COMETS
Comets are quite different from planets in how they move around the solar system. While the planets tend to have fairly circular orbits, the orbits of comets are very elliptical, so that they are stretched out from near the Sun to the very edges of our solar system. Some of the orbits aren't even elliptical - the comets just do one passage in toward the solar system and then are gone forever. The orbits are also rather randomly oriented to the ecliptic; they could come in toward the Sun at pretty much any angle relative to the ecliptic.
We know that comets are composed primarily of many varieties of ice, including water, carbon dioxide, methane and ammonia ice. There is also a bit of dirt mixed in, usually in the form of carbon. This makes them appear as dirty snowballs, which is actually the name for the model that is proposed for their compositions. The best way to think of a comet is that it is like a big chunk of ice, dirt and slush that gets stuck to the wheel well on your car during the winter.
Comets may appear as huge objects in the sky, but they are typically only about 10 km in diameter, much smaller than many other objects going around the Sun. This core or nuclei is how most comets appear when located in the outer solar system (beyond Jupiter's orbit). However, they don't always remain as small, dirty, frozen icebergs. Comets change as they orbit around the solar system, especially when they get into the inner solar system. As a comet moves closer to the Sun, the heat from the Sun will start to evaporate the ices that make up the core of the comet.
The material is then in a gaseous state and will form around the core of the comet as a coma, or head of the comet. As the comet gets closer to the Sun, the gas starts getting blown off by the solar wind. Not only is the gas blown off, but also the heavier, dusty material gets blown away. Due to the motion of the comet, which is pretty fast, and the force exerted by the solar winds, the trail that this evaporated material leaves can grow quite large and will develop into tails. The coma can be thousands of times (or more) larger than the cometary nuclei, while the tails can be up to 1 A. U. in size (remember, 1 A. U. is about 100 million miles!).
Two tails are usually seen. These include the gas tail (also called the ion tail), which is made up of material that is blown straight back by the solar wind. This is generally made of the really lightweight gases. Within the gas tail you find stuff such as water vapor, CO, CO2, N2, ammonia and methane gases and particles. The gas tail has a rather ragged appearance and is sometimes rather bluish. It is always pointed directly away from the Sun. The other tail, the dust tail, is made up of heavier particles and is not as greatly affected by the solar wind. It has a very fuzzy appearance, often looking rather yellow-ish or whitish. This is, of course, made up of mainly dust (rocks and silicates).
This is much heavier material, so it is not pushed into a straight line like the gas tail but often has a curved shape that is sort of symbolic of comets. Both tails get longer as the comet gets closer to the Sun. Actually, the tails start developing when the comet is still quite a ways from the Sun, well beyond the orbit of the Earth.
Another thing about comets that people are confused by is how fast they move. Comets orbit the Sun, so they obey Kepler's laws, just like the planets. Over the course of an evening it is possible to see tiny motions of a comet relative to the stars, especially if it is close to perihelion, but they don't go streaking across the sky as is often portrayed in cartoons.
No, because from our view point it looks like it is hardly moving along, but if you look at it the next evening, you'll see it is in a different location relative to the stars. The same is true for comets; their motions may be apparent from one night to the next, but to see motion with your eyes over the course of a few minutes during one evening would be difficult, if not impossible.
A comet is made up of material that gets evaporated easily by the Sun, so comets lose mass with each passage around the Sun. As much as one percent of their masses can be blown away. Comets may start out very icy, but this is not how they'll look for long, since the ice is the first thing to go. After a while, their nuclei will look very dark and dirty, since the dark, dirty material (mainly carbon) will not get blown away as easily. Comets that can't withstand the strong solar winds can also shatter apart. This has been observed recently in the case of Comet LINEAR (C/1999 S4), and Comet Schwassmann-Wachmann 3, both of which broke apart into smaller pieces. Other comets that pass very close to the Sun can either completely disintegrate or actually hit the Sun!
Comets were originally thought to come from the Oort Cloud, a spherical region that extends about 50,000 AU from the Sun where the cores of comets reside. Every once in a while, the comets are perturbed by a passing star or collision with other comets and some fall in toward the solar system. These comets tend to have orbits that are very elongated. In some cases the comet will only pass near the Sun once. In other cases the comet will have its path altered, usually by going too close to Jupiter, and it will become trapped into a shorter period around the Sun.
The existence of the Oort Cloud is based on the characteristics of comets. Many comets tend not to be aligned with the ecliptic, so it makes sense that they originate from a place that surrounds the solar system in all directions. Also, the very long period (greater than 1000 years) comets have paths that stretch out to such great distances that it is logical that they originated at great distances from the Sun. It is estimated that there are about 100 billion comet cores out in the Oort cloud.
