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.)