A comet is a small cosmic object similar to an asteroid, but composed primarily of ice. They are very often described as “dirty snowballs”, although according to recent research, this only applies to the surface of the comet’s core and is largely made up of frozen carbon dioxide, methane and water with mixed dust and various mineral aggregates. These aggregates hold together only due to their own gravity. Comets are thought to have formed in a distant cloud known as the Oort cloud. According to older theories, the Oort cloud originated in the protoplanetary disk from which the entire solar system was formed. However, according to a new theory developed by astronomers in 2010, some comets may have formed in the orbit of stars such as the Sun. The capture of these bodies by the Sun could have taken place at a time when the Sun was still a member of its native star cluster (because stars form in groups) relatively close to neighboring stars. This new view of the formation of the Oort cloud of comets arose after a more detailed investigation showed that the Oort cloud formed only as a remnant of a protoplanetary disk should be much smaller.
A comet traditionally consists of a nucleus, a coma and a tail. Although the coma and tail are the most characteristic properties of a comet, thanks to which it can almost certainly be distinguished from other bodies, but most comets do not have these components for most of their trajectory. Coma and tail form only near the star under the influence of its radiation.
Comets typically move in very elliptical orbits, the gloom of which (farelium) may be much more distant than Pluto’s orbit.
A distinction is made between an active comet and a sleeping comet. The active comet has a surface that releases material (escapes). The sleeping comet has an inactive surface (does not escape) at any distance from the star.
About every decade, a comet becomes bright enough to be observable to a normal observer – such comets are often referred to as large comets.
Of the thousands of known comets, some are unusual. Comet Encke has an orbit between the orbits of Jupiter and Mercury, while Comet 29P / Schwassmann-Wachmann has an unstable orbit between Jupiter and Saturn. Comet 2060 Chiron, whose unstable orbit remains between Saturn and Uranus, was initially classified as an asteroid until its faint coma was detected. Similarly, Comet Shoemaker-Levy 2 was initially designated the 1990 UL3 asteroid. Some near-Earth asteroids are considered to be extinct nuclei of comets from which no more gases are released.
Comet Hale-Bopp, bearing the designation C / 1995 O1, was probably the most observed comet of the 20th century and one of the brightest comets in decades. It was visible to the naked eye for a record 19 months, twice as long as the previous record holder, the Great Comet of 1811. Comet Hale-Bopp is one of the long-period comets.
Halley’s Coma is a periodic comet with a period of about 75.3 years. It is the most famous periodic comet. She was last in the perihelion in 1986, when she was visited by the Vega 1, Vega 2 and Giotto spacecraft. Her next return to the Sun is expected on July 28, 2061.