FIRE ON EARTH

Here is an able summary of the growing body of evidence that Earth has sustained a number of collisions with various large objects from space. John Gribbin (In the Beginning, 1993, etc.) and his wife are a versatile British team of popular-science writers, and the story of asteroid collisions calls on their wide range of scientific knowledge. The discovery of high concentrations of the element iridium (rare on Earth, but common in meteors) in the geological strata marking the end of the Cretaceous Period led a team of chemists and geologists to the conclusion that a large meteor impact had occurred at that time, probably causing the death of the dinosaurs. This theory initially met with widespread skepticism, but the evidence for it has continued to accumulate. The authors summarize the Tunguska event, a probable meteor impact in Siberia in 1908, and another that took place in the same region in 1947. There have been a few spectacular near- misses—one fireball seen over Montana in 1972, another over the Pacific in 1994. Impact craters on the moon and other planets testify that over geological time spans these events are far from rare. And the collision of Comet Shoemaker-Levy with Jupiter made it abundantly clear how powerful such a collision would be. The volume of space around the Earth as well as beyond our solar system contains a large number of comets, meteors, and small asteroids. Large objects, the size of the dinosaur killer, seem to arrive with a periodicity on the order of 30 million years. The Gribbins also discuss the possibility of a periodic swarm of smaller objects every few thousand years, causing widespread damage and disrupting civilization. Finally, they discuss various plans being made to detect and possibly deflect an incoming object—concluding that such efforts are unlikely to be of value in the foreseeable future. A well-written and comprehensive discussion of a sobering but inevitably fascinating subject.