Category Science

          Because earth passes through meteor streams at roughly the same time each year, meteor showers can be predicted highly accurately. Astronomers have now even worked out which comets are responsible for each annual shower. Two meteor showers come from the trail left by Halley’s Comet: the Orionids in October and the Eta Aquarids in May. Although meteors in a shower fall to Earth over a large distance, perspective makes them seem to be falling from the same point in the sky, called the radiant.

           Most ‘predictions’ of the rate of meteors per hour during meteor showers are based on both theory and observation. Essentially, a computer model is built containing the trajectories of every known comet – since it is the debris from comets that forms the ‘stream’ of particles we see during a meteor shower.

          This model contains information on the rate that these comets release material, along with the sizes, directions and velocities at which they are released, as well as the gravitational forces that determine their subsequent trajectories through space. The trajectory of the Earth and the conditions of the Earth’s atmosphere are also inputted into the computer model.

          By watching how Earth moves through the meteor stream it is possible to estimate the likely number of meteors that will be visible during a given shower for a given location. But different astronomers use different models. Plus, these models are partly based on difficult measurements of the meteoric particles in the Solar System, so their predictions are often only approximate. But generally, they can be used to reliably predict when a meteor shower is likely to be more or less intense than the average.

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          The average meteorite enters the Earth’s atmosphere at around 50km/s (31mi/s), but particles in the atmosphere cause the speeding rocks to slow down. All but the largest meteorites are decelerated to around 150km/h (93mph) by the time they impact. Larger meteorites will not be slowed by atmospheric friction and hit the ground travelling at deadly speed.

          The term meteor comes from the Greek meteoron, meaning phenomenon in the sky. It is used to describe the streak of light produced as matter in the Solar System falls into Earth’s atmosphere creating temporary incandescence resulting from atmospheric friction. A meteoroid is matter revolving around the sun or any object in interplanetary space that is too small to be called an asteroid or a comet. Even smaller particles are called micrometeoroids or cosmic dust grains, which includes any interstellar material that should happen to enter our solar system. A meteorite is a meteoroid that reaches the surface of the Earth without being completely vaporized.

          Meteor’s come in a range of sizes, from dust-sized which we see as reflected sunlight in the orbital plane of the Solar System (called zodiacal light) to house-sized.

          When a meteor enters the atmosphere friction causes ablation of its surface (i.e. it burns up). If the meteor is small (fist-sized) it vaporizes before hitting the ground. If larger it survives to impact on the ground, although it will be reduced in size during entry into the atmosphere. About 25 million meteors enter the Earth’s atmosphere every day (duck!). Most burn up and about 1 million kilograms of dust per day settles to the Earth’s surface.

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          A great deal of the material that makes up meteorites comes from short-period comets. As comets travel close to the Sun, they lose material, creating a trail of debris behind them. These trails, called meteoroid streams, can take many hundreds of years to form, but gradually build up to contain a large amount of loose dust and rock fragments. If Earth’s orbit carries it through one of these streams, then hundreds of meteoroids will enter the atmosphere in a very short time, creating a meteor shower.

          Meteor showers occur when the earth’s orbit and that of a comet intersect. What you are seeing is the bits of dust that the comet left behind colliding with the atmosphere at high speed. The friction with the earth’s atmosphere heats up the particles of dust to thousands of degrees until they either vaporize or strike the surface of the earth. In some cases they particles a deflected back into space like a stone skipping on water. When the earth is not in a recent orbital path of a comet there are still loose particles all around the solar system so there is a base rate of about 5 visible meteors an hour in ideal viewing conditions. You can see one strike the atmosphere if its dark enough and you happen to be looking in the right direction. The orbital paths of the earth and many comets intersect once every year and the meteor rate can be much higher when the earth passes through these debris fields. The process of small particles loose in space being swept up by larger bodies like the earth has been going on for billions of years and is what created the sun, planets and comets to begin with. The earth captures 40,000 metric tons of space dust a year currently which is much less than the rate it was 4 billion years ago when the planets were first forming. This makes sense logically as the dust clears the collision rate falls. The geologic record on earth and the moon also support this hypothesis.

          Meteor showers associated with particular comet orbits occur at about the same time each year, because it is at those points in the earth’s orbit that the collisions occur. However, because some parts of the comet’s path are richer in debris than others, the strength of a meteor shower may vary from one year to the next. Typically a meteor shower will be strongest when the earth crosses the comet’s path shortly after the parent comet has passed.

