Space Station Info::Space Comets
Comets are small, easily broken, erratically shaped bodies composed mostly of a mixture of water ice, dust, and carbon- and silicon-based compounds. They have highly oval orbits that frequently bring them very close to the Sun and then swing them into space. Comets have three distinct parts: a nucleus, a coma, and a tail. The solid core is called the nucleus, which develops a coma with one or more tails when a comet sweeps lock to the Sun. The coma is the dirty, nebulous cloud around the nucleus of a comet, and the tail extends from the comet and points away from the Sun. The coma and tails of a comet are brief features, present only when the comet is near the Sun.
Formation Of Comets
Our entire solar system, counting comets, formed from the collapse of a giant, disperse cloud of gas and dust about 4.5 billion years ago. When cloud was continuing its collapse, it was revolving very slowly. But the cloud began to heat up and spin faster as it shrank, just as twirling ice skaters spin earlier by bringing their arms lock to their bodies. The fast rotation helped make sure that not all of the material chops into the core. Instead, the material in the fast-spinning cloud spread out into a trodden disk. Meanwhile, the temperature in the dense, central core was heating up.
The core ultimately became so hot that it ignited nuclear fusion, creating the Sun. The disk's external regions, however, were quite cold. The low temperatures permitted water to ice over onto dust grains, which grew in size to make clumps. Some clumps in time reached a size of numerous kilometers in diameter. The clumps then began amalgamation, most likely by collisions, and formed the planets. Many theories abound about how these clumps became planets. This topic is at the forefront of technical research. Whatever the details, large planets were created from the buildup of clumps of matter and gas from the nearby cloud. But some of this matter did not merge into planets.
Within the last decade, for example, astronomers discovered leftover clumps, called planet, in a region beyond Neptune, although no large planets formed beyond that planet. These bodies form an outer asteroid belt at the edge of the solar system, called the Edgeworth-Kuiper belt, named for the scientists who proposed its existence in the 1950's. Recent calculations show that this asteroid-rich Kuiper belt (as it is now known) is most likely the source of most of the short-period comets, such as Halley's comet, which orbits the Sun every 76 years.
A comet's tail is its most typical feature. As it approaches the Sun it develops an enormous tail of polished material that extends for millions of kilometers away from the Sun. When far from the Sun, a comet's nucleus is very cold and its fabric is frozen. Water ice, as well as other compounds such as carbon dioxide and carbon monoxide ice, may be found in the nucleus. This icy nucleus changes fundamentally when a comet approaches the Sun. The strong solar wind from the Sun transforms the solid nucleus directly into a vapor, bypassing the liquid phase. This process is called sublimation. The vapor helps stir things up in the nucleus, forcing the core to form a cloud-like mixture of gas and dust around it, called the coma. There, sunlight and the solar wind relate with the ingredients, creating the tails.
Features Of Comet
The ingredients in the coma determine the types and number of tails. Some comets may emerge to have no tails, but they really do. They are simply very faint. Scientists can identify these tails by using special filters that are aware to dust or gas emissions. Other comets, which could be seen from Earth in 1997, have very prominent tails. Although Hale-Bopp's tails could be seen apparently from Earth, scientists using sensitive cameras familiar a much more difficult tail structure. One of these images revealed a long, curving dust tail. Other pictures showed dust and gas ion tails. There was even an image of a dust tail and two gas ion tails. The different tails provide scientists with important information about the internal chemistry and structure of a comet's nucleus.
Types Of Comets
There are two types of comet tails: dust and gas ion. A dust tail, which is usually yellow, contains small, solid particles that are about the same size as those found in cigarette smoke. This tail forms because sunlight acts on these small particles, gently pushing them away from the comet's nucleus. Because the pressure from sunlight is relatively weak, the dust particles end up forming a diffuse, curved tail. A gas ion tail, which is usually blue, forms when ultraviolet sunlight rips one or more electrons from gas atoms in the coma making them into ions (a process called ionization). A solar wind then carries these ions straight outward away from the Sun. The resulting tail is straighter and narrower. Both types of tails may extend millions of kilometers into space. As a comet heads away from the Sun, its tail dissipates, its coma disappears, and the matter contained in its nucleus freezes into a rock-like material. Recent observations of the very bright comet Hale-Bopp pinpointed a tail made of sodium (Na), a relative of the gas ion tail. This tail forms when sunlight pushes on sodium atoms released from the nucleus.