Comets are balls of dust and gas that orbit the Sun in highly elongated paths. When approaching the Sun, their volatile ices sublimate and form an atmosphere called the coma; when exposed to its radiation pressure, dust particles break off into long tails of gas and dust visible only through telescopes.
The Nucleus
There are billions of comets orbiting our Sun; some occasionally streak through our inner Solar System. Their visible part is their tail – an expanse of dust and gas stretching millions of miles – while their nucleus contains solid rock-like material filled with ice that forms its tail when close to our Sun. Scientists speculate that their surface ices contribute to this effect.
Volatile ices on a comet nucleus are known as volatiles because their cold temperature enables them to transform into gasses such as water vapor, carbon dioxide, methane, or formaldehyde that can become gaseous when heated by the Sun. Water and volatiles make up about 75% of its mass;
Once a comet approaches the Sun, its ices and gases expand into an expanding cloud of dust and ions surrounding its nucleus – this cloud, known as its “coma,” gives rise to its characteristic tail. Gravitational forces pull on these dust particles, but their heat causes them to escape and fly away, producing two tails: dust tail and ion tail extending from its nucleus.
Scientists have also noted that comet nuclei appear dark because complex organic molecules absorb sunlight that heats a comet’s surface ice and gases and emits heat rays that heat the comet from within.
Rotational forces cause various parts of a comet’s nucleus to face the Sun, activating certain sections and inactivating others. When an active comet approaches the Sun, its ices may melt and release material onto its surface, leaving behind a tail of gas and dust that stretches for millions of miles.
As comets get farther from the Sun, their icy surface and volatiles gradually cool off, and their tail dissipates. Scientists believe some comets may even split apart due to thermal stress or internal gas pressure; examples include 3D/Biela in 1846, Shoemaker-Levy 9 in 1992, and 53P/Van Biesbroeck from 1995-2006 as examples.
The Coma
At the center of every comet is a solid core composed of ice, dust, and small amounts of gas – often less than 10 miles (16 kilometers) across and about the size of a small town. When comets orbit farthest from our solar system, like those found in Kuiper Belt or Oort Cloud regions, scientists believe there may only be this frozen core present; when closer to Sun, heat from our star can cause surface ice to turn into gas as it warms the nucleus, potentially producing jets of gas that burst forth bringing dust along their path forming what’s known as coma clouds around its nucleus called Coma Coma Coma Coma Comas.
Astronomers know that the comet’s coma can often be seen from Earth using a large telescope even when it is far from the Sun. This explains why astronomers strive to ensure maximum coverage of newly discovered comets. They can determine whether it qualifies as a comet rather than simply another asteroid using factors such as its brightness and tail shape.
As the comet nears the Sun, its coma continues to expand and brighten due to the melting ice on its surface, which expand out into space to form a hydrogen-rich atmosphere around it called a coma. Solar winds then carry away dust particles ionized by ultraviolet radiation into long tails of gas and dust that lead back away from its path.
Once a comet reaches perihelion, its coma becomes so thick and bright that it’s easily seen even from Earth. At this point, its appearance may resemble that of a teardrop shape, with jets of gas emanating from within its core, creating long tails which extend millions of miles from its head.
Astronomers utilize several instruments to study the coma and tails of comets. Since their appearance constantly shifts as ice material vaporizes or is caught by solar winds, images submitted to the COMa Morphology Campaign at the Joint Astronomy Laboratory at the University of Arizona are included as coauthors on any papers resulting from these observations.
The Tails
Comets spend much of their time out in the cold outer reaches of our solar system, coming close to it only every few thousand or million years. However, we can see them thanks to their coating – made up of different kinds of ice (water ice), rock, and complex organic molecules known as “come” that give off an intense glow like dusty snowballs.
As comets approach the Sun, their surface heats up rapidly. Some of its ice begins to “sublimate,” turning from solid into a gas and creating an extended tail of gases and particles extending millions of miles behind it. This phenomenon is called sublimation; once sublimated into gas, it makes an expansive bottom of particles known as a coma, often white or pink in hue, encasing it all.
Comets’ ion and dust tails have two distinctive features; an ion tail is always pointed away from the Sun while their dust tail follows in an arching path behind. Both seats make this comet even more spectacular and are particularly interesting to scientists as they could contain remnants from when our solar system first formed!
The ion tail and dust tail of comets are formed when the solar wind interacts with gases and particles in its coma, both producing electrically charged gas molecules (ions) which blow them back, while solar radiation pressure pushes outward to create dust tails.
Comet tails can reach extraordinary lengths, often exceeding one astronomical unit (the distance from Earth to Sun), filled with glowing gas that illuminates when sunlight hits them. This creates an incredible display that lasts months while the comet travels its highly eccentric orbit around our sun, making comets one of the most stunning objects visible in the night sky.
What We Learn About Comets
Comets are primitive bodies that offer us insight into the early history of our solar system. As one of the first solids to form from solar nebula — an interstellar cloud composed of dust and gas from which planets form — comets offer us glimpses into its development. Additionally, because their orbits were far away from Sun’s heat, they have undergone fewer altering processes that alter asteroids or other rocks closer to it in structure or composition.
Comets pass close enough to the Sun for their nuclei (and, sometimes, multiple cores) to heat up, emitting gases and dust that form long tails extending millions of miles. This process causes meteor showers here on Earth and is the source of many regular meteor showers. Scientists also believe comets were essential in transporting water across our inner solar system and planet Earth.
Comets have several components, including the nucleus, coma, hydrogen envelope, and dust/plasma tails. Scientists can learn about their composition, size, and location by studying these elements; specifically the light-scattering properties of their constituent gases that compose the coma when passing nearer the Sun reveal its chemical makeup through its spectrum spectra.
Comet nuclei comprise frozen molecules like water, carbon dioxide, and ammonia that have frozen together. As low-pressure conditions of space vaporize, some of these sublimated molecules sublimate into space’s low-pressure conditions resulting in gradual changes to their appearance over time, sometimes to such an extent that they become visible against the night sky.
Astronomers have long observed and tracked comets. Comets can generally be divided into short-periodic comets with orbits lasting less than 200 years and regularly returning to our inner solar system, and long-periodic comets originating either in the Kuiper Belt or further out in Oort Cloud.
