A comet is a tiny, frozen entity in the Solar System that heats up and begins to emit gases as it approaches the Sun, a process known as outgassing. This results in a visible atmosphere or coma, as well as a tail in rare cases”the impact of solar radiation and the solar wind on the nucleus of the comet cause these events. Comet nuclei are made up of loose collections of ice, dust, and tiny rocky particles that range in size from a few hundred meters to tens of kilometers wide. The coma may be up to 15 times the size of Earth, and the tail could be more than one astronomical unit long. A comet may be observed without a telescope from Earth if it is brilliant enough, and it can cover an arc of 30° (60 Moons) across the sky. Many civilizations and faiths have seen and documented comets from ancient times.

Comets have extremely eccentric elliptical orbits with a wide variety of orbital durations ranging from a few years to possibly millions of years. Short-period comets are born beyond Neptune’s orbit in the Kuiper belt or its associated dispersed disc. The Oort cloud, a spherical cloud of ice bodies spanning from outside the Kuiper belt to halfway to the closest star, is considered to be the source of long-period comets. Gravitational perturbations generated by passing stars and the cosmic tide propel long-period comets from the Oort cloud towards the Sun. Before being hurled into interstellar space, hyperbolic comets may travel through the inner Solar System once. An apparition is a name given to a comet’s appearance.

The existence of an extensive, gravitationally unbound atmosphere surrounding the core nucleus distinguishes comets from asteroids. The coma (the core region immediately around the nucleus) and the tail (a usually linear segment comprised of dust or gas blasted out of the coma by the Sun’s light pressure or out streaming solar wind plasma) are two distinct sections of this atmosphere. Extinct comets that have traveled close to the Sun numerous times, on the other hand, have lost almost all of their volatile ices and dust and may resemble tiny asteroids. Asteroids are considered to be distinct from comets in that they formed within Jupiter’s orbit rather than in the outer Solar System. The discovery of main-belt comets and active centaur minor planets has made the boundary between asteroids and comets increasingly blurry. The finding of several small bodies with long-period comet orbits but features of inner solar system asteroids in the early twenty-first century was dubbed Manx comets. Comets like C/2014 S3 are still categorized as comets (PANSTARRS). Between 2013 and 2017, 27 Manx comets were discovered.

There are 4595 known comets as of April 2021, a figure that is continuously growing as more are discovered. However, because the reservoir of comet-like entities in the outer Solar System (the Oort cloud) is believed to be one trillion, this represents just a small percentage of the total possible comet population. Every year, around one comet, is visible to the human eye, though many of them are dim and uninteresting. “Great comets” are very brilliant examples. Unmanned probes like the European Space Agency’s Rosetta, which was the first to land a robotic spacecraft on a comet, and NASA’s Deep Impact, which blew a crater on Comet Tempel 1 to investigate its innards, have both visited comets.

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