Comets are basically frozen masses of water, carbon dioxide, ammonia, methane, dust, and chunks of rock that orbit the Sun. They have been referred to as "dirty snowballs." The main body of a comet is called a nucleus, which may be several miles in diameter. As a comet approaches the Sun, some of the frozen material is heated and sublimes (sublimation is the change of state from a solid to a gas). The gases expand to form a cloud-like structure known as the coma. As the comet gets closer, these gases are illuminated by sunlight, making them visible to us if the comet is either especially large or especially close to the Earth. Comet Hyakutake is not all that big but will pass very near the Earth (about 10 million miles or so, or about 1/9 the distance between the Earth and the Sun).

The hallmark of a comet is its bright tail or tails. For some reason comets are associated with speed, but the word comet means "long haired" in reference to the streaming tails of some comets, which may extend tens of millions of miles from the nucleus. The tails are simply extensions of gas and dust pushed back from the nucleus and coma by the solar wind.(If you remember from a science question last fall about auroras, the solar wind is an endless stream of atomic particles generated by the Sun) Even though the nucleus is the brightest part of the comet, it's the tail that seems to draw our gaze. There are two distinct types of tails. The longer plasma tail (also called the gas or ion tail) always points away from the Sun and is made of, for the most part, gas molecules. A second tail is sometimes visible. This is the dust tail and is composed chiefly of silicate grains (dirt). It trails behind the comet like smoke from a freight train. It's predicted that both tails should be visible on Comet Hyakutake.

Comets have their orgin in a region well beyond the orbit of Pluto, and perhaps half way to the nearest stars. This region is at the outer limits of our solar system and is called the Oort Cloud. There are possibly hundreds of millions of comets in waiting here. Occassionally, a passing star or even a comet already in orbit around the Sun will nudge one of these dirty snowballs. If it's jossled toward the Sun, it will eventually have an orbit all of its own. This is probably what happened to Comet Hyakutake about 5000 years ago. Comets that arrive from the Oort Cloud are called long period comets. Sometimes these comets can be influenced by the gravity of one of the planets as they travel around the Sun. Jupiter is large enough to alter the orbit of a comet if it ventures too near. This most likely happened to Halley's Comet several thousand years ago. As a result, it now takes only 76 years to orbit the Sun. Comets that return in less than 200 years are called short period comets.

How is it known that Comet Hyakutake takes about 10,000 years to orbit the Sun if it has never been seen before? Once a comet has been observed and its position plotted over a period of a few days or weeks, astronomers are able to predict its orbit by measuring its velocity (the distance traveled divided by the time taken). This is not a simple task, since comets, like all orbiting bodies, move faster as they get closer to the Sun. Comets have extremely exaggerated orbits. Whereas the planets have relatively circular orbits, the orbit of a comet, is markedly oblong or elliptical. The Sun's tremendous size and mass, in effect, reels in the comet and then slings it back to whence it came.

On each passage around the Sun a comet uses-ups some of its matter. So, with every successive visit they lose some of their pizazz, and eventually they won't reappear. However, if you recall from an earlier science question, we can still see their final gasps for glory when the Earth passes through their orbit and we are treated to a shower of meteors.

All indications are that Comet Hyakutake will be the most impressive comet since Comet West twenty years ago. It may not be so stunning as to take your breath away, but it should be bright enough and big enough (about the size of the full moon) to see with the unaided eye. It make look something like a stationary search light overhead that changes position each night. The best viewing will be the next few nights - from tonight (Mar. 22) through about Mar. 27. After this time it will gradually get dimmer but should be visible until early April. For best results, get away from street lamps and look to the east of the Big Dipper tonight and tomorrow night. On Sunday and Monday it will be in the vicinity of the Little Dipper (near the North Star). If for some reason you miss it, another comet (Comet Hale-Bopp) is predicted to put on a good show in April of 1997.

One of the neat things about comets, especially for a new comet, is that unlike most of the other things we can see (planets, moons, stars, eclipses, sunrises and sunsets etc.), there aren't any tables or guides to tell us much about them. Also, when you think about it, it's pretty amazing that we're able to see an object close-up that at one time was perhaps more than a light year away. Whether or not this is visually a great comet remains to be seen. Exceptional comets, ones that blaze in the night sky for days at a time, are rare indeed. These are the comets that evoked awe and brought fear into the hearts of people in centuries past. It's easy to understand how something that wasn't predicted and hangs ghost-like in the night sky could have been thought to signify some remarkable event. In 1066, for example, King Harold of England saw what we know now was Halley's Comet to be a bad omen. He was right. William the Conqueror saw it as a good omen. He was also right, and the new King of England.