METEOROIDS
A meteoroid can be an object from the size of a grain of sand to the bulk of a boulder that is traveling in space. There are several official definitions of what is a meteoroid . They all differ based on the largest objects to be included in the group. If a meteoroid enters the atmosphere of Earth or any other body the visible trail that it leaves is called a meteor. Once it impacts the surface it is called a meteorite. These are commonly called shooting stars.
What's the difference between these things? Here's a quick definition of the three
It surprises many people that meteor showers are not due to asteroids but are instead caused by comets. Comets are objects that travel through the inner solar system, so some of their paths intersect the Earth's orbital path. The chance of a collision is not that great, but because the trail of debris left by the comet's passage is rather large, the Earth does pass through that. As the debris is swept up by the Earth and passes through our atmosphere, we have all sorts of bits of ice and rock burning up - that's what makes a meteor shower. This is also how we can predict when the showers are going to occur, since we know where the comet's orbital path is located and when the Earth will pass through it. It is possible to see individual meteors at just about any time of the year, but the dates of showers are when people make an effort to view them. The meteors tend to originate from one location in the sky (since that is where the cometary debris is), so the names of meteor showers are based upon the constellation they are centered upon. Here are some of the main showers, approximate dates, intensity estimates and the comets that are thought to have produced them.
If you are interested in watching a meteor shower, then you should do a few things. First of all, get far from city lights. You will also have to do some careful planning, since the best observations of meteor showers take place after midnight. You should set up a lawn chair or something comfortable to sit on, since you could spend quite some time looking up at the sky. You may want to bring along something or someone to keep you warm. You do not want to use things like binoculars or a telescope, since you want to view as much of the sky as possible. You just have to sit back and watch the show! Meteors are actually quite a ways up the atmosphere, typically dozens of miles. Most of the objects that are meteors don't survive their passage through the atmosphere but are completely vaporized. However, if objects are large enough and dense enough, they can make it to the surface of the Earth, at which point they become meteorites.
Most pieces of cometary debris evaporate in the atmosphere, so most meteorites are thought to originate from asteroids. Just as there are different types of asteroids, there are also different types of meteorites. There are actually three main types. These are -
Meteorites have been floating around in space for so long that they contain material that originates from the time when the solar system formed, and they have not been altered significantly since their formations. Meteorites provide very important information about the early solar system, and the processes that went into the formation of the planets. Meteorites have a wide variety of minerals within them, and the requirements for the formation of those minerals (temperature, density, composition), help us understand the early solar system characteristics. Some of these minerals are very ordinary, like calcite, sulfur and pyrite (fool's gold), but things like diamonds (yes, real diamonds) have been found in meteorites. There are even some minerals that are found only in meteorites. Meteorites also help us determine the age of the solar system through the process of radioactive decay. There is even a group of meteorites discovered in Antarctica that are thought to be from Mars. How is that possible? It is possible that an impact in the past on Mars threw enough material up and away from Mars that it could have eventually made its way to the Earth. The main feature that distinguishes these meteorites from others is the amount of certain gases found in them, which is similar to the atmosphere of Mars in many respects. A similar process has brought some pieces of the Moon to the Earth as well.
They are produced by the heating and cooling of the metal. They are very distinctive and not found in Earth rocks or metals. It also helps if you find your suspected meteorite in a place where there are not too many rocks. Places like Antarctica and large deserts are good places for meteorite hunting, though they are not very nice places to visit.
It is also rather easy to find meteorites by just setting out a collecting dish in your back yard. Huh? I didn't say they would be big meteorites. Pretty much any location on Earth will have micro-meteorites fall on it. These are really small meteorites that are sort of like dust specks that are continually raining down onto the surface of the Earth. If you set out a bowl of water or a petri dish with petroleum jelly in it, you'll catch of a few meteorites after some time - along with bugs, pollen, regular dust, etc. You'd have to use a microscope to find the micrometeorites. They tend to very spherical.
Finding meteorites on the Earth may seem like a difficult task, just imagine how difficult it is to find a meteorite on another planet. Or is it? The Mars rover Opportunity has found a pretty good sized meteorite on the surface of Mars, which is about 2 feet across. Since the rover has the ability to chemically analyze rocks, it was able to determine that this meteorite (shown here) is an iron meteorite.
