They orbit the sun in highly elliptical orbits that can take hundreds of thousands of years to complete. As a comet approaches the sun, it heats up very quickly causing solid ice to turn directly into gas via a process called sublimation, according to the Lunar and Planetary Institute. The gas contains water vapor, carbon monoxide, carbon dioxide and other trace substances, and is eventually swept into the distinctive comet tail.
Scientists sometimes call comets dirty snowballs or snowy dirtballs, depending on whether they contain more ice material or rocky debris according to NASA. Though billions more are thought to be orbiting the sun beyond Neptune in the Kuiper Belt and the distant Oort cloud far beyond Pluto. Occasionally, a comet streaks through the inner solar system; some do so regularly, some only once every few centuries. Many people have never seen a comet, but those who have won't easily forget the celestial show.
A comet primarily consists of a nucleus, coma, hydrogen envelope, dust and plasma tails. Scientists analyze these components to learn about the size and location of these icy bodies, according to ESA. The nucleus is the solid core of a comet consisting of frozen molecules including water, carbon monoxide, carbon dioxide, methane and ammonia as well as other inorganic and organic molecules — dust.
According to ESA the nucleus of a comet is usually around 10 kilometers across or less. As a comet gets closer to the sun, the ice on the surface of the nucleus begins turning into gas, forming a cloud around the comet known as the coma. According to science website howstuffworks. Surrounding the coma is a hydrogen envelope that can be up to 6. As the comet gets closer to the sun, the hydrogen envelope gets bigger. There are two main types of comet tails, dust and gas.
Comet tails are shaped by sunlight and the solar wind and always point away from the sun according to Swinburne University of Technology. According to NASA, comet tails get longer as a comet approaches the sun and can end up millions of miles long. The dust tail is formed when solar wind pushes small particles in the coma into an elongated curved path. Whereas the ion tail is formed from electrically charged molecules of gas.
Comet tails may spray planets, as was the case in with Comet Siding Spring's close encounter with Mars. We can see a number of comets with the naked eye when they pass close to the sun because their comas and tails reflect sunlight or even glow because of energy they absorb from the sun.
However, most comets are too small or too faint to be seen without a telescope. A comet warms up as it nears the Sun and develops an atmosphere, or coma. The Sun's heat causes the comet's ices to change to gases so the coma gets larger. The coma may extend hundreds of thousands of kilometers. The pressure of sunlight and high-speed solar particles solar wind can blow the coma dust and gas away from the Sun, sometimes forming a long, bright tail.
However, some comets, called sungrazers, crash straight into the Sun or get so close that they break up and evaporate. Scientists have long wanted to study comets in some detail, tantalized by the few images of comet Halley's nucleus. NASA's Deep Space 1 spacecraft flew by comet Borrelly in and photographed its nucleus, which is about 8 kilometers 5 miles long. NASA's Stardust mission successfully flew within kilometers miles of the nucleus of Comet Wild 2 in January , collecting cometary particles and interstellar dust for a sample return to Earth in The photographs taken during this close flyby of a comet nucleus show jets of dust and a rugged, textured surface.
Analysis of the Stardust samples suggests that comets may be more complex than originally thought. Minerals formed near the Sun or other stars were found in the samples, suggesting that materials from the inner regions of the solar system traveled to the outer regions where comets formed. In July , the impactor was released into the path of the nucleus of comet Tempel 1 in a planned collision, which vaporized the impactor and ejected massive amounts of fine, powdery material from beneath the comet's surface.
En route to impact, the impactor camera imaged the comet in increasing detail. Two cameras and a spectrometer on the flyby spacecraft recorded the dramatic excavation that helped determine the interior composition and structure of the nucleus.
After their successful primary missions, the Deep Impact spacecraft and the Stardust spacecraft were still healthy and were retargeted for additional cometary flybys. Comet naming can be complicated. Comets are generally named for their discoverer—either a person or a spacecraft. They range from a few miles to tens of miles wide, but as they orbit closer to the Sun, they heat up and spew gases and dust into a glowing head that can be larger than a planet. This material forms a tail that stretches millions of miles.
Comets are cosmic snowballs of frozen gases, rock, and dust that orbit the Sun. When frozen, they are the size of a small town. When a comet's orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of miles.
There are likely billions of comets orbiting our Sun in the Kuiper Belt and even more distant Oort Cloud. The current number of known comets is:. Key Science Targets. Comets orbit the Sun just like planets and asteroids do, except a comet usually has a very elongated orbit. As the comet gets closer to the Sun, some of the ice starts to melt and boil off, along with particles of dust.
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