Comets | Research & Encyclopedia Articles

Comets

Comets are small astronomical bodies that are composed chiefly of dust and ice crystals that orbit the Sun along highly elliptical orbital paths. As comets approach the Sun a portion of the comet is vaporized to form a characteristic head and tail.

Observations of comets date back to the fifth century B.C., when the Greeks mistook comets for clusters of stars. Aristotle (384-322 B.C.) argued that comets arose from the Earth. During the 18th century French mathematician Joseph-Louis Lagrange (1736-1813) postulated that comets were expelled from planets. Significant theories of comets include the hypothesis of British astronomer R. A. Lyttleton in 1948, that a comet was actually a "flying gravel-bank" of dust particles. In 1950, American astronomer Fred L. Whipple countered with the "dirty iceball" theory. Whipple believed the nucleus of a comet was a conglomerate of rocky fragments and "ices" made from frozen methane, ammonia, carbon dioxide, water, and other substances. Even before the international efforts to observe Halley's Comet in 1985 and 1986, evidence indicted that Whipple's dirty iceball model of the comet was correct, partly because of observations made of Kohoutek's Comet in 1974.

Until the twentieth century, most new comets were discovered with the naked eye or with a telescope. In contemporary astronomy, the majority of comets are seen on photographic plates or electron detectors.

The overall mass of a comet is very small in comparison with a planet and spectroscopic analysis is utilized to determine the actual composition of comets. In 1864, Italian astronomer Giovanni Battista Donati was first astronomer to study the spectrum of a comet contained within the reflected solar light returned from a comet.

While some cometary processes are still not completely understood, the typical comet appears to contain a nucleus that may measure a few miles in diameter in larger comets. The nucleus is surrounded by a coma composed of small particles of material such as iron, nickel and magnesium in an icy mixture with frozen water, ammonia and methane. The coma contains ions, molecules that have lost at least one electron. These give off emissions after being excited by solar radiation.

A tail extends from the coma, but not every comet will have an observable tail. The tail is formed when icy particles evaporate or when small particles are driven out of the nucleus. The tail is composed of gaseous ions and/or dust. Comet tails are actually formed by different processes. When gases from a comet mix with solar wind, ion tails are created. In almost every case, the tail will point away from the sun. Most astronomers believe the direction of a comet's tail is influenced by a form of solar wind that consists of electrified low-energy particles streaming from Sun. However, the details of this process remain a subject of astronomical investigation.

Ultraviolet observations have revealed that some comets are surrounded by large clouds of hydrogen. Comet clouds have been seen on space probes, first in 1968 by Orbiting Astronomical Observatory No. 2, which observed the Tago-Sato-Kosaka Comet to have a hydrogen cloud 1 million miles in diameter.

In particular, Halley's Comet has long fascinated both scientists and the general public. There are records of the comet's appearance dating back to 240 B.C.. Until the 17th century the comet's appearance was often interpreted as a omen or sign from God. The name of the comet recognizes that in 1682 English astronomer Edmund Halley predicted the return of the comet after reading records and recognizing the periodic return of the same comet. Halley also demonstrated that the orbit of the comet was in accord with English physicist Sir Isaac Newton's (1642-1727) law of gravitational attraction. The return of the comet in 1910 spurred some hysteria over whether gases from the comet might prove dangerous as the Earth passed through the path of the comet tail. In fact, although some of the gases in a comet can be toxic in dense form, they are many millions of times more rarefied than air, and thus what few molecules might survive entry into Earth's atmosphere would be harmless. As part of a concerted international study, in 1986 six spacecraft made specific studies of Halley's Comet.