The tail of the giant comet, as it travels out in space, meets almost no resistance. It is composed of debris attracted to trail after the comet not only from the days when the comet formed, but as it travels, picking up stragglers in space attracted to its gravitational pull. The tail has dust, gases, stones, boulders, and moons, depending on the size and composition of the entourage at any given point. Nothing much affects this out in space, on the long arc the 12th Planet makes as it zooms away from Earth and floats into the darkness of outer space. All this changes when the comet enters your Solar System. The Solar wind, a bombardment of radiation your eye cannot see except as light, pushes the tail outward, away from the Sun. Thus the comet's tail sweeps the Earth - dust, gases, stones and boulders all. What is the effect? The comet's moons hug close, so do not come in range, but all else is a massive onslaught on the Earth's atmosphere.
Meteors, or falling stars, are not unknown to mankind, who can see them nightly if they persevere in their nightly watch. These flaming streaks represent a burning meteor, heated by friction and set afire by the Earth's oxygen mantel. But what if there were no oxygen? During the comet's passage, there is such an onslaught to the Earth's atmosphere that the available oxygen in places is completely consumed. Heated gas in the comet's tail form petrol chemicals, which when falling through the atmosphere burst into flame, a sheet of flame falling to Earth. This does more than burn every living thing it lands on - it consumes the oxygen. What has been reported by the author Velikovsky is that the Mastodon has been found frozen in the Siberian tundra, grass between its teeth, in perfect though frozen condition, as though they simply stopped breathing. They did. Without oxygen, the blackness of unconsciousness descends quickly. When the available oxygen is consumed in the upper atmosphere, any meteors falling to Earth have less resistance.
The tail of the comet is composed of lighter material toward the tip, as any heavier substances that far from the great comet's gravitational influence are drawn in other directions at one point or another. Thus, the gases and dust curl toward the Earth, and are first noticeable as a fine red iron dust, turning the water a bitter blood red. Does this dust not burn in the available oxygen, and end as so many tiny flying star specs? This dust, already oxygenated, does not burn. As the comet approaches for its loop around the Sun, the next notice is of a fine gravel, dropping in places like hail stones. Why does this not burn, as meteors regularly do? Some do, but their number overwhelms the Earth's defenses, the majority not igniting at all. By this time the Earth's upper atmosphere is tearing away, dispelled into space and no longer snug as a mantle around the Earth. Now comes an occasional boulder, falling without resistance on the hapless Earth.
Those who would escape the wicked lick of this tail are advised to take shelter against cliffs, in caves, in valleys, or under metal roofs. Its passage is swift, a matter of days, and the ending abrupt.
Note: below added during the Dec 21, 2002 Live ZetaTalk IRC Session.
During every passage, there are some debris which land on the surface of Earth, as the atmosphere is stripped away to a great extent and the tail of the passing comet contains much large debris. Boulders drop, thudding to Earth and crushing quickly anything beneath them. It is reported in folklore that hail stones, gravel, lashes the Earth, but those reporting this have sought shelter and are thus alive to report. During the last passage the reports stated that crops and small trees were utterly destroyed by these hail storms. That some craters are in the news lately is not because they have just been discovered, but because there is increasing talk about Star Wars, asteroid shields, Near Earth Orbit passes, and this has awakened concerns. The establishment wants this heightened concern, as they still hope to erect some magic shield to zap out anything coming close, which is fantasy. In the first place, such a shield itself would be torn from the skies, ineffectual, absent, and probably crashing to Earth itself as a source of destruction. In the second place, the trash is so intense that any zapping laser would not keep up with it. The best defense for humans is to huddle near cliffs, out of the wind, not in open fields, as most missiles that crash to Earth, large boulders, will not drop straight down! They come in at an angle, and will glance off hilltops, bouncing about. Thus, those in ravines are unlikely to experience a direct hit.