act ranges from 100 meter 5 to 1 kilometer and strike the Earth an average of about once every 5000 years. One of the most frightening scenarios is based on Category 3 impacts. These impactors, ranging from 1 kilometer to 5 kilometers in diameter and traveling at speeds of tens of kilometers per second, were more prolific in early days of the solar system, as seen with the many larger impact craters seen on the Moon, Mercury, Mars and Venus (Morrison 36).In July 1994 Jupiter experienced the crash of Comet Shoemaker-Levy 9. After fragments of Comet Shoemaker-Levy 9 exploded in Jupiter's atmosphere they produced huge fireballs of hot gas and dust that rose high above the planet's clouds. Jupiter's atmosphere was literally blown away from above the impact site and hot gasand dust were funneled up to altitudes of 3,500 kilometers.But in Jupiter's stronggravity, whatever went up had to come back down. So for about twenty minutes after each impact, the dust-laden plumes fell back into the atmosphere, reentering with a horrendous release of energy. The heat from this reentry was so intense it was easily detected from Earth (Morrison 37).Although no known asteroids are currently on a potential collision course with Earth, 108 are nevertheless worrisome enough in the long term for the Minor PlanetCenter to catalog them as “Potentially Hazardous Asteroids” (PHAs). The purpose of the list is to identify objects that astronomers should routinely watch (Goldman 36). This does not mean however that Earth has not experienced this phenomenon before. In 1908 the crash of a large NEO occurred in the Tunguska River Valley of Central Siberia. It flattened trees for many miles around and killed a herd of reindeer. Additionally, Earth is spotted with dozens of visible craters that testify to similar impacts, and there surely are thousands of craters that vanished long ago from erosion. It is widely believed the impact of a giant NEO caused a ...
