My Science School

          The sun is a large ball of gas. It is so large that, if it was hollow, one million Earth-sized planets could fit inside it! Sun is the largest and the most massive object in the solar system, but it is just a medium-sized star among the hundreds of billions of stars in the Milky Way galaxy.

          The sun is nearly a perfect sphere. Its equatorial diameter and its polar diameter differ by only 6.2 miles (10 km). The mean radius of the sun is 432,450 miles (696,000 kilometers), which makes its diameter about 864,938 miles (1.392 million km). You could line up 109 Earths across the face of the sun. The sun's circumference is about 2,713,406 miles (4,366,813 km). It may be the biggest thing in this neighborhood, but the sun is just average compared to other stars. Betelgeuse, a red giant, is about 700 times bigger than the sun and about 14,000 times brighter.

          We have found stars that are 100 times bigger in diameter than our sun. "We have also seen stars that are just a tenth the size of our sun.” It's possible that the sun is even larger than previously thought. Xavier Jubier, an engineer and solar eclipse researcher, creates detailed models of solar and lunar eclipses to determine precisely where the moon's shadow would fall during the solar eclipse. But when he matched actual photos and historical observations with the models, he found precise eclipse shapes only made sense if he scaled up the sun's radius by a few hundred kilometers.

          Even missions like NASA's Solar Dynamics Observatory (SDO) and measurements of the inner planets across the face of the sun don't refine the star's radius as precisely as desired. "It's harder than you think just to put a ruler on these images and figure out how big the sun is — [SDO] doesn't have enough precision to nail this down," NASA researcher Ernie Wright told Space. com. "Similarly, with the Mercury and Venus transits, it turns out [a measurement based on those is] not quite as precise as you'd like it to be."

          Wright said different papers using a variety of methods have produced results that differ by as much as 930 miles (1,500 km). That could be a problem if you are planning to skirt the edges of the next solar eclipse.

          "For most people, yes, it doesn't really matter; it won't change everything," Jubier said. "But the closer you get to the edge of the [eclipse] path, the more risk you take."

          Sunquakes are violent eruptions on the Sun around areas of hot gas. These explosions send out shockwaves more powerful than the detonation of a billion tonnes of high explosives.

          Earthquakes start deep below the surface, when two blocks of the crust suddenly slips, releasing enormous amounts of energy. The motion creates seismic waves that ripple throughout our planet. There are quakes on the Sun as well. But they originate above the surface, in giant outbursts of magnetic energy.

          As the Sun rotates, it produces a strong magnetic field. Over time, the lines of magnetic force become tangled and twisted. Those lines occasionally snap, creating explosions of energy or geysers of charged particles. These outbursts originate above the Sun’s visible surface, and they direct most of their energy outward. But some of it is directed back toward the Sun. That heats particles in the Sun’s lower atmosphere, creating a pressure wave that penetrates deep into the Sun. The wave then reflects back to the surface, causing a “Sunquakes.” Hot gas ripples outward from the site of the quake, and seismic waves travel deep into the Sun.

          As you might expect, Sunquakes are massive events — they can be thousands of times more powerful than the deadly quake that struck Japan a couple of years ago. And like earthquakes, they can reveal what’s going on below the surface. The sound waves reverberate throughout the Sun and reflect back to the surface, causing it to jiggle. Astronomers can measure these jiggles and use them to probe conditions below the surface — thanks to the power of Sunquakes.

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          The surface of the sun often appears to be dotted with small dark patches. These are called sunspots. They form when the Sun’s magnetic field blocks the heat rising from inside the Sun. Sunspots are actually very bright but appear dark because of their surroundings.

          The surface of the Sun is a very busy place. It has electrically charged gases that generate areas of powerful magnetic forces. These areas are called magnetic fields. The Sun’s gases are constantly moving, which tangles, stretches and twists the magnetic fields. This motion creates a lot of activity on the Sun's surface, called solar activity. Sometimes the Sun’s surface is very active. Other times, things are a bit quieter. The amount of solar activity changes with the stages in the solar cycle. Solar activity can have effects here on Earth, so scientists closely monitor solar activity every day.