There is also evidence that many of the short period comets do not come from the Oort Cloud but from a closer reservoir of cometary material that was previously mentioned (in the last set of notes), theKuiper Belt. This region is much closer than the Oort Cloud, extending from about 40 AU out to a few hundred AU at most. The characteristics of Kuiper Belt Objects can help explain the shorter period comets (less than 1000 years), especially those with orbits that tend to be closer to the ecliptic. Remember, if the distance from the Sun is smaller, the time for an orbit is smaller - Kepler's Third Law is action.
It seems that there are really two types of comets, with the difference in them evident in the periods of the orbits. Those with long periods (thousands to millions of years) and large orbits (1000 to 30,000 AU) also tend to have very elliptical orbits and to have orbits randomly oriented relative to the plane of the ecliptic. These are likely from the Oort Cloud. Those with shorter periods (a few years to a few hundred years) and smaller orbits (a few AU to 50 AU) could have originated in the Kuiper Belt.
Throughout history comets have scared, inspired or awed people around the world. There are some comets that are well known for various reasons. Also, comets are a popular target for amateur astronomers, since if you find one, it is named after you. This is one of the few things in astronomy that you can have named after you and many amateur astronomers have been lucky enough to find a few. However, this would require years of work, and even if you search for years you might not discover one.
Comet Halley was named after Edmund Halley, who first determined that comets are objects that return to the inner solar system on periodic bases and are therefore predictable. He did not discover the comet that is named after him, but because he "demystified" comets, the comet whose orbit he determined and whose return he predicted is named after him. The comet came by the inner part of the solar system in 1986 and won't come back again until 2061. Its passage in 1986 wasn't very spectacular, and the 2061 trip will probably not be very good either. This comet has gone around the Sun quite a few times, so it has been getting smaller and less spectacular with each passage.
Due to its fame, Halley's comet was the target of several spacecraft during its passage through this part of the solar system in 1986-1987. One of the spacecraft was named Giotto after the Renaissance painter who incorporated a comet in one of his frescoes. Giotto flew close to the core of the comet, only about 600 km from it. An animation of the approach can be seen here. At that distance it was able to obtain many images of the comet nucleus, showing it to be a very dark object.
This makes sense, because humans have observed Halley's comet for more than 2000 years, and with each passage by the Sun it looses more and more of its ice, leaving behind the dark dirt. Other features observed on the comet were strong venting events, where material was evaporated in a rather explosive manner. These are seen as the bright features in the Giotto image shown above. The power of such vents is pretty great, enough to even alter the comet's orbital path slightly.
Comet Hale-Bopp passed by during the Spring of 1997 and came within 1.315 A.U. of the Earth. Even though this is not a really close passage, it was very bright due to its unusually large size, with a core of about 40 km wide. Hale-Bopp is easily the most photographed comet in history. Millions of people observed it since it was easily visible to the naked eye for several months. It was also the third brightest in recorded history. If you missed it, then you must have had your eyes closed, since this was one of the most prominent features in the sky during the spring of 1997.
The way that we currently study comets is changing quite a bit. Previously we just were able to only sit and watch them pass by. But recently we've sort of gotten "in their face". The first close encounters were with Comet Halley, when the Giottospace craft flew close to the core. This was followed in January 2004, when theStardust flew very close to the core of comet Wild 2.
During the close encounter with the core, it was able to obtain a sample of cometary material. The spacecraft then flew past the Earth in 2006 and successfully dropped off the collected samples. Currently astronomers are analyzing the material, which is microscopic, to determine the conditions under which comets formed. In general we had assumed that they formed out of only "cold" material, but scientists were surprised to discover several different minerals that only form under conditions of very high temperatures! Also, some icy particles were found that form only in condistion with very low temperatures.
It would appear that early in the history of our solar system there was mixing of material from the inner to the outer solar system, so that some of the high temperature material was incorporated into comets. Stardust continued on its journey after dropping off the sample from Wild 2, and visited the comet Tempel 1 (see below) to check on the "damage" caused by another spacecraft. They actually renamed the spacecraft Stardust-NExTsince this was a "new" mission. After the encounter with Tempel 1, the spacecraft was sent off into an orbit that would prevent it from hitting the Earth.
The most dramatic comet encounter occured in July 2005 when the Deep Impact spacecraft sent a probe slamming into the surface of comet Tempel 1. Astronomers wanted to break "into" the comet to see what sort of material was below the surface, since the surface of a comet is altered by constant exposure to the Sun and radiation from other sources.
Results from the impact are still being studied, but there have already been some surprises, such as the how the dust on the surface of the comet is very fine and powdery, something that wasn't expected. The material also seems to be rich in organic matter, material that is rich in carbon. There isn't a great deal of water ice visible on the comet's surface, which means it looks more like a dirt ball with some ice on it, rather that a dirty snowball. It is thought that there is still quite a bit of ice inside of the comet, below the surface. The impact caused the comet to brighten slightly, but that didn't last very long. And like the Stardust spacecraft was re-used to visit Tempel 1, the Deep Impact spacecraft also was sent on a journey to another comet, in this case Hartley 2. To see the view from the spacecraft as it flew by the comet, just follow this link.