Obviously, if some meteors can make it all the way to the surface of the Earth and become meteorites, it is possible for even larger objects to strike the Earth. This has happened quite a bit in the past and it will happen in the future - we just don't know when. Even within the last 100 years, people have had some near-misses with meteorites. In 1938 a woman in Illinois heard a sound in her garage and went out to find a meteorite sitting on top of her car. A few years later a woman in Alabama was hit by a ricocheting meteorite that gave her a really nasty bruise on her hip. In 1971 a house in Wethersfield, Connecticut was hit by a meteorite. Eleven years later, a house about one mile away was hit by another meteorite. One of the best recorded impacts was of an object that smashed through the trunk of a woman's 1980 Chevy Malibu. This happened in 1992. The woman was offered $69,000 for her wrecked car, which was a real bargain (since the car was 12 years old and rather run down). The meteorite that did the damage was 30 pounds. The path that this particular piece of space debris took was recorded by many observers in the eastern US, and one person filmed it (they were actually at a high school football game when they caught the images, which can be seen here).
These are all really small things; what if a really big thing hit the Earth? If you want to see the effects of very large impacts, you need only go to Arizona to see a large impact crater. There are also craters visible in other parts of the world, like Australia, Canada and Africa. However, these are pretty old impacts, most pre-dating human history. A big impact that occurred not too long ago took place in remote Siberia. At about 7:17 AM on June 30, 1908, an object struck the Tunguska region of Siberia. The glow of the fire from the impact was visible 1000 km from the site. People 60 km from the impact site were knocked down by the force of the shock wave. The night sky was kept bright by the fires ignited by this event. Actually, people as far away as England noted the overly bright night sky during that summer. Some records indicated that the sky was so bright that you could read a newspaper at midnight. Barometers around the world measured a change in air pressure as the blast wave traveled around the Earth several times. It wasn't until the 1930s that a team of scientists finally reached the site, since it was located in remote swamp lands. No meteorite fragments were found, but the forests in the region were either flattened or burned. Also, based upon the tree pattern, it appeared that the object did not actually hit the ground but exploded in the atmosphere. It is most likely that the object was a comet, since it would have shattered much more easily than an asteroid and would have been easily dissolved. If the object was an asteroid, there should have been more pieces of meteoritic material in the area. Either way, it was pretty devastating. The main casualties in the area were herds of reindeer and one man who was knocked to the ground by the blast and died of internal bleeding. The impact had the strength of roughly a 40 megaton nuclear bomb (about 1000 times as powerful as the bomb that fell on Hiroshima).
Of course, we have been seeing evidence of damage on other worlds. Only recently have we been lucky enough to see a major impact on a world. In 1993, a comet named Shoemaker-Levy 9 (SL9 for short) was discovered. There's nothing too unusual about that, but when further observations showed the motion of the comet to be in orbit about Jupiter (and not the Sun), interest increased. Interest in SL9 intensified when it was determined that this comet wasn't just in orbit about Jupiter but was actually on a collision course with the planet. A previous passage around Jupiter had caused the comet to break into about 20 chunks. This meant that for about one week, we could watch 20 pieces of cometary debris, some about 1 km in diameter, smash into the cloud layers of Jupiter from the safety of the Earth - and boy, was it a cool show! Infrared cameras showed large explosions occurring as each fragment hit, and as Jupiter rotated around, we could see dark cloud features, sort of like cosmic black eyes, on the clouds. The largest fragment had an impact energy of 6 million megatons of TNT (think of that - it was 12 times more powerful than the Manson impact!) The temperatures produced by each explosion was much greater than the temperature of the Sun's surface. The spots that were left by the impact were larger than the Earth in size. Jupiter wasn't damaged by the impact, and evidence of the impacts was gone in a few months, but for a while it was the best show around.
If you missed it, odds are you won't be able to see such an event again. Such an impact event has the chance of occurring only once every thousand years... Or so we thought. In July of 2009 an amateur astronomer noticed a new spot on Jupiter that appeared literally overnight and looked very similar to the spots left by Shoemaker-Levy 9. So perhaps the "once-in-a-life-time" estimate that we thought wasn't quite correct. The Hubble Space Telescope took some images of the new spot, which will likely last only a few months. You can see those images here. And it also appears that someone has caught an impact on video - not what you'd see on America's Funniest Home Video, but still very interesting. Here is a link to the video.
A meteoroid can be an object from the size of a grain of sand to the bulk of a boulder that is traveling in space. There are several official definitions of what is a meteoroid . They all differ based on the largest objects to be included in the group. If a meteoroid enters the atmosphere of Earth or any other body the visible trail that it leaves is called a meteor. Once it impacts the surface it is called a meteorite. These are commonly called shooting stars.