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          There are three main types of meteorite. More than 90% of meteorites found on Earth are made of stone. Stony meteorites are divided into chondrites, which contain particles of solidified rock, and achondrites, which do not. Iron meteorites are composed of iron and nickel. Less than 1% of all meteorites are a mixture of rock and iron, and are called stony-iron meteorites.

Stony Meteorites

Stony meteorites are made up of minerals that contain silicates—material made of silicon and oxygen. They also contain some metal—nickel and iron. There are two major types of stony meteorites: chondrites and achondrites.

Chondrites themselves are classified into two major groups: ordinary and carbonaceous. Ordinary chondrites are the most common type of stony meteorite, accounting for 86 percent of all meteorites that have fallen to Earth. They are named for the hardened droplets of lava, called chondrules, embedded in them. Chondrites formed from the dust and small particles that came together to form asteroids in the early solar system, more than 4.5 billion years ago. Because they were formed at the same time as the solar system, chondrites are integral to the study of the solar system’s origin, age, and composition. 

Ordinary chondrites can be classified into three main groups. The groups indicate the meteorite’s quantity of iron. The H chondrite group has a high amount of iron. The L chondrite group has a low amount of iron. The LL group has a low amount of iron and a low amount of metal in general.

Carbonaceous chondrites are much rarer than ordinary chondrites. Astronomers think carbonaceous chondrites formed far away from the sun as the early solar system developed. As their name implies, carbonaceous chondrites contain the element carbon, usually in the form of organic compounds such as amino acids. Carbonaceous chondrites also often contain water or material that was shaped by the presence of water.

Achondrites do not contain the lava droplets (chondrules) present in chondrites. They are very rare, making up about 3 percent of all known meteorites. Most achondrites form from the brittle outer layers of asteroids, which are similar to Earth’s crust.

There are many classifications of achondrites. The “primitive achondrites” group, for instance, has a very similar mineral composition to chondrites. Lunar meteorites are achondrites that crashed to Earth from the Moon, while Martian achondrites crashed to Earth from our neighbor planet, Mars.

Iron Meteorites

Iron meteorites are mostly made of iron and nickel. They come from the cores of asteroids and account for about 5 percent of meteorites on Earth.

Iron meteorites are the most massive meteorites ever discovered. Their heavy mineral composition (iron and nickel) often allows them to survive the harsh plummet through Earth’s atmosphere without breaking into smaller pieces. The largest meteorite ever found, Namibia’s Hoba meteorite, is an iron meteorite.

Stony-Iron Meteorites

Stony-iron meteorites have nearly equal amounts of silicate minerals (chemicals that contain the elements silicon and oxygen) and metals (iron and nickel). 

One group of stony-iron meteorites, the pallasites, contains yellow-green olivine crystals encased in shiny metal. Astronomers think many pallasites are relics of an asteroid’s core-mantle boundary. Their chemical composition is similar to many iron meteorites, leading astronomers to think maybe they came from different parts of the same asteroid that broke up when it crashed into Earth’s atmosphere.

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          Space is teeming with millions of tiny pieces of rock and dust left over from the formation of the Solar System 4.6 billion years ago. These fragments are called meteoroids. They range in size from minuscule dust particles no larger than one-millionth of a gram to large rocks weighing many tonnes. Meteoroids travel through space and are often caught by Earth’s gravitational pull. When a meteoroid enters Earth’s atmosphere, it begins to heat up because of friction. As it heats up, it starts to glow, becoming a meteor — better known as a shooting star. Most meteors burn up in the atmosphere before they reach the ground. Those that hit the Earth’s surface are called meteorites.

          So, they start as a meteoroid in the sky. Then, they fall as a meteor flashing light. Next, when it lands on Earth, we call it a meteorite.

• Meteoroids are far up in the sky.
• Meteorites have already landed on Earth.
• Meteors are falling down to Earth streaking light when they break down in the atmosphere.

So, they start as a meteoroid in the sky. Then, they fall as a meteor flashing light. Next, when it lands on Earth, we call it a meteorite.

Meteoroids

Meteoroids are stony or metallic debris travel through outer space – some directed to Earth. Meteoroids are smaller than asteroids and contain less water and ice than comets. In terms of location, meteoroids are way out in our solar system. They aren’t in Earth’s atmosphere and they haven’t. Because meteoroids are in the solar system, they can interfere with spacecraft operations. This is why considers the risk of meteoroids beyond Earth’s orbit.

