What are they?
Comets are mixtures of ice and dust. They have earned the nickname of "dirty snowballs" by some astronomers. They have many parts, as you can see in the diagram:
Nucleus--Consists of mostly ice and gas, with some dust and other solids.
Coma--A dense water cloud, made up of carbon dioxide and neutral gases, that comes off of the nucleus.
Hydrogen cloud--A huge section of hydrogen.
Dust tail--The most visible part of the comet made of dust particles from the nucleus. Dust tails are very long and smoke-like.
Ion tail--Even longer than the dust tail, consists of plasma. This is the part of the tail that interacts with the solar wind.
Where do they come from?
Comets are actually collections of materials left over from the formation of the solar system. This makes them especially interesting to astronomers because the solar system formed around 4,600 million years ago! Studying the composition of comets can tell us something about the history of the solar system.
What do they have to do with the Sun?
Without the Sun, we wouldn't be able to see comets very easily, maybe at all! This is because they reflect the light from the Sun, rather than having their own source of light. This makes the sun vital to the observation of comets.
The Sun is actually what "makes" the various parts of the comet listed above. As a comet approaches the Sun, the radiation evaporates the ice. This causes gases and dust to drag from the nucleus--the coma. Radiation from the Sun blows the dust away from the head of the comet, creating the dust tail. The solar wind drags ionized gas from the comet, creating the ion tail.
Where do they go?
Comets orbit around the Sun just like planets, but their orbits are more elongated and take longer. Most of them go beyond Pluto! Periodic comets are those that have an orbit of less than 200 years, but they make up the minority of known comets.
How long do they last?
That depends on the comet's orbit. A comet loses some of its ice and gases every time it goes by the Sun. After about 500 passes, most of the ice and gas is lost, and the comet turns into something close to an asteroid. So the amount of time it takes a comet to orbit the Sun determines how long the comet will last.
Why are comets so important?
Comets are actually more than just objects flying through space that are neat to look at. They may have had a huge effect on Earth! Many astronomers believe that impacts with comets brought the large amounts of water to Earth that we have today. Water allowed life to begin on Earth. Comets could be the reason for our existence! Many scientists also believe that a comet caused the extinction of the dinosaurs. If this is true, what would have happened if a comet had not collided with the Earth? Would we be here today if the dinosaurs had not gone extinct?
Before they were understood, comets caused quite a stir throughout history. People have blamed them for all sorts of tragedies, from the start of wars, to plagues, illnesses, and deaths of leaders. In recent times, with the onset of the new millennium, there has been a lot of hype about "doomsday" comets, which are comets that people believe will hit the Earth. The reality is that comets only impact the Earth every few million years. There are also several telescopes devoted to scanning the solar system to look for potentially dangerous objects, so even if a comet looked like it was heading our way, we would be able to detect it ahead of time and possibly change its course. Although this does not rule out the possibility of a comet impact with Earth, the chance it will happen while humans are still alive on this planet is unlikely.
There are some groups of people that promote fear of comets by trying to "prove" that one will hit the Earth. Gary Kronk gives a good commentary about why comets shouldn't be feared. (Check out his comet home page, too!)
Here are some other great comet links:
Comets and Meteor Showers--from OSR articles
Comet Introduction--includes a good comet tutorial
the Comet's Orbit--an entertaining look at comets
Here are some specific comet links:
Comet Hale-Bopp Home Page (JPL)
Comet Hyakutake Home Page (JPL)
Comet Shoemaker-Levy Home Page (JPL)
Hale-Bopp for Kids
April 14, 2002
Comet Ikeya-Zhang is now a morning comet, too! It is climbing higher in northern skies as it recedes from the Sun and approaches our planet.
April 1, 2002
If you have dark skies and you can see the western horizon, then get ready to see Comet Ikeya-Zhang just after sunset on April 4th. Although it will be visible to the naked eye, you will get a better look if you use binoculars.
March 18, 2002
Comet Ikeya-Zhang's orbit brought if close to the Sun, its closest approach to it in almost four centuries. Solar heating made the comet extremely visible.
Click here for the most recent image of the comet.
January 22, 2002
The comet passed close to the Sun on January 22nd and then flared. It is now pretty easy to see for Southern sky watchers. Try looking in the hour before dawn on the eastern horizon, near the Sagittarius constellation.
January 7, 2002
Comet 96P/Machholz will be coming close to the Sun this week, as it does every 5.24 years. Although it can't be seen by the human eye because it is so close to the Sun, coronagraphs make it possible to view the comet.
Click here for an animation of the comet swinging by the Sun.
July 13, 2001
The comet LINEAR flared on the 13th, the faded a great deal throughout the weekend, but now it is back as a naked eye object. Although at the moment it is difficult to see, who knows what it will do in the days ahead?
June 16, 2001
Comet LINEAR has brightened once again. It is heading on a path which will bring it close to Earth at the end of the month. It is only visible in the south right now, but will be visible to northern viewers by June 30. The comet is so bright now, however, that it can be seen with the naked eye in the eastern sky around 4 AM.
May 10-11, 2001
The comet LINEAR that split recently has brightened suddenly. The comet may be visible to those in the Southern hemisphere with binoculars after sunset. The comet may get brighter as the day approaches when it will have a close encounter with the Sun, May 24th.
May 3, 2001
It was reported on Tuesday, May 2nd that the nucleus of Comet LINEAR has split in two! It is brightening as it approaches the Sun and may soon become visible to the naked eye. The comet will become extremely close to the Sun on May 24th. The split comet may be visible to those watching through a telescope around sunset tonight, somewhere near the feet of the constellation Orion.
Click here for some more information about Comet LINEAR and an image.