Meteoroids, Meteors, Meteorites
- Meteoroid - a small object in space. This can be in many different forms and could be a comet, an asteroid, or even something that is artificial (man-made). It is just a general term for anything small that is out there.
- Meteor - when an object from space enters the Earth's atmosphere and burns up. It usually leaves a trail of "smoke" in the sky and appears as a bright streak. This is a meteor - the burning up of a meteoroid. The phrase meteor shower comes when a lot of these happen in a short time span. A meteor is also known as a shooting star, though, of course, it has nothing to do with stars. As in the case of a meteoroid, the origin of the object (what it originally was) is not used to categorize it, since it is not always possible to determine what it was made of or where it originally came from. A meteor could be a piece of a comet, asteroid, spacecraft, etc. that is falling through the atmosphere.
- Meteorite - an object that actually makes it to the Earth. Again, the origins of such objects can vary, though it is generally easier to have things that are rather rocky or metallic end up all the way down at the surface. Icy objects usually don't make it.
It surprises many people that meteor showers are not due to asteroids but are instead caused by comets. Comets are objects that travel through the inner solar system, so some of their paths intersect the Earth's orbital path. The chance of a collision is not that great, but because the trail of debris left by the comet's passage is rather large, the Earth does pass through that. As the debris is swept up by the Earth and passes through our atmosphere, we have all sorts of bits of ice and rock burning up - that's what makes a meteor shower. This is also how we can predict when the showers are going to occur, since we know where the comet's orbital path is located and when the Earth will pass through it. It is possible to see individual meteors at just about any time of the year, but the dates of showers are when people make an effort to view them. The meteors tend to originate from one location in the sky (since that is where the cometary debris is), so the names of meteor showers are based upon the constellation they are centered upon. Here are some of the main showers, approximate dates, intensity estimates and the comets that are thought to have produced them.
| Shower | Dates | Meteors/hour | Comet |
| Quadrantids | January 3 | 40 | unknown |
| Lyrids | April 22 | 15 | Thatcher |
| eta Aquarids | May 4 | 20 | Halley |
| delta Aquarids | July 30 | 20 | |
| Perseids | August 12 | 80 or more | Swift-Tuttle |
| Orionids | October 21 | 20 | Halley |
| Taurids | November 4 | 15 | Encke |
| Leonids | Nov 16 | 15 | Tempel-Tuttle |
| Geminids | Dec 13 | 50 | Phaethon (asteroid) |
| Ursids | December 22 | 15 | Tuttle |
If you are interested in watching a meteor shower, then you should do a few things. First of all, get far from city lights. You will also have to do some careful planning, since the best observations of meteor showers take place after midnight. You should set up a lawn chair or something comfortable to sit on, since you could spend quite some time looking up at the sky. You may want to bring along something or someone to keep you warm. You do not want to use things like binoculars or a telescope, since you want to view as much of the sky as possible. You just have to sit back and watch the show! Meteors are actually quite a ways up the atmosphere, typically dozens of miles. Most of the objects that are meteors don't survive their passage through the atmosphere but are completely vaporized. However, if objects are large enough and dense enough, they can make it to the surface of the Earth, at which point they become meteorites.
Most pieces of cometary debris evaporate in the atmosphere, so most meteorites are thought to originate from asteroids. Just as there are different types of asteroids, there are also different types of meteorites. There are actually three main types. These are -
- Stony - Just basic stony, rocky material, often with a burned crust on the outside. These are the most common type of meteorite (95%), but are hard to find (it is hard to tell whether an object on the ground is a plain old rock or a meteorite). One of the rare varieties of these is known as thecarbonaceous chondrite, an carbon rich object thought to originate from c-type asteroids. The neat thing about these is that some have been found to contain things like water and amino acids. In case you forgot, amino acids are the building blocks of proteins - one of the basic cornerstones of life! Life from a meteorite? - it's possible. However, most of the stony meteorites aren't so interesting, and are very similar to the s-type asteroids.
- Stony-Irons - As the name implies, these are a mix of stone and metals. These are actually quite rare (1%).
- Irons - While the name implies these are made of iron, they are actually a mix of mainly iron and nickel. This often helps distinguish them from plain old iron from the Earth, since often industrial by-products don't have nickel in them. About 4% of meteorites come in this form.
