Most of the comets take thousands or even millions of years to go round the Sun, and their orbits are so elongated or ‘eccentric’ that they travel far beyond Pluto.
Very few become bright enough to see until they are as close to Mars. That is why some comets appear without warning every year. Not all comets are surprise visitors. The famous Halley’s Comet , for example , has a period of 76 years and has been observed regularly since 239 BC. It will be seen again in 2062.
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