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So… what is a comet?
Well, unofficially, many people describe comets as ‘dirty snowballs’ that orbit our solar system. Here is a more scientific description of our heavenly visitors…
A comet 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 comets, 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. In addition, 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 make up living things.
Parts of a comet
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 nucleus, spewing gas and dust particles into space. This gas and dust forms 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 16km or less across. Some comas can reach diameters of nearly 1.6 million km (the Earth’s diameter is 12 715.43 km – so a comet’s coma can be as much as 130 times bigger than Earth. That’s BIG. Some tails extend to distances of 160 million km (the average distance between the Earth and the Moon is 384 403 km – so a comet’s tail can be about 416 times longer than the distance between us and our nearest celestial body – and event greater than the average distance between Earth the Sun – 149 million km.
The life of a comet
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 of 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.
Studying 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 15km long. The nucleus contains equal amounts of ice and dust. About 80% of the ice is water ice, and frozen carbon monoxide makes up another 15%. Much of the remainder is frozen carbon dioxide, methane, and ammonia. Scientists believe that other comets are chemically similar to Halley’s Comet.
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.
Like a great potato
Another comet nucleus that has been seen by spacecraft cameras 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 24 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 197 million km 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 30 to 40km across.
In 2004, the US 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.
Contributor: Donald K. Yeomans, Ph.D., Senior Research Scientist, Jet Propulsion Laboratory.
Source: NASA World Book. NWB recommends the following format: Yeomans, Donald K. ‘Comet.’ World Book Online Reference Center. 2005. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar125580.
The space probe Giotto passed near Halley’s Comet on March 14, 1986. Giotto returned dramatic close-up images of the comet, including this one. (Image: European Space Agency)
