Sungrazing Comets

After centuries of speculation that the early records of observations of bright comets near the sun involved repeated returns of a single sungrazing object, there came in the 1880s the realization, first by Kirkwood, and then most notably by Kreutz, that the observations could be explained by the existence of a group of related comets that had separated from each other at some earlier perihelion passage. After the appearance of further bright sungrazers in the mid-twentieth century, attempts were made to understand the specific manner in which the various members of this Kreutz Group of comets had evolved from each other. Beginning in 1979, coronagraphic observations from space allowed the detection of numerous additional Kreutz members that were completely vaporized as they grazed the sun. Since 1996, the Solar and Heliospheric Observatory (SOHO) coronagraphs have revealed some 700 of these comets, giving rise to the thought that there is a constant stream of small members, much of the break-up having occurred instead near aphelion and even quite recently. Whereas consideration of break-up only near perihelion requires that the observed dispersion of the orbital parameters would take many millennia, recent calculations have shown that the evolution can be substantially sped up by allowing fragments to be rotationally spun off at heliocentric distances of many tens of AU. The SOHO observations have also allowed the recognition of three other substantial groups of sungrazing comets, as well as some pairs arriving only hours apart.