Comet Anatomy

• FIELDS OF STUDY: Astronomy; Cosmology; Astrochemistry
• ABSTRACT: A comet is an icy celestial body comprised mainly of frozen gases, rock, and dust that has an elliptical orbit around the sun. The main part of a comet is the nucleus. When a comet’s orbit takes it near the sun, the nucleus begins to sublimate, and gas and dust form a cloud around it called a coma. When the comet comes even closer to the sun, solar wind and heat push the cloud behind the nucleus, creating tails.

A Dirty Snowball

A comet is a celestial body made of frozen ice, rock, and dust that orbits the sun. Because of its composition, a comet is often called a "dirty snowball." Scientists believe that comets were created during the formation of our solar system about 4.6 billion years ago in either the Kuiper Belt or the Oort Cloud regions. A comet has three parts: a nucleus, a coma, and a tail. The frozen body of a comet is called the nucleus.

A comet has an elliptical orbit, so at times it is close to the sun. When a comet approaches the sun, its nucleus begins to sublimate (go directly from solid to gas) and release gas and dust, forming a coma, or cloud, around it. When the comet comes very close to the sun, solar winds blow the dust and gas in the coma away from the sun, which creates the coma’s tail. Comets that have a bright tail can be seen from Earth.

The size of a comet’s orbit determines how often it can be seen from Earth. A comet with a smaller orbit (short period) is visible more often. The famous Comet Halley (or Halley’s Comet) travels around the sun approximately every 76 years. People on Earth will see Halley again in 2061. The comet Hale-Bopp has a very large orbit. Hale-Bopp was visible to the naked eye for eighteen months in 1995 and 1996, but will not be seen again for about 2,400 years.

The Oort Cloud comet, also known as C/2023 A3 Tsuchinshan-ATLAS, approached the inner solar system for the first time in recorded human history in 2023, according to NASA in 2024. Astronomers believed that the Oort Cloud comet's path may take it out of the solar system so it will not be seen again. Scientists theorized that the comet might break up as it passed by the sun because of the sun's intense heat, but it remained mostly intact. The comet was first spotted by the International Space Station (ISS) when it was 44 million miles away.

A Comet’s Anatomy

The main part of a comet is the nucleus, which is a large chunk of ice and other components. The ice may be frozen gas, such as carbon dioxide, nitrogen, ammonia, and methane; other elements include rock, iron, and dust. The composition of a comet’s nucleus varies. When a comet is far from the sun, it consists only of a nucleus. A comet’s nucleus may range from a few hundred feet to many miles wide. Hale-Bopp, a very large comet, has a nucleus that is 30 to 40 kilometers (19 to 25 miles) wide.

The ice in a comet’s nucleus begins to melt when the comet is about 3 to 4 astronomical units (AU) from the sun. Gas, water, and dust "boil off" or are expelled from the nucleus and form a cloud around it. This process is referred to as sublimation. The gas portion of the cloud that forms around the nucleus is called the gas coma. A dust coma is the dust portion of the cloud. An invisible layer of hydrogen surrounds the coma. It is called the hydrogen cloud or hydrogen envelope. A comet’s coma grows and can become very large—occasionally even bigger than the sun. The nucleus and the coma are referred to as the "head" of a comet.

As the comet comes closer to the sun, solar wind and heat push the coma behind the nucleus. This forms tails that reflect sunlight, making the comet a beautiful sight in the night sky. Dust and gas each form their own tail. A comet’s tail is sometimes hundreds of millions of kilometers long.

When a comet orbits the sun, it leaves behind some of its material. In time, it will not have enough material to form a coma and a tail. When this happens, the comet becomes extinct and may be classified as an asteroid.

The Study of Comets

Throughout history people were both fascinated and terrified by the comets they saw in the night sky. They did not understand the existence and workings of the solar system. Comets seemed to appear and disappear randomly. People worried that a comet might be a sign of bad luck to come, such as a storm or an earthquake. They tried to use comets to predict the future.

Scientists in modern times study comets to learn about the past. Because comets formed so long ago, they can help scientists learn about the formation and evolution of our solar system. They hope that spacecraft and probes will help them learn more about the chemistry of comets. They theorize that learning more about comets may help them find life in other places within the universe.

When their orbit takes them close to Earth, some comets can be seen at night with the naked eye. However, comet chasers often use binoculars and telescopes to view them more closely.

PRINCIPAL TERMS

• comet: an icy body made primarily of frozen gases and dust that orbits the sun.
• dust coma: a cloud of dust that surrounds the nucleus of a comet when it approaches the sun.
• gas coma: a cloud of gas that surrounds the nucleus of a comet when it approaches the sun.
• hydrogen cloud: an invisible layer of hydrogen that surrounds a comet’s coma.
• nucleus: the frozen body of a comet.
• tail: ionized gas and dust that is pushed away from the head of a comet.

Bibliography

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Peterson, Carolyn Collins. Astronomy 101: From the Sun and Moon to Wormholes and Warp Drive, Key Theories, Discoveries, and Facts about the Universe. Avon: Adams, 2013. Print.

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