Solar system has objects {comet}| that start outside Pluto orbit. Famous comet is Halley's comet, which neared Sun in 1986 and has 76-year orbit.
composition
Comets have frozen water, iron, silicate, carbon, and nickel, plus ammonia, carbon dioxide, methane, cyanogen, and hydrogen cyanide.
parts
Comets have central ball {nucleus, comet} and coma.
size
Comets have average diameter 2000 meters.
number
More than 1,000,000,000,000 comets exist, with total mass 1 to 1000 times Earth mass.
orbit
Comets have elliptical orbits, averaging 10,000 years, in same motion direction as planets, extending up to 1/5 distance to nearest star. Comet orbits change as they come close to Sun, because hot side evaporates more than cold side and spin interacts with Sun gravity.
tail
As comets approach Sun and evaporate, they leave curved dust tails along motion path. As comets approach Sun, solar wind creates straight ion tails, away from Sun.
meteor showers
Comet dust causes meteor showers, which happen 600 times a year. Meteor-shower names are the name of the constellation in which they appear: Quadrantids in January, Lyrids in April, Aquarids in May, Perseids in August, Draconids in October, Orionids in October, Taurids in November, and Geminids in December.
Comets have a central nucleus surrounded by a dust and gas cloud {coma, comet}.
Most comets lie in circular orbits {Oort cloud}| beyond Pluto, where they formed. From there, passing star can push them into elliptical orbit. Some comets lie in Kuiper belt.
Comet debris causes faint patch {counterglow} {gegenschein}|, 10 degrees diameter, in night sky in direction opposite Sun.
Comet debris causes faint glow {zodiacal light}| along ecliptic, which is 10 degrees above horizon, in eastern sky before sunrise or in western sky after sunset.
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Date Modified: 2022.0225