Nucleus (Comet Anatomy)
The nucleus of a comet is its solid core, primarily composed of ice, dust, and rocks. This central body plays a crucial role in a comet's structure and behavior as it orbits the sun. When far from the sun, the nucleus remains frozen, but as it approaches, the heat causes the ice to sublime—transforming directly into gas—resulting in the formation of a coma, a surrounding cloud. The coma can expand significantly, sometimes reaching over 96,000 kilometers (60,000 miles) in diameter, and as the comet continues its journey, the dust and gas are pushed back to create distinct tails that can be visible from Earth.
Comets are categorized based on their orbits; short-period comets originate from the Kuiper Belt, while long-period comets come from the distant Oort Cloud. Notable examples include Halley's Comet, which has a 76-year orbital period, and Hale-Bopp, which has a much longer cycle of over 2,500 years. Scientific interest in comet nuclei has led to numerous space missions, revealing that many nuclei, like Halley's, consist largely of dust and ice, predominantly water. Recent discoveries, such as the largest known comet nucleus measured at approximately 80 miles (129 kilometers) across, underscore the ongoing exploration and understanding of these fascinating celestial bodies.
Nucleus (Comet Anatomy)
FIELDS OF STUDY: Astronomy; Astrophysics; Cosmology
ABSTRACT: The nucleus is the frozen body of a comet. When a comet’s orbit takes it close to the sun, the icy nucleus begins to melt and forms a cloud. Comets reside in either the Kuiper Belt or in the Oort Cloud. Comets are believed to have existed since the formation of the solar system.
The Body of a Comet
The nucleus is the solid body of a comet. It is made primarily of ice along with dust and rocks. Comets orbit the sun and have existed since the formation of the solar system. Short-period comets come from the Kuiper Belt beyond the planet Neptune. Long-period comets come from the Oort Cloud far outside of Pluto’s orbit, about 5,000 to 100,000 astronomical units (AU) from the sun. One AU is about 93 million miles (150 million kilometers).
A Comet’s Transformation
A comet has an elliptical, or oval-shaped, orbit. When a comet is far from the sun, the frozen nucleus is the only part that exists. When the comet’s orbit takes it closer to the sun, it begins to melt and the ice, rocks, and dust sublime. When a solid sublimes, it becomes a gas without first turning into liquid. When the solids in a comet sublime, they form a coma, or cloud, around the comet. The coma grows and can become very large: 96,000 kilometers (60,000 miles) or more across.
As the comet gets closer to the sun, the gas coma and the dust coma are pushed behind the nucleus. The moving dust coma forms a dust tail that reflects sunlight and can been seen in the night sky. A comet’s ion tail is sometimes hundreds of millions of kilometers long.
A comet with a smaller orbit is visible from Earth more often. Halley’s Comet travels around the sun every seventy-six years and will again be visible from Earth in 2061. Comet Hale-Bopp, on the other hand, has a very large orbit of over 2,500 years. It was last visible in 1997.
Studying the Cometary Nucleus
Scientists are trying to learn as much as possible about comets and their nuclei. More than twenty space missions have been sent to explore comets and learn about the formation and evolution of the solar system. When Halley’s Comet orbited close to Earth, five unmanned spacecraft flew near it to gather data. Scientists learned that its nucleus is about half dust and half ice, 80 percent of which is water and 15 percent is carbon monoxide. Most comets are similar to Halley’s.
In 2022, astronomers using NASA’s Hubble Space Telescope discovered the largest comet nucleus ever observed. The nucleus is about 80 miles (129 kilometers) across, fifty times the size of an average comet. The comet is orbiting about two billion miles from the sun, and will not get closer than the orbit of Saturn.
PRINCIPAL TERMS
- comet: an icy body made primarily of frozen water and gases, as well as dust and rocks, that orbits the sun.
- dust coma: the cloud of dust particles that come off a comet’s nucleus when its gas molecules sublime.
- dust tail: a visible stream of dust that flows off a comet’s nucleus and forms an arc behind it as the comet approaches the sun.
- ion tail: a stream of ions that flows out behind a comet when it approaches the sun and its gas coma interacts with the solar wind.
- gas coma: the cloud of molecules that are sublimed from the comet’s nucleus when it approaches the sun and begins to heat.
Bibliography
"Anatomy of a Comet." Rosetta. NASA Jet Propulsion Laboratory, n.d. Web. 30 Mar. 2015.
"Comets Overview." Solar System Exploration. NASA, 20 Oct. 2014. Web. 30 Mar. 2015.
"Hubble Confirms Largest Comet Nucleus Ever Seen." National Aeronautics and Space Administration, 12 Apr. 2022, www.nasa.gov/feature/goddard/2022/hubble-confirms-largest-comet-nucleus-ever-seen/. 14 June 2022.
"Kuiper Belt & Oort Cloud." Solar System Exploration. NASA, 19 Nov. 2014. Web. 30 Mar. 2015.
Meierhenrich, Uwe. Comets and Their Origin: The Tools to Decipher a Comet. Weinheim: Wiley, 2015. Print.
"Mission Science Goals." Rosetta. NASA Jet Propulsion Laboratory, n.d. Web. 30 Mar. 2015.
Petersen, Carolyn Collins. Astronomy 101. Avon: F+W Media, 2013. Print.