Comet definition: what is a comet?


Comets, these small bodies with impressive “hair”, are remnants of the solar system. These frozen remnants can offer a sublime astronomical spectacle, sometimes visible to the naked eye. What is a comet made of?

More than 3,700 comets have been identified. Perhaps you have already had the chance to see one “plunge” towards the Earth, like the comet NEOWISE which was visible to the naked eye during the summer of 2020. The astronomical spectacle offered by these celestial bodies can be striking. Some of them are also at the origin of swarms of shooting stars. But what exactly is a comet?

What is a comet?

These are remains approximately 4.6 billion years old. The size of these relics of the young solar system is variable, from barely a few kilometers to several tens of kilometers in width. Comets revolve around the Sun, following elliptical orbits (more “elongated” than a perfect circle).

It is suspected that comets may have played a role in the habitability of the Earth. It is not impossible that these bodies brought water and organic compounds to our planet when it was still young, or even to other bodies in the solar system.

When will the next comet be visible?

Comets are often unpredictable objects. It is very difficult to predict in advance what will be the next comet visible to the naked eye from our planet. Even a very promising comet may end up reacting unexpectedly to the approach of the Sun, and not being visible.

The National Radio Astronomy Observatory (NRAO) points to a list of comets that may become visible, both in the Northern Hemisphere and in the Southern Hemisphere, through 2025. These are just suggestions of the next comets that may, perhaps -to be, to become visible with or without a telescope.

What was the last visible comet?

The last major visible comet of the 21st century is comet C/2020 F3 NEOWISE. After its discovery on March 27, 2020, it had become visible during the summer. She made a passage in the constellation of the Big Dipper at the end of July. Comet NEOWISE is not expected to return to our planet for about 6,800 years.

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What is a comet made of?

You have to imagine comets as “ frozen remnants of the formation of the solar system composed of dust, rock and ice “, describes NASA. At certain times in their trajectory, comets are closer to the Sun than at others: this is when they heat up and release gases and dust.

The ejected material forms an atmosphere and an impressive tail (not necessarily directed behind the comet), which can extend over very large distances, millions of kilometers – the Earth thus seems very small when compared to the interstellar comet Borisov . If we want to be more precise, we must say that comets actually have two tails: one of dust, the other made up of ions. There are even anti-tails, formed by projection effects.

What is the difference between a comet and an asteroid?

Comets and asteroids have things in common: these objects are part of the solar system, orbit the Sun, have irregular shapes. But unlike an asteroid, which is rocky, a comet is made up of ice and dust. As it approaches the Sun, the ice and dust lodged in a comet vaporize.

Periodic or non-periodic comets: what’s the difference?

It is estimated that there are billions of comets orbiting the Sun, both in the Kuiper belt (beyond the orbit of Neptune) and in the Oort cloud (even farther from the Sun than the Kuiper belt). Many of them are called periodic comets, that is to say that they take less than 200 years to make a turn around the Sun. We can therefore try to anticipate the next appearance of certain specimens. But others take much longer to travel around the Sun: for some comets in the Oort Cloud, it takes up to 30 million years to complete a single orbit. They are non-period or long-period comets.

What are the most famous comets?

Comets are numerous, but some are better known than others.

Halley’s Comet

The most famous of all is probably 1P/Halley, also called Halley’s Comet. Thanks to this object, astronomers have understood that comets can return to visit our sky several times. Its appearances have been recorded in observations dating back more than 2,000 years. The last time this comet was seen was in 1986. It takes 76 years to circle the Sun. Astronomers therefore expect to see it again in 1961.

Comet Leonard

Discovered in January 2021, comet C/2021 A1 (Leonard) passed closest to the Sun exactly one year after its discovery. She was quite difficult to see with the naked eye. Unfortunately, this bright comet that marked the year 2021 ended up disintegrating.

Comet NEOWISE

The observers who were lucky enough to see comet C/2020 F3 (NEOWISE) in the summer of 2020 could not forget this magnificent spectacle. You could see her without an instrument. After its passage near the Sun, its activity increased considerably.

Other famous comets include comet Shoemaker-Levy 9, famous for having been observed hurtling towards Jupiter in 1994, or comet Hale-Bopp, the most observed comet of the 20th century.

What are comets called?

By tradition, the names of comets often take the name of the person who discovered them (or several people, if there are several discoverers). Astronomers have realized, however, that this method leads to ambiguities, as some scientists have discovered multiple comets. The International Astronomical Union has therefore implemented a nomenclature since 1995.

When a new comet is observed, it receives as annual reference the number of the current year, followed by a capital letter corresponding to the fortnight of the current month. », Details the Institute of Celestial Mechanics and Calculation of Ephemerides. A number also indicates the order of discovery of the comet in the fortnight. The name is preceded by a “P/” or a “C/” to indicate whether the comet is periodic or not. Comet NEOWISE is thus officially named C/2020 F3.

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