Where do meteorites come from? Where do meteorites come from? Where do meteorites come from? Where do meteorites come from? Exhibition Text All meteorites come from inside our solar system. Most of them are fragments of asteroids that broke apart long ago in the asteroid belt, located between Mars and Jupiter. Such fragments orbit the Sun for some time-often millions of years-before colliding with Earth. Meteorites can be huge: the biggest one ever found weighs around 60 tons, roughly twice as much as the Ahnighito meteorite at the center of this room. People have also found meteorites that are quite small, about the size of beach pebbles or even grains of sand. Asteroids The vast majority of meteorites are fragments of shattered asteroids. Asteroids are rocky bodies found mostly in the asteroid belt, between Mars and Jupiter. Jupiter is the largest planet in our solar system, and its gravity is very strong. Asteroids, which are much smaller than planets, are sometimes pulled out of the asteroid belt by the force of Jupiter's gravity. Many of these asteroids then travel toward the inner solar system—where they can collide with Earth.

Planets A small number of meteorites are pieces of rock from the surfaces of other planets. These fragments were likely blasted off planets when they were hit by a large asteroid or comet. People have found meteorites that are definitely from the planet Mars, some of which are on display in this hall. Some meteorites might be from Mercury, but researchers are still investigating this claim. Moon The most famous moon rocks are those collected by astronauts who walked on the Moon. But small pieces of the Moon also occasionally reach Earth as meteorites. Such "lunar meteorites" are identical in composition to the astronauts' moon rocks, although they come from different locations-possibly even from the far side of the Moon, which never faces Earth. Comets Meteorites might also come from comets. Made of dust, rock and ices, comets are typically found in the outer reaches of our solar system, beyond the orbit of the planet Neptune. Scientists have identified several meteorites that might be fragments of the rocky cores of comets.

Keywords: Meteorites, Astrophysics, Astronomy, Rocks, Moon, Planets, Astrogeology, Asteroids, Comets Audience: General NASA Scientists Find Diamonds and Other Treasures in Gold Rush Meteorite | NASA NASA Scientists Find Diamonds and Other Treasures in Gold Rush Meteorite November 7, 2014 [image-114] [image-69] Researchers digging deeper into the origins of the Sutter's Mill meteorite, which fell in California's Gold Country in 2012, found diamonds and other "treasures" that provide important new insight into the early days of our solar system. They report their results in thirteen papers in the November issue of Meteoritics & Planetary Science. "Sutter's Mill gives us a glimpse of what future NASA spacecraft may find when they bring back samples from a primitive asteroid,” said consortium lead Peter Jenniskens of the SETI Institute in Mountain View, California, at NASA’s Ames Research Center in Moffett Field, California. "From what falls naturally to the ground, much does not survive the violent collision with Earth's atmosphere." Jenniskens found one of the first and one of the most unusual of the Sutter's Mill meteorites before rain hit the area.

In his search, Jenniskens was guided by Marc Fries of NASA's Johnson Space Center, in Houston, who describes in detail how Doppler weather radar enabled the rapid and pristine collection of the meteorites. "The two 10-micron diamond grains we found in this meteorite are too small to sparkle in a ring," said Mike Zolensky, space scientist of Johnson, working with associate professor Yoko Kebukawa late of Hokkaido University, Japan, "But their size is much larger than the nanometer-sized diamonds commonly found in such meteorites." Nanodiamonds are thought to originate in the atmospheres of stars. The larger diamonds found in Sutter's Mill may have had another origin closer to home. "We suspect that these diamonds are so-called xenoliths,” says Kebukawa. "Bits and pieces that originated in the interior of other much larger parent bodies." University of California (UC), Davis researchers Akane Yamakawa and Qing-Zhu Yin studied the different forms of the element chromium, called isotopes.