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Comets
Comets are small Solar System bodies, typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.
Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as Hale–Bopp, are believed to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent.Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.at governed the Earth. Additionally, the invention of the telescope led to the discovery of further planets and moons. In more recent times, improvements in the telescope and the use of unmanned spacecraft have enabled the investigation of geological phenomena such as mountains and craters, and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.
We know that comets are composed primarily of many varieties of ice, including water, carbon dioxide, methane and ammonia ice. There is also a bit of dirt mixed in, usually in the form of carbon. This makes them appear as dirty snowballs, which is actually the name for the model that is proposed for their compositions. The best way to think of a comet is that it is like a big chunk of ice, dirt and slush that gets stuck to the wheel well on your car during the winter.
Comets may appear as huge objects in the sky, but they are typically only about 10 km in diameter, much smaller than many other objects going around the Sun. This core or nuclei is how most comets appear when located in the outer solar system (beyond Jupiter's orbit). However, they don't always remain as small, dirty, frozen icebergs. Comets change as they orbit around the solar system, especially when they get into the inner solar system. As a comet moves closer to the Sun, the heat from the Sun will start to evaporate the ices that make up the core of the comet.
The material is then in a gaseous state and will form around the core of the comet as a coma, or head of the comet. As the comet gets closer to the Sun, the gas starts getting blown off by the solar wind. Not only is the gas blown off, but also the heavier, dusty material gets blown away. Due to the motion of the comet, which is pretty fast, and the force exerted by the solar winds, the trail that this evaporated material leaves can grow quite large and will develop into tails. The coma can be thousands of times (or more) larger than the cometary nuclei, while the tails can be up to 1 A. U. in size (remember, 1 A. U. is about 100 million miles!).
Two tails are usually seen. These include the gas tail (also called the ion tail), which is made up of material that is blown straight back by the solar wind. This is generally made of the really lightweight gases. Within the gas tail you find stuff such as water vapor, CO, CO2, N2, ammonia and methane gases and particles. The gas tail has a rather ragged appearance and is sometimes rather bluish. It is always pointed directly away from the Sun. The other tail, the dust tail, is made up of heavier particles and is not as greatly affected by the solar wind. It has a very fuzzy appearance, often looking rather yellow-ish or whitish. This is, of course, made up of mainly dust (rocks and silicates).
This is much heavier material, so it is not pushed into a straight line like the gas tail but often has a curved shape that is sort of symbolic of comets. Both tails get longer as the comet gets closer to the Sun. Actually, the tails start developing when the comet is still quite a ways from the Sun, well beyond the orbit of the Earth.
Another thing about comets that people are confused by is how fast they move. Comets orbit the Sun, so they obey Kepler's laws, just like the planets. Over the course of an evening it is possible to see tiny motions of a comet relative to the stars, especially if it is close to perihelion, but they don't go streaking across the sky as is often portrayed in cartoons.
No, because from our view point it looks like it is hardly moving along, but if you look at it the next evening, you'll see it is in a different location relative to the stars. The same is true for comets; their motions may be apparent from one night to the next, but to see motion with your eyes over the course of a few minutes during one evening would be difficult, if not impossible.
A comet is made up of material that gets evaporated easily by the Sun, so comets lose mass with each passage around the Sun. As much as one percent of their masses can be blown away. Comets may start out very icy, but this is not how they'll look for long, since the ice is the first thing to go. After a while, their nuclei will look very dark and dirty, since the dark, dirty material (mainly carbon) will not get blown away as easily. Comets that can't withstand the strong solar winds can also shatter apart. This has been observed recently in the case of Comet LINEAR (C/1999 S4), and Comet Schwassmann-Wachmann 3, both of which broke apart into smaller pieces. Other comets that pass very close to the Sun can either completely disintegrate or actually hit the Sun!
Comets were originally thought to come from the Oort Cloud, a spherical region that extends about 50,000 AU from the Sun where the cores of comets reside. Every once in a while, the comets are perturbed by a passing star or collision with other comets and some fall in toward the solar system. These comets tend to have orbits that are very elongated. In some cases the comet will only pass near the Sun once. In other cases the comet will have its path altered, usually by going too close to Jupiter, and it will become trapped into a shorter period around the Sun.
There is also evidence that many of the short period comets do not come from the Oort Cloud but from a closer reservoir of cometary material that was previously mentioned (in the last set of notes), theKuiper Belt. This region is much closer than the Oort Cloud, extending from about 40 AU out to a few hundred AU at most. The characteristics of Kuiper Belt Objects can help explain the shorter period comets (less than 1000 years), especially those with orbits that tend to be closer to the ecliptic. Remember, if the distance from the Sun is smaller, the time for an orbit is smaller - Kepler's Third Law is action.
