Taking a cue from "encounters," Asimov categorizes catastrophes leading to the destruction of human life into five classes. "First class" catastrophes are changes affecting the universe, inimical to life anywhere. Class two are events threatening our solar system; Class three, those threatening the earth itself; Class four, those that would destroy the human race; and Class five, those that would wipe out civilization, leaving a few survivors to lead "nasty, brutish, short-lived existences." Such an embarrassment of poornesses allows the massive Asimovian index file to adduce theory, evidence, and probabilities for disasters ranging from black-hole approaches to Andromeda strains. A Class one catastrophe could occur, for example, because the universe is running down, according to the second law of thermodynamics, leading to ultimate "heat-death." Earlier, however, catastrophes could occur either by the universe expanding indefinitely (the open universe theory) or by gravitational collapse to the cosmic egg (closed or oscillating universe theory). At the other end of the spectrum, Class five annihilation of civilization could come about through the well-known routes of overpopulation, pollution, limited resources, or war. But Asimov is no Cassandra. In the first three classes, indeed, the highest probabilities are given to a collapsing universe (which would take billions of years); to the sun's evolving to a blazing red giant (seven billion years remaining); to the earth's suffering a new ice age or, alternatively, a melting and flooding (in several thousand years)--lots of time to Do Something About It. What must be dealt with now is the threat of thermonuclear war, a Class four catastrophe, and the multiple Class five problems. Here, again, Asimov trusts in science and technology. As ever, this is clearcut exposition, leading the reader expertly down paths of entropy or recombinant DNA; only the optimism seems strained, with too much belief in sweet reason, and insufficient evidence as to how it might prevail.