Universe Today

The Perseid Meteor Shower is caused by debris coming from the Comet Swift-Tuttle (not to be confused with Comet Tempel-Tuttle which causes the equally spectacular Leonid Meteor Shower).

One of the main components of comets is ice. So when a comet passes near the Sun, a part of it vaporizes and is ejected from the main body. The ejected components form a stream of particles that follow the outline of the comet's orbit. When the Earth intersects with this path (or in other cases, come close to it), the particles then enter the Earth's atmosphere.

Once in the atmosphere, gravity pulls them downward into a high-speed plunge. Because they are mostly very small, virtually all of them readily ignite and disintegrate. We then observe them as streaks of light as they burn up. They can number by the hundreds to hundreds of thousands per hour, hence the name meteor shower.

The Perseid meteor shower is always seen from Earth as if coming from the constellation Perseus. It is for this reason that the Perseid meteor shower, and all other meteor showers for that matter, is named as such. The apparent source of the shower is more commonly known in astronomy circles as the radiant.

Observed since 2,000 years ago, the Perseid meteor shower is also known to some Catholics as the 'tears of Saint Lawrence' because the time when it peaks (early August) usually coincides with the saint's martyrdom. The event actually begins in mid-July but usually peaks at around August 10-12. This is the time when our planet is bathed inside the densest portion of the debris stream.

In the non-peak times, you may catch less than 10 meteors per hour. But during the peak times, 60+ of them can be visible in just one hour.

The meteor shower is most visible in the Northern hemisphere. However, for as long as you have a clear sky and standing in a dark area, you can easily spot the streaks of light wherever you are on Earth. If you're lucky, you can even catch a fireball. A bright moon can also prevent you from viewing the shower in all its magnificent glory.

Sometimes, a brilliant streak of light caused by an Iridium flare can be mistaken for a fireball. Newbie astronomers can easily be fooled because the Perseid meteor shower can cover a big part of the sky. The Iridium flares actually come from any of the 66 satellites that make up the Iridium Satellite Constellation.

Allow us to share previous announcements here at Universe Today regarding the Perseid Meteor Shower:

2009 Perseid Meteor Shower – Double Peaks This Year!

Perseid Radio Astronomy from the Fourth Plinth With Chris Lintott

From NASA, there's a very recently published article entitled: The Perseids are coming. Another informative article, published in 2007 and titled: Great Perseids, can also be found there.

Astronomy Cast has the following related episodes:
Meteor Showers. Yes, the sky is falling.
Getting Started in Amateur Astronomy

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In the movie “Deep Impact” the chance of a comet hitting Earth is explored. The world is saved when a spacecraft is built to deliver a system of destroying the comet before an Extinction Level Event(ELE) could happen. Is this pure science fiction or is there a chance of a comet hitting Earth? The fact is that there are comets and other solar debris hitting the Earth on a yearly basis. The most recent was in 2008 when a small asteroid hit our atmosphere. It burned up immediately and left nothing to hit the ground, but it goes to show that these events happen on a yearly basis. The chances of an ELE are very rare and only happen many millions of years apart.

Small objects hit the Earth with an amazing regularity. Astronomers have even gone to the trouble to predict these comet hitting Earth events. Objects with a 1 km diameter impact the Earth every 500,000 years on average. Large collisions with five kilometer objects happen approximately once every ten million years. Objects with diameters of 5-10 m impact the Earth's atmosphere approximately once per year, but break up and most of the material is vaporized in the upper atmosphere. Objects of diameters of over 50 meters strike the Earth approximately once every thousand years. It is easy to believe these numbers when you take into account the millions of objects that are out there, then couple that with gravity anomalies, etc. The surprise is actually that you would expect more frequent impact events.

A comet hitting Earth helps us to learn many new things about solar objects, but could be very dangerous for the peoples of the world if we are caught off guard. The complexity of the SENTRY and NEAT programs will prevent any unexpected strikes and will help us to defend any threatened cities. The movies have embellished on what might happen to prevent an impact, but they might not be too far off from the truth.

