What Is A Comet?

    Published: 06-16-2009
    Views: 11,201
    Science expert Emerald Robinson explains what a comet is.

    Emerald Robinson: Hi! I am Emerald Robinson. In this "What Is" video, we explore comets, those unique streaks of light in our night sky that rarely visit us.

    Comets are icy objects, typically ranging a few miles in size, which orbit our Sun. Their highly elliptical orbits carry them from close to the Sun all the way to the outer edges of the solar system.

    When a comet gets close to the Sun, its outer layers of ice melt and evaporate creating an atmosphere of gas and dust around the comet. This atmosphere is called a coma. Radiation from the Sun pushes some of this atmosphere into a long tail. A comet's tail always points away from the Sun, no matter which way the comet is moving.

    Gases in the coma and tail of a comet reflect the light from the Sun giving comets their familiar appearance. But Comets are very dark and hard to see when they aren't near the Sun.

    The time between one visit near the Sun and the next is called the comet's period. The first comet whose period was known was Halley's Comet. Its period is 75 years. Halley's Comet last traveled through the inner solar system in 1986.

    Comets with periods of 200 years or less are called short period comets. Short period comets are from a region beyond the orbit of Neptune called the Kuiper Belt. The Kuiper Belt is home to comets, asteroids, and dwarf planets.

    Long period comets have periods of thousands or even millions of years. Most long-period comets come from a very distant region of the solar system called the Oort cloud. The Oort cloud is about 50,000-100,000 times the distance from the Sun to Earth.

    Comets carry materials in from the outer solar system. Some scientist believes that water may have been brought to early Earth from a collision with a comet carrying water.