Comets: Origin & Properties

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  • 0:01 What Is a Comet?
  • 1:06 The Properties of Comets
  • 3:01 The Origin of Comets
  • 5:12 The Oort Cloud and Kuiper Belt
  • 6:22 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will define and discuss comets, a coma, dust tail, gas tail, sublimation, the composition of comets and their origins in the Oort cloud and Kuiper belt.

What is a Comet?

A comet is a small, icy body that orbits the sun and develops a tail and glowing head.

Ancient people, including the famous Aristotle, did not believe comets were celestial objects. Instead, they used to believe that comets were things that occurred within the Earth's atmosphere.

Back in those days, people also believed that comets were good or bad omens. For instance, after Julius Caesar was brutally murdered by Brutus, games were held in honor of his deification. On the first day of these games, a comet appeared and hung around for about a week. People thought the comet was really Caesar's soul going up into heaven.

Even as recently as 1910, many people around the world thought the world was going to end when Halley's comet appeared that year.

As interesting as all the historical facts about comets are, we need to get down to the meat of this lesson. So, let's busy ourselves with the important properties of comets and their origins.

The Properties of Comets

A comet is composed of several different parts. The first part of the comet is known as the nucleus; it is the icy heart of a comet, composed of frozen water, methane, ammonia, and carbon dioxide, as well as dust and rock embedded within the ice.

You can think of the nucleus of a comet as a dirty snowball of sorts, as it's made up of a mixture of ices (mainly water ice) with dust and rock in it.

When a comet nucleus approaches the sun, it will begin to form a coma and a tail.

The bright head of a comet, made up of a spherical cloud of gas and dust around the nucleus of a comet, is known as the coma. The coma can reach a diameter of over a million kilometers (larger than the sun).

And the last major part of a comet is probably the most famous one: the tail. But did you know there are actually two different tails to a comet?

The gas tail (aka ion or plasma tail) is a comet tail that's made up of ionized gas. As the ices in a comet's nucleus are vaporized or sublimated by the sun, the gas is carried away from the nucleus by the solar wind. The atoms of this gas tail are ionized by the sun's U.V. rays and glow as a result.

Sublimation, by the way, is the change of a frozen solid directly into a gas, bypassing the liquid state.

The dust tail is a comet tail made of dust that is released by the comet's nucleus. So, as the ice of a comet's nucleus is sublimated, the dust previously embedded within it is released. This dust is then pushed outwards by the pressure of sunlight.

Some of the dust particles released by a comet encounter Earth's atmosphere and are seen as meteors in our sky.

The Origin of Comets

As I mentioned not too long ago, the nuclei of comets are made of a mix of ices, like that of water, carbon dioxide, methane, and ammonia. Such material could've clumped together only in the cold outer reaches of the solar nebula. It's simple really; I mean, you know heat melts ice. So, the only way to have it form is to be far away from the sun.

Thus, astronomers are pretty sure that comets are actually ancient samples of the same dust and gas from which the Jovian planets formed. In other words, they are little bits and pieces of history about the beginning of our very own solar system.

But it's not that simple, really (sorry about that). I'm going to throw in a little kink as food for thought here; here it goes.

A comet that enters the inner solar system over and over again cannot survive forever. Remember, it's a ball of dirty ice. The heat from the sun will vaporize literally millions of tons of ice with every passage around the sun.

This doesn't mean that a comet will be completely vaporized during one pass through the inner solar system.

It's like those big mounds of dirty snow and ice you see piled up in parking lots. They don't melt all in one day; but with every warm day, little by little, the mounds melt away for good.

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