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Only a few comets return in a time measureable in human terms (shorter than a century), like Comet Halley does; these are called short-period comets. Many short-period comets have had their orbits changed by coming too close to one of the giant planets—most often Jupiter (and they are thus sometimes called Jupiter-family comets). Most comets have long periods and will take thousands of years to return, if they return at all. As we will see later in this chapter, most Jupiter-family comets come from a different source than the long-period comets (those with orbital periods longer than about a century).
Observational records exist for thousands of comets . We were visited by two bright comets in recent decades. First, in March 1996, came Comet Hyakutake, with a very long tail. A year later, Comet Hale-Bopp appeared; it was as bright as the brightest stars and remained visible for several weeks, even in urban areas (see the image that opens this chapter, [link] ).
[link] lists some well-known comets whose history or appearance is of special interest.
| Some Interesting Comets | ||
|---|---|---|
| Name | Period | Significance |
| Great Comet of 1577 | Long | Tycho Brahe showed it was beyond the Moon (a big step in our understanding) |
| Great Comet of 1843 | Long | Brightest recorded comet; visible in daytime |
| Daylight Comet of 1910 | Long | Brightest comet of the twentieth century |
| West | Long | Nucleus broke into pieces (1976) |
| Hyakutake | Long | Passed within 15 million km of Earth (1996) |
| Hale–Bopp | Long | Brightest recent comet (1997) |
| Swift-Tuttle | 133 years | Parent comet of Perseid meteor shower |
| Halley | 76 years | First comet found to be periodic; explored by spacecraft in 1986 |
| Borrelly | 6.8 years | Flyby by Deep Space 1 spacecraft (2000) |
| Biela | 6.7 years | Broke up in 1846 and not seen again |
| Churyumov-Gerasimenko | 6.5 years | Target of Rosetta mission (2014–16) |
| Wild 2 | 6.4 years | Target of Stardust sample return mission (2004) |
| Tempel 1 | 5.7 years | Target of Deep Impact mission (2005) |
| Encke | 3.3 years | Shortest known period |
When we look at an active comet, all we normally see is its temporary atmosphere of gas and dust illuminated by sunlight. This atmosphere is called the comet’s head or coma. Since the gravity of such small bodies is very weak, the atmosphere is rapidly escaping all the time; it must be replenished by new material, which has to come from somewhere. The source is the small, solid nucleus inside, just a few kilometers across, usually hidden by the glow from the much-larger atmosphere surrounding it. The nucleus is the real comet, the fragment of ancient icy material responsible for the atmosphere and the tail ( [link] ).
The modern theory of the physical and chemical nature of comets was first proposed by Harvard astronomer Fred Whipple in 1950. Before Whipple’s work, many astronomers thought that a comet’s nucleus might be a loose aggregation of solids, sort of an orbiting “gravel bank,” Whipple proposed instead that the nucleus is a solid object a few kilometers across, composed in substantial part of water ice (but with other ices as well) mixed with silicate grains and dust. This proposal became known as the “dirty snowball” model.
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