NASA has a good article about monitoring near Earth objects here. Here on Universe Today there are two good articles on the topic. One is about meteors hitting the Earth and the other is about the Tunguska event being caused by a comet hitting Earth. Astronomy Cast offers a good episode about destruction in our solar system.

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A comet coma is like a comet's atmosphere. It is made up of the heated gases and dust that makes up the comet's nucleus. The nucleus of a comet is a ball of ice and rocky dust particles that resembles a dirty ice ball. 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.

Scientists learned much about comets by studying Comet Halley 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 coma. The spacecraft found a roughly potato-shaped nucleus measuring about15 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.

There are interesting articles about comet coma here and here. We have a couple of great articles here on Universe Today. One covers comets in general and the other talks about some interesting facts about comets. Astronomy Cast has a good episode about losing contact with rovers and shooting lasers into space.

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A comet nucleus is the solid, central part of the comet. Some people refer to it as the dirty snowball aspect of a comet, but it is really rock, dust, and frozen gases that are covered by a shallow crust. NASA sent the space probe Deep Space 1 closer to Comet Borrelly than any probe had ever been. The probe found that a comet's nucleus can have mountains, valleys, and smooth plains. Many of the materials in a comet have different reflective properties. They look similar to asteroids when viewed close up.

When a comet nucleus is heated by the Sun, the gases transition for solid to gas without a liquid state and produce a temporary atmosphere surrounding the nucleus called the coma. The forces exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an albedo of 0.04. Cometary nuclei are among the darkest objects known to exist in the solar system. Some comet nuclei reflect less than 4% of the light that falls on them. Blacktop(asphalt) reflects 7% of the light that falls on it. It is thought that complex organic compounds make up the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.

Most cometary nuclei are thought to be no more than about16 km across. But cometary nuclei up to 40km across are known to exist. The shapes of comets is very inconsistent. Comets lack the needed gravity to become spherical, so the are often irregular, somewhat potato shaped in appearance.

NASA has a short page about comet nucleus here. There is an in depth article on the topic here. Here on Universe Today we have an article about the interesting facts about comets that has a lot of info on the nucleus and another on an asteroid/comet hybrid.

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A comet tail does not appear on a comet until it approaches the Sun. As the Sun's radiation heats up the dust and gases there are actually two tails formed. The dust forms the anititail and the gases form the true tail. In the outer solar system, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. As a comet approaches the inner solar system, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit so that it often forms a curved tail called the antitail. At the same time, the ion tail, made of gases, always points directly away from the Sun. This gas is more strongly affected by the solar wind than dust and follows magnetic field lines instead of an orbital trajectory. Parralax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.

Ion tails have been observed to extend 1 AU (150 million km) or more. The observation of antitails contributed significantly to the discovery of solar wind. The ion tail is formed as a result of solar ultra-violet radiation acting on particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge which in turn gives rise to an induced magnetosphere around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. As the relative orbital speed of the comet and the solar wind becomes supersonic, a bow shock is formed upstream of the comet, in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions congregate and act to load the solar magnetic field with plasma, such that the field lines drape around the comet forming the ion tail.

There is a good article about the comet tail here. Here on Universe Today we have an article about the strange acting Comet Lulin. Astronomy Cast has a good episode about space dust in general.

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On January 30, 1996, Comet Hyakutake was discovered using 25×150 binoculars. The comet was designated Comet C/1996 B2 (Hyakutake). As subsequent observations of the new comet were obtained, the IAU Central Bureau was able to compute the comet's orbital elements, and these computations indicated that the comet passed as close as 0.10 AU from the Earth on March 25, 1996! The comet became a bright unaided-eye object and remained so in March, April and May in 1996. The comet had exceeded expectations, becoming the brightest comet since Comet West in 1976. A long tail of up to 100 degrees was reported, and small fragments have been observed to break off the main nucleus. Comet Hyakutake was indeed the Great Comet of 1996.

