David Morrison of the National Aeronautics and Space Administration's Ames Research Center, replies:

"The effects of an impact on the earth depends critically on the size, and hence the energy, of the impact. For example, a comet less than 100 meters across will explode high in the atmosphere and probably do no harm whatsoever. If the comet is 10 kilometers across or larger (that is, if the impact carries an energy of more than about 100 million megatons), the resulting global environmental damage will be so extensive that it will lead to a mass extinction, in which most life forms die. This is what happened 65 million years ago at the end of the Cretaceous era, when the dinosaurs went extinct. The environmental consequences of a giant impact take many forms; the worst effects come from a global firestorm ignited by backfalling impact debris, and from a global pall of darkness that lasts for many months due to dust suspended in the stratosphere."

Gerrit L. Verschuur, an astrophysicist and radio astronomer at Rhodes College in Memphis, Tenn., has been investigating this topic. He fills in more of the disconcerting details:

"A collision between a comet and the earth would be a calamitous event. Based on the best available computer simulations, the impact of an object a kilometer or more in size would probably trigger the end of civilization. Such a body might not wipe out our species, but if the incoming object were more than about five kilometers across, it is very unlikely that Homo sapiens would survive.

"Broadly speaking, the initial impact creates a vast fireball that kills anyone who can see it. In the aftermath, ejecta (material hurled into the air) from the impact blasts into space and showers a large area--half of the world, perhaps--with flaming debris. (Something like this was seen when Comet Shoemaker-Levy 9 collided with Jupiter in 1994.) The sky becomes filled with gazillions of meteors, a shower of particles that burn up in the atmosphere on their way down. It is as if the sky itself has caught fire. The heat unleashed by this rain of incandescent debris ignites forests and cities, burning them to embers.

"Then dust from the impact and smoke from the fires girdles the earth, plunging our planet into a so-called impact winter. (Again, a similar effect was seen on Jupiter, where huge black bruises appeared after each fragment of Shoemaker-Levy struck.) Sunlight completely disappears for a month or so after a collision with a one-kilometer object; a 10-kilometer object might block out the sun for a year or more. Temperatures drop, and there is no light for photosynthesis. Plants die, animals die. Animals that happen to live underground, where roots and other food sources are not destroyed, have a relatively good chance of survival.

"During the impact, the comet is blown to smithereens. It unleashes billions of tons of sulfur-laden dust, some of it from the comet itself, some of it from sulfur-rich rocks that may lie beneath the impact point. The heat of the impact also creates nitrogen oxides. The sulfurous stuff and nitrogen oxides make a corrosive acid rain that strips bare any surviving vegetation. The acid rain runs into the oceans and kills marine organisms, especially along the continental shelves.

"Nitrogen oxides and debris tossed into the stratosphere by the initial blast would destroy the ozone layer within days. The impact and subsequent fires also release a tremendous amount of carbon dioxide into the atmosphere. After the initial impact winter is all over, there is so much carbon dioxide in the air that a centuries-long greenhouse effect may result.

"Scientists are still divided on whether the object that struck the earth 65 million years ago was a comet or an asteroid; further drilling in the presumed impact region in the Yucatan will help settle the question. Either way, the consequences were so dire that it is a wonder that any species survived at all. But some did. After the dust settled, they emerged to spread over the newly shaped world and multiplied and found new niches in which to fulfill their destinies.

"More details are to be found in my book 'Impact: The Threat of Comets and Asteroids,' which will be published by Oxford University Press in September 1996.