Comets: what are they and how are they formed?

Comets are spectacular beyond a shadow of a doubt. Throughout the history of our civilization, these objects have long been viewed with suspicion. They were considered harbingers of doom and calamity, due to their sudden appearance in the starry skies, in stark contrast to the regular harmony of the cosmos.

Today, thanks to astronomers, we know that comets are part of the family of objects in the Solar System.

The time has come to find out what comets are!


Comets can look very similar to asteroids. They are minor celestial bodies of the Solar System, whose dimensions vary between hundreds of meters and tens of kilometers.

The most accredited theory states that most of these objects are located in a kind of bubble with a radius of about 1 light year (about 60,000 Astronomical Units), which surrounds the entire Solar System, called the Oort Cloud . How do comets reach the depths of the solar system, where are we?

It is possible that some gravitational perturbations caused by other stars could cause the ” fall ” of one or more comets towards the Sun. The comets at this point embark on a journey that can last thousands or millions of years, towards the Inner Solar System .

Astronomers also believe that there is a second “reservoir” of comets, identified as the Kuiper Belt . It is a remote region of the Outer Solar System, located beyond the orbits of Neptune and Pluto. Many “trans-Neptunian” objects have been discovered here, such as the dwarf planets Makemake and Eris.

So how many comets could exist just within the domain of our Star? There is no 100% certain estimate. The Kuiper Belt could contain millions of comets. A huge number, sure, but that pales in the face of the tens, perhaps hundreds, of billions of comets that could be found in the Oort Cloud.


Comet NEOWISE taken from Trentino-Alto Adige.

As we said earlier, comets can look very similar to asteroids. Their “bodies”, the nuclei , are mainly composed of dust, rocks and metals. However, the nuclei are also composed of ices of water and carbon dioxide , along with compounds such as methane and ammonia . Their composition is the main reason why astronomers refer to these celestial bodies as ” dirty snowballs “.

Astronomers have discovered a lot of information about these celestial objects, thanks to ground-based observations and space missions. The Halley’s Comet , for example, received the visit of the Vega and Giotto probe. How can we not mention the spectacular mission Rosetta of ESA , who gave the world the stunning images of the comet 67 / P Churyumov-Gerasimenko .

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The two characteristics that distinguish comets are the tail (in some cases, more than one) and the coma , which develop when a comet comes close to the sun.

As a comet approaches the Inner Solar System, the heat from our star causes the material that composes it to sublimate . The particles of the solar wind thus lead to the formation of a tail of dust, always oriented in the opposite direction to the Sun. The ultraviolet radiation, through the ionization process, can also lead to the formation of a second tail, called a gas tail , or tail of ions .

The main tail can reach extraordinary dimensions, in the order of tens or hundreds of millions of kilometers.

Comet tails may also be responsible for the origin of meteor showers that we observe every year. The Perseids, for example, are due to the comet Swift-Tuttle , when the Earth crosses the cloud of debris that the comet has dispersed into space.


Every now and then it happens to read a piece of news such as “New comet discovered …” and then read a name full of strange acronyms, such as C / 2020 F3 (NEOWISE) or C / 2020 S3 (ERASMUS).

This nomenclature system was developed by the International Astronomical Union to uniquely classify comets. Let’s take comet Neowise as an example.

The “C /” indicates a non-periodic comet. In other cases, we might find the letters “P” or “X”. In the first case, the letter indicates a periodic comet, while the letter “X /” would indicate a comet for which it has not been possible to obtain detailed information on its orbit.

“2020 F3” indicates the period in which the comet was discovered: year, month and number of the comet. For example, a comet with the acronym A2 would be the second to have been discovered in the first half of January.

So A – first half of January, B – second half of January, C – first half of February etc.

NEOWISE, ERASMUS, Lovejoy (…) indicate the mission, space or terrestrial, or the people who discovered the comet.

Therefore C / 2020 F3 NEOWISE was the third comet to be discovered in the second half of March by the NEOWISE space observatory ( Near-Earth Objects Wide-fild Infrared Survey Explorer ).


