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Comets only have tails when they are close to the sun. When they are far from the sun, comets are extremely dark, cold, icy objects. The icy body is called the nucleus. Nuclei are made of various types of ices, dirt and dust. As comets get closer to the sun in their journeys through the solar system, they start to warm up.

As they reach an area roughly the same distance from the sun as Jupiter, the ices start to heat up and vaporize, releasing the gases and embedded dust particles that form a cloud or atmosphere -- called a coma -- around the comet. As comets continue traveling closer to the sun, the dust particles and other bits of debris in the coma are blown away from the sun due to the pressure of sunlight. This process forms a dust tail.

If this tail is bright enough, we can see it from Earth when the sunlight is reflected off the dust particles. Comets usually have a second tail too. This one is called an ion or gas tail and it is formed when the ices of the nucleus are heated and turn directly into gases without passing through the liquid stage -- a process called sublimation. The gas tail forms when charged particles from the sun, called the solar wind, push the cometary gas particles directly away from the sun. The gas tail is visible because its particles glow after being excited by solar radiation.

Once comets start moving away from the sun, their activity decreases. Their tails fade and the coma disappears. They return to just the icy nucleus again. When comets' orbits eventually bring them back towards the sun, the coma and tails begin to form again.

Comets come in a wide range of sizes. The largest nucleus observed is about 25 miles, or 40 kilometers, in diameter. The dust and ion tails can be enormous. Comet Hyakutake's ion tail stretched 360 million miles, or about 580 million kilometers.

About 4 ½ billion years ago, our sun was just beginning to take shape as a star. As the sun got warmer and warmer, a cloud of ice and dust surrounded the early sun. As the cloud formed, it started rotating. It also slowly changed from a spherical shape to one that was flatter, shaped more like a donut with the sun in the hole. The cloud whirled around and around and the rotating cloud got flatter and denser. In the area of the cloud most distant from the sun, dust and ice particles came together and stuck to each other. Over millions of years, these particles got a little bigger. Eventually these particles grew to the size of basketballs and then cars and buildings. Finally, these particles grew to the size of city blocks. Many of these objects in the distant solar system became known as comets.

In some cases, these large particles kept sticking together, eventually forming the cores for the giant planets Jupiter, Saturn, Uranus and Neptune. A similar process occurred closer to the sun but the cloud contained very little ice. The inner planet (Mercury, Venus, Earth and Mars) formation process began when tiny pieces of dust sticking together, grew to the rocky inner planets with the leftover bits and pieces being what we now call asteroids.

Comets originate in the outer solar system. Some comets head toward the inner solar system from a region called the Kuiper Belt, which is beyond the orbit of Neptune. These comets orbit the sun in less than 200 years. Most comets come from a region that is at the extreme outer edge of our solar system. This area is called the Oort cloud. Comets that head toward the sun from here have much longer orbital periods. Comets from this area may take more than 30 million years to orbit the sun just once.

Diagram showing the parts of a comet.
Anatomy of a comet.

The nucleus of the comet is its solid part that is embedded in a cloud of gas and dust called the coma. The term arises from our perception of the comet as an observational phenomenon, of a fuzzy ball of light with a tail. The nucleus would then be the center of the phenomenon. Until 1986 when the Giotto spacecraft flew past Halley's comet, we had never seen the nucleus of any comet.

Now that we can get close to a comet and study it in more detail, we start to talk about the parts of a nucleus going into its interior. We ask if there is a 'crust' of the nucleus which is the top layer that has been processed by devolatilization (the removal of icy materials). Is there a 'mantle', which is a layer that is denser than the crust. Since the layering implies some processes involving heat and alteration of material, we then can hypothesize a cometary 'core', which is the very center of the solid part of the nucleus. We don't know the nature of the crust, mantle or core of the comet or even if they exist. In the Deep Impact mission we plan on finding out if layering exists and if it does, to study the crust and mantle of the comet.

We don't know how many comets there are since most have never been seen. There could be one hundred billion in the Oort cloud. As of 2010, astronomers have discovered about 4,000 comets in our solar system.

In general, comets can be a thing of wonder to see from Earth. It is a great sight to see a comet in the night sky. Also, scientists think that comets helped bring important things like water and other materials that would help life originate and evolve on Earth.

In extremely rare cases, comets can cause problems on Earth. Most scientists believe that a very large asteroid or comet hit Earth about 65 million years ago. The resulting changes on Earth led to the extinction of the dinosaurs. Very large asteroids, as well as very large comets, could possibly cause major damage if one was to hit Earth. However, scientists believe that large impacts like the one that killed off the dinosaurs occur once very few hundred million years.

The path comets take as they orbit the sun may get altered for a few reasons. If they pass close enough to a planet, the tug of that planet's gravity can slightly change the comet's path. Jupiter, the largest planet, seems the most likely planet to alter a comet's path. Telescopes and spacecraft instruments have taken images of at least one comet -- Shoemaker-Levy 9 -- breaking up and hitting Jupiter's atmosphere. Also, comets head towards the sun and in some cases, fall right into it.

Over millions of years, most comets run into the sun or another planet, are gravitationally ejected from the solar system by a major planet or lose their ices and disintegrate during their travels through the inner solar system near the sun.

Actually, early cultures where not even aware of comets as being icy bodies in the Solar System. So every comet they saw was a 'new' demon in their belief system. It wasn't until the late 1600's that astronomers thought that some comets that had been observed through the centuries might be the same comets seen over and over. In fact, the "first" comet that is defined as periodic, meaning that it is seen on a regular basis, is comet Halley. It is named after Edmund Halley who noticed that every 76 years there was a comet visible. He was able to predict when that one comet would return. Unfortunately, he died before he could see his prediction come true, but the comet was still named after him. How was he able to predict the return of this comet? Well, every comet has a very distinctive orbit or elliptical path around the sun. That orbit can be defined with a set of numbers that we refer to as "orbital elements." Since the path of the comets (and the planets) are in three dimensional space, we need at least 3 numbers to orient that path, a few others to define the size and shape, and a few to define the position in the orbit. Using these orbital elements, we can then calculate a comet's past and future positions to predict when it will next be observable. If a comet is found by chance, one has to observe it long enough to trace its path on the sky, fit it to an ellipse to determine the orbital elements, then compare those values to a table of known comet orbital elements to identify it.

Good question. Water vapor in the vacuum of space and when exposed to sunlight further breaks down in a process called photodissociation, into H atoms and OH molecules. The OH molecules fluoresce in the presence of sunlight and is detected with spectrometers sensitive to Ultraviolet light. In fact, we can measure the abundance of water in a comet's nucleus by measuring the intensity of the emission bands and applying some scaling factors.

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Last Updated: 23 Jun 2015