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22.23: Comets

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    Is this comet going to hit?

    Astronauts on board the International Space Station witness incredible things. This photo of Comet Lovejoy was taken in December 2011. Although it looks like its going to strike Earth, it was not even visible from our planet by the naked eye. The comet is now traveling far out into space.


    Comets are small, icy objects that have very elliptical orbits around the Sun. Their orbits carry them from the outer solar system to the inner solar system, close to the Sun. Early in Earth’s history, comets may have brought water and other substances to Earth during collisions.

    Comet tails form as the comet flies close to the Sun and the outer layers of ice melt and form a glowing coma. Particles streaming from the Sun push this gas and dust into a long tail. The tail always points away from the Sun. The coma is bright because it reflects light from the Sun (Figure below). Comets appear for only a short time when they are near the Sun. They seem to disappear as they move back to the outer solar system.

    Comet Hale-Bopp, also called the Great Comet of 1997

    Comet Hale-Bopp, also called the Great Comet of 1997. The comet has two visible tails: a bright, curved dust tail and a fainter, straight tail of ions (charged atoms) pointing directly away from the Sun.

    The time between one appearance of a comet and the next is called the comet’s period. Halley’s comet, with a period of 75 years, will be seen next in 2061. The first mention of the comet in historical records may go back as much as two millennia.

    Where Comets Come From

    Short-period comets have periods of about 200 years or less. These comets come from a region beyond the orbit of Neptune called the Kuiper belt (pronounced “KI-per”). The Kuiper belt also contains asteroids and at least two dwarf planets.

    Comets with periods as long as thousands or even millions of years come from a very distant region of the solar system. This region is called the Oort cloud. It is about 50,000-100,000 AU from the Sun (50,000–100,000 times the distance from the Sun to Earth).


    • Comets are icy objects that have very elliptical orbits around the Sun.
    • Comet tails form as ice vaporizes and glows in the Sun's light.
    • Short-period comets come from the Kuiper belt beyond Neptune, and long-period comets come from the Oort cloud far out away from the Sun.


    1. Why do comets only have tails when they are near the Sun?
    2. Where is the Kuiper belt, and what is found in it?
    3. Why do most comets appear in regular, predictable time periods?


    Image Reference Attributions

    [Figure 1]

    Credit: User:Mizusumashi/Wikimedia Commons and User:Autiwa/Wikimedia Commons
    License: Public Domain

    [Figure 2]

    Credit: Flickr:sungod17;User:Mizusumashi/Wikimedia Commons and User:Autiwa/Wikimedia Commons;Courtesy of H Weaver (JHU/APL), A Stern (SwRI), and the HST Pluto Companion Search Team
    Source: ; ;
    License: CC BY 2.0; Public Domain

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