Comet | Research & Encyclopedia Articles

Comet

Generally consisting of a nucleus, head, and tail, comets are celestial bodies, similar in appearance to stars, whose origins and structures scientists are still in the process of determining. Because of their unusual shape and sudden appearances, comets were commonly viewed in ancient times as warnings or forerunners of great news. Aristotle was among those who believed comets were atmospheric phenomena because they were found outside the zodiac and they appeared to be different from planets. However, he also suggested that comets were weather signs, a statement that kept fear of comets alive for centuries.

When printing began, every comet appearance brought forth pamphlets with such titles as "News of the Terrible and Fearsome Comet." Halley's Comet (before it was so named) was especially seen in this light when it returned in 1456, just three years after the Turks had conquered Constantinople.

Nonetheless, there were those who looked scientifically at this phenomenon. Johann Müller (1436-1476), who was also called Regiomontanus, observed a bright comet in 1472. He followed its path in the sky and made such accurate measurements that Edmond Halley was able to use them much later to calculate the comet's orbit. Two other early observers, Girolamo Fracastoro (c. 1478-1553) and Peter Bennewitz (1501-1552), published reports that stated that the comets they had observed always kept their tails pointed away from the sun. Johannes Kepler, who observed the comet of 1607, concluded that comets followed straight lines, coming from and disappearing into infinity. Johannes Hevelius (1611-1687) thought comets followed slightly curved lines. Georg Samuel Dörffel (1643-1688), contradicting both, suggested that comets might follow a parabolic course. When the comets of 1680 and 1682 had appeared, Halley set out to definitively calculate their orbits. He looked back at past comet appearances, as described by careful observers, and noticed that a few orbits were the same. He claimed the comet of 1682 was the same one seen in 1456, 1531, and 1607. He predicted it would return in 1758. Even though he died before its return, Halley lived on in the name of the comet. His comet and others had elliptical orbits, so astronomers of the time believed there were two types of comets with differing orbits: periodic comets that had elliptical orbits and nonperiodic comets that were one-time visitors and had parabolic orbits.

Further study revealed that comets followed elliptical paths much more frequently than parabolic ones. Parabolic paths were believed to be caused by gravitational forces of stars and planets the comet encountered or passed near during its flight. Depending upon the velocity and proximity of the two bodies at the crucial juncture, the comet's path might be altered only slightly or drastically. Comets, therefore, appeared to be residents of the solar system, though occasional disturbances such as gravity might cause some to exit the solar system or disintegrate.

Modern scientists speculate that there might be a reservoir of comet nuclei surrounding the solar system. Jan Oort pioneered this theory with the assertion that these inactive comets would have a very low temperature and number in the billions; such comets would remain inactive until a significant astronomical occurrence--the passing of star, for example--would pull a comet into the solar system by means of its gravitational force. This reservoir is now referred to as the Oort Cloud.

Astronomers have spent a great deal of time considering the composition of comets. They used spectroscopes to analyze comets, but the results in the late 1800s proved inconclusive because the spectrogram indicated mostly reflected sunlight. Harvard astronomer Fred Whipple (1906-) coined the description "dirty snowball" to explain his understanding of comets, now generally accepted as bits and pieces of dust and rocky material mixed with frozen methane, ammonia, and water.

In March 1986 five spacecraft passed near the long-awaited Halley 's Comet. The most daring craft, Giotto, flew in toward the comet's center and sent back the first pictures of Halley's nucleus. These shots revealed a potato-shaped object darker than coal measuring 7.5 miles (12 km) in length. Two bright jets of dust were seen extending 9 miles (14.5 km) from the nucleus toward the sun. These jets were caused by the sun's heat, and they are responsible for causing comets to deviate from their scheduled appearances.

The mid-1990s produced a bumper crop of interesting comets. In 1994, comet Shoemaker-Levy 9 collided spectacularly with Jupiter. The comet, which had been broken into numerous fragments during a previous orbit of the giant planet, became elongated into a string of pieces that impacted Jupiter in the summer of 1994. Astronomers around the world spent the months preceding the event debating whether or not the impacts would visibly disrupt Jupiter's atmosphere. Once the huge blemishes in Jupiter's clouds put that issue to rest, they spent the months afterward studying the results.

In 1996, comet Hyakutake passed a mere 9 million miles from Earth, its tail stretching some 40 degrees across the sky. In 1997, the more distant but intrinsically brighter comet Hale-Bopp swung by. Both these comets follow large orbits that will not bring them back to the inner solar system for thousands of years. However, the modern, detailed observations that were made of them yielded important new information on the composition and rotation periods of comets.