What's covered here: What exactly are comets? Where do comets come from? What are asteroids? Why aren't they planets? What types of asteroids are there? What's the difference between meteor, meteoroid and meteorite? What causes "meteor showers"? What types of meteorites are there? What sort of objects have hit the Earth? How much damage can these objects do? When is the next chance for an Earth impact? Finally, we come to the last bit of detail involving the solar system - the junk. This is really just all of the little bits that can't be put into the categories of planet or satellite. We'll start off with the very spectacular comets, then the elusive asteroids, followed by the meteoroids-meteors-meteorites and end up with the problems that occur when worlds collide. Get ready for a bumpy ride! Comets Image of comet Hale-Bopp taken by Dr. Morgan, 1997.
Comets are quite different from planets in how they move around the solar system. While the planets tend to have fairly circular orbits, the orbits of comets are very elliptical, so that they are stretched out from near the Sun to the very edges of our solar system. Some of the orbits aren't even elliptical - the comets just do one passage in toward the solar system and then are gone forever. The orbits are also rather randomly oriented to the ecliptic; they could come in toward the Sun at pretty much any angle relative to the ecliptic. We know that comets are composed primarily of many varieties of ice, including water, carbon dioxide, methane and ammonia ice. There is also a bit of dirt mixed in, usually in the form of carbon. This makes them appear as dirty snowballs, which is actually the name for the model that is proposed for their compositions. The best way to think of a comet is that it is like a big chunk of ice, dirt and slush that gets stuck to the wheel well on your car during the winter.
Figure 1. The orbital paths of comets are very elongated (elliptical) and randomly oriented to the ecliptic.
Comets may appear as huge objects in the sky, but they are typically only about 10 km in diameter, much smaller than many other objects going around the Sun. This core or nuclei is how most comets appear when located in the outer solar system (beyond Jupiter's orbit). However, they don't always remain as small, dirty, frozen icebergs. Comets change as they orbit around the solar system, especially when they get into the inner solar system. As a comet moves closer to the Sun, the heat from the Sun will start to evaporate the ices that make up the core of the comet. The material is then in a gaseous state and will form around the core of the comet as a coma, or head of the comet. As the comet gets closer to the Sun, the gas starts getting blown off by the solar wind. Not only is the gas blown off, but also the heavier, dusty material gets blown away. Due to the motion of the comet, which is pretty fast, and the force exerted by the solar winds, the trail that this evaporated material leaves can grow quite large and will develop into tails. The coma can be thousands of times (or more) larger than the cometary nuclei, while the tails can be up to 1 A. U. in size (remember, 1 A. U. is about 100 million miles!).
Two tails are usually seen. These include the gas tail (also called the ion tail), which is made up of material that is blown straight back by the solar wind. This is generally made of the really lightweight gases. Within the gas tail you find stuff such as water vapor, CO, CO2, N2, ammonia and methane gases and particles. The gas tail has a rather ragged appearance and is sometimes rather bluish. It is always pointed directly away from the Sun. The other tail, the dust tail, is made up of heavier particles and is not as greatly affected by the solar wind. It has a very fuzzy appearance, often looking rather yellow-ish or whitish. This is, of course, made up of mainly dust (rocks and silicates). This is much heavier material, so it is not pushed into a straight line like the gas tail but often has a curved shape that is sort of symbolic of comets. Both tails get longer as the comet gets closer to the Sun. Actually, the tails start developing when the comet is still quite a ways from the Sun, well beyond the orbit of the Earth.
Another thing about comets that people are confused by is how fast they move. Comets orbit the Sun, so they obey Kepler's laws, just like the planets. Over the course of an evening it is possible to see tiny motions of a comet relative to the stars, especially if it is close to perihelion, but they don't go streaking across the sky as is often portrayed in cartoons. Think of it this way - you know that the Moon moves relative to the background stars, right? Can you actually see its motion if you sit there looking at the Moon for some time? No, because from our view point it looks like it is hardly moving along, but if you look at it the next evening, you'll see it is in a different location relative to the stars. The same is true for comets; their motions may be apparent from one night to the next, but to see motion with your eyes over the course of a few minutes during one evening would be difficult, if not impossible.
A comet is made up of material that gets evaporated easily by the Sun, so comets lose mass with each passage around the Sun. As much as one percent of their masses can be blown away. Comets may start out very icy, but this is not how they'll look for long, since the ice is the first thing to go. After a while, their nuclei will look very dark and dirty, since the dark, dirty material (mainly carbon) will not get blown away as easily. Comets that can't withstand the strong solar winds can also shatter apart. This has been observed recently in the case of Comet LINEAR (C/1999 S4), and Comet Schwassmann-Wachmann 3, both of which broke apart into smaller pieces. Other comets that pass very close to the Sun can either completely disintegrate or actually hit the Sun!
Where do comets come from? Comets were originally thought to come from the Oort Cloud, a spherical region that extends about 50,000 AU from the Sun where the cores of comets reside. Every once in a while, the comets are perturbed by a passing star or collision with other comets and some fall in toward the solar system. These comets tend to have orbits that are very elongated. In some cases the comet will only pass near the Sun once. In other cases the comet will have its path altered, usually by going too close to Jupiter, and it will become trapped into a shorter period around the Sun. The existence of the Oort Cloud is based on the characteristics of comets. Many comets tend not to be aligned with the ecliptic, so it makes sense that they originate from a place that surrounds the solar system in all directions. Also, the very long period (greater than 1000 years) comets have paths that stretch out to such great distances that it is logical that they originated at great distances from the Sun. It is estimated that there are about 100 billion comet cores out in the Oort cloud.
There is also evidence that many of the short period comets do not come from the Oort Cloud but from a closer reservoir of cometary material that was previously mentioned (in the last set of notes), the Kuiper Belt. This region is much closer than the Oort Cloud, extending from about 40 AU out to a few hundred AU at most. The characteristics of Kuiper Belt Objects can help explain the shorter period comets (less than 1000 years), especially those with orbits that tend to be closer to the ecliptic. Remember, if the distance from the Sun is smaller, the time for an orbit is smaller - Kepler's Third Law is action.
Figure 4. The two comet sources are shown. First is the Oort cloud, located much further from the solar system, and the second is the Kuiper belt, located just beyond the orbit of Neptune. Image from Calvin J. Hamilton.
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