There is increasing evidence that the disruption or disintegration of cometary bodies such as that witnessed with Comet Shoemaker-Levy 9 (that later impacted on Jupiter in July 1994) is a more common occurrence than previously suspected. All comets leave trails of debris as they travel through the solar system, and these cause annual "meteor showers" as the Earth passes through their path. Meteors are caused by small particles, most no larger than pebbles, but there is a strong possibility that larger fragments may be interspersed with them. It is unlikely to have been a coincidence that the 1908 Tunguska event occurred at the height of the Taurid meteor shower, associated with the periodic Comet Encke. The same comet has been linked with a number of probable events that have occurred during recorded history.

Should a large comet nucleus shatter, for whatever reason, as seen with Comet Shoemaker-Levy 9, it will produce a stream of fragments in its wake. These will, over time, spread out along the orbital path, and are likely to vary in size from a few microns to a few hundreds or thousands of metres in diameter. Such a debris stream could pose a significant and recurring threat to the Earth.

There is increasing evidence (Bailey and Emel-Yanenko, 1997) that there might be a significant population of "dead" comets occupying Halley type orbits. Once a comet has outgassed all of the available volatiles, its coma and tail will disappear, and the remaining, inert nucleus will take on the appearance of a low albedo asteroid. Finding such bodies could present new challenges to search programmes, requiring the use of infrared technology.

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