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This article offers a look at four sediment deposits called chevrons at the southern end of Madagascar, which contain microfossils that have been fused together with metals formed by cosmic impacts. Each chevron points in the same direction, which leads to scientists' estimation that a large asteroid or comet collided with Earth in the Indian Ocean 4,800 years ago, producing a tsunami.

Features the two-stage spacecraft called Deep Impact by the U.S. National Aeronautics & Space Administration, which is about to blast a crater in the nucleus of comet Tempel 1 and provide the first look at a comet's icy inner material.

Reports that researchers from the U.S. have calculated the threshold size for a comet or asteroid impact enough to ignite global wildfires.

Focuses on asteroids colliding with the Earth. Research announced at an international meeting on asteroids and comets at Cornell University in Ithaca, New York; Size range for asteroids that researchers say could cause a catastrophic change in global climate; Approaches to spotting near-Earth objects; How observing conditions prevent astronomers from keeping track of newly found asteroids.

Reports on the evidences presented during a convention of Geological Society of America which support the theory that a comet crashed in southern Nevada. Rough estimate of the crash site; Alleged physical characteristic of the comet; Effect of the crash on life forms.

Discusses the theory, announced in May 1997, that many thousands of comets are bombarding the Earth every day. Possibility that the theory, if verified, could settle questions concerning the origin of Earth's waters; Dr. Louis Frank as the main discoverer of the theory of cosmic snowstorms; Some space and planetary scientists as vigorously attacking the theory; Comments of scientists; Explanation of the theory.

Looks at the results released by an ozone-watching satellite, which appear to support a theory that Earth is being bombarded daily by thousands of house-sized comets. Release of the satellite by the space shuttle Discovery on August 7, 1997; Comments from Doctor Robert Conway, a scientist of the Naval Research Laboratory.

Focuses on a report of scientists, that snowballs from outer space are bombarding the Earth's atmosphere every day, adding water to the Earth's air and seas. Role of extraterrestrial slush in filling the oceans and nurturing life on the planet; Description of the snowballs; Comments from Dr. Louis A. Frank, a physicist at the University of Iowa. INSET: Water from space..

Discusses with astronomists the accuracy of actions taken to protect the earth from a comet in the movie `Deep Impact.' What is necessary to discover and predict the course of a comet; The amount of explosive power which would be necessary to disrupt a comet enough to prevent a collision; Why an understanding of a comet's structure would be pivotal in determining the methods and equipment necessary to change its course. INSET: Who plays Paul Revere when a comet is coming?.

Mentions research which indicates that thousands of tiny comets enter Earth's atmosphere every day. Photographs presented by Louis A. Frank which show tiny comets disintegrating in the upper atmosphere; Frank's hypothesis that the tiny comets add organic chemicals and water to the Earth's surface.

Suggests that there are water-bearing small comets that are bombarding the Earth's atmosphere. The size of the comets; The images acquired by an ultraviolet camera aboard the National Atmospheric and Space Administration's (NASA) Polar spacecraft; Details about the author's search for the small comets; The opposition of editor Alexander Dessler. INSET: Small comets, big questions..

The article reports that the devastation caused by natural explosions could be far higher than one by nuclear explosions. According to Joseph V. Smith, a professor of geophysical sciences at the University of Chicago in Chicago, Illinois, the dangers of natural catastrophes like volcanoes, earthquakes and collisions with comets are greater in degree and intensity than nuclear explosions. He asked, therefore, the scientists to intensify their efforts to predict such catastrophes.