What Is a Comet? All About The Cosmic “Dirty Snowballs”
If you just want to spot a comet, you can skip this article and download Star Walk 2 instead. This app lets you find any comet in the sky, including C/2023 A3. Just click this link to get it. But if you're here to read more, we're glad you stayed! There's lots of interesting stuff ahead. And after you're done, don't forget to try out Star Walk 2 — we'd really appreciate it.
Contents
- What is a comet?
- Basic types of comets
- More comet types to know about
- Where do comets come from?
- Observing comets: all-round guide
- Famous comets
- Facts about comets
- Comets F.A.Q.
- Comets: to sum up
What is a comet?
A comet is a space object of ice and dust that orbits the Sun. When a comet is in the inner Solar System, it can be seen in the sky as a blurry spot with a tail.
What are comets made of?
Comets are sometimes referred to as “dirty snowballs” because they are composed mostly of ice, rock, gas, and dust left over from the beginning of our Solar System.
Nucleus
At the heart of every comet, there is a solid core called nucleus, typically composed of ice, dust, and rocky materials. It can range in size from a few hundred meters to tens of kilometers across. When a comet approaches the Sun, the heat causes the nucleus to sublimate, releasing gas and dust.
Coma
Surrounding the nucleus is the coma, a fuzzy, glowing envelope of gas and dust that forms when the comet gets close to the Sun. The coma can be thousands of kilometers wide and makes comets appear bright in the night sky.
Hydrogen envelope
Beyond the coma lies the hydrogen envelope, which consists of neutral hydrogen atoms emitted from the comet. This layer can extend up to 10 million kilometers into space. As the comet approaches the Sun, the hydrogen envelope gets bigger.
Tails
Comets are famous for their spectacular tails, which can stretch millions of kilometers. There are typically two types of tails that are caused by the solar wind's impact on the comet:
- The dust tail is made of small dust particles that reflect sunlight. It is broad and often slightly curved. The dust tail points generally (but not precisely) away from the Sun.
- The ion tail is formed by charged gas particles. It is thin and linear, always pointing directly away from the Sun.
As the gas released by the comet cools, it becomes invisible. Meanwhile, the remaining dust particles keep on drifting along the comet’s path around the Sun. When the Earth intersects with the comet’s orbital plane, part of that dust gets re-illuminated by sunlight, so we can see an anti-tail – a bright streak that seems to be directed towards the Sun, opposite to the other two tails. It is not actually part of a comet but an optical illusion. Most comets do not evolve enough for the anti-tail to be visible.
Fun fact: the Great Comet of 1744 had as many as six tails. This might have happened because the comet started breaking apart when it got really close to the Sun and had at least three active nuclei. Another theory is that its tails were an example of “dust striae,” which are lines sometimes seen in comet tails.
How do comets form?
Astronomers believe comets formed more than 4.5 billion years ago in the same disk of gas and dust that created the Solar System planets. As they originated in the cold regions far from the Sun, they contain significantly more ice. These icy bodies can be thought of as cosmic time capsules: they are made of materials and ices that date back to the formation of the early Solar System. It means they hold clues about how our planets formed and what conditions were like back then.
Basic types of comets
Comets are generally classified based on the length of their orbital period.
Short-period comets
- Description: Have orbits that are relatively small and return to the inner Solar System frequently.
- Orbital period: Typically less than 200 years
- Example: Halley's Comet (orbital period 75 years)
Long-period comets
- Description: Follow elliptical orbits, which are longer than those of short-period comets but shorter than those of non-periodic comets.
- Orbital period: Generally between 200 and 1000 years
- Example: C/1983 H1 (orbital period 960 years)
Non-periodic comets
- Description: Either have extremely long orbital periods or follow hyperbolic or parabolic paths that aren’t closed orbits.
- Orbital period: Over 1000 years/no period (only appear in the Solar System once)
- Example: C/1811 F1, or the Great Comet of 1811 (orbital period 2,974 years)
More comet types to know about
Sungrazers
Sungrazers are a specific type of comet that pass extremely close to the Sun, which subjects them to intense solar radiation and gravitational forces. Smaller sungrazers burn up entirely as they approach the Sun, but larger ones (with a nucleus radius larger than 2–3 km) can survive many perihelion passages.
As these comets get close to the Sun, the heat causes their ices to vaporize, often resulting in spectacular outbursts of gas and dust. Many sungrazers even become great comets, visible to the unaided eye.
One of the most well-known groups of sungrazing comets is the Kreutz family, which is believed to have originated from a single larger comet that broke apart over a millennium ago. The famous comet Ikeya-Seki (C/1965 S1), sometimes referred to as the Great Comet of 1965, belonged to this group. One of the Kreutz sungrazers is now hitting the headlines: the newly discovered comet C/2024 S1 (ATLAS) is expected to reach naked-eye visibility by late October! According to some reports, the comet has disintegrated, but this has not yet been officially confirmed.
