Tail (Comet Anatomy)

• FIELDS OF STUDY: Astrometry; Astrophysics; Cosmology
• ABSTRACT: Comets are large objects that travel through space. Comet tails are the long projections often seen behind comets. Tails can be made from a variety of substances and appear to be different lengths depending on the comet’s position in relation to the sun. Historically, comet tails have been poorly understood. In the past, people considered comet tails powerful omens or curses.

What Is a Comet?

Sometimes called "dirty snowballs," comets are frozen balls of ice, gas, dust, and dirt. Some scientists think comets are made of undisturbed material dating to the formation of the solar system. Comets have large orbits that occasionally pass through the solar system. They come in a variety of shapes and sizes. The largest comet observed was 80 miles (128 kilometers) across. Because many of their orbits are elliptical, comets can return at predictable intervals. Some comets return quickly, while some will not return for thousands of years.

When comets pass close to the sun, some of the material on the surface of the comet evaporates. The evaporated material trails behind the comet, forming a long tail that can be visible from Earth. Comet tails sometimes reach millions of miles long.

Comet Tails

Comets have two types of tails: the dust tail and the ion tail. The dust tail is composed of solid matter that the sun’s heat causes to radiate off the comet. The ion tail is composed of various gases, including methane, ammonia, water, carbon dioxide, and carbon monoxide, that evaporate from the surface of the comet. Sunlight ionizes these gases, causing them to glow.

Comet tails always point away from the sun. The ion tail points in a straight line, directly away from the sun. The dust tail curves toward the path of the comet’s orbit. Ion tails are significantly thinner, longer, and brighter than dust tails, and they tend to glow a pale blue color.

When a comet passes by Earth, its tail leaves a trail of tiny, leftover pieces of the comet. Sometimes, Earth passes through these trails. Larger pieces of the comet can burn brightly when they enter Earth’s atmosphere, causing meteor showers.

Researchers have solved a 90-year-old astronomical puzzle regarding comet tails. They had long wondered why comet heads tend to emit green light, while the tail appears white or blue. The scientific explanation, which was published in the Proceedings of the National Academy of Sciences (PNAS) in 2021, involves the manner in which the molecule dicarbon (C₂) gets blown apart by sunlight as the comet travels. This state was observed accidentally when scientist Jun Jiang, then a graduate student at MIT University, was observing the properties of the molecule acetylene while it was excited by a high-power frequency-turntable UV laser.

Comet Tails in History

Comet tails visible from Earth can be large, dramatic displays. Some even look like giant balls of fire in the sky. Consequentially, ancient people sometimes considered their appearance a significant, magical event. In one instance, when Halley’s comet appeared in 1066 CE, it was believed to be a bad sign for the Anglo-Saxon king Harold II, but a good sign for William the Conqueror, duke of Normandy. William went on to famously defeat Harold at the Battle of Hastings.

In more modern times, astronomers in the early twentieth century noted that Earth was going to pass through the tail of Halley’s Comet upon its return in 1910. After cyanogen gas was detected in the comet’s tail, a French astronomer theorized the poison could prove fatal to life on Earth. Although some people panicked and began wearing gas masks, the substances in the comet’s tail posed no danger to humans, and nothing happened.

PRINCIPAL TERMS

• comet: a celestial body mostly made of carbon dust and ice.

Bibliography

"About Comets." Lunar and Planetary Institute. USRA, 31 Oct. 2012. Web. 9 Mar. 2015.

"Anatomy of a Comet." Rosetta. Jet Propulsion Laboratory, Caltech, n.d. Web. 19 Feb. 2015.

Apple, Charles. "We Didn’t Die." Spokesman-Review, 2022, www.spokesman.com/stories/2020/may/19/through-tail-halleys-comet-and-15-other-notable-ma/. Accessed 3 Mar. 2025.

Borsovszky, Jasmin, et al. "Photodissociation of Dicarbon: How Nature Breaks an Unusual Multiple Bond." Proceedings of the National Academy of Sciences (PNAS), vol. 118, no. 52, 20 Dec. 2021, doi.org/10.1073/pnas.2113315118. Accessed 3 Mar. 2025.

Choi, Charles Q. "Comets: Facts about the ‘Dirty Snowballs’ of Space." Space.com. Purch, 15 Nov. 2014. Web. 9 Mar. 2015.

"Comets." Solar System Exploration. NASA, 3 Feb. 2014. Web. 12 Mar. 2015. "Comet Facts for Students." JPL Education. NASA, 2 Nov. 2010. Web. 19 Feb. 2015.

Koberlein, Brian. "What Are Comet Tails?" Universe Today. Universe Today, 21 July 2014. Web. 9 Mar. 2015.

Koe, Crystal. "Why Are Comet Heads Green—But Not Their Tails?" MIT News, 31 Jan. 2022, news.mit.edu/2022/comet-heads-green-but-not-tails-mystery-0131. Accessed 3 Mar. 2025.

"What Are the Types of Comet Tails?." HubbleSite. Space Telescope Science Inst., n.d. Web. 12 Mar. 2015.