Comet Hyakutake led to several scientific discoveries. Most surprising to cometary scientists was the first discovery of X-rays being emitted from a comet. They are believed to have been caused by ionized solar wind particles interacting with neutral atoms in the coma of the comet. The Ulysses spacecraft unexpectedly crossed the comet's tail at a distance of more than 500 million km from the nucleus, showing that Comet Hyakutake had the longest tail known for a comet. Hyakutake is a long period comet. Its orbital period was 17,000 years, but the gravitational influences of the large planets this trip through the inner solar system have extended that to every 100,000 years.

Comet Hyakutake became visible to the naked eye in early March 1996. By mid-March, the comet was still fairly unremarkable, shining at 4th magnitude with a tail about 5 degrees long. As it neared its closest approach to Earth, it rapidly became brighter, and its tail grew in length. After its close approach to the Earth, the comet faded to about 2nd magnitude. It reached perihelion on May 1, 1996, brightening again and exhibiting a dust tail in addition to the gas tail seen as it passed the Earth. By this time, however, it was close to the Sun and was not seen as easily. It was observed passing perihelion by the SOHO Sun-observing satellite, which also recorded a large coronal mass ejection being formed at the same time. Its distance from the Sun at perihelion was 0.23 AU, well inside the orbit of Mercury.

There are many good images and some good information on the JPL page for Comet Hyakutake. Here is a good article about the comet. Here on Universe Today there is more information about the X-rays that Hyakutake emitted. Astronomy Cast offers an episode about what the Christmas Star may have been. Possibly a comet?

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What are comets made of? Good question! Comet nuclei are loose collections of ice, dust and small rocky particles, ranging from a few kilometers to tens of kilometers across. As a comet approaches the inner solar system, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the radiation pressure of the Sun and solar wind cause a tail to form. The tail always points away from the sun.

In order to understand what are comets made of, we need to break down the three main parts of the comet: the nucleus, coma, and tail. Comet nuclei are known to range from about 100 meters to more than 40 kilometers across. They are composed of rock, dust, ice and frozen gases such as carbon monoxide, carbon dioxide, methane, and ammonia. Sometimes called dirty snowballs, recent studies have shown that the ice of a comet is covered by a crust. Comets also contain a variety of organic compounds as well as the gases already mentioned. Some of these are methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane. More complex molecules such as long-chain hydrocarbons and amino acids may also be in comets. Because of their low mass, comets cannot become spherical under their own gravity, and will thus have irregular shapes.

The coma is the the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on a highly elliptical orbit. As the comet warms, parts of it turn from solid to gas(sublimate). Larger charged dust particles are left along the comet's orbital path while smaller charged particles are pushed away from the Sun into the comet's tail by solar wind. This helps astronomers distinguish comets from stars because it creates a fuzzy appearance.

The tail is illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionization. The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail. At the same time, the ion tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. Paralax viewing from the Earth may sometimes mean the tails appear to point in opposite direction.

Understanding the three parts of the comet is essential to know what are comets made of. Here is an article with a little more detail. Here on Universe Today there is a great article on a comet/asteroid hybrid. Astronomy Cast has another outstanding episode about solar dust.

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What are comets? The answer to that question is a simple two sentence answer, but it will take a whole article to even partially explain. A comet is a small solar system body that orbits the Sun. The nucleus of a comet is made up of dust, ice, and small rocky particles. They can range in size from a few km to tens of km across. Now that you know the easy definition of what are comets, let's get a little more in depth.

