WHAT YOU SHOULD KNOW ABOUT COMETS:

A Comet is a small icy body in space that sheds gas and dust. Like rocky asteroids, icy comets are ancient objects left over from the formation of the solar system about 4.6 billion years ago. Some comets can be seen from Earth with the unaided eye.

Comets typically have highly elliptical (oval-shaped), off-center orbits that swing near the Sun. When a comet is heated by the Sun, some of the ice on the comet’s surface turns into gas directly without melting. The gas and dust freed from the ice can create a cloud (coma) around the body (nucleus) of the comet. More gas and dust erupt from cracks in the comet’s dark crust. High-energy charged particles emitted by the Sun, called the solar wind, can carry the gas and dust away from the comet as a long tail that streams into space. Gas in the tail becomes ionized and glows as bluish plasma, while dust in the tail is lit by sunlight and looks yellowish. This distinctive visible tail is the origin of the word comet, which comes from Greek words meaning “long-haired star.”

Humans have observed comets since prehistoric times. Comets were long regarded as supernatural warnings of calamity or signs of important events. Astronomers and planetary scientists now study comets for clues to the chemical makeup and early history of the solar system, since comets have been in the deep-freeze of outer space for billions of years. Materials in comets may have played a major role in the formation of Earth and the origin of life. Catastrophic impacts by comets may also have affected the history of life on Earth, and they still pose a threat to humans.

WHAT COMETS CONTAIN AND THEIR ORIGIN:

Close observations of comets by spacecraft confirm that comets have a rotating solid nucleus made of icy material (mainly water ice) mixed with dust and rock. This “dirty snowball” model of comets was first proposed by American astronomer Fred L. Whipple in a 1950 paper. Earlier theories suggested that comets were made up almost entirely of gas and lacked a large solid core, or were a pile of rubble.

The size of the nucleus of a comet may vary, but it is typically a few kilometers across and irregular in shape. For example, the Giotto space probe in 1986 revealed that Halley’s Comet has a dust-blackened nucleus about 15 by 7 km (about 9 by 3.6 mi) in size. The hazy coma of gas and dust released around the head of an active comet may exceed the planet Jupiter in size, however.

Observations from telescopes on Earth and in space indicate that most of the gases in the coma and tail of a comet are fragmentary molecules, or radicals, of the most common elements in space: hydrogen, carbon, nitrogen, and oxygen. The radicals, for example, of CH, NH, and OH may be broken away from the stable molecules CH₄ (methane), NH₃ (ammonia), and H₂O (water), which may exist as ices or more complex, very cold compounds in the nucleus.

All comets were once believed to be made up almost entirely of primitive icy

material that existed in the colder outer reaches of the huge cloud of dust and gas that collapsed to form the solar system about 4.6 billion years ago. According to a widely held theory of how the solar system formed, the dust and gas coalesced into tiny clumps that contained differing amounts of ice depending on the distance from the early Sun. A “snow line” in the disk around the Sun meant that objects in the region from Jupiter outward must have contained a much larger proportion of ice than objects closer to the Sun.

Over time, these tiny objects clumped together to form planetesimals—the building blocks of planets. Larger and larger objects formed as planetesimals collided and clumped together in a process called accretion, leading to the formation of planets. The gas-giant planets Jupiter and Saturn, and the ice-giant planets Uranus and Neptune, along with their moons, were built from mainly icy material, while the inner planets Mercury, Venus, Earth, and Mars were built from mainly rocky material. Rocky planetesimals that were left over from the planet-building period became asteroids, which now mainly orbit between the planets Mars and Jupiter in a region called the asteroid belt. Icy planetesimals became comets. Gravitational interactions among the giant outer planets as they moved into their modern orbits likely threw most of the icy planetesimals into the distant parts of the solar system where they are now found.

The material in comets supposedly remained frozen and unchanged since that time. However, data from space probes and other research indicate that important differences may exist in the composition of objects that become comets. Astronomers have found that some short-period comets contain large amounts of material that is similar to minerals in rocky asteroids—material that has been heated and chemically altered near the Sun. Dynamic processes may have mixed material from the inner solar system with icy debris in the outer regions as the Sun and planets formed. Additionally, short-lived radioactive isotopes of mineral elements may have heated some comets internally, creating liquid water and leading to the formation of clays and other compounds.

From Microsoft Encarta Encyclopedia.