Dead comets
Dead comets, often referred to as “extinct comets” or “dormant comets”, are comets that have lost most or all of their volatile materials, such as water ice and gases. While dead comets no longer produce a visible coma or tail, they can still be detected as solid bodies in space. For example, the near-Earth asteroid 3552 Don Quixote is, in fact, a dead comet: the faint cometary tail was found in infrared images.
Where do comets come from?
Short-period comets typically originate from the Kuiper Belt, a region beyond Neptune filled with icy bodies. Long-period comets usually originate from the Oort Cloud, a distant spherical shell surrounding the Solar System. Recently, some comets were found in the main asteroid belt. Also, there are exocomets spotted beyond the Solar System.
What brings comets near the Earth?
The gravity of a planet or star can pull comets from the Kuiper Belt or Oort Cloud. This gravitational tug redirects a comet toward the Sun. As it accelerates, its path becomes elongated, resembling a long oval. Some comets dive directly into the Sun, never to be seen again.
Observing comets: all-round guide
When are comets visible?
Comets are most visible when they are in the inner Solar System. While passing close to the Sun, a comet warms up and begins to release gases (this process is called outgassing). That’s when a small, dark rock turns into a large, fluffy cloud of gas and dust and becomes much more visible through telescopes or even to the naked eye. Comets’ brightness can vary based on their distance from the Earth and the Sun.
How rare is seeing a comet?
Bright naked-eye comets are relatively rare – they appear at irregular intervals but, on average, about once per decade. Each comet like this becomes a big event – for example, we are now looking forward to January 2025, when comet C/2024 G3 (ATLAS) should reach its maximum brightness.
Fainter comets can be observed several times a year with binoculars and telescopes. Visit our list of upcoming comets to learn what comets can be seen in the sky soon!
Naked-eye comet: how bright should it be?
When talking about comets, one important thing to consider is their visual brightness, known as apparent magnitude. It determines how easily we can see it without a telescope.
Under ideal conditions, the average human eye can detect objects with a magnitude up to about 6. However, comets are not pinpoint sources of light (like stars) but rather diffuse blobs (like galaxies or nebulae). This means they don't focus light into a single point, making them appear less bright than a star of the same magnitude. Therefore, comets need to be brighter (around magnitude 4) to stand out in the night sky and be easily seen with the naked eye.
Dimmer comets can appear brighter because of a phenomenon called forward scattering. This occurs when sunlight reflects off tiny particles in the comet's coma and tail, scattering toward the observer. Forward scattering enhances the comet's visibility, making it more striking against the background of space.
How to see a comet?
-
Check for upcoming comets: Look for announcements from astronomical organizations or websites that track comet sightings. Comet Observation database, TheSkyLive website, as well as astronomers Gideon van Buitenen and Seiichi Yoshida, often provide updates. Also, you can check articles on our website.
-
Choose the right time: Comets are best viewed during specific times when they are at perigee (closest to the Earth in space) or at perihelion (closest to the Sun in space). It is better to check the best viewing time for each individual comet you are going to observe. For example, around perihelion, when a comet is close to the Sun in the sky, it’s good to start observations before sunrise or just after sunset to keep the comet from being blocked by sunlight.
-
Find a dark location: Light pollution can hinder your view, so try to find a location away from city lights. Parks, rural areas, or elevated locations can be good choices.
-
Use binoculars or a telescope: While some comets can be seen with the naked eye, binoculars or a telescope can enhance your viewing experience and help you see more detail.
-
Check the weather: Clear skies are essential for good visibility. Check the weather forecast to ensure optimal conditions.
-
Know where to look: locate the comet in the night sky via star charts or stargazing apps, like Star Walk 2. You can use exact coordinates, constellations, or sky objects (planets, stars, etc.) as a reference point.
-
Be patient: It takes time to spot a comet, especially if it's faint. Allow yourself plenty of time to scan the sky.
Famous comets
1P/Halley
Halley's Comet is perhaps the most famous comet, visible from the Earth approximately every 76 years. Halley's Comet has been recorded since 240 BC by various cultures throughout history and is often seen as an omen or a herald of significant events. In 1066, the comet appeared before King Harold II of England was defeated in the Battle of Hastings. In 1910, scientists discovered the harmful gas cyanogen in the tail of Halley's Comet, which led to a public panic, with people rushing to buy gas masks, fake “anti-comet pills”, and “anti-comet umbrellas”. Its last appearance was in 1986, and it will return in 2061. Looking forward to it!
Shoemaker-Levy 9
Shoemaker-Levy 9 made headlines in 1994 when it collided with Jupiter, creating a series of spectacular explosions that were observed from the Earth. The collision demonstrated the potential destructive power of comets and significantly advanced our understanding of impact events and their effects on planetary bodies.
