Hundreds of tiny comets pass through the inner Solar System every year, but very few are actually noticed by people who are not astronomers. About every 10 years, a comet becomes bright enough to be noticed by a casual observer, these comets are often designated Great Comets.
An image of Halley's Comet taken in 1986. CREDIT: NASA
In the past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. Some of the more recent comets have also caused problems. In 1910, during the passage of Halley's Comet, the earth passed through the comet's tail. False reports spread fear that cyanogens in the tail could kill millions of people.
Predicting just how bright a comet will be is not perfect. Many factors can cause a comet to be brighter, or dimmer, than predicted. Several comets, in the past, have been predicted to be bright, but have failed to perform. One of these was Comet Kohoutek in 1973.
Comet Kohoutek - Image Taken in January 1974 at the Stony Ridge Observatory by Sara Martin and Brenda Noah
It fulfilled all the criteria to be a bright comet and was expected to become spectacular, but failed to do so. Comet West, three years later, had much lower expectations, but became an extremely impressive comet.
Great comets have been rare over the last 30 years. In the late 90s two great comets became visible, Comet Hyakutake in 1996, followed by Hale–Bopp, which reached maximum brightness in 1997. The first comet in the 21st century was Comet McNaught in 2007 (C/2006 P1) and was visible to the naked eye in January 2007. McNaught was the brightest comet in 40 years.
Comet McNaught - Photo source: John Drummond - Royal Astronomical Society of New Zealand Comet and Meteors
Here is a list of past great comets:
Great Comets of the past two millennia include the following:
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