Concern exists among an increasing number of scientists throughout the world regarding the possibility of a catastrophic event caused by an impact of a large Earth-crossing object (ECO) on the Earth-Moon System (EMS), be it an asteroid or comet. Such events, although rare for large objects (greater than 1 km diameter), are not unprecedented. Indeed, the great upheaval and resulting ice age that marked the extinction of the dinosaurs is thought to have been caused by the impact of a 10 km diameter asteroid. Such an event is thought to occur approximately every millennia. In 1908 a stony asteroid of approximately 50 meters diameter exploded in the air above the Tunguska River in Siberia, producing an equivalent yield of 15-30 megatons (mt) of TNT, leveling over 2000 square miles of dense forest. Such an event is thought to occur approximately every century.

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