          Sunspots are areas that appear dark on the surface of the Sun. They appear dark because they are cooler than other parts of the Sun’s surface. The temperature of a sunspot is still very hot though—around 6,500 degrees Fahrenheit!

          Why sunspots are relatively cool? It’s because they form at areas where magnetic fields are particularly strong. These magnetic fields are so strong that they keep some of the heat within the Sun from reaching the surface. The magnetic field lines near sunspots often tangle, cross, and reorganize. This can cause a sudden explosion of energy called a solar flare. Solar flares release a lot of radiation into space. If a solar flare is very intense, the radiation it releases can interfere with our radio communications here on Earth. Solar flares are sometimes accompanied by a coronal mass ejection (CME for short). CMEs are huge bubbles of radiation and particles from the Sun. They explode into space at very high speed when the Sun’s magnetic field lines suddenly reorganize.

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          Temperatures in and around the Sun vary considerably. On the Sun’s surface, called its photosphere, the temperature is around 5500 °C (10,000°F). Above this lies a hotter section of the atmosphere called the chromosphere, where temperatures can reach 15, 000 °C (27, 000°F). Temperatures in the core of the Sun can exceed an incredible 15 million °C (27 million °F).

          The sun, a massive nuclear-powered source of energy at the center of the solar system, generates the heat and light that sustain life on Earth. But how hot is the sun?

          The answer is different for each part of the sun. Arranged in layers, the sun varies in temperature: It is hottest at its center and cooler in its outer layers— until it strangely reheats at the fringes of the sun's atmosphere. At the sun's core, gravity causes intense pressure, and temperatures of up to 27 million degrees Fahrenheit (15 million degrees Celsius). This generates the nuclear fusion responsible for the star's energy.

          That energy then radiates outward in the sun's inner radiative zone, which lacks the heat and pressure to cause fusion. In that zone, temperatures drop from 12.6 million to 3.6 million F (7 million to 2 million C). In the next zone, called the convective zone, plasma bubbles carry heat to the surface. This zone hits about 3.5 million F.

          Next, energy reaches the surface of the sun, or photosphere, producing the light visible from Earth, and a comparatively chilly 10,000 F (5,500 C). Hydrogen atoms get compressed and fuse together, creating helium. This process is called nuclear fusion. Nuclear fusion produces huge amounts of energy. The energy radiates outward to the sun's surface, atmosphere and beyond. From the core, energy moves to the radiative zone, where it bounces around for up to 1 million years before moving up to the convective zone, the upper layer of the sun's interior. The temperature here drops below 3.5 million degrees F (2 million degrees C). Large bubbles of hot plasma form a soup of ionized atoms and move upward to the photosphere.

          Like all stars, the Sun generates nuclear energy. In the Sun’s core, the temperature is so high that particles of gas cannot form completely. Instead, atomic nuclei and electrons travel around at very high speeds, moving so fast that if they collide, they join to form new particles. This process is called nuclear fusion, and it converts hydrogen into helium whilst also releasing vast amounts of energy. The Sun converts over four million tonnes of matter into energy every second.

          The interior of the sun is a kind of thermonuclear bomb of fusing material, mainly of hydrogen atoms under extreme pressure and temperature controlled at a giant scale, because of its enormous amount of particles interacting at high energy, it generates an electromagnetic field that helps maintaining it for an extremely long time. In the Sun there are trillions of particles in constant rotation colliding, in constant fission and fusion, mainly using hydrogen ions to convert them in helium ions in a chain reaction. In the sun it can be identified different layers that vary in density, temperature, pressure and behavior: the “thermonuclear core”, the “radiative zone”, the “convection zone”, the “photosphere”, the “chromosphere” and “solar corona”. The plasma is transparent to its own radiation.

