Halley’s Comet becomes visible to the unaided eye about every 76 years as it nears the sun. A comet (KOM iht) is an icy body that releases gas or dust. Most of the comets that can be seen from Earth travel around the sun in long, oval orbits. A comet consists of a solid nucleus (core) surrounded by a cloudy atmosphere called the- coma and one or two tails. Most comets are too small or too faint to be seen without a telescope.
Some cornets, however, become visible to the unaided eye for several weeks as they pass close to the sun. We can see comets because the gas and dust in their comas and tails reflect sunlight. Also, the gases release energy absorbed from the sun, causing them to glow.
Astronomers classify comets according to how long they take to orbit the sun. Short-period comets need less than 200 years to complete one orbit, while long-period comets take 200 years or longer.
Astronomers believe that comets are leftover debris from a collection of gas, ice, rocks, and dust that formed the outer planets about 4.6 billion years ago. Some scientists believe that comets originally brought to Earth some of the water and the carbon-based molecules that makeup living things.
The nucleus of a comet is a ball of ice and rocky dust particles that resembles a dirty snowball. The ice consists mainly of frozen water but may include other frozen substances, such as ammonia, carbon dioxide, carbon monoxide, and methane. Scientists believe the nucleus of some comets may be fragile because several comets have split apart for no apparent reason.
As a comet nears the inner solar system, heat from the sun vaporizes some of the ice on the surface of the nudeus, spewing gas and dust particles into space. This gas and dust form the comet’s coma. Radiation from the sun pushes dust particles away from the coma. These particles form a tail called the dust tail.
At the same time, the solar wind — that is, the flow of high-speed electrically charged particles from the sun-converts some of the comet’s gases into ions (charged particles). These ions also stream away from the coma, forming an ion tail. Because comet tails are pushed by solar radiation and the solar wind, they always point away from the sun.
Most comets are thought to have a nucleus that measures about 10 miles (16 kilometers) or less across. Some comas can reach diameters of nearly 1 million miles (1.6 million kilometers). Some tails extend to distances of 100 million miles (160 million kilometers).
Comets that pass near the sun come from two groups of comets near the outer edge of the solar system, according to astronomers. The disk-shaped Kuiper belt contributes comets that orbit the sun in fewer than 200 years. The Kuiper belt lies beyond Pluto’s orbit, which extends to about 4.6 billion miles (7.4 billion kilometers) from the sun. The Oort cloud provides comets that take longer to complete their orbits. The outer edge of the Oort cloud maybe 1,000 times farther than the orbit of Pluto.
Scientists think that short-period comets come from a band of objects called the Kuiper belt, which lies beyond the orbit of Pluto. The gravitational pull df the outer planets can nudge objects out of the Kuiper belt and into the inner solar system, where they become active comets. Long-period comets come from the Oort cloud, a nearly spherical collection of icy bodies about 1,000 times farther away from the sun than Pluto’s orbit. Gravitational interactions with passing stars can cause icy bodies in the Oort cloud to enter the inner solar system and become active comets.
Comets lose ice and dust each time they return to the inner solar system, leaving behind trails of dusty debris. When Earth passes through one of these trails, the debris becomes meteors that burn up in the atmosphere. Eventually, some comets lose all their ices. They break up and dissipate into clouds of dust or turn into fragile, inactive objects similar to asteroids.
The long, oval-shaped orbits of comets can cross the almost circular orbits of the planets. As a result, comets sometimes collide with planets and their satellites. Many of the impact craters in the solar system were caused by collisions with comets.
Scientists learned much about comets by studying Halley’s Comet as it passed near Earth in 1986. Five spacecraft flew past the comet and gathered information about its appearance and chemical composition. Several probes flew close enough to study the nucleus, which is normally concealed by the comet’s coma. The spacecraft found a roughly potato-shaped nucleus measuring about 9 miles (15 kilometers) long. The nucleus contains equal amounts of ice and dust. About 80 percent of the ice is water ice, and frozen carbon monoxide makes up another 15 percent. Much of the remainder is frozen carbon dioxide, methane, and ammonia. Scientists believe that other comets are chemically similar to Halley’s Comet.
The space probe Giotto passed near Halley’s Comet on March 14, 1986. Giotto returned dramatic dose-up images of the comet, including this one. Scientists unexpectedly found the nucleus of Halley’s Comet to be extremely dark black. They now believe that the surface of the comet, and perhaps most other comets, is covered with a black crust of dust and rock that covers most of the ice. These comets release gas only when holes in this crust rotate toward the sun, exposing the interior ice to the warming sunlight.
Another comet nucleus that has been seen by spacecraft camera is is that of Comet Borrelly. During a flyby in 2001, the Deep; Space 1 spacecraft observed a nucleus about half the size of the nucleus of Halley’s Comet. Borrelly’s nucleus was also potato-shaped and had a dark black surface. Like Halley’s Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight.
In 1994, astronomers observed a comet named Shoemaker-Levy 9, which had split into more than two dozen pieces, crashing into the planet Jupiter. One of the most active comets seen in more than 400 years was Comet Hale-Bopp, which came within 122 million miles (197 million kilometers) of Earth in 1997. This was not an especially close approach for a comet. However, Hale-Bopp appeared bright to the unaided eye because its unusually large nucleus gave off a great deal of dust and gas. The nucleus was estimated to be about 18 to 25 miles (30 to 40 kilometers) across.
In 2004, the U.S. spacecraft Stardust passed near the nucleus of Comet Wild 2 and gathered samples from the comet’s coma. Stardust was scheduled to return the samples to Earth in 2006. Also in 2004, the European Space Agency launched the Rosetta spacecraft, which was to go into orbit around Comet Churyumov-Gerasimenko in 2014. Rosetta carried a small probe designed to land on the comet’s nucleus.