Comets have a variety of different orbital periods. Some are only a few years, others are hundreds of thousands of years, while others are believed to pass through the inner Solar System only one time before being thrown out into interstellar space. Short-period comets are thought to originate in the Kuiper Belt or the associated scattered disc, both of which lie beyond the orbit of Neptune. Long-period comets are believed to originate in the Oort Cloud, consisting of debris left over from the condensing of the solar nebula. The cloud is located well-beyond the Kuiper Belt. Comets are thrown from these outer reaches of the Solar System towards the Sun by gravitational disturbances from the outer planets or nearby stars, and, on occasion, as a result of collisions between objects within these regions. Comets are distinguished from asteroids by the presence of a coma or tail, though very old comets that have lost all their volatile materials may come to resemble asteroids because they lack that distinguishing tail. Asteroids are also believed to have a different origin from comets, having formed in the inner Solar System rather than the outer Solar System, but recent findings have somewhat blurred the distinction between asteroids and comets.

You can't explain what are comets without describing the nucleus. Comet nuclei are known to range from about 100 meters to more than 40 kilometers across. They are composed of rock, dust, ice, and frozen gases such as carbon monoxide, carbon dioxide, methane, and ammonia. They are often popularly described as "dirty snowballs", but recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath a crust. Comets also contain a variety of organic compounds in addition to the gases already mentioned, these may include methanol, hydrogen, hydrogen cyanide, formaldehyde, ethanol, and ethane. Perhaps they may contain more complex molecules such as long-chain hydrocarbons and amino acids. Because of their low mass, comets cannot become round under their own gravity and will have irregular shapes. Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto space probe found that Halley's comet nucleus reflects approximately 4% of the light that falls on it and Deep Space 1 discovered that Borrelly's Comet's surface reflects 2.4–3.0% of the light that falls on it. In comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their out-gassing.

Now you know what are comets. There is a nice article here that gives you more information about comets. Here on Universe Today we have a great article about comets holding the secrets to the origins of life and another that lets you know the differences between asteroids and comets. Astronomy Cast has an episode on submillimeter astronomy.

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The chance of a comet impact are 100%. The big question is when? Every object in the Solar System shows evidence of multiple impacts. The Earth has over one hundred verified impact craters and several more that have not been verified as of yet. The most recent was in 2008 when a small object hit our atmosphere. It burned up immediately and left nothing to hit the ground, but it goes to show that these events happen on a yearly basis.

Small objects hit the Earth with an amazing regularity. Astronomers have even gone to the trouble to predict these comet impact events. Objects with a 1 km diameter impact the Earth every 500,000 years on average. Large collisions with five kilometer objects happen approximately once every ten million years. Objects with diameters of 5-10 m impact the Earth's atmosphere approximately once per year, but break up and most of the material is vaporized in the upper atmosphere. Objects of diameters of over 50 meters strike the Earth approximately once every thousand years. It is easy to believe these numbers when you take into account the millions of objects that are out there, then couple that with gravity anomalies, etc. The surprise is actually that you would expect more frequent impact events.

Comet impact events help us to learn more about space born objects and their content, but could be very dangerous for the peoples of the world if we are caught off guard. The complexity of the SENTRY and NEAT programs will prevent any unexpected strikes and will help us to defend any threatened cities. The movies have embellished on what might happen to prevent an impact, but they might not be too far off from the truth.

There is a nice article on NEA monitoring here. Here on Universe Today there is a great article on a recent comet impact event on Jupiter. Astronomy Cast has a good episode on ice in space and death from the skies. Enjoy them all.

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The Hale Bopp Comet a.k.a. the Great Comet of 1997 is perhaps the most observed comet ever, having been visible to the naked eye for 18 months. Coincidentally, it is also one of the brightest to have passed near our planet in recent years.

The Hale Bopp is a combination of the names of the two observers who first saw the comet – Alan Hale and Thomas Bopp. Hale saw it from New Mexico, while Bopp chanced upon it from Stanfield Arizona.

