ABSTRACT

In part due to the Tunguska impact 100 years ago in Siberia, cosmic impacts are now recognized as a major factor in the history of the Earth and other planets. Most dramatic was the discovery that the KT mass extinction of 65 million years ago was caused by the impact of a 15-km-diameter asteroid or comet. The sensitivity of the biosphere to such an impact came as a surprise, and prompts us to ask whether other mass extinctions were also triggered by cosmic impacts. Recent research on mass extinctions has demonstrated that they happened suddenly (in geological terms) and thus injected a catastrophic element into evolutionary history. Similar studies of the frequency and environmental effects of impacts can be used to evaluate the contemporary hazard. Of particular interest at the small end of cosmic impacts is the 10-megaton Tunguska event, which has only in the past 25 years been recognized for what it is – the impact of a rocky object roughly 50 meters in diameter. This talk concludes with discussion of the current Spaceguard Survey to discover and characterize potentially threatening near-Earth-asteroids, and of the technologies and policy implications of efforts to protect our planet by deflecting future impactors before they hit.