They found that at least five different stellar sources composed of mixtures of 54-chromium-rich and -poor materials must have contributed matter to the nascent solar system four and half billion years ago. Some of these materials remained in the Sutter's Mill meteorite. "The formation of the solar system did not fully erase and homogenize these signatures and Sutter’s Mill provides the clearest record yet," said Yin, also a co-lead of the Sutter’s Mill Meteorite Consortium. Some fragments escaped alteration by liquid water, while others where heavily altered. Based on isotopes of the elements manganese a nd chromium, NASA Earth and Space Science Fellow Christine Jilly and colleagues now report when that happened. "We determined that minerals such as calcium and magnesium carbonate likely precipitated from liquid water 2.4 to 5.0 million years after the formation of the solar system," says Jilly. "It is possible that radioactive compounds in the meteorite's parent body heated the nascent planet to the point that water ice became liquid." In primitive meteorites like Sutter's Mill, some grains survive from what existed in the cloud of gas, dust and ices that formed the solar system.

In Sutter's Mill, the liquid water appears to have destroyed the silicate type of these, according to Xuchao Zhao of the Chinese Academy of Sciences, working with NASA and UC Davis colleagues. The meteorite also was heated, perhaps as recent as during entry in Earth's atmosphere. Using a technique called thermoluminescense, Derek Sears, senior research scientist at Ames, determined the rock was heated to 572 degrees Fahrenheit (300 degrees Celsius) in the last 100,000 years. Other materials were heated on the parent body, differently at different locations. Zolensky found unusual materials not seen in any other meteorites, including a very unusual calcium sulfide called oldhamite, also suspected to be a xenolith. "I don't know of any similar meteorites that contain both heated and unheated materials," said Zolensky. "Some materials were heated to 932 degrees Fahrenheit (500 degrees Celsius), while others clearly were not." Michel Nuevo and Scott Sandford, both astrophysicists at Ames probed numerous grains with infrared spectroscopy, as did Mehmet Yesiltas and colleagues at the University of Central Florida.

They confirmed that some portions of the Sutter's Mill meteorites were heated more than others, removing volatile organic compounds and causing chemical changes in the remainder. "We still found small amounts of amino acids - building blocks of proteins - in the pristinely collected stone by Jenniskens" says Aaron Burton, then at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, now at Johnson. "Indigenous amino acids were much less abundant in the Sutter's Mill samples than in other such meteorites.” For more information, visit: Meteorite | Asteroid | Mars | Moon | Center for Meteorite Studies | ASU Center for Meteorite Studies Where do meteorites come from? Most meteorites are believed to originate in the asteroid belt between Mars and Jupiter, and were formed early in the history of the Solar System ~4.56 billion years ago. These fragments of asteroids were either knocked out of their orbit of the Sun, and into Earth-crossing orbits, through collisions with other objects, or through the interaction of gravitational forces exerted by the Sun and Jupiter. Image credit: NASA/JPL-Caltech How do we know meteorites come from space?

Meteorites that come from the asteroid belt are about the same age as the solar system, approximately 4.5 billion years old. No Earth rocks are this old, because they have all been repeatedly broken down and reformed by terrestrial geological processes such as erosion and plate tectonics. Which asteroids do they come from? While it is difficult to pinpoint specific asteroids as parent bodies of specific meteorite types, comparison of meteorite data with data from asteroids from Earth-based observations and spacecraft can help to define likely sources of some meteorite types. For example, data collected by NASA’s Dawn spacecraft has strengthened the theory that the HED (Howardites, Eucrites and Diogenites) group of achondrites formed in the crust of asteroid 4-Vesta (This mosaic synthesizes some of the best views the Dawn spacecraft had of the giant asteroid Vesta. Image credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA). Meteorites from a tracked asteroid The small asteroid 2008 TC3 was first discovered October 6th, 2008, by an automated telescope at Mount Lemmon in Tucson, Arizona.

