The Earth does not exist in the solar system in isolation, but is subjected to the impact of interplanetary bodies.  About 150 terrestrial impact structures are currently known.  However, throughout geologic time the Earth had received more impacts than the moon, and the relatively unscarred appearance of the Earth reflects a highly active surface which removes and obscures impact structures.  Planetary exploration has highlighted the importance of impact on the terrestrial planets as a process for catastrophic change.  Large impact basins (> 1000 km) must have formed on the early Earth and would have resulted in massive melt pools (> 10^7 km cubed).  This may partially explain the early differences between continental and oceanic crust.  Large impacts probably also affected the early biological evolution on Earth, and more recently at least one global extinction event (Cretaceous-Tertiary) is linked ot a major impact.  It is possible that other major impacts were responsible for sudden climatic excursions in the geologic record.  The currently popular theory for the origin of the moon is that it is condensed and reaccreted from vaporized ejecta that resulted from the collision of a Mars-sized body with the proto-Earth.  Without the moon, there would be no tides and the terrestrial biological evolutionary record would be much different.  Impacts have also played a role in the economic development of the Earth.  A number of hydrocarbon producing impact structures occur around the world and two world-class mining camps - Sudbury, Canada, and Witwatersrand Basin, S. Africa, are located within large, ancient impact structures.  Finally, there is currently much concern over the hazard that extraterrestrial impact constitutes to human civilization, with events capable of producing the equivalent of a global nuclear winter occurring with a frequency of every 2-4 million years.