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March 21, 2021
Comets are believed to have two regions of origin in the solar system. Short-period comets (with periods of up to 200 years) originate from the Kuiper belt or scattered disk. Long-period (with periods of more than 200 years) arrive in the inner region of the solar system from the Oort cloud.
When approaching the Sun, the comet forms a coma and, sometimes, a tail of gas and dust. Over time, ices of comets of various compositions sublimate under the influence of solar radiation. In this case, non-volatile compounds are deposited downward, forming a thick crust, which blocks the access of solar energy to the deep layers. As a result, when approaching the Sun, the comet no longer forms a coma and a tail. Such comets are called degenerate, hidden, or dormant. Degenerate comets are small, dark celestial bodies that are very difficult to detect even with the most powerful telescopes.
The discovery of degenerate comets introduced some confusion about which objects are considered asteroids and which are comets, since their main criterion was the presence or absence of a coma or tail. As a result, in 2006, the International Astronomical Union decided to assign two designations to some objects, since at first they were classified as asteroids, but then, when they found cometary activity, they also received a cometary designation. Comets tend to move in more elongated orbits than asteroids, so asteroids with high orbital eccentricities are more likely to be degenerate comets. Such objects include, for example, the asteroids Don Quixote and Adonis.
The term "sleeping comet" is used to describe inactive comets that can become active as they approach the Sun.
Scientists estimate that about 6% of all asteroids approaching the Earth are degenerate comets that have completely depleted their reserves of volatile components.
ROSCOSMOS Press Release: https://www.roscosmos.ru/30411/
Images, Text, Credits: ROSCOSMOS/Orbiter.ch Aerospace/Roland Berga.
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