Comets: Icy Messengers and Guardians of the Origins of Life

Scientific Summary

This article presents comets, celestial bodies composed of ice, dust, and rocks, which follow elliptical orbits around the Sun. They are considered primitive remnants of the Solar System's formation, containing materials preserved for 4.6 billion years. The article discusses several famous comets: Halley (76-year period), Hale-Bopp (visible for 18 months in 1996-1997), Hyakutake (1996), Holmes (spectacular outbursts), Hartley 2 (flyby by the EPOXI mission in 2010), Neowise (2020), Tsuchinshan-ATLAS (great comet of 2024), and Linear (disintegration in 2000), as well as the interstellar comet 3I/ATLAS (2025), the third known visitor from another stellar system. Comets have a dual tail (dust and ionic) and are messengers of the cosmic past.

What Are Comets and Which Are the Most Famous?

Comets are fascinating celestial bodies composed of ice, dust, and rocks, which follow elliptical orbits around the Sun. Often described as "dirty snowballs," they are primitive remnants of the Solar System's formation, 4.6 billion years ago, containing preserved materials that offer a window into the cosmic past. Once perceived as harbingers of doom, their image transformed in the 19th century with the separation of science and religion. The article presents several famous comets: Halley's Comet (76-year period, next passage in 2061), Hale-Bopp (visible to the naked eye for 18 months in 1996-1997), Hyakutake (1996, one of the brightest of the 20th century), Holmes (spectacular outbursts), Hartley 2 (flyby by the EPOXI mission in 2010), Neowise (2020, with two spectacular tails), Tsuchinshan-ATLAS (2024, the brightest since Hale-Bopp), and Linear (disintegration observed in 2000). Since 2025, astronomers have also been tracking 3I/ATLAS, only the third confirmed interstellar object after 'Oumuamua (2017) and Borisov (2019). Comets generally have two tails: a dust tail, wide and curved, and an ionic tail, thin and straight, formed of electrically charged molecules. These celestial wanderers are true messengers of space, spreading primordial matter.

What Is a Comet?

Comets are fascinating celestial bodies composed of ice, dust, and rocks. They follow elliptical orbits around the Sun and can offer magnificent displays when they approach Earth. Comets illuminate the collective memory of humanity. Once perceived as harbingers of doom, disasters, and upheavals, they embodied our imaginary fears as well as our real ailments. However, they are not messengers of doom but rather guardians of the past, holding the keys to our origins. In the 19th century, with the separation of science and religion, the image of comets ceased to be negative. They can be observed for weeks before disappearing, only to return decades, centuries, or even millennia later. Comets are the wanderers of the sky, the messengers of space, akin to dirty snowballs spreading primordial matter.

The Hyakutake Comet

The Hyakutake Comet (C/1996 B2), discovered on January 30, 1996, by Japanese amateur astronomer Yuji Hyakutake, passed closest to Earth on March 25, 1996, at about 15 million kilometers, before reaching its perihelion on May 1, 1996. Its long tail, visible to the naked eye and spanning nearly 80°, made it one of the brightest comets of the 20th century.

The Ikeya Comet

The Ikeya Comet, or C/1963 A1, was discovered on January 2, 1963, by Japanese amateur astronomer Kaoru Ikeya, then aged 19. It is famous for its long tail of dust and gas, which stretched across several degrees in the sky. It reached its perihelion on March 21, 1963. Ikeya later co-discovered, with Tsutomu Seki, the Ikeya-Seki Comet (C/1965 S1), a "sungrazer" visible in daylight and considered one of the brightest comets of the 20th century.

The Linear Comet

The Linear Comet, or C/1999 S4, was discovered in 1999 by the LINEAR asteroid search program. It underwent a spectacular disintegration in July 2000, fragmenting into several pieces. This disintegration was observed by many astronomers and provided valuable insights into the composition of comets.

The Hale-Bopp Comet

Discovered in 1995 by Alan Hale and Thomas Bopp, the Hale-Bopp Comet is one of the most famous comets in recent history. It was visible to the naked eye for 18 months and reached its perihelion in April 1997. Its great brightness and long tail made it a privileged object of observation for amateur and professional astronomers alike.

Halley's Comet

Halley's Comet, or 1P/Halley, is probably the most famous comet of all. It has an orbital period of about 76 years and has been observed numerous times throughout history. Its last close approach to Earth was in 1986, and its next passage is expected in 2061.

The Holmes Comet

The Holmes Comet, or 17P/Holmes, is a periodic comet discovered in 1892 by Edwin Holmes. It is known for its spectacular outbursts that can significantly increase its brightness. In 2007, it underwent an outburst that made it visible to the naked eye for several weeks.

The Hartley 2 Comet

The Hartley 2 Comet, or 103P/Hartley, was discovered in 1986 by Malcolm Hartley. It was flown by on November 4, 2010, by the Deep Impact probe, reused as part of the extended EPOXI mission, which provided detailed images of its surface and gas jets. This mission helped better understand the composition and dynamics of comets.

