Comets are usually small objects, the solid cores only a few kilometres across. They usually have highly eccentric orbits, taking some far out from the Sun. They are thought to be like dirty "snowballs", that is they are a conglomerate of rock dust and frozen gases like water, ammonia and other volatiles. When a comet approaches the Sun it warms up and the volatiles start to evaporate, so that the comet becomes surrounded by a large halo, or coma, of vapour and dust. The coma can be pushed out as an elongated tail by the solar wind.
The coma of a comet may be tens of thousands of kilometres across so that, visually, the comet is a small, diffuse object. Because the amount of gassing which will occur is very hard to predict, magnitude predictions for comets can be quite unreliable. They may be much fainter than expected, or sometimes brighter, especially if there is an outburst of vapour. The magnitudes given are for the whole object, but since the comet is not a star like point, the apparent brightness is lower, making the comet more difficult to see than a star of the same magnitude.
The material left behind in the comet's tail will continue to orbit the Sun and some may eventually enter and burn up in the Earth's atmosphere resulting in a meteor shower.
Information will be provided on these pages about bright comets predicted to occur during the year. As comet discoveries are continually being made, it is always possible an unpredicted comet will become visible.
General information on comets is available at
Gary Kronk's comet pages.
Ephemeris details of a large number of comets are available at the IAU Minor Planet Centre. Users of the site can generate their own ephemerides for a any of a large number of comets and download orbital elements.
The majority of the orbital information presented on this site is based on elements from the IAU Minor Planet Centre.
Only two comets are predicted to reach magnitude 10 or brighter and be visible from the southern hemisphere in 2011. It is always possible others may be discovered.
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