from Illustrated London News, 1985
There can be few people who have not heard of comets, but there are still a great many non-scientists who have no real idea of what a comet is. The most popular mistake is to assume that a comet streaks  across the sky and disappears in a few seconds.

In fact, all comets are very distant – far beyond the top of the earth’s atmosphere – and you cannot see them moving. If you see an object moving visibly, it certainly cannot be a comet. It will be either an artificial satellite, thousands of which have been launched since the Space Age opened  with Russia’s Sputnik 1 in October, 1957, or else a meteor.

Comets ' parts
The parts of comet

Of course, it can also be a weather balloon or a high-flying aircraft. Comets belong to the Sun’s family, or solar system, but they are quite unlike planets. They are not solid and rocky; a comet consists of an icy central part (or nucleus), a head (or coma) and a tail or tails  made up of tiny particles of ‘dust’ together with extremely thin gas.

The Comets
The Comet


Comets may be enormous (the head of the Great Comet of 1843 was larger than the Sun), but they are very light since the nucleus, the only relatively massive part of a comet, cannot be more than a few miles in diameter. If a comet fell to the earth, it would only cause local  damage.

NEAR MISS - Comet Siding Spring
Comet Siding Spring is racing toward Mars for a close encounter in October 2014


Comets move around the Sun. In almost all cases their paths (or orbits) are elliptical, and except for Halley’s Comet, all the really bright comets take thousands or even millions of years to complete one circuit. This means that we cannot predict them. During the last  century, several were seen but in our own time they have been extremely rare.

Comets and Spacecraft
Comets Visited By Spacecraft


The last really ‘great’ comet was that of 1910, though there have been others which have become bright enough to be seen with the naked eye. Halley’s Comet is unique because it appears every 76 years, and it has been seen regularly since well before the time of  Christ; there is even a Chinese record of it dating back to 1059 B.C. However, it was only recently that astronomers realised that there was something unusual about it.

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