How Comets Work

Comets have fascinated mankind since humans first noticed the distinctive tail streaking across the night sky. We mark the date that we saw a comet that comes around only once a century (or even once in four centuries), and we remember the sight for the rest of our lives.

Astronomers find comets fascinating, too. They're remarkable pieces of our universe's past, and they tell us a great deal about how the universe was formed.

Almost every year, we are visited by comets from the outer reaches our solar system, like ISON or LINEAR. In this edition of How Stuff Works, we will examine the fascinating world of comets. We will find out what comets are, what they are made of, where they come from and how to observe them. You can learn where to look for comets and maybe even discover one yourself.

What Is a Comet?

Comets are small members of the solar system, usually a few miles or kilometers in diameter. They have been described as "dirty snowballs" by astronomer Fred Whipple and are thought to be made of:

dust
ice (water, ammonia, methane, carbon dioxide)
some carbon-containing (organic) materials (e.g., tar)
a rocky center (some comets)

Comets are thought to be made from the earliest materials of the solar system. When the sun first formed, it blew lighter material (gases, dust) out into space. Some of this material (mainly gas) condensed to form the outer planets (Jupiter, Saturn, Uranus, Neptune) and some remains in orbit far from the sun in two areas:

Oort Cloud - a sphere about 50,000 AUs from the sun; named after the Dutch astronomer Jan Oort who proposed it
Kuiper Belt - an area within the plane of the solar system outside the orbit of Pluto

The Path of a Comet

Comets are thought to orbit the sun in either the Oort cloud or Kuiper belt. When another star passes by the solar system, its gravity pushes the Oort cloud and/or Kuiper belt and causes comets to descend toward the sun in a highly elliptical orbit with the sun at one focus of the ellipse. Comets can have short period orbits (less than 200 years such as Halley's comet) or long period orbits (greater than 200 years such as comet Hale-Bopp).

As the comet passes within six AUs of the sun, the ice begins to go directly from the solid to the gas state (sublimation) much like the way fog is formed. When the ice sublimes, the gas and dust particles flow away from the sun to form the comet's tail.

Parts of a Comet

As a comet approaches the sun, it warms up. During this warming, you can observe several distinct parts:

nucleus
coma
hydrogen envelope
dust tail
ion tail

The nucleus is the main, solid part of the comet. The nucleus is usually 1 to 10 kilometers in diameter, but can be as big as 100 kilometers. It can be composed of rock.

The coma is a halo of evaporated gas (water vapor, ammonia, carbon dioxide) and dust that surrounds the nucleus. The coma is made as the comet warms up and is often 1,000 times larger than the nucleus. It can even become as big as Jupiter or Saturn (100,000 kilometers). The coma and nucleus together form the head of the comet.

Surrounding the coma is an invisible layer of hydrogen called the hydrogen envelope; the hydrogen may come from water molecules. It usually has an irregular shape because it is distorted by the solar wind. The hydrogen envelope gets bigger as the comet approaches the sun.

The comet's dust tail always faces away from the sun. The tail is made of small (one micron) dust particles that have evaporated from the nucleus and are pushed away from the comet by the pressure of sunlight. The dust tail is the easiest part of the comet to see because it reflects sunlight and because it is long, several million kilometers (several degrees of the sky). The dust tail is often curved because the comet is moving in its orbit at the same speed that the dust is moving away, much as water curves away from the nozzle of a moving hose.

Comet Halley Pictures — **Comet Halley as it appeared in several images from the 1910 apparition. The comet's tail gets bigger as it gets closer to the sun and then decreases as it moves away from the sun.**
Photo courtesy NASA/JPL

Comets often have a second tail called an ion tail (also called the plasma or gas tail). The ion tail is made of electrically charged gas molecules (carbon dioxide, nitrogen, water) that are pushed away from the nucleus by the solar wind. Sometimes, the gas tail disappears and later reappears when the comet crosses a boundary where direction of the sun's magnetic field is reversed.

Comets Can Break Apart

As comets pass through the inner solar system, they can be broken into pieces by Jupiter's gravity. Comet Shoemaker-Levy 9 was broken into 20 pieces, each of which collided with Jupiter in one of the most spectacular examples of interplanetary impacts in recorded history.

Comet Shoemaker-Levy 9 and Jupiter — **Artist's rendering of the collision of comet Shoemaker-Levy 9 and Jupiter**
Photo courtesy NASA/JPL

Jupiter planet image — **Here is a Hubble Space Telescope image of Jupiter after pieces of comet Shoemaker-Levy 9 hit the planet. The dark spots are the impact sites.**
Photo courtesy NASA/JPL

Recently, comet LINEAR was also broken into fragments by the sun's gravity as it passed the sun.

NASA's Stardust Mission

NASA has launched a mission called Stardust to comet Wild-2 to return pieces of the comet. The spacecraft will fly into the comet's tail and catch particles in a gel called aerogel, which is mounted on the panels of the spacecraft. Once captured, the particles will be returned to Earth in 2006. By studying the particles, scientists hope to learn more about comets and the composition of the early solar system.

Observing Comets

Many comets are actually discovered by amateur astronomers. To look for comets, here are things to keep in mind:

Go to a place where there are few lights.
Learn what a comet looks like (observe as many comets as you can) and what a comet does not look like (observe other deep sky objects because they also appear as small fuzzy objects).
Use binoculars or a telescope (low magnification, 20-40x).
Look toward the east about 30 minutes before sunrise or to the west about 20 minutes after sunset because comets are often spotted by their tails.
Sweep the sky slowly near the horizon.

Comets will appear as small, fuzzy objects. This type of observing takes discipline, long hours and patience. On average, comet hunters spend several hundred hours of observing time to find a new comet. However, comets are named after their discoverers, so many people think it is worth the effort. For a discussion of comet hunting, consult The Sky: a User's Guide by David H. Levy, who has discovered several comets including comet Shoemaker-Levy 9 that hit Jupiter.