungrazer group are two families of objects which most likely represent the fragmented remains of two giant comets in the current era. SOHO has recently discovered many new members of the Kreutz group which were previously unknown.
The Kreutz progenitor was injected into a retrograde orbit and attained the sungrazing state at a high inclination to the ecliptic. Hence the debris of its "children" does not pose a threat to the Earth. The Taurid progenitor on the other hand ended up in a short-period low-inclination prograde orbit. This is why the Earth can encounter its debris with potentially calamitous results.
What would happen should the Earth pass through the orbit of a disintegrating giant comet just before or after the comet passes that same point? Since larger fragments tend to cluster close to the nucleus of the comet, chances would increase that the Earth would be bombarded by these larger fragments. The severity of this comet fragment shower would far exceed any ordinary meteor shower. Not only would "shooting stars" and bright fireballs caused by small debris appear, but so too would large airbursts and possibly ground impacts. These would result in significant destruction should they occur over an inhabited area. If a large enough fragment struck in the ocean -- say, 200 meters or so in diameter -- it would raise tsunamis even at a great distance that would sweep away coastal habitations.
Duncan Steel, a colleague of Clube and Napier, refers to this process as coherent catastrophism. Widespread destruction derives from the coherent arrival of many impactors within a few days, as opposed to the sporadic arrival of objects spread randomly in space. The shower repeats for a period of years until the cometary orbit precesses so that the Earth no longer encounters the dense part of the debris field. (Of course, sporadic debris unrelated to the disintegrating comet may impact at any time as well.)
I believe that coherent catastrophism, as enunciated by Clube, Napier, Steel, and their colleagues, provides the best physical model for recent astronomical catastrophes, although I do not necessarily agree with all of their historical ideas.
For a contrary viewpoint you should read Is The Sky Falling? by David Morrison which appeared in Skeptical Inquirer magazine which admirably summarizes the majority view on the impact hazard. The article also provides reviews of books on the impact hazard published in the 1990s. A preliminary version of a review of Duncan Steel's book by Clark Chapman also offers useful criticisms of the coherent catastrophist viewpoint. The final version appeared in the journal Meteoritics and Planetary Science, v.31 (1996), pp. 313-314. Morrison and Chapman strongly disagree with the Clube/Napier/Asher/Steel idea of coherent catastrophism.
A common criticism levelled against Clube et al's giant comet hypothesis is that it uses a "Velikovskian" approach to mythological and historical evidence as a primary basis. It does not. Even should every single one of the mythological interpretations offered by Clube and Napier in The Cosmic Serpent or Cosmic Winter prove to be incorrect, this says nothing whatever about the correctness of the giant comet hypothesis and coherent catastrophism. The correctness of these depends solely on physical evidence. Mythological evidence might at best be supporting evidence. The same cannot be said of many other versions of non-orthodox catastrophism, e.g., Velikovsky's, which seek to rewrite physics and astronomy based upon ancient myths.
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Last modified by pib on May 12, 2009.