Comets are truly one of the Universe's great spectacles. If you were aware and had the opportunity to observe the passing of the great comet Hale-Bopp in late 1996 and early 1997, you were observing a remarkable event. This comet could be seen during daylight for a period of time.
These beautiful interlopers are believed to originate in the Oort Cloud region of space which is far beyond the orbit of Pluto, perhaps beginning at 30,000 AU (One Astronomical Unit= Earth-Sun distance, 93,000,000 miles) and extends out to 100,000 AU. Some astronomers believe that this region may extend as far as half way or more to our next closest star Proxima Centauri (4.23 Light Years distant or 24.6 trillion miles). The Oort Cloud region is a comet nursery and contains trillions of comets of all sizes.
We, on Earth, are presented the opportunity to view these magnificent comets when certain events occur. As everything moves in the Universe and gravity influences all bodies in the Universe, comets in the Oort Cloud region are influenced by the gravity of passing stars, asteroids and meteors—not to mention any large comets within the Oort Cloud region itself. They begin their journey through space. This journey takes an incredibly long time to enter our solar system so that we can observe them. Thousands, tens of thousands of years pass before we can view them in telescopes, or better yet, the naked eye. It is always a celebrated event to observe a comet.
The structure of a typical comet has three parts: the nucleus, the coma, and the tail. The nucleus is where the substance of the comet is located; the coma is the dust and gas filled atmosphere immediately surrounding the nucleus; and the tail is the dust and gas of the comet that follows the coma. Tails of comets always point the opposite direction from the Sun. Our star has what is called a solar wind, and this is what pushes the tail from the comet. As the comet approaches the Sun, the material from the nucleus and coma burn off and leave generally long sweeping tails for millions of miles. Tails can be single, double, or triple. The white portion of the tail is the debris coming off of the comet, and the blue tail (Ion Tail) is gas that is sublimating from the nucleus and coma.
Comets are not dense like our planet or our moon. They are spongy in design, which is why they shed material from the nucleus upon entering our solar system. Halley's Comet (passed Earth in 1986) was first recorded in 240 B.C. and has a period of between 74-79 years before it returns for observation. Each time a comet passes the Sun, more and more material are lost, and eventually comets will burn out. A short-period comet is one that returns in under 200 years. Long period comets, such as Kohoutek, may have a period of 70,000 years, and when it reaches aphelion (farthest distance from the sun), it may be gravitationally influenced by another object and never return again.
In the case of comet Shoemaker-Levy 9, it was discovered in 1993 in a two-year orbit around Jupiter. This discovery was a most remarkable find, and it had practically every telescope on Earth and in space pointed in its direction. Jupiter has tremendous gravity. And when this comet came into Jupiter's reach, the planet tore the comet into twenty pieces. They crashed successively into Jupiter over a period of six days in July of 1994. Infrared fireballs and dark clouds were produced in Jupiter's atmosphere. I had the opportunity to meet David Levy late last year, and he says that he still hunts for comets every chance he gets. We wish him the best, and maybe he will discover another one, although it will be hard to top Shoemaker-Levy 9.