On the Cover: Sometime in the distant past, an asteroid or comet struck Saturn's moon Mimas with nearly enough force to break the satellite into pieces, leaving a giant crater over 100 kilometers across. Another wayward asteroid or comet may have struck Earth 65 million years ago, causing the extinction of the dinosaurs. This computerized image of Mimas accentuates the crater, appearing here in relief against the darkened limb of the satellite. Alan Ames, I.M.A.G.E.S. Computer Graphics System, Columbia College, Hollywood
3 Planets and Perils: Owen B. Toon takes us on a tour of of catastrophic events in Earth's history.
4 Volcanoes and Atmospheres: Catastrophic Influences on Planets: Susan Werner Kieffer discusses the lessons of volcanic eruptions on Earth and elsewhere in the solar system.
7 A Talk With Eugene Shoemaker: Charlene Andersonsits down with one of the world's experts in impact cratering events to discuss the impact thought to have killed the dinosaurs.
12 Greenhouses and Glaciers: Climate Change and the Continuously Habitable Zone Around the Sun: James F. Kastinglooks at the complicated factors of habitability.
16 Precessing Worlds: Planetary Spin and Climatic Stability: William R. Wardexplains the effects of a wobbling planet.
20 Intelligence and Catastrophe: The Cosmic Quarantine Hypothesis: Why haven't we discovered any extraterrestrial civilizations? Steven Soter investigates.
11 News & Reviews Triton and Pluto; Ionian volcanism.
18 World Watch Europe and the U.S. propose joint space ventures.
19 Society Notes Hans Mark rejoins board; New Millennium Committee scholarships.