In 1966 the Leonids returned. ... The meteor count rate was over 500 per minute for almost exactly one hour, peaking at about 2,500 per minute (150,000/hr). These few lucky observers had witnessed the most spectacular meteor shower in recorded history, surpassing even the 1833 Leonids. And that meteor storm is in an orbit that brings it back every thirty-three years.
John S. Lewis:1996 46-47
It is not too late for you to make your plans for the night of November 16-17,1999... The Leonids and Andromedids are not especially noted for their displays of very brilliant fireballs, but some other streams are. Historical surveys of meteor data reveal that another prominent November meteor shower, the Taurids, was once extremely conspicuous for its numberous brilliant fireballs. During the eleventh century the brilliant Taurid fireballs were the most notable meteoric phenomenon of all. At that time the two modern active branches of the stream, with radiants about 16 debrees apart, were both prominent, suggesting derivation from two distinct cometary bodies in similar orbits. From about 1100 to 1869 the Taurids were lost, or so inconspicuous as to evade detection. November of 1920 brought another brief flurry of bright fireball activity. In the l960s careful orbital studies of the Taurids revealed that there are indeed two distinct streams, but with essentially identical orbital planes. Thus the case for a common origin was greatly strengthened. The orbits suggest, but do not prove, that at least two major ejection events from Enke's comet, in the fifth century AD and around 2700 BC, were responsible for the Taurids. Enke is unusual in that it is the active comet with the shortest known orbital period. Because of the predictability of these annual showers, it is conceivable that Anaxgoras's real feat was the prediction of the date of a display of brilliant fireballs that just happened to drop a meteorite, or that accidentally coincided with the date of fall of an asteroidal meteorite. No cometary meteor shower has ever produced a meteorite fall.
Enke is also the parent of the particularly elusive Beta Taurid meteor shower that spans the entire time from about June 5 to July 18, with a peak near July 1. Since this shower occurs on the dayside of Earth it is generally observed only by radio techniques (usually, by bouncing radar pulses off the ion trails left by the meteors as they burn up). This very broad and old stream crosses the orbits of all the terrestrial planets nearly in the plane of the ecliptic, thus providing endless opportunities for dynamical complexity and collisional mischief. Encounters of this stream with Earth therefore can occur at two distinct, widely separated time of year. The combination of all of Enke's family members, including the north and south Taurid streams of November and the Beta Taurids of June, is appropriately kown as the Taurid complex. The cometary events responsible for these multiple episodes of fragmentation are clearly important to the terrestrial planets....
When the long-delayed demise of the dinosaurs finally occurred, the crash was spectacular. Of all the global discontinuities in the geological record, the best-studied has been the end of the Cretaceous (K) period and the beginning of the Tertiary (T) period. (The letter C had already been appropriated by the Cambrian period.) Although the Cretaceous extinctions were not the largest in biological history, the event was so recent (only 65 million years ago!) that evidence is relatively easy to gather. The Cretaceous/Tertiary (K/T) boundary layer tells us an astonishing story.
When geologists read the pages just before the Cretaceous-Tertiary boundary they find the score of a typical uniformitarian opera; a long list of overly familiar characters playing tiny variations on a throughly familar story, with arias and recitatives that all somehow inspire deja vu, running on and on with soporific effect. But suddenly, marking the transition to the Tertiary, there is a sharp global discontinuity in everything. The sediments change from the local norm, found in hundreds of variations throughout the world, reflecting the humdrum processes of the last few million uneventful years. Suddenly, over the entire world, at every site where ocean or lake sediments have been preserved, there is a millimeters-thick layer of gray clay. The thickness of the layer varies little from place to place, usually between one and ten millimeters, and its composition is strikingly uniform, largely unrelated to the local chemistry. Only in the vicinity of Mexico and the Caribbean Sea is the layer systematically thicker. Worldwide, this layer contains a staggering several trillion tons (10 to the 18th power grams) of clay, or about 1,000 cubic kilometers of material. The thinness of the layer, and its uniformity, suggest that it was deposited in a single event, with a duration possibly less than a year. In this layer, physicist Luis Alvarez and his colleagues in 1980 found an enormous concentration of certain metals that are very rare in Earth's crust, notably the platinum-group metals. The first of these metals to be documented in the K/T boundary layer was iridium but we now know that a number of other metals are also enriched. Interestingly, the pattern of relative abundances of these metals is very different from that found in Earth's crust, but indistinguishable from the pattern of average solar system abundances. The pattern is not similar to any terrestrial source, but matches average meteorite material very well. It is, in effect, an alian fingerprint. Mixed in with the global iridium-bearing clay layer are tiny particles of heavily shocked quartz. In addition, in the Caribbean basin, the boundary layer is thicker and contains larger glassy spherules and droplets that have been largley, but not completely, altered by seawater.
Above the boundary, the world quickly quiets down to a new mode of functioning. As the transients die out, a sadly reduced cast of characters improvises its way into a new stable ecology. The intensity of the experience dwindles again as the inquiring geologist, like the audience at the premiere of Madama Butterfly, begins to notice apparent quotations and paraphrases. The tendency to stand as they did and shout, "Give us something new!" bubbles up inside us. Uniformity is back.
It is as if a score by Philip Glass were inadvertently printed by a sloppy publisher with a page of the 1812 Overture inserted. The orchestra, discovering the error after the first cannon blast, struggles to regain control of the concert. Meanwhile, the cognitive dissonance of the experience sould shock any uniformitarian in the audience fully awake.
John S. Lewis:1996 103-104
A simple calculation would have revealed that any impact with enough energy to change Earh's axis detectably would have completely overwhelmed the biosphere, probably sterilizing the surface of the planet.
Whether or not one accepts the notion of regular periodic impact storms, the idea of episodic clusters of impacts remains highly plausible. A scenario built by by Victor Clube and Bill Napier of Oxford University favors the idea of periodic extinctions, but rejects the Nemesis hypothesis as untenable. They propose in their book, The Cosmic Serpent, that periodicity is imposed on the impact rate by the periodic oscillation of the Sun through the plane of the Milky Way galaxy as it pursues its orbit about the galactic center. Each passage of the solar system through the galactic plane is likely to be disruptive of the Oort cloud of comets around the Sun. Many large comets would be perturbed into orbits that penetrate the inner solar system. Each such large comet, once it has encountered Jupiter and been perturbed into a short-period orbit, may then shatter into a hail of fragments with dynamically similar orbits, many of which may present a hazard to Earth.
Several NEAs, including 1990 SM, 2212 Hephaistos, 4197 1982 TA, and 1993 KA2, have orbits that are similar to the orbit of Enke's comet. It is possible that they are dormant fragments of comet cores from earlier breakup episodes of Enke or Enke's parent. These orbits have precessed apart, attesting to the passage of many millennia since they all traveled together as a compact swarm. Shortly after the fragmentation of such a comet or asteroid, the fragments would all be following closely similar, closely aligned orbits that would cross Earth's orbit on one or two precisely defined dates each year. Thus it would be possible for an observer with years' worth of data to predict the likely recurrence of spectacular meteor showers, or even meteorite falls, on the same date each year. It may be that this is what Anaxagoras really did.