Most comets inhabit Neptune orbits and move through elliptical orbits with a significant eccentricity. At the end of the 20th century, inside the orbit of the main belt of asteroids found relatively small bodies with visual characteristics resembling comets – coma, gas or dust tails.
In 2006, the British-American astronomer David Jihitt offered to call these bodies “Comets of the main belt” or “MBC objects” (from English Main-Belt Comets). The period of circulation around the Sun such a comet is 5-6 years. By 2006, only a few such objects were reliably confirmed. The term “comet of the main belt” implies that these comets must exist only within its limits and have mainly ice composition. However, the opening of new objects of this type indicates that it is not always the case. Therefore, they are also called “active asteroids”.
The first active asteroid was discovered in 1979. At first, it was opened as an asteroid, but already in 1996, Astronomers Eric Elst and Guido Pissero, watching this object in the La Silly Observatory in Chile, saw a comettle tail when the asteroid was close to the perihelion. In 2001, the picture was repeated. The asteroid demonstrated cometary activity for 5 months. So an asteroid 7968 ELST-PIZARRO received a cometic name – 133p / Elst-Pizarro.
In 1999, as part of the project on the search for near-earth asteroids in the Lincoln Laboratory in the United States (English. Abbreviation Linear) was opened by the asteroid (118401) linear, so the object was named after this project. In 2005, an active asteroid 238p / Read was opened, in 2008 – P / 2008 R1 (Garradd), in 2010 – P / 2010 R2 (La Sagra). In December 2010, cometary activity was discovered from the asteroid 596 Scheila in the main belt, but, despite the official double status, according to the rules of the International Astronomical Union, all such objects in databases pass as asteroids.
Until recently, the “cometary” theory of water formation on Earth was very popular. According to this hypothesis, water on Earth appeared as a result of bombardments by comets that arrived from the external areas of the solar system. However, according to the latest research, in the water of classic comets almost 3 times more than the deuterium than in the water of the Earth. It is possible that the comets of the main belt served as a source of water on Earth.