          The “thermonuclear core” has a spherical shape due to the action of gravity on the particles compressing towards the center, with a radius of 170,000 km. which represents 10% of the Sun’s mass and 25% of its radius. It is 530,000 kilometers deep from its surface. The most central part of the core is already 60% helium, although here is generated 99% of the energy emitted by the sun (in form of highly energized shortwaves); none of the fusion products of the center have risen to the photosphere. At the core of the Sun’s gravity pulls all atoms to the center. Pressure is 340 billion atmospheres of Earth, thus generating a tremendous vibration and rubbing of particles so that the temperature reaches over 15 million ° C. Matter is in form of ultra-dense plasma (150 t/m3) on this layer.

          Each particle in the core has its own rotation. By adding the emitted charges of all the particles as a gigantic electromagnetic field rotation product of the rotation in the center of the sun, that will seek to be fed back through the poles, but being so close together, will add each other to generate a huge electromagnetic field of plasma around the core. The “radiative zone” can include the core so that together it is credited a radius of 580,000 km, accounting for 80% of the radius of the sun. Without considering the core, this layer would be 410 000 km thick. There is great compression on this layer, which is a bit less dense (20 tons/m3 to 200 kg/m3), but the pressure and the energy from the core of energized atoms generates vibrations emanating short electromagnetic wavelengths that transport heat and light to the surface.

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          We know that earth is bombarded by thousands of meteorites every day, none of which does our planet much damage. Any meteorite up to 10m (33ft) in diameter will normally burn up in the atmosphere before it reaches Earth, separating into tiny fragments. If a meteorite larger than this falls to Earth, it can cause considerable damage — impacting with the energy of five nuclear warheads. Approximately once every 1000 years, a larger meteorite does fall to Earth, and several large craters caused by such impacts can still be seen. One such was the nickel - iron meteorite that created the Barringer Crater in Arizona, USA. The meteorite was an incredible 45m (148ft) wide, creating a crater nearly 1.5km (1 mile) in width. However, it would take an impact by an object roughly 5km (3 miles) wide to cause mass extinctions and threaten life on Earth.

          Most meteorites that are found on the ground weigh less than a pound. While it may seem like these tiny pieces of rock wouldn't do much damage, a 1-lb. (0.45 kilograms) meteorite traveling upward of 200 mph (322 km/h) can fall through the roof of a house or shatter a car windshield. 

          When the Grimsby meteorite landed in Ontario, Canada in 2009, for example, it broke the windshield of an SUV. In another incident, meteorites crashed into the back end of a Chevy Malibu in Peekskill, New York, in 1992, Cooke and Moorhead said. Thankfully, no one was injured during these events. 

          However, the pieces of rock falling from the sky are not even the greatest concern regarding meteor impacts, Cooke said.

          "What causes the most damage is the shock wave produced by the meteor when it breaks apart in [Earth's] atmosphere," Cooke said. "So, you don't have to watch for the falling rocks — you have to worry about the shockwave."

          For example, the Chelyabinsk meteor — an asteroid the size of a six-story  building that entered Earth's atmosphere in February 2013 over Russia — broke apart 15 miles (24 km) above the ground and generated a shock wave equivalent to a 500-kiloton explosion, Cooke said. It injured 1,600 people.

          Another major collision was the Tunguska meteorite, which was larger than Chelyabinsk and 10 times more energetic. The meteorite exploded over the Tunguska River on June 30, 1908, and flattened 5000,000 acres (2,000 square km) of uninhabited forest. Because of its remote location, the event is an example of a meteorite that would have gone undetected had it not been so large, Cooke and Moorhead explained. 

