A comet is an icy body that releases gas or dust. They are often compared to dirty snowballs, though recent research has led some scientists to call them snowy dirtballs. Comets contain dust, ice, carbon dioxide, ammonia, methane and more. Astronomers think comets are leftovers from the gas, dust, ice and rocks that initially formed the solar system about 4.6 billion years ago.

Some researchers think comets might have originally brought some of the water and organic molecules to Earth that now make up life here. To research this hypothesis, the Rosetta mission, which landed a probe on a comet on Nov. 12, 2014, is studying its nucleus and environment, observing how it changes as it approaches the sun.

Comets orbit the sun, but most are believed to inhabit in an area known as the Oort Cloud, far beyond the orbit of Pluto. Occasionally a comet streaks through the inner solar system; some do so regularly, some only once every few centuries. Many people have never seen a comet, but those who have won't easily forget the celestial show.

The solid nucleus or core of a comet consists mostly of ice and dust coated with dark organic material, according to NASA, with the ice composed mainly of frozen water but perhaps other frozen substances as well, such as ammonia, carbon dioxide, carbon monoxide and methane. The nucleus may have a small rocky core. As a comet gets closer to the sun, the ice on the surface of the nucleus begins turning into gas, forming a cloud known as the coma. Radiation from the sun pushes dust particles away from the coma, forming a dust tail, while charged particles from the sun convert some of the comet's gases into ions, forming an ion tail. Since comet tails are shaped by sunlight and the solar wind, they always point away from the sun. At first glance, comets and asteroids may appear very similar. The difference lies in the presence of the coma and tail. Sometimes, a comet may be misidentifiedas an asteroid before follow-up observations reveal the presence of either or both of these features.

The nuclei of most comets are thought to measure 10 miles (16 kilometers) or less. Some comets have comas that can reach nearly 1 million miles (1.6 million km) wide, and some have tails reaching 100 million miles (160 million km) long.

We can see a number of comets with the naked eye when they pass close to the sun because their comas and tails reflect sunlight or even glow because of energy they absorb from the sun. However, most comets are too small or too faint to be seen without a telescope.

Comets leave a trail of debris behind them that can lead to meteor showers on Earth. For instance, the Perseid meteor showeroccurs every year between August 9 and 13 when Earth passes through the orbit of the Swift-Tuttle comet.

Astronomers classify comets based on the durations of their orbits around the sun. Short-period comets need roughly 200 years or less to complete one orbit, long-period comets take more than 200 years, and single-apparition comets are not bound to the sun, on orbits that take them out of the solar system, according to NASA. Recently, scientists have also discovered comets in the main asteroid belt these main-belt comets might be a key source of water for the inner terrestrial planets.

Scientists think short-period comets, also known as periodic comets, originate from a disk-shaped band of icy objects known as the Kuiper Belt beyond Neptune's orbit, with gravitational interactions with the outer planets dragging these bodies inward, where they become active comets. Long-period comets are thought to come from the nearly spherical Oort Cloud even further out, which get slung inward by the gravitational pull of passing stars.

Some comets, called sun-grazers, smash right into the sun or get so close that they break up and evaporate.

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Comets: Facts About The Dirty Snowballs of Space