They are produced by the heating and cooling of the metal. They are very distinctive and not found in Earth rocks or metals. It also helps if you find your suspected meteorite in a place where there are not too many rocks. Places like Antarctica and large deserts are good places for meteorite hunting, though they are not very nice places to visit.
It is also rather easy to find meteorites by just setting out a collecting dish in your back yard. Huh? I didn't say they would be big meteorites. Pretty much any location on Earth will have micro-meteorites fall on it. These are really small meteorites that are sort of like dust specks that are continually raining down onto the surface of the Earth. If you set out a bowl of water or a petri dish with petroleum jelly in it, you'll catch of a few meteorites after some time - along with bugs, pollen, regular dust, etc. You'd have to use a microscope to find the micrometeorites. They tend to very spherical.
Finding meteorites on the Earth may seem like a difficult task, just imagine how difficult it is to find a meteorite on another planet. Or is it? The Mars rover Opportunity has found a pretty good sized meteorite on the surface of Mars, which is about 2 feet across. Since the rover has the ability to chemically analyze rocks, it was able to determine that this meteorite (shown here) is an iron meteorite.
Impacts
These are all really small things; what if a really big thing hit the Earth? If you want to see the effects of very large impacts, you need only go to Arizona to see a large impact crater. There are also craters visible in other parts of the world, like Australia, Canada and Africa. However, these are pretty old impacts, most pre-dating human history. A big impact that occurred not too long ago took place in remote Siberia. At about 7:17 AM on June 30, 1908, an object struck the Tunguska region of Siberia. The glow of the fire from the impact was visible 1000 km from the site. People 60 km from the impact site were knocked down by the force of the shock wave. The night sky was kept bright by the fires ignited by this event. Actually, people as far away as England noted the overly bright night sky during that summer. Some records indicated that the sky was so bright that you could read a newspaper at midnight. Barometers around the world measured a change in air pressure as the blast wave traveled around the Earth several times. It wasn't until the 1930s that a team of scientists finally reached the site, since it was located in remote swamp lands. No meteorite fragments were found, but the forests in the region were either flattened or burned. Also, based upon the tree pattern, it appeared that the object did not actually hit the ground but exploded in the atmosphere. It is most likely that the object was a comet, since it would have shattered much more easily than an asteroid and would have been easily dissolved. If the object was an asteroid, there should have been more pieces of meteoritic material in the area. Either way, it was pretty devastating. The main casualties in the area were herds of reindeer and one man who was knocked to the ground by the blast and died of internal bleeding. The impact had the strength of roughly a 40 megaton nuclear bomb (about 1000 times as powerful as the bomb that fell on Hiroshima).
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Of course, we have been seeing evidence of damage on other worlds. Only recently have we been lucky enough to see a major impact on a world. In 1993, a comet named Shoemaker-Levy 9 (SL9 for short) was discovered. There's nothing too unusual about that, but when further observations showed the motion of the comet to be in orbit about Jupiter (and not the Sun), interest increased. Interest in SL9 intensified when it was determined that this comet wasn't just in orbit about Jupiter but was actually on a collision course with the planet. A previous passage around Jupiter had caused the comet to break into about 20 chunks. This meant that for about one week, we could watch 20 pieces of cometary debris, some about 1 km in diameter, smash into the cloud layers of Jupiter from the safety of the Earth - and boy, was it a cool show! Infrared cameras showed large explosions occurring as each fragment hit, and as Jupiter rotated around, we could see dark cloud features, sort of like cosmic black eyes, on the clouds. The largest fragment had an impact energy of 6 million megatons of TNT (think of that - it was 12 times more powerful than the Manson impact!) The temperatures produced by each explosion was much greater than the temperature of the Sun's surface. The spots that were left by the impact were larger than the Earth in size. Jupiter wasn't damaged by the impact, and evidence of the impacts was gone in a few months, but for a while it was the best show around.
If you missed it, odds are you won't be able to see such an event again. Such an impact event has the chance of occurring only once every thousand years... Or so we thought. In July of 2009 an amateur astronomer noticed a new spot on Jupiter that appeared literally overnight and looked very similar to the spots left by Shoemaker-Levy 9. So perhaps the "once-in-a-life-time" estimate that we thought wasn't quite correct. The Hubble Space Telescope took some images of the new spot, which will likely last only a few months. You can see those images here. And it also appears that someone has caught an impact on video - not what you'd see on America's Funniest Home Video, but still very interesting. Here is a link to the video.
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