Time was when esoteric theories were argued in the pages of professional journals. Today, they are headline news. Now, a current ""hot"" dispute over whether the dinosaurs disappeared rapidly 65 millions ago because the earth was hit by a massive object from space forms the basis of this lively work by U. Cal (Berkeley) astronomer Goldsmith. Goldsmith, fresh from a seminar which included the major prinicipals involved, lays out the bare bones of the theory and moves on to the larger ""Shiva"" hypothesis it has spawned. Named for the Hindu goddess of destruction and rebirth, the Shiva hypothesis suggests that every 26 million (or maybe 30-32 million) years, the earth is struck by a sufficently large object (on the order of a few kilometers in diameter) to effect mass extinctions. Of course anything in the immediate path of the object would be incinerated. However, worldwide extinctions would occur because the impact of the object would cause extensive scattering of dust in the upper atmosphere; photosynthesis would all but cease; the food chain would lose its base, and, in the subsequent chill and famine, major die-outs would occur. In a few months' time the skies would clear and what was left would find a new world to conquer or new niches to fill. With respect to the dinosaurs, such a scenario would mean that pint-size mammals, mostly nocturnal species, would at last come into their own. Among supporters of Shiva (and one of its namers) is Harvard paleontologist Stephen Jay Gould, who sees in it a neat explanation for what he and colleague Niles Eldridge have termed the ""punctuated equilibria"" theory of evolution--which postulates short periods of rapid proliferation of many new species between longish intervals of relative stasis. Opponents argue that the dinosaurs didn't die all at once and not precisely at the stratum cited as evidence for Shiva. This stratum is a thin iridium-rich clay layer that marks a boundary between geological periods. The iridium is suggestive of an extraterrestial origin because any iridium present when the earth formed would presumably have by now sunk below the crust. Questions of what could cause massive upheavals and how they might occur periodically occupy Goldsmith in later chapters. Again, he skillfully presents the pros and cons for asteroid visitors and comet showers, coming to grips with what kinds of solar system or galactic objects could regularly perturb these flotsam. One candidate is ""Nemesis""--a postulated brown dwarf-star that might be earth's companion, far far away, encircling earth in a particularly eccentric orbit. All these theories are neatly presented in prose suited to the general reader along with the reminder that out of such debate can come good science. In addition--and probably the ingredient that has caused more than academic interest in the business--is the fact that the model of species extinction wrought by earth-darkening is the same scenario projected by scientists concerned with nuclear winter.