Meteors

When you observe a meteor shower in the sky, these are meteors burning up in Earth’s atmosphere. During a meteor shower, we often call meteors “shooting stars”.

Meteors flash light through the sky because of Earth’s atmosphere. Specifically, meteors break due to friction in our mesosphere. They often leave a tail behind them in the direction they are traveling in. After all, meteor showers are among the most beautiful sites we can observe in our night’s sky. Most meteors never make it to the Earth and break down in the atmosphere. Specifically, they break down in the mesosphere. But the ones that reach the ground, we call them “meteorites”.

Meteorites

Meteorites are something that we all can see because they are the ones that crash down to Earth. For example, the Barringer Crater in Arizona is an old artifact from a stony meteorite. Stony meteors like this one are the most abundant. We know this from all the meteorites that we count in the ice of Antarctica. When you look at the moon, you can see all the impacts from meteors. Back in primeval days, Earth had the same number of meteor impacts. So why can we see so many meteors on the moon but not on Earth?

          One of the key differences is how much water we have on Earth. Because the Earth is mostly water, we don’t see a lot of the meteorites that reach the Earth. But how about ones that crash on land? Over the years, weathering, erosion and mass wasting has erased many craters, mountains and terrain on Earth.

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          Most comets have very long orbits that cover millions of kilometres. They travel into the Solar System from about one light year away, before swinging round the Sun and heading back out into space for thousands of years. These are called long-period comets. Some comets, particularly those that are trapped by the gravity of large planets, orbit the Sun in less than 200 years. These are called short-period comets.

          You may remember from the Origins section, that most comets are very far from the Sun and the center of the solar system. Where do comets spend their time? Why do some comets come near the Sun and become bright? What makes these comets different? And how is it that some comets, like Comet Halley, return again and again?  In order to understand this, we must understand comet orbits.

          Since comets were created from the same spinning cloud of gas and dust as the planets, they continue that motion, revolving around the Sun like the planets and everything else in the solar system. Like the planets, each comet travels on a regular path, called an orbit. The planets’ orbits are very nearly circular, but not quite. Each orbit has the shape of a slightly stretched-out circle, called an ellipse. 

          Earth’s orbit is so close to a circle that if you could look at it from space, you couldn’t really tell the difference. But many comets revolve along more stretched-out ellipses with the Sun near one end instead of in the center. It’s as if the Sun were twirling each comet on a stretchy rubber band, that gets longer and then shorter again, each time the comet comes back around the Sun. 

          Having an elliptical orbit means there is a point for each comet where it is closest to the Sun. At this point we say that it is at perihelion; “peri” means close, “helio” is the root word for Sun. There is also a point where the comet is farthest away from the Sun. At this point, we say that it is at aphelion. 

          In the Oort cloud, a comet’s orbit can be changed over many years by gravity, until it is long and thin, with the Sun very close to one end. These comets travel all the way from the Oort Cloud to a point inside the Kuiper Belt and back out again.  If the orbit’s perihelion is close enough to the Sun, say, less than 5 AU, it then receives enough solar energy to become bright and be seen by the naked eye. 

          Since these comets still travel from the Oort Cloud all the way around the Sun and back, they can take from hundreds of years, to over a hundred thousand (100,000) years to revolve once around the Sun. What would you call these comets? Right! They are known as long period comets.  Comet Hale-Bopp, which appeared in 1997, is a long period comet. It won’t return to its perihelion near the Sun again for almost 2500 years: a long time to us, but a short time for a comet. About five out of every six comets that have been observed are long period comets. The comets that led Oort to develop his theory of the comet cloud were all long-period comets.

          There are also comets whose entire orbit lies within the region of the Kuiper Belt, the “waistband” of comets that is just beyond the planet Neptune. These comets have periods of about 200 years or less. Because of this, they are known as short period comets. Comet Halley, which last appeared in 1986, is a short period comet.  

          It takes 76 years for Comet Halley to complete one trip around the Sun. But as you saw, in the Origins section, many objects in the Kuiper Belt have nearly round orbits. Only a few have long flat orbits that come close to the Sun at one end. This picture shows that Halley’s orbit doesn’t even go beyond that of Pluto.  You can just see the comet and its tail inside the orbit of Venus near perihelion.  The planets are NOT drawn to scale. They are shown bigger so the viewer can recognize them.

Picture Credit : Google