Because of its extreme brightness, Hale Bopp was visible from Earth despite the comet's very far distance from it. At 7.2 AU from the Sun, the comet's position when it was seen, a typical comet would have exhibited very faint brightness. What's even more amazing is that an image taken in 1993 from the Anglo-Australian telescope showed the Hale Bopp Comet even while it was still at 13 AU from the Sun.

Astronomers hoped that, because of its brightness, Hale Bopp would be very visible once it reached perihelion. Sure enough, they weren't disappointed. As a matter of fact, neither were the general public, who were treated to its awesome display on January of that year.

The timing of the comet's fly-by couldn't have been better – it came at a time when the Internet was rapidly gaining popularity. Hence, numerous websites were found featuring it. As a result, more people knew about it than any other comet in history.

Unfortunately, the excitement was also accompanied by paranoia, panic, and even a false sense of euphoria as many people believed it signaled the end of the world. The mass suicide of the Heaven's Gate (an American UFO cult) members drummed up even more doomsday stories.

During its fly-by, the Hale Bopp Comet was able to provide a treasure trove of scientific information.

Scientists discovered a third type of comet tail on it. While it is known that comets have 2 types of tails, gas and dust, this one also had a sodium tail. This tail, which extended up to 50 million kilometers from the nucleus, was found to contain neutral atoms. It positioned itself between the dust tail, which followed the comet's trajectory, and the gas tail, which pointed away from the Sun.

It was also discovered that the Hale Bopp Comet carried a huge amount of deuterium and contained argon. Argon has never been detected on any other comet.

We have some related articles here in Universe Today. Here are the links:

• Astrophoto: Comet Hale-Bopp
• Amino Acid Found in Stardust Comet Sample

Here are the links of two more articles from NASA:

• Comet Hale-Bopp
• ISTP Diverts Its Eyes to Hale-Bopp

Here are two episodes at Astronomy Cast that you might want to check out as well:

• Comets, Our Icy Friends From the Outer Solar System
• Asteroid Belt

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Comets are small solar system bodies that orbit the Sun. When they are close enough to the Sun a comet displays a visible coma and a tail. Both are the results of the acting of solar radiation upon the comets nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, ranging from a few kilometers to tens of kilometers across. Aristotle first used the derivation komets to depict comets as "stars with hair." The astronomical symbol for comets accordingly consists of a disc with a hairlike tail.

Comets have a variety of different orbital periods. Some are only a few years, others are hundreds of thousands of years, while some are believed to pass only once through the inner Solar System before being thrown out into interstellar space. Short-period comets are thought to originate in the Kuiper Belt or the associated scattered disc, both of which lie beyond the orbit of Neptune. Long-period comets are believed to originate in the Oort Cloud, consisting of debris left over from the condensing of the solar nebula. The cloud is located well-beyond the Kuiper Belt. Comets are thrown from these outer reaches of the Solar System towards the Sun by gravitational perturbations from the outer planets or nearby stars, or on occasion as a result of collisions between objects within these regions. Comets are distinguished from asteroids by the presence of a coma or tail, though very old comets that have lost all their volatile materials may come to resemble asteroids because they lack that distinguishing tail. Asteroids are also believed to have a different origin from comets, having formed in the inner Solar System rather than the outer Solar System, but recent findings have somewhat blurred the distinction between asteroids and comets.

Comet nuclei are known to range from about 100 meters to more than 40 kilometers across. They are composed of rock, dust, ice, and frozen gases such as Carbon monoxide, carbon dioxide, methane, and ammonia. They are often popularly described as "dirty snowballs", though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath a crust. Comets also contain a variety of organic compounds in addition to the gases already mentioned, these may include methanol, hydrogen, hydrogen cyanide, formaldehyde, ethanol, and ethane. Perhaps they may contain more complex molecules such as long-chain hydrocarbons and amino acids. Because of their low mass, comets cannot become round under their own gravity and will have irregular shapes. Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto space probe found that Halley's comet nucleus reflects approximately 4% of the light that falls on it and Deep Space 1 discovered that Borrelly's Comet's surface reflects 2.4–3.0% of the light that falls on it. In comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.

