Comets are cold, small solar system objects. When they approach the sun, they warm up and begin to release gas. This process is called degassing. This creates a visible atmosphere or coma, and sometimes a tail. These phenomena are due to the influence of solar radiation and the solar wind acting on the comet nucleus. Cometary nuclei range from hundreds of meters to tens of kilometers and consist of loose collections of ice, dust, and small rock particles. Coma can be up to 15 times the diameter of the earth, and the tail can stretch an astronomical unit. If it's bright enough, a comet can be seen from the earth without the aid of a telescope, and the comet may be in 30 ° arc (60 satellites) in the sky. Since ancient times, many cultures have observed and recorded comets.

Comets usually have highly eccentric elliptical orbits, and their orbital periods range from a few years to possibly millions of years. Short period comets originate in the Kuiper belt or its associated disk, which lies outside the orbit of Neptune. Long period comets are thought to have originated in the Oort cloud, a cold, spherical body that extends from the Kuiper belt to half of the nearest star. Long period comets move from the Oort cloud to the sun due to gravitational disturbances caused by tides of stars and galaxies. Hyperbolic comets may pass through the inner solar system at one time and then hurl into interstellar space. The appearance of comets is called mirage.

The difference between comets and asteroids is that there is an extended, gravity free atmosphere around their central core. This atmosphere has a part called coma (the central part tightly surrounding the nucleus) and a part called tail (usually a linear part, made up of dust or gas blown out of coma by the light pressure of the sun or solar wind plasma). But extinct comets that have been close to the sun many times have lost almost all the volatile ice and dust, and may be like asteroids. Asteroids and comets have different origins. They are formed inside the orbit of Jupiter rather than outside the solar system. The discovery of the main comet and the active Centauri obscures the difference between asteroids and comets.

As of November 2014, there were 5253 known comets, which is growing steadily with the discovery. However, this is only a small part of the total number of potential comets, as the reserves of comet like objects in the outer solar system (in the Oort cloud) are estimated to be 1 trillion. The naked eye can see about one comet a year, although many of them are faint and unobtrusive. A particularly bright example is called a "big comet.". Comets have been visited by unmanned probes, such as the European Space Agency's Rosetta (the first robotic spacecraft ever to land on a comet) and NASA's deep impact, which blasted the crater of comet Temple 1 to study its interior.