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Comet Impacts on Earth

Comets are chunks of ice, frozen gases and dust left over from the formation of the solar system. They have often been called "dirty snowballs." Most comets are in orbit far from the sun and are entirely frozen. When a comet enters the inner solar system, the heat of the sun begins to melt the ice and frozen gases, which stream behind the main body of the comet and form the characteristic glowing tail.

Classification
- Scientists classify comets as either long-period or short-period. The name refers to the period of time between approaches to the sun. Long-period comets appear to originate in the Oort Cloud, a cloud of comets theorized to surround the solar system almost a light year from the sun. Long-period comets exhibit irregular orbits which are difficult to predict. A second source of comets, the Kuiper Belt, lies just beyond Neptune. Short-period comets apparently originate in the Kuiper Belt. Short-period comets have regular, predictable orbits.
Risk Factors
- Comets have hit Earth in the past, but not frequently. Most comets are long-period objects and only approach Earth at intervals of 200 or more years. Comets are estimated to have hit Earth in the past at a rate of one every 32 million years, and comets are believed to account for less than 10 percent of the significant past impacts on the Earth.
  
  The size of the head, or nucleus, of comets varies greatly. Comet Hale-Bopp, which neared the sun from 1995 to 1997, has a nucleus diameter of 25 to 50 miles. Most comets, however, have a diameter of 1 to 2 miles. NASA estimates that an object more than 1/2 mile in diameter would cause serious damage if it hit the Earth.
Effects
- According to NASA's Ames Research Center, objects ranging in size from 1/2 to 1 mile in diameter would cause severe local damage, and objects 1.2 miles in diameter or larger would have a serious effect on climate worldwide. The largest impacts of the past may have caused mass extinction events. Many scientists believe that a comet impact 65 million years ago led to the disappearance of the dinosaurs by causing a global "impact winter."
Detection and Protection
- Since 1998 NASA has operated the Spaceguard Survey and the Near Earth Object program, efforts to find large objects in space near the Earth, calculate their orbits and provide warning of future impacts. These particular programs are not set up to track long-period objects. In the future, however, NASA plans to extend the NEO program with the Comet/Asteroid Protect system which would give at least a year of warning before the impact of a long-period comet. When completely in place the proposed system would include ways to divert dangerous objects before they hit the Earth.
  
  The Spaceguard Foundation, a private organization headquartered in Europe, also works to coordinate efforts to identify possible impact risks. Beyond coordination, the Foundation works to raise public awareness of the issue and promotes international cooperation to find ways to prevent impacts.

Astronomy