Comet definition: What is a comet?

More than 3,800 comets have been identified. Perhaps you have already had the opportunity to observe a “dive” towards Earth. One of them has just suddenly appeared in the northern hemisphere: Comet Nishimura. It’s hard to see, but you have to take advantage of it because this comet won’t come back for hundreds of years.

Other comets put on a beautiful display, such as comet ZTF in early 2023 and comet NEOWISE in summer 2020. The astronomical spectacle offered by these celestial bodies can be impressive. Some of them are also the origin of shooting star swarms. But what exactly is a comet?

What is a comet?

These are remains that are around 4.6 billion years old. The size of these relics of the young solar system varies from a few kilometers to several dozen kilometers wide.

It is thought that comets may have played a role in Earth’s habitability. It is not impossible that these bodies transported water and organic compounds to our young planet or even to other bodies in the solar system.

Where do comets occur?

Comets orbit the Sun in elliptical orbits (more “elongated” than a perfect circle). Some comets come from the Kuiper Belt (beyond the orbit of Neptune), others from the Oort Cloud (the outer edge of the solar system).

When will the next comet be visible?

Comets are often unpredictable objects. It is very difficult to predict which comet will be the next one that will be visible to the naked eye from our planet. Even a promising comet may end up reacting unexpectedly as it approaches the sun and not be visible.

The National Radio Astronomy Observatory (NRAO) links to a list of comets that could become visible in the Northern Hemisphere and Southern Hemisphere by 2025. These are just suggestions for the next comets that might become visible with or without a telescope.

What was the last visible comet?

In 2023, a comet surprised us and became visible to the naked eye in January. This was comet C/2022 E3 (ZTF). From now on, this comet will never come back to see us again. However, you can admire the photos of this green comet, each more beautiful than the other.

The other large visible comet of the 21st century is comet C/2020 F3 NEOWISE. After its discovery on March 27, 2020, it became visible over the summer. At the end of July it crossed the constellation Ursa Major. Comet NEOWISE is not expected to return to our planet for another 6,800 years.

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What is a comet made of?

We have to think of comets as “ frozen remnants of the formation of the solar system, consisting of dust, rock and ice », Describes NASA. At certain times in their trajectory, comets are closer to the Sun than at other times: at this point they heat up and release gas and dust.

The ejected materials form an atmosphere and an impressive tail (not necessarily directed behind the comet) that can extend over very large distances, millions of kilometers. The Earth therefore appears very small compared to the interstellar comet Borisov. If we want to be more precise, we have to say that comets actually have two tails: one made of dust, the other made of ions. There are even anti-tails created by projection effects.

How do you recognize a comet?

We recognize comets in the sky by their bright heads and long tails. We might imagine comets crossing the sky, a bit like shooting stars. This is not how comets move: only by observing the comet from one night to the next (and better yet, taking a photo of it) can you see that it is moving slowly across the starry sky background.

What is the difference between a comet and an asteroid?

Comets and asteroids have something in common: These objects are part of the solar system, orbit the sun and have irregular shapes. But unlike an asteroid, which is rocky, a comet is made of ice and dust. As it approaches the Sun, the ice and dust contained within the comet evaporate.

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 further from the Sun than the Kuiper Belt). Many of them are so-called periodic comets, meaning that they take less than 200 years to orbit the sun. We can therefore try to predict the next appearance of certain specimens. But others take much longer to orbit the sun: some comets in the Oort cloud take up to 30 million years to complete a single orbit. These are aperiodic or long-period comets.

What are the most famous comets?

There are many comets, but some are better known than others.

Halley’s Comet

The best known of all is probably 1P/Halley, also called Halley’s Comet. Thanks to this object, astronomers realized that comets could return to our sky several times. Its appearances have been documented in observations dating back more than 2,000 years. The last time this comet was seen was in 1986. It takes 76 years to orbit the sun. Astronomers therefore assume that it will be seen again in 2061.

Comet Leonard

Comet C/2021 A1 (Leonard), discovered in January 2021, came closest to the Sun exactly one year after its discovery. It was quite difficult to see with the naked eye. Unfortunately, this brilliant comet that marked 2021 disintegrated.

Comet NEOWISE

Observers who had the opportunity to see Comet C/2020 F3 (NEOWISE) in the summer of 2020 could not forget this magnificent spectacle. You could see it without an instrument. After its flyby near the Sun, its activity increased significantly.

The ZTF comet

The year 2023 began with a nice surprise for professional and amateur astronomers. The green-haired comet C/2022 E3 (ZTF) became visible to the naked eye around January 20th. This small body, like many other comets, comes from the boundaries of the solar system. We could even see its “anti-tail,” a disturbing optical illusion.

The famous comets also include comet Shoemaker-Levy 9, which was observed on its way to Jupiter in 1994, and comet Hale-Bopp, the most frequently observed comet of the 20th century.

What are comets called?

Traditionally, the names of comets often bear the name of the person who discovered them (or several people if there are multiple discoverers). However, astronomers realized that this method led to confusion, as some scientists discovered several comets. The International Astronomical Union has therefore been introducing a nomenclature since 1995.

“ When a new comet is observed, it is given the current year number as a year reference, followed by a capital letter corresponding to the two weeks of the current month », Details about the Institute for Celestial Mechanics and Ephemeris Calculation. A number also indicates the order in which the comet was discovered over the two weeks. The name is preceded by a “P/” or a “C/” to indicate whether the comet is periodic or not. Comet NEOWISE is therefore officially named C/2020 F3.

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