Figure 25.34: A lunar meteorite ( 27 )they move back to the outer solar system. The time between one appearance of a comet and
the next is called the comet’speriod. For example, the first comet whose period was known,
Halley’s comet, has a period of 75 years. It last traveled through the inner solar system in
1986, and will appear again in 2061.
Where Comets Come From
Comets that have periods of about 200 years or less are considered short period comets.
Most short-period comets come from a region beyond the orbit of Neptune. This area,
which contains not only comets but also asteroids and at least two dwarf planets, is called
theKuiper belt.(Kuiper is pronounced “KI-per,” rhyming with “viper.”)
Some comets have much longer periods, as long as thousands or even millions of years. Most
long-period comets come from a very distant region of the solar system called theOort cloud,
which is about 50,000–100,000 AU from the Sun (50,000–100,000 times the distance from
the Sun to Earth). Comets carry materials from the outer solar system to the inner solar
system. Comets may have brought water and other substances to Earth during collisions
early in Earth’s history.