67P/Churyumov-Gerasimenko
67P/Churyumov-Gerasimenko gained fame due to the European Space Agency's Rosetta mission, which successfully orbited and landed a probe (Philae) on the comet in 2014. The data collected has provided insights into the origins of comets and their role in the Solar System's history. 67P is particularly notable for its distinctive duck-like shape and its active jets of gas and dust, making it a fascinating subject for ongoing research.
This list could go on: we can think of a dozen more comets that have become famous for some reason. The most common comets to become famous are the so-called great comets, which become particularly bright and can be seen with the naked eye. Visit our dedicated article to learn more about these special comets.
Facts about comets
“Comet” means “hairy star”
The word “comet” originates from the Greek word “comētēs”, which translates to “long-haired star”. The Greeks also called comets “bearded stars”.
Comets are the source of “shooting stars”
“Shooting stars”, or meteors, are often linked to comets. When a comet travels through the Solar System, it leaves behind a trail of dust and debris. When the Earth passes through this debris field, tiny particles enter our atmosphere at high speeds, burning up and creating the bright streaks we see as meteors.
Comets may have brought life to the Earth
Scientists speculate that comets, composed of ice, organic compounds, and other essential elements, could have collided with our planet billions of years ago, providing amino acids that make up proteins – the building blocks necessary for life. This hypothesis suggests that comets might have played a crucial role in the origin of life.
Comets are often seen as bad omens
Long ago, people thought comets predicted bad events, like the death of kings or big disasters. The Great Comet of 1811 was one of the brightest ever observed and was associated with various disasters, including the Napoleonic Wars and natural calamities. Comet Biela was believed to have caused several major fires that occurred simultaneously in America in October 1871, including the Great Chicago Fire, the Great Michigan Fire, and the Peshtigo Fire. The arrival of Comet Hale-Bopp in 1997 also provoked the Heaven's Gate mass suicide, where followers believed that the comet was a sign to leave the Earth.
There are tons more interesting facts about comets. Do you know which comet was visible to the naked eye for over 18 months? What does the Great Comet of 1811 have to do with cognac? Pass our quiz about the great comets and learn more about these exceptional space objects!
Comets F.A.Q.
How many comets are there?
As of September 2024, there are around 4,000 comets discovered. Nevertheless, this is only a tiny portion of the overall potential comet population, as the reservoir of comet-like bodies in the outer Solar System (within the Oort cloud) is estimated to contain around one trillion comets.
When is the next bright comet?
We’re keeping our fingers crossed for comet C/2024 G3 (ATLAS): if it passes perihelion and doesn't fall apart, it could reach naked-eye brightness (magnitude 0.6) by January 2025.
How do comets get their names?
Comets are often named after their discoverers; for example, comet McNaught, also known as the Great Comet of 2007, is named after British-Australian astronomer Robert H. McNaught. Ground-based and space-based telescopes and observatories are also good at finding comets, so the names of many comets include their names, such as ISON or Tsuchinshan (Purple Mountain Observatory).
Some comets are given a designation that includes the year of discovery and a letter or number indicating the order in which they were discovered that year. For instance, the name C/2023 A3 (Tsuchinshan-ATLAS) indicates it is a non-periodic comet (designated with a "C") discovered in 2023 in the first half of January (this corresponds to the letter A in the IAU comet naming system), and it was the third comet discovered in the same period (hence the number 3). Tsuchinshan-ATLAS means that the discovery was made using the telescopes of the Purple Mountain Observatory (Zijinshan Astronomical Observatory) and the Asteroid Terrestrial Impact Last Alert System (ATLAS).
What causes a comet to have a tail?
When a comet is in the inner Solar System, the Sun warms it up, causing the comet's ice to vaporize and release dust and gas particles. The solar wind blows those particles off the comet, and we see the dust and ion tails.
How long do comets last?
Comets can last for thousands to millions of years, but their nuclei gradually erode with each pass near the Sun. Eventually, they may break apart or lose all their volatile materials.
Can a comet hit the Earth?
While it's theoretically possible for a comet to collide with the Earth, such events are extremely rare. Studies estimate that comets measuring 0.3 kilometers in diameter strike Earth approximately once every 500 years, whereas those with a diameter of 1.6 kilometers impact our planet about once every 6,000 years. The likelihood of a comet the size of Shoemaker-Levy 9 (1.5-2 kilometers) colliding with the Earth is estimated to occur once every 10 million years. If this comet had impacted Earth instead of Jupiter, it could have potentially eradicated life as we know it. Scientists actively monitor near-Earth objects to assess any potential threats.
Comets: to sum up
A comet is a space object primarily composed of ice, dust, and rocky material. Comets orbit the Sun and are often categorized based on their orbital periods: short-period comets take less than 200 years to orbit the Sun, while long-period comets take more than 200 years. They are considered remnants of the early Solar System and can provide valuable insights into its formation and evolution. Download Star Walk 2 and easily track comets in the sky!