Astronomy is one of the few fields of science where amateur astronomers can make important contributions in various fields . The discovery of new comets is one of these fields.

Many enthusiasts spend an incredible amount of time observing the sky, hoping to discover celestial bodies such as comets or asteroids. The reward is very attractive: the thrill of discovery, being the first to have observed a new comet combined with the privilege of having one’s name associated with that object.

What would happen if, for example, you happen to discover a comet ?

First, you need to check and record the position of the object in question. In fact, we must be sure that it is something that is not already present in the various astronomical catalogs. Subsequently, it is necessary to prepare new observations, to verify the orbital motion of the suspected comet.

Finally, if we are sure that we are in front of a comet never observed before, we must contact the Central Bureau for Astronomical Telegrams at the Harvard-Smithsonian Center for Astrophysics, which will arrange all the necessary verifications.

Be careful: finding comets requires patience and technique. As we said, amateurs spend hundreds of hours making careful and precise observations.

Comets are among the most characteristic and fascinating celestial bodies, thanks to their long luminous tail. They have always fascinated and frightened man by their appearance and their sudden appearance in heaven. According to ancient popular beliefs, comets were carriers of misfortunes, plagues and wars.
In reality, they are nothing more than harmless “dirty snowballs”, made up of rocks mixed with frozen gas, water, methane, ammonia and dust.

Comet Hyakutake, which appeared in the spring of
1996. (H. Mikuz)

  Comets come from a set of millions and millions of rocky bodies, called the “Oort cloud”. This cloud, shaped like a spherical shell, is located at the edge of the Solar System and extends up to fifty thousand times the Earth-Sun distance.
Comets have been in the Oort cloud since the Solar System was formed and have remained the same as then, as in a large “cosmic refrigerator”. Occasionally, when something disturbs their orbit, one of these pieces of icy rock escapes from the cloud and approaches the Sun at great speed. It enters a very elongated orbit and becomes a comet.
Some comets travel in a closed orbit, elliptical in shape, so they recur periodically, while others travel in an open orbit and therefore pass only once in proximity to the Sun.


The different types of comet orbits.

As soon as the comet approaches a few hundred million kilometers from the Sun, the ice it contains begins to vaporize, forming a spheroidal cloud of gas and dust, called coma , around the rocky core .

In fact, the name “comet” derives from the Latin “coma” which means coma. When the comet approaches less than two to three hundred million km from the Sun, the solar radiation affects the dust particles of the coma, exerts pressure on them and pushes them away along the opposite direction to the Sun. The dust therefore forms a tail , facing the opposite side of the Sun.
The combination of the motion of the comet and the thrust of the radiation causes the tail to assume a slightly arched shape.

A comet can take thousands of years to make an orbit around the Sun, but it is visible only when it is closest to it, that is, for a few weeks or a few months.


  The radiation of the Sun ionizes the gas in the coma, that is, it strips their electrons from the gas atoms. The gas then becomes a plasma , that is a set of atomic nuclei and free electrons. It too is pushed away by the pressure of the solar radiation, in the opposite direction to the Sun, and forms a tail of ions . However, being lighter than dust, it does not “lag behind” as the comet moves: the ion tail is therefore straight. This is the reason why two separate tails are observed in comets.

The different appearance of a comet’s tail as it moves along its orbit. As the comet approaches the Sun, the intensity and pressure of the solar radiation increase: the amount of gas and dust that vaporises increases, so the tail lengthens.

Comets are not so bright only because they emit their own light, but above all because the particles that make up the coma and the tail diffuse the light of the Sun. The tail of a comet can reach 150 million kilometers, equal to the distance Earth – Sun! But as the comet moves away, the tail gets shorter and shorter until it is no longer visible. The comet returns to the deep darkness where it came from.

The nucleus of a comet has dimensions of a few kilometers, but with each subsequent passage in the vicinity of the Sun, a good part of the material of which it is composed is dispersed in space. Comets then “consume” themselves: they are not eternal, but after a certain number of passages they disintegrate.


by Abdullah Sam
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