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          Unlike many of the planets, moons and smaller bodies in the Solar System, Earth appears to be covered by very few craters. In the early days of the Solar System, Earth was as much a target for meteorites as any other planet, and suffered intensive cratering in the first one billion years of its existence. However, unlike bodies such as Mercury and the Moon, Earth has many geological processes that “hide” craters. Constant weathering and erosion from winds and water wear away or cover up craters. Some may also be hidden by vegetation or lie under the sea, although in the last hundred years, aerial photography and other forms of imaging have given us a clearer view of many remaining craters.

          Impact craters leave quite an impression on the surface of planets and moons — just think of Earth’s moon, which gets its distinctive appearance from millions of encounters of asteroids over the centuries. But Earth is a different story altogether, with only 128 impact craters recorded in the most recent count. That can’t be right, can it?

          He reports that a new study shows that the low number found by past scientists isn’t “just the result of lazy searching”: it’s the surprising truth about a planet that’s astonishingly crater-free.

          The study looked at the ways Earth erosion affects existing craters and concluded that the current count of 70 craters larger than 6 km (3.7 miles) in diameter should be just about right. That’s a rare instance of a complete geologic record, writes Hand — and one that may discourage people on the hunt for new craters.

          But don’t put away your crater-catching gear just yet. The study’s authors note that just because we’ve already found all of the likely large impact craters on Earth don’t mean there aren’t more to discover. The real opportunity, they write, lies in smaller craters: they estimate that more than 90 craters between .6 miles and 3.7 miles in diameter should still be undiscovered and more than 250 between 0.1 miles and .6 miles.

          NASA notes that Earth is equipped with three processes that eat up craters relatively quickly: erosion, tectonics, and volcanism. These forces leave only the largest scars from meteorites or asteroids — unlike, say, the moon, which can’t gobble up craters. Hand writes that the parameters of the study also play a part in the low number — it looks at just surface craters, not those that lie beneath sediment. And the study also didn’t look at volcanic craters, which formed some of Earth’s most distinctive basins and lakes.

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          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.

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          Amazingly, thousands of rocks from space hit the surface of Earth each day. Every year our planet puts on nearly 10,000 tonnes in weight due to meteoroids entering the atmosphere. Many of these are minuscule grains of dust, but some can be many metres in length. The world’s largest known meteorite was discovered in Namibia, Africa, in 1920. It weighs an incredible 55,000 kg (120, 000Ibs).

          To date, there have been nearly 1,100 recovered falls (meteorites seen to fall) and nearly 40,000 finds (found, but not seen to fall). It is estimated that probably 500 meteorites reach the surface of the Earth each year, but less than 10 are recovered. This is because most fall into the ocean, land in remote areas of the Earth, land in places that are not easily accessible, or are just not seen to fall (fall during the day). From a model animation, it appears that lots of small asteroids/large meteoroids pass close to the Earth each day. Most of these are not detected, but recently, three 5–10 meter “asteroids” have been discovered and have passed well within the orbit of the Moon. Also recently, an asteroid about 500 meters in diameter passed about 2 million km from the Earth (five times the distance to the Moon). It is estimated that each day one or two 5–10 meter objects pass within the Moon’s orbit and that there are probably 30 million near-Earth objects! Most of these are too small to ever cause any damage. Five to ten meters is probably the smallest object that would likely survive passage through the Earth’s atmosphere.

          While large impacts are fairly rare, thousands of tiny pieces of spaces of space rock, called meteorites, hit the ground each year. However, the majority of these events are unpredictable and go unnoticed, as they land in vast swathes of uninhabited forest or in the open waters of the ocean, Bill Cooke and Althea Moorhead of NASA's Meteoroid Environments Office told Space.com. 

          In order to understand meteorite impacts on Earth, it is important to know where the chunks of rock come from. Meteoroids are rocky remnants of a comet or asteroid that travel in outer space, but when these objects enter Earth's atmosphere, they are considered meteors. Most (between 90 and 95 percent) of these meteors completely burn up in the atmosphere, resulting in a bright streak that can be seen across the night sky, Moorhead said. However, when meteors survive their high-speed plunge toward Earth and drop to the ground, they are called meteorites. 

