After a large part of the Earth's human population is rendered blind by a brightly coloured comet shower, triffids begin escaping confinement and easily kill large numbers of blinded people.

Triffids also become more aggressive, as a comet shower has blotted out the sun and thus necessitates them to increase their nutritional intake.

A comet shower was said to have scared the Nangnang soldiers, however, and they retreated.

Instead of a comet shower, the researchers say, the event causing the dusty conditions was probably a major collision in the asteroid belt.

A similar but more controversial explanation proposes that "passages of the [hypothetical] solar companion star Nemesis through the Oort comet cloud would trigger comet showers."

In the 1980's, a few speculated that comet showers might produce strikes on the earth over a period of a million years or so to bring on extinctions.

In 1.4 million years Gliese 710 will be only 1.1 light years from Earth and might catastrophically perturb the Oort cloud, possibly resulting in a comet shower.

Readers looking for a discussion of the recent controversial proposition that periodic comet showers are triggered by the regular passage of a so-called death star will be disappointed as well.

Among other things, this showed that if a comet hit Earth, it was a lone object, not part of a comet shower (which would have been accompanied by more space dust).

The initial stimulant was, of course, Alvarez's theory of extinction triggered by extraterrestrial impact, but the discussion has moved well beyond errant asteroids to comet showers, putative 26-million-year cycles, and mathematical models for genuine catastrophe.