It seems that there are really two types of comets, with the difference in them evident in the periods of the orbits. Those with long periods (thousands to millions of years) and large orbits (1000 to 30,000 AU) also tend to have very elliptical orbits and to have orbits randomly oriented relative to the plane of the ecliptic. These are likely from the Oort Cloud. Those with shorter periods (a few years to a few hundred years) and smaller orbits (a few AU to 50 AU) could have originated in the Kuiper Belt.
Throughout history comets have scared, inspired or awed people around the world. There are some comets that are well known for various reasons. Also, comets are a popular target for amateur astronomers, since if you find one, it is named after you. This is one of the few things in astronomy that you can have named after you and many amateur astronomers have been lucky enough to find a few. However, this would require years of work, and even if you search for years you might not discover one.
Comet Halley was named after Edmund Halley, who first determined that comets are objects that return to the inner solar system on periodic bases and are therefore predictable. He did not discover the comet that is named after him, but because he "demystified" comets, the comet whose orbit he determined and whose return he predicted is named after him. The comet came by the inner part of the solar system in 1986 and won't come back again until 2061. Its passage in 1986 wasn't very spectacular, and the 2061 trip will probably not be very good either. This comet has gone around the Sun quite a few times, so it has been getting smaller and less spectacular with each passage.
Due to its fame, Halley's comet was the target of several spacecraft during its passage through this part of the solar system in 1986-1987. One of the spacecraft was named Giotto after the Renaissance painter who incorporated a comet in one of his frescoes. Giotto flew close to the core of the comet, only about 600 km from it. An animation of the approach can be seen here. At that distance it was able to obtain many images of the comet nucleus, showing it to be a very dark object.
This makes sense, because humans have observed Halley's comet for more than 2000 years, and with each passage by the Sun it looses more and more of its ice, leaving behind the dark dirt. Other features observed on the comet were strong venting events, where material was evaporated in a rather explosive manner. These are seen as the bright features in the Giotto image shown above. The power of such vents is pretty great, enough to even alter the comet's orbital path slightly.
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Comet Hale-Bopp passed by during the Spring of 1997 and came within 1.315 A.U. of the Earth. Even though this is not a really close passage, it was very bright due to its unusually large size, with a core of about 40 km wide. Hale-Bopp is easily the most photographed comet in history. Millions of people observed it since it was easily visible to the naked eye for several months. It was also the third brightest in recorded history. If you missed it, then you must have had your eyes closed, since this was one of the most prominent features in the sky during the spring of 1997.
The way that we currently study comets is changing quite a bit. Previously we just were able to only sit and watch them pass by. But recently we've sort of gotten "in their face". The first close encounters were with Comet Halley, when the Giottospace craft flew close to the core. This was followed in January 2004, when theStardust flew very close to the core of comet Wild 2.
During the close encounter with the core, it was able to obtain a sample of cometary material. The spacecraft then flew past the Earth in 2006 and successfully dropped off the collected samples. Currently astronomers are analyzing the material, which is microscopic, to determine the conditions under which comets formed. In general we had assumed that they formed out of only "cold" material, but scientists were surprised to discover several different minerals that only form under conditions of very high temperatures! Also, some icy particles were found that form only in condistion with very low temperatures.
It would appear that early in the history of our solar system there was mixing of material from the inner to the outer solar system, so that some of the high temperature material was incorporated into comets. Stardust continued on its journey after dropping off the sample from Wild 2, and visited the comet Tempel 1 (see below) to check on the "damage" caused by another spacecraft. They actually renamed the spacecraft Stardust-NExTsince this was a "new" mission. After the encounter with Tempel 1, the spacecraft was sent off into an orbit that would prevent it from hitting the Earth.
The most dramatic comet encounter occured in July 2005 when the Deep Impact spacecraft sent a probe slamming into the surface of comet Tempel 1. Astronomers wanted to break "into" the comet to see what sort of material was below the surface, since the surface of a comet is altered by constant exposure to the Sun and radiation from other sources.
Results from the impact are still being studied, but there have already been some surprises, such as the how the dust on the surface of the comet is very fine and powdery, something that wasn't expected. The material also seems to be rich in organic matter, material that is rich in carbon. There isn't a great deal of water ice visible on the comet's surface, which means it looks more like a dirt ball with some ice on it, rather that a dirty snowball. It is thought that there is still quite a bit of ice inside of the comet, below the surface. The impact caused the comet to brighten slightly, but that didn't last very long. And like the Stardust spacecraft was re-used to visit Tempel 1, the Deep Impact spacecraft also was sent on a journey to another comet, in this case Hartley 2. To see the view from the spacecraft as it flew by the comet, just follow this link.
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