          The Perseid meteor shower — one of the most popular meteor showers of the year — is expected to put on a particularly breathtaking show Aug. 11 and 12, when the Earth passes through the trail of debris created by Comet Swift-Tuttle. However, viewers should not expect to find any meteorites lying on the ground after this spectacular meteor shower. "Perseids come from Comet Swift-Tuttle and are very fragile, being an ice-dust mix," Cooke said. "They are not strong enough to survive passage through the atmosphere at 132,000 mph (212,433 km/h) and so never produce meteorites — they are totally vaporized by the time they make it to 50 miles (80 km) altitude." 

Picture Credit : Google

          Several more probes have been designed and built to visit comets in the near future.

          Stardust, the 4th Discovery mission launched in February 1999, will collect coma samples from the recently deflected comet 81P/Wild 2 on 2 January 2004 and return them to Earth on 15 January 2006 for detailed laboratory analyses. Stardust will be the first mission to bring samples back to Earth from a known comet and also the first to bring back contemporary interstellar particles recently discovered. These samples should provide important insights into the nature and amount of dust released by comets, the roles of comets in planetary systems, clues to the importance of comets in producing dust in our zodiacal cloud as well as circumstellar dust around other stars, and the links between collected meteoritic samples with a known commentary body. Samples are collected in newly invented continuous gradient density silica aerogel. Stardust is facilitated by a magnificent trajectory designed to accomplish a complex and ambitious flyby sample return mission within the Discovery program restrictions. The remaining science payload, which provides important context for the captured samples, includes a time?of?flight spectrometer measuring the chemical and isotopic composition of dust grains; a polyvinylidene fluoride dust flux monitor determining dust flux profiles; a CCD camera for imaging Wild 2 coma and its nucleus; a shared X band transponder providing two?way Doppler shifts to estimate limits to Wild 2 mass and integrated dust fluency; and tracking of the spacecraft's attitude sensing for the detection of large particle impacts. The graphite composite spacecraft brings the collected sample back to Earth by a direct reentry in a capsule.

          Stardust, the fourth NASA Discovery mission, launched on 7 February 1999, now circles the Sun in an orbit that will cause a close encounter on 2 January 2004 with the comet 81P/Wild 2. Stardust will collect coma dust at 150 km from Wild 2's nucleus and return it to Earth for detailed laboratory analysis on 15 January 2006. Figure 1 shows an artist's rendition of the Stardust spacecraft encountering the comet Wild 2 with the sample collector fully deployed. The Halley Intercept Mission (HIM) proposed in 1981 for the last comet Halley apparition inspired the near 2?decade quest for this comet coma sample return mission, Stardust.

          In addition, along the way to Wild 2, the backside of the Wild 2 sample collector will be used to capture interstellar particles (ISP) as bonus science. Besides the primary sample instrument, Stardust also makes in situ investigations to provide important context to the return samples: a time?of?flight spectrometer, a dust flux monitor, an optical navigation camera, an X band transponder for determining integrated dust flux and an estimate of the mass of Wild 2, and monitoring of spacecraft attitude control disturbances for large particle impacts.

          The return of lunar samples by the Apollo program provided the first opportunity to perform detailed laboratory studies of ancient solid materials from a known astronomical body. The highly detailed study of these samples revolutionized our understanding of the Moon and provided fundamental insights into the remarkable and violent processes that occur early in the history of moons and terrestrial planets. This type of space paleontology is not possible with astronomical and remote sensing. Despite these advantages, however, the last US sample return was made by Apollo 17 over 30 years ago! Now, 3 decades later, Stardust is leading a new era of sample return missions, including missions to return samples of solar wind [Burnett et al .,2003], asteroid [Fujiwara et al., 1999], and Mars [Garvin, 2002].

Picture Credit : Google