As a comet approaches the inner solar system, radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun.

Both the coma and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionization. The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the anti-tail. At the same time, the ion tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. While the solid nucleus of comets is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend 1 AU or more.

There is a lot more information to be had about comets. Here is a good article to start with. Universe Today has several great articles about comets. Here is one on the difference between comets and asteroids and here is another on smaller comets being released from Comet Holmes. Astronomy Cast offers a nice episode on the role of molecules in space.

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NASA scientists studying the comet samples returned by the Stardust spacecraft have discovered glycine, a fundamental building block of life. Stardust captured the samples from comet Wild 2 in 2004 and returned them to Earth in 2006. "Glycine is an amino acid used by living organisms to make proteins, and this is the first time an amino acid has been found in a comet," said Dr. Jamie Elsila of NASA's Goddard Space Flight Center. "Our discovery supports the theory that some of life's ingredients formed in space and were delivered to Earth long ago by meteorite and comet impacts."
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Back in 1996, astronomers discovered a strange object in the asteroid belt. They decided it was either a "lost" comet or an icy asteroid, as it ejected dust like a comet but had an orbit like an asteroid. No one had ever seen anything like the object, called 133P. Ever since it was found, astronomers have wondered if it was just an oddity — one of a kind. We now know it is not, and the discovery of more of these half asteroids/half comets means there is a new class of objects in our solar system.
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The main difference between an asteroid and a comet is what they are made of. Asteroids are made up of metals and rocky material, while comets are made up of ice, dust and rocky material. Both of these space objects were formed during the earliest times of the solar system, around 4.5 billion years ago. Asteroids formed much closer to the Sun, where it was too warm for ices to remain solid. Comets formed farther from the sun where ices would not melt. Comets, which approach the Sun, lose material with each orbit because some of their ice melts and vaporizes to form a tail.

A few other important differences between an asteroid and a comet exist. Obviously, comets have tails and asteroids do not. The heat from the Sun causes ice and other materials on a comet's surface to heat up until they vaporize. That vapor is what is seen as the comets tail. Another difference is in their orbital patterns. Comets tend to have very extended and elongated orbits, many times going more than 50,000 AU from the Sun (1 AU, or astronomical unit, equals the distance from the Earth to the Sun).

Asteroids tend to have shorter, more circular orbits and they seem to want to group together in belts.

Another difference between an asteroid and a comet is in the numbers of each. There are only 3,572 known comets, but there are many millions of asteroids. Some as small as dust particles.

While the main difference between an asteroid and a comet is what they are made up of, there are a few interesting differences. Asteroids may not have the flash of a tail, they do have many scientists studying them for many purposes, including the possibility of them hitting the Earth. The mining of asteroids is becoming another venture that scientists and industrialists are keeping their eyes on.

We have written many articles about both asteroids and comets for Universe Today. Here's an article about how we might keep asteroids and comets away from the Earth. And here's an article about the strange asteroid BD 2009.

Want more resources on asteroids? Here's NASA's Lunar and Planetary Science Page on asteroids. And here's NASA's World Book on Asteroids.

We have recorded two episodes of Astronomy Cast about asteroids. There's Episode 55: The Asteroid Belt, and here's Episode 29: Asteroids Make Bad Neighbors.

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On December 2^nd, 1995 a large joint ESA and NASA mission was launched to gain an insight to the dynamics of the Sun and its relationship with the space between the planets. 12 years on, the Solar and Heliospheric Observatory (SoHO) continues to witness some of the largest explosions ever seen in the solar system, observes beautiful magnetic coronal arcs reach out into space and tracks comets as they fall to a fiery death. In the line of duty, SoHO even suffered a near-fatal shutdown (in 1998). As far as astronomy goes, this is a tough assignment.
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