What Are Comets And What Are They Made Of?

by Carson

Comets are icy objects, sometimes with long tails, and you might have seen some of them. Let’s find out more about comets, their structure, and their compositions in this article.

What Are Comets?

There is a brief explanation above, but let’s go a little deeper in this section. Like asteroids, they are remnants of the Solar System’s formation, and they are formed by the accretion of small particles in the Solar System. However, unlike most asteroids, they are made of ice, dust, carbon dioxide, ammonia, methane, etc., which is why they are called “dirty snowballs”. This is the comet’s nucleus and is the basis of the tail that emerges sometimes.

Parts of a Comet

A comet is just a nucleus when it is far from the Sun, but as it nears the Sun, other parts will emerge as it evaporates gradually. Firstly, a cloud of gases around the comet nucleus is formed, called the coma. Next, the solar wind pushes the coma in the direction opposite to the Sun, forming two tails, which are the dust tail and the ion tail.

As the comet passes perihelion and gets farther from the Sun, it cools down, and the evaporation stops. No coma or tail is visible at that time, and the comet becomes a lonely rock in vast space once again.

The Orbit of a Comet

This is the most interesting part because a comet’s orbit can be very extreme. They can get thousands of astronomical units away from the Sun or so close to our star that it completely disintegrates on perihelion. But before learning about these complex orbits, let’s sort them first.

Short-period comets orbit the Sun once in less than 200 years, while long-period ones have even larger orbits. While Jupiter-family comets orbit the Sun in less than about 20 years, Halley-type comets’ orbital periods are between 20 and 200 years, like Halley’s comet.

Some long-period comets venture so far away from the Sun that no spacecraft can reach their farthest point from the Sun in the lifetime of a person. The area is called the Oort Cloud, and it is the home for some comets. They either settle down there due to gravity assists from the planets or be captured by the Sun. In fact, the Oort Cloud stretches so far that its outer edge might be over a light-year away from the Sun!

At this point, the objects are so loosely bound to the Sun that they only return to the inner Solar System when external forces, like the force exerted due to differences in densities of areas in the Milky Way itself, are applied to them.


In this article, we briefly explained what comets are, their structure, and their spectacular orbits. If you want to know more about them, please visit the websites in the references below.

References and Credits

  1. NASA Solar System Exploration. (2019, December 19). Comets. Retrieved July 4, 2021, from https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/in-depth/
  2. (n.d.). The Outer Planets: Comets. Retrieved July 4, 2021, from https://lasp.colorado.edu/outerplanets/kbos_comets.php
  3. Ethan Siegel. (2017, December 9). Ask Ethan: Why Don’t Comets Orbit The Same Way Planets Do?. Retrieved July 5, 2021, from https://www.forbes.com/sites/startswithabang/2017/12/09/ask-ethan-why-dont-comets-orbit-the-same-way-planets-do/
  4. NASA Solar System Exploration. (2019, December 19). Oort Cloud. Retrieved July 5, 2021, from https://solarsystem.nasa.gov/solar-system/oort-cloud/in-depth/
  5. (n.d.). Short-period Comets. Retrieved July 5, 2021, from https://astronomy.swin.edu.au/cosmos/s/short-period+comets
  6. Charles Q. Choi. (2017, October 23). Comets: Facts About The ‘Dirty Snowballs’ of Space. Retrieved July 5, 2021, from https://www.space.com/53-comets-formation-discovery-and-exploration.html

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