On the Edge of Extinction

Abstract

On the morning of June 30, 1908, civilization may have suffered the worst piece of luck in its history. A small cometlike object exploded in the atmosphere above the Tunguska river valley in Siberia. It did little more than scorch and flatten trees for 20 kilometers in all directions and kill a thousand reindeer. However, if that object had struck a heavily populated region, we would not now dwell under any illusion concerning how close to the edge of extinction the human species actually hovers. Because the Tunguska missile missed a populated area, the threat of impact did not really begin to enter the public imagination until after the 1980 announcement of the discovery of the iridium in the K/T boundary layer. Had the Tunguska object struck a large city, a million people or more might have perished, and the phenomenon would have raised everyone’s awareness to the threat of comet impact. Instead, nearly a century later, the threat of comet and asteroid impact is regarded as little more than an interesting anecdote. Very slowly the nature of the threat is being recognized, but only because of the somewhat esoteric discovery that the dinosaurs were wiped out by a major impact 65 million years ago. Such huge collisions are infrequent, perhaps about once every 50 to 100 million years. It is the smaller impacts that pose the greatest danger, and they occur far more frequently. About 800 years ago the South Island of New Zealand suffered widespread fires, which leveled the island and led to the extinction of the Moa bird. Maori legend says that a big explosion in the sky was the cause of the strange fire. Duncan Steel of the University of Adelaide and Peter Snow from Otago in New Zealand have pieced together a fascinating scenario that suggests that the Maori were correct. The fireball created by a comet impact may have ignited the forests of South Island. Near the town of Tapanui in the province of Otago there exists a crater that geologists have been slow to identify as extraterrestrial in origin.