Comet

A comet is an icy small Solar System body or interstellar object that warms up and releases gases when it gets close to the Sun. This process is called outgassing. It creates a glowing cloud of gas called a coma. Sometimes, comets also have a beautiful tail made of gas and dust. These tails can stretch for millions of miles and are pushed away from the Sun by solar wind and radiation.

Comet nuclei are made of loose ice, dust, and rocky pieces. They can range from a few hundred meters to tens of kilometers across. When bright comets come close to Earth, we can sometimes see them without a telescope. These amazing objects have fascinated people for thousands of years and have been recorded by many cultures.

Comets travel in very stretched-out oval paths called eccentric orbits around the Sun. Some comets take just a few years to complete their journey, while others may take millions of years. Most come from distant areas far beyond the planet Neptune, such as the Kuiper belt or the huge spherical Oort cloud. Scientists have sent spacecraft to visit comets to learn more about what they are made of and how they behave.

Etymology

The word comet comes from the Old English word cometa, which came from the Latin word comēta or comētēs. This Latin word is a romanization of the Greek word κομήτης, meaning 'wearing long hair'. In Greek, people used this word to describe something like a 'long-haired star'. The word comes from κομάω (koman), meaning 'to wear the hair long', which itself comes from κόμη (komē), meaning 'the hair of the head'. This is why comets are often thought of as having a tail.

Comets have a special symbol used in writing, which is U+2604 ☄ COMET. This symbol shows a small circle with three lines coming out from it, like hair, representing the tail of a comet.

Physical characteristics

The solid middle part of a comet is called the nucleus. Comets are made of rock, dust, water ice, and frozen gases like carbon dioxide, carbon monoxide, methane, and ammonia. This is why they are sometimes called "dirty snowballs." As comets move closer to the Sun, they get warmer and let out gases and dust. This makes a fuzzy area around the nucleus called a coma and sometimes a tail.

Main article: Coma (comet)

The coma is a big, thin layer of dust and gas around the nucleus. When a comet gets near the Sun, the solar wind and sunlight push this material away, making a tail that always points away from the Sun. The coma can sometimes get bigger than the Sun, but it is usually too faint to see without a telescope.

Orbital characteristics

Most comets travel in stretched-out paths around the Sun. These paths are called elliptical orbits. They take comets close to the Sun at one point and then far away into the outer parts of the Solar System.

Comets are grouped by how long it takes them to go around the Sun once. Those with shorter orbits, less than 200 years, are called short-period comets. These comets usually move in the same plane as the planets and often come from areas beyond Jupiter. Some short-period comets, like Encke's Comet, have very short orbits and stay closer to the Sun. Others, like Halley's Comet, take between 20 and 200 years to orbit the Sun.

Long-period comets have orbits that can take thousands or even millions of years. They come from a distant region called the Oort cloud and have very stretched-out paths. Some of these comets may only visit the inner Solar System once before disappearing into space.

Effects of comets

When a comet gets close to the Sun, it warms up and lets out gases and dust. This can leave trails of tiny pieces in space. When Earth passes through these trails, we see lovely streaks of light in the night sky called meteor showers. For example, the Perseid meteor shower happens every year when Earth moves through bits left by Comet Swift–Tuttle.

Scientists think that a long time ago, comets may have brought water to Earth, helping to fill our oceans. They might have also carried important molecules that helped life start. Comets hitting the Moon might have left ice there too. Long ago, some people felt worried when they saw comets, thinking they might mean big changes were coming.

Fate of comets

Some comets leave our Solar System if they move very fast. These are called hyperbolic comets. They often get a push from a big planet like Jupiter. For example, Comet C/1980 E1 changed its path after it passed close to Jupiter in 1980.

Over time, many comets lose their ice and become like small, dark rocks. They may start to look and act like asteroids. Sometimes comets can break apart because of pressure from the Sun or inside them. One famous example is Comet Shoemaker–Levy 9, which broke into pieces and hit the planet Jupiter in 1994. These events help scientists learn more about how objects move and change in space.

Main article: Extinct comet

Nomenclature

Main article: Naming of comets

Comets have been named in many different ways over time. In the past, they were often named after the year they appeared, like the "Great Comet of 1680" or the "Great Comet of 1882".

Later, some famous comets were named after the scientists who studied them, like Halley's Comet. Today, most comets are named after the people or tools that first found them. For example, a comet discovered in 2019 by astronomer Gennadiy Borisov was named 2I/Borisov.

History of study

Main article: Observational history of comets

People have seen comets for thousands of years. Long ago, many believed comets were special signs.

Early scientists tried to explain what comets were. Some thought they were close to Earth, while others thought they were far away in space. With better tools, we learned that comets are made of ice and dust and come from far beyond our planet.

In the 1600s and 1700s, scientists discovered that comets travel around the Sun in long paths. A famous comet, Halley's Comet, was expected to return in 1758 and it did, showing that scientists were right. Today, spacecraft have flown by comets, taking pictures to help us learn more about them.

Classification

Comets are grouped by scientists based on their paths around the Sun and their appearance. Some comets become very bright and easy to see, called great comets. These happen about once every ten years. Predicting which comets will become great comets is tricky because many things can change their brightness.

Some comets come very close to the Sun and are called sungrazing comets. Most of these belong to groups that came from a single large comet that broke apart. Other comets have unusual paths or appearances, like orbiting between planets or looking like asteroids until they start to show a fuzzy atmosphere.

Observation

A comet can be discovered using a wide-field telescope or with binoculars. Even without special equipment, people can find sungrazing comets by looking at images from satellite observatories such as SOHO.

Lost

Main article: Lost comet

Some periodic comets found in the past are now called lost comets. This is because their orbits were not well enough known to predict when they would return, or because they broke apart. Sometimes, a newly discovered comet turns out to be one of these lost comets. For example, Comet 11P/Tempel–Swift–LINEAR, found in 1869, was lost until it was rediscovered in 2001.

In popular culture

See also: Comets in fiction and Category:Fiction about impact events

Comets have been used in stories as signs of big changes. Famous comets like Halley's Comet have inspired many books and tales.

In science fiction, comets are sometimes shown crashing into Earth, causing problems for heroes to fix. Stories like Deep Impact, Armageddon, and Lucifer's Hammer explore these ideas. Other stories, such as Jules Verne's Off on a Comet and Arthur C. Clarke's 2061: Odyssey Three, imagine adventures with comets.

In literature

The long-period comet was first seen by Pons in Florence on July 15, 1825. It inspired a funny poem by Lydia Sigourney called The Comet of 1825. In her poem, the stars and planets talk about why the comet appeared and what it might be for.