20.17 Comets

Why do comets have tails?

The ball of white in the lower left portion of the image is a comet, Comet Holmes. Comets do not have tails out in space, only when they are close to the Sun. The spiral shaped light in the image is the Andromeda Galaxy.

Comets

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 the outer layers of ice melt and evaporate as the comet flies close to the Sun. The ice from the comet vaporizes and forms a glowing coma, which reflects light from the Sun. Radiation and particles streaming from the Sun push this gas and dust into a long tail that always points away from the Sun (Figure below). Comets appear for only a short time when they are near the Sun, then seem to disappear again as they move back to the outer solar system.

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 next be seen 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, with periods of about 200 years or less, come from a region beyond the orbit of Neptune called the Kuiper belt (pronounced “KI-per”). It contains not only comets, but also 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 called the Oort cloud, about 50,000 — 100,000 AU from the Sun (50,000 – 100,000 times the distance from the Sun to Earth).

Summary

• 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.

Review

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 does Halley's comet appear to earthlings every 75 years?