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Comets 101

By Hal Weaver, cometary scientist at Johns Hopkins University

What's in a Comet

A comet is composed of a nucleus, a coma, and two kinds of tails. The nucleus, which is typically only about 1 to 10 miles across, is at the center of the comet. Fred Whipple, the Harvard astronomer who is the father of cometary science, dubbed the nucleus a giant ``dirty Nucleus of Halley's comet snowball.'' That description was thought to be appropriate because cometary nuclei were known to contain a mixture of ices (the ``snowball'' component) and meteoritic material (the ``dirt''). However, based on recent infrared observations, some astronomers now think that the nucleus is more like an ``icy dirtball'' with a much higher proportion of dirt to ice than was originally thought. Except when various spacecraft flew near the nucleus of Comet Halley in 1986, no one has ever seen a comet nucleus directly.

As the nucleus moves closer and closer to the Sun, it gets warmer and warmer, which causes its ices to evaporate. When the ices evaporate, they drag Dust particles (i.e., the meteoritic material) off of the surface of the nucleus. Sunlight reflecting off of these dust particles produces a coma, which is primarily what you see when you look at a comet.

The dust particles leaving the nucleus are pushed by light from the Sun into a dust tail. The gas molecules (like water, carbon monoxide, and carbon dioxide) that evaporated from the ices are ionized by sunlight (the sunlight tears off one of their electrons) and are pushed by the solar wind into an ion tail.

Where Do Comets Come From?

Comets are members of our Solar System. But unlike the Earth and other planets, which always stay at approximately the same distance from the Sun, most comets are great travelers that spend most of their time on the outskirts of the Solar System, way beyond Pluto and then wisk in briefly for a close pass near the Sun.

The comets that pass close to the Sun originally came from one of two places: either the Oort Cloud or the Kuiper Belt. You can think of the Oort Cloud as a giant spherical shell surrounding the solar system that's filled with about 1 million million comets (a 1 followed by 12 zeros). Its inner and outer boundaries are not very well defined, but the cloud's extent is roughly from about 900 thousand million miles (a 9 followed by 11 zeros) from the Sun to about 9 million million miles (a 9 followed by 12 zeros) from the Sun. The outer boundary is almost halfway to the nearest star and, for all practical purposes, is the outside edge of our Solar System. Because they are so far from the Sun, the comets in the Oort Cloud take over 1 million years to make a single revolution around the Sun. Occasionally one of these comets feels a tug by another star in our galaxy and gets pushed closer to the Sun. Since such comets are probably making their first such trip near the Sun, they are called "new" comets. Approximately a dozen "new" comets are discovered every year.

The Kuiper Belt refers to a roughly disk-shaped region that lies in the same plane that contains the Earth and the other planets and which extends from just beyond Pluto's orbit out to about twice Pluto's orbit. (Pluto is about 40 times farther from the Sun than we are.) Astronomers believe that most of the comets that travel around the Sun in fewer than about 200 years originally came from the Kuiper Belt. These so-called "periodic," or "old," comets slowly drifted closer to the Sun over time. When they approach Jupiter, their paths are disturbed in a very chaotic way.

It is often difficult to tell whether a comet originally came from the Oort Cloud or the Kuiper Belt. For example, Halley's comet has a period of only 76 years, but many astronomers believe that Halley was once in the Oort Cloud.

What Makes Comets So Important?

Since comets have spent most of their existence in the outer regions of the solar system, they are essentially ``cosmic refrigerators.'' They are so cold that they have been able to preserve a record of the physical and chemical conditions of the solar system as it existed 4.6 billion years ago, during the time when the planets and Sun were formed. When we observe comets today, in a very real sense we are looking back into the past and learning about the origin of the solar system.

Although there are still many things about comets that we do not understand, scientific investigations over the past couple of decades have enabled us to identify at least some of the ices that are contained in comets. For example, we know that plain old water ice is the most abundant constituent of a comet. Since comets frequently collided with the Earth in the first billion years after the solar system was formed, it's quite possible that comets provided most of the water on our planet. Life as we know it requires abundant water in order to be viable, so cometary bombardments may have been key to the development of life on the Earth.

Of course, cometary bombardments can also be hazardous to life that already exists. A large comet or asteroid that impacted the Earth just off the Yucatan peninsula in Mexico approximately 65 million years almost certainly caused global-wide catastrophes, which probably contributed to the extinction of the dinosaurs. While such collisions are few and far between, efforts are being made to map all of the comets and asteroids that pass close to the Earth's orbit so that we can produce a reliable quantitative assessment of the impact hazard. Whether or not we can do anything to prevent catastrophe is a more difficult issue that we will ultimately have to face.

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