The Neowise Comet

The Neowise Comet (C/2020 F3), discovered on March 27, 2020, by the NEOWISE space telescope, reached its perihelion on July 3, 2020. Like most comets, it has two tails: one of dust and one of electrically charged molecules, according to NASA. The data processed by the WISPR instrument on NASA's Parker Solar Probe shows details in the twin tails of comet NEOWISE as observed on July 5, 2020. The lower, wider tail corresponds to the comet's dust tail, while the upper, thinner tail corresponds to the ionic tail of the comet. (Credits: NASA/Johns Hopkins APL/Naval Research Lab/Parker Solar Probe/Guillermo Stenborg)

The Tsuchinshan-ATLAS Comet

The Tsuchinshan-ATLAS Comet (C/2023 A3), nicknamed the "Great Comet of 2024," was discovered on January 9, 2023, by China's Purple Mountain Observatory (Tsuchinshan), and independently rediscovered on February 22, 2023, by the South African survey of the ATLAS network. Originating from the Oort Cloud, it reached its perihelion on September 27, 2024, at 0.39 AU from the Sun, before becoming, with an apparent magnitude of about -4.9, the brightest comet observed from the Northern Hemisphere since Hale-Bopp in 1997.

3I/ATLAS, the Third Known Interstellar Comet

Detected on July 1, 2025, by the ATLAS telescope network in Chile, 3I/ATLAS (also designated C/2025 N1) is only the third confirmed interstellar object, after 'Oumuamua (2017) and Borisov (2019). Its highly hyperbolic orbit, with an eccentricity of about 6.1, proves that it is not gravitationally bound to the Sun and originates from another stellar system. It reached its perihelion in late October 2025, at about 1.36 AU from the Sun, before permanently leaving the Solar System. Observations, particularly by the Hubble Space Telescope, have shown a nucleus a few kilometers across surrounded by primitive ice preserved beneath a crust altered by billions of years of exposure to cosmic rays.

References

FAQ: Everything You Need to Know About Comets

What Exactly Is a Comet?

A comet is a small celestial body composed of ice (water, methane, ammonia, CO₂), dust, and rocks. It follows a highly elongated elliptical orbit around the Sun. When it approaches the Sun, the heat causes the ice to sublimate, releasing gases and particles that form a diffuse atmosphere (the coma) and often a spectacular tail that extends for millions of kilometers. Comets are primitive remnants of the Solar System's formation, 4.6 billion years ago.

What Is the Difference Between a Comet and an Asteroid?

The main difference lies in their composition and origin:
• Comets are composed of ice (volatiles) and dust, and mainly come from the outer regions of the Solar System (Kuiper Belt, Oort Cloud). When they approach the Sun, they develop a coma and a tail.
• Asteroids are primarily rocky or metallic, with no significant ice, and come from the main belt between Mars and Jupiter. They do not develop a tail.
Some objects are intermediate (e.g., centaurs), and the distinction is not always absolute.

Which Are the Most Famous Comets?

Among the most famous comets:
• Halley's Comet (1P/Halley): 76-year period, observed since antiquity, last passage in 1986, next in 2061.
• Hale-Bopp (C/1995 O1): visible to the naked eye for 18 months in 1996-1997, one of the brightest of the 20th century.
• Hyakutake (C/1996 B2): discovered in 1996, offered a magnificent display with its long tail.
• Neowise (C/2020 F3): visible in 2020, with two spectacular tails photographed by the Parker probe.
• Tsuchinshan-ATLAS (C/2023 A3): Great Comet of 2024, the brightest since Hale-Bopp.
• Holmes (17P/Holmes): known for its sudden brightness outbursts.
• Hartley 2 (103P/Hartley): flown by the EPOXI mission in 2010.

Why Do Comets Have Two Tails?

Comets generally develop two distinct tails when they approach the Sun:
• The dust tail: composed of solid particles (dust) released by the sublimation of ice. It is wide, curved, and follows the comet's trajectory. Its color is often yellowish because it reflects sunlight.
• The ionic tail (or gas tail): composed of electrically charged molecules (ions) blown by the solar wind. It is thin, straight, and always oriented away from the Sun, regardless of the comet's direction. Its color is often bluish.
These two tails can extend for millions of kilometers.

What Is the Orbital Period of Halley's Comet?

Halley's Comet (1P/Halley) has an orbital period of about 76 years. It has been observed many times throughout history, with recorded passages dating back at least to 240 BCE. Its last perihelion (closest approach to the Sun) was in 1986, and its next passage is expected in 2061. It is the most famous periodic comet and the first for which the return was predicted (by Edmond Halley).

Where Do Comets Come From?

Comets mainly come from two reservoirs located at the outskirts of the Solar System:
• The Kuiper Belt: located beyond Neptune (between 30 and 50 AU), it houses short-period comets (less than 200 years), such as Halley's Comet.
• The Oort Cloud: a vast sphere (up to 100,000 AU) surrounding the Solar System, containing long-period comets (more than 200 years), such as Hale-Bopp, Hyakutake, or Tsuchinshan-ATLAS. These comets can take thousands or even millions of years to complete one revolution.
Gravitational perturbations (passing stars, galactic tides) can send these comets toward the Sun.
A third, very rare group comes from outside the Solar System: interstellar comets. Since 2017, three such objects have been confirmed—'Oumuamua, Borisov, and 3I/ATLAS—moving at hyperbolic speeds and trajectories that prove they are not gravitationally bound to the Sun and will never return.

Have Comets Ever Struck Earth?

Yes, it is certain that comets have struck Earth in the past. The most famous impact is probably the one that contributed to the extinction of the dinosaurs 66 million years ago (Cretaceous-Paleogene). It is generally attributed to an asteroid, but some scientists suggest it could have been a comet. More recently, the Tunguska event in 1908 (Siberia) may have been caused by a comet fragment or an asteroid. Comets, due to their size and speed, pose a real impact risk, although the probability is low on a human timescale.