Category The Universe, Exploring the Universe, Solar System, The Moon, Space, Space Travel

          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

          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.” 

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          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].

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          If a comet collided with Earth, the results could be disastrous — possibly meaning the end of all life on our planet. Comets can often be caught by the strong gravitational pulls of planets. In 1994, the Shoemaker-Levy 9 comet crashed into Jupiter’s atmosphere. It impacted at more than 200,000km/h (124,000mph), creating balls of fire larger than Earth.

          An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effect. When large objects impact terrestrial planets such as the Earth, there can be significant physical and biospheric consequences, though atmospheres mitigate many surface impacts through atmospheric entry. Impact craters and structures are dominant landforms on many of the Solar System’s solid objects and present the strongest empirical evidence for their frequency and scale.

          Impact events appear to have played a significant role in the evolution of the Solar System since its formation. Major impact events have significantly shaped Earth’s history, have been implicated in the formation of the Earth-Moon system, the evolutionary history of life, the origin of water on Earth and several mass extinctions. The prehistoric Chicxulub impact, 66 million years ago, is believed to be the cause of the Cretaceous-Paleocene extinction event.

          The Comet Shoemaker-Levy 9 impact provided the first direct observation of an extraterrestrial collision of Solar System objects, when the comet broke apart and collided with Jupiter in July 1994. An extrasolar impact was observed in 2013, when a massive terrestrial planet impact was detected around the star ID8 in the star cluster NGC 2547 by NASA’s Spitzer space telescope and confirmed by ground observations. Impact events have been a plot and background element in science fiction.

          In April 2018, the B612 Foundation reported “It’s 100 per cent certain we’ll be hit [by a devastating asteroid], but we’re not 100 per cent certain when.” Also in 2018, physicist Stephen Hawking, in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the “National Near-Earth Object Preparedness Strategy Action Plan” to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.

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