A Dummy’s Guide to Comets: 3 amazing things you NEED to know

A celestial object with a majestic tail, like a divine creature roaming the boundless universe –  seems like an apt description for a comet. It is one of the most beautiful marvels to observe from the earth. But then the question arises, what exactly are these? Are they rocks, much like tiny asteroids? Or are they broken bits of larger stars that are coursing through space?

The answer is much simpler: they are masses of frozen gas and rocks, about the size of a small town. When their orbits take them too close to a star, they heat up and they swell up many times over until they become larger than most planets. The ensuing chaos gives out a tail of dust and gases, resulting in the beautiful streaking comets that we’ve all seen in pictures and, if one is lucky enough, in real life. 


Composition and the Tail 

The core of any comet is made up of dust and ice which are coated in dark organic matter. The Rosetta Mission, which inspected a comet, found that around 40% of that comet was made up of organic material. Some comets may have a small rocky core, though that is not conclusively proven. This nucleus is often no wider than a few kilometers in diameter.

When comets pass close to the sun, the ice on the surface of their nucleus melts and forms gas which is the “atmosphere” of the comets, called the coma, which has a diameter of hundreds of kilometers. This coma contains gases such as carbon monoxide, carbon dioxide, methane, and ammonia. A very intriguing fact about the ice of the comets is that it is heavier than that found on earth since it contains more deuterium which is a heavier hydrogen isotope.

As the comets get closer to a star, the radiation from the hot stars pushes away dust particles from the coma, forming the dust tail. On the other hand, charged particles from them ionize gases in the coma, forming a gas tail. Since these are shaped by the star, pushing particles away from the coma, the tails always point away from the stars.


Myths – Comets as Divine Omens 

Like anything which has existed in space and is seen by humans, comets are sure to have myths of gods and prophecies surrounding them. This object has evoked extreme responses in people throughout major civilizations and cultures of humanity throughout history, from joyous celebrations to visceral hysteria.

Throughout the Roman Civilization, comets symbolized the advent of major events, good or bad. The appearance of a comet after the death of Julius Cesar was interpreted as his ascension to godhood (called deification or apotheosis). 

In the Norse myths, comets were believed to be part of the giant Ymir’s skull. It was said that when Odin and his brothers killed Ymir and fashioned the Earth out of it, comets were the remnants of his skull falling towards the Earth and disintegrating. 

At the time of the Chinese Empire, comets were an important part of their astrology, with the appearance of one always interpreted as disastrous. Under the Wu Xing Theory ( or the Theory of the Five Elements), comets were a bad omen and symbolized the Yin and Yang imbalance.


Myths aside, Astronomers believe that our observable comets emerged from the protoplanetary disk, a donut-shaped cloud of debris surrounding the sun as it was born. Over the outer edges of this disk, clumps of ice-coated dust formed, which clumped together with other such clumps, again and again, to form ice-rich rocks which then evolved into the nucleus of the comets. 

Comets observed by us from the earth come from the Kuiper Belt, a region of dust and debris that are the leftovers of our solar system as it formed. They also come from the Oort Cloud which exists beyond Pluto, the Cloud is a region of icy objects, including comets.

Occasionally, due to the effect of gravity, comets from the Oort cloud and the Kuiper Belt are thrown into closer orbits much closer to the sun, which enable us to see them.

Explorations (for your further reading)

Some of the missions undertaken by NASA to understand comets better are:

  1. The Stardust Mission
  2. Extrasolar Planet Observation and Characterization (EPOCh) investigation
  3. Deep Impact

Enjoyed reading this? Consider reading: Neutron Stars: 3 amazing fates of Stellar objects

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