There was a time when science refused to believe that meteors actually fell from the sky; now, scientists soberly calculate the possibility that one of those falls could destroy a city--or the entire human race. Lewis (codirector for science of the NASA/Univ. of Arizona Space Engineering Research Center) attempts to place the threat of cosmic bombardment in down-to-earth perspective by systematically building up evidence. Most early accounts of meteorite falls were ignored by later scientists; even Meteor Crater in Arizona was long considered a volcanic cone, despite the absence of volcanic rock in the area. The Tunguska event--a meteor explosion over Siberia in 1908--was not properly investigated for nearly 20 years. Only with the exploration of space did the full truth become evident: Every planetary surface we have examined shows proof of massive bombardment from space, although erosion has obliterated many of the traces on Earth's surface. Not all traces, however--numerous craterlike features show the geological stigmata of high impacts, such as shocked quartz crystals and the tiny glass beads known as microtektites. A crater near Yucatn is now believed to be the remnant of the impact that destroyed the dinosaurs. The dramatic impact of Comet Shoemaker-Levy 9 on Jupiter converted most of the remaining skeptics. Lewis draws parallels with nuclear explosions to explain the possible effect of large impacts on human populations; at the same time, he points out that even the largest nuclear device ever exploded (60 megatons) was far less damaging than what we might expect from the impact of a million-ton asteroid, of which there are tens of thousands in orbits that threaten Earth. In his final chapter, Lewis proposes a space-going capability to divert the most threatening asteroids and to exploit the mineral resources of the richest. An apocalyptic vision that should be taken with the utmost seriousness by anyone concerned with the long-range fate of the human race.