The asteroid was tracked closely as it entered Earth's atmosphere and exploded approximately 37 km above northern Sudan’s Nubian Desert early the following morning. The meteorites recovered from this fall, named Almahata Sitta , are extremely interesting, as the specimens exhibit multiple distinct lithologies, ranging from ordinary chondrite to bencubinnite. The stones from this unprecedented event continue to provide key information about the composition of asteroids. Do we have meteorites from other planets? Some meteorites come from the Moon and Mars. These lunar or martian crustal rocks were ejected into space when another (asteroidal or cometary) object collided with the Moon or Mars with enough force to launch some of the impact-produced debris into Earth-crossing orbits. How do we know they’re from Mars? Martian meteorites are distinguished from Earth rocks and other meteorite types by their chemical and mineral composition, as well as their age. Moreover, gases trapped in shock glass in martian meteorites have been matched to measurements of the martian atmosphere taken by the NASA Viking mission in 1976. Nakhla, martian achondrite. Image ⓒ ASU/CMS How do we know they’re from the Moon?

Because we’ve been there! Lunar meteorites are identified by their unique chemical and mineral characteristics indicative of their origin on the Moon, mainly based on comparison to lunar rocks returned to Earth by the Apollo and Luna missions. Did you know that the first ever carbon analysis of lunar rocks was done at the ASU Center for Meteorite Studies? Learn more about it here! Do we have any confirmed meteorites from Venus or Mercury? Not yet, but there could be pieces of both planets in existing meteorite collections, waiting to be identified. Data from NASA’s MESSENGER mission could help to identify potential mercurian meteorites, while venusian meteorites might be distinguished using data from the Vega 1 and 2 landers, and the Magellan spacecraft. Venus. Image credit: SSV, MIPL, Magellan Team, NASA Meteorite | COSMOS Meteorite Dar al Gani meteorite showing the black fusion crust. Credit : photograph by R. Pelisson, SaharaMet If an incoming meteoroid survives its passage through the atmosphere as a meteor and actually impacts the Earth, it is known as a ‘meteorite’.

Of the several thousand tonnes of interplanetary debris to enter the Earth’s atmosphere every day , only about 1 tonne makes it to the Earth’s surface, and the majority of this is in the form of microscopic dust particles. In particular, objects smaller than 10-6 m are slowed by collisions with molecules in the upper atmosphere to a degree where ablation does not occur during their fall to Earth. These land as micrometeorites. Meteorites (which can measure metres across), are mostly fragments of asteroids and can be traced back to the different classes of asteroids that exist in the main asteroid belt . Their survival through the atmosphere depends on their initial mass , composition and speed of entry, and their superheated arrival means that they are typically smooth and encased in a black ‘fusion crust’. Meteorites are classified according to their composition and whether their parent body was differentiated or not. The three primary classes are: Stony meteorites are primarily composed of silicates, but up to 25% of their mass can come from iron and nickel. They make up about 95% of all the meteorites known to have fallen to Earth, and can be differentiated (achondrites) or non-differentiated (chondrites).

Chondrites are particularly interesting as they appear to have experienced very little change since their formation in the early Solar System. Iron meteorites are composed of iron and nickel and are thought to originate from the cores of differentiated asteroids. They are relatively easy to find since they look very different to normal rocks, and their metallic composition makes them extremely heavy and resistant to erosion. They are also usually larger than other meteorite types since they are less affected by ablation during their passage through the atmosphere. This is reflected in the statistic that although iron meteorites make up only 4% of the meteorites witnessed falling to Earth, they account for almost 20% of all of the meteorites found. Stony-iron meteorites are roughly equal parts silicate and iron-nickel by mass. They account for only about 1% of the meteorites known to have fallen to Earth and also originate from differentiated asteroids. A chondritic meteorite discovered in Antarctica. Where do meteors come from? | Reference.com Where do meteors come from? A: Quick Answer According to the Center for Meteorite Studies at the Arizona State University, meteorites are believed to come from the asteroid belt between Mars and Jupiter.

Hubblesite defines a meteorite as a meteor or a piece of meteor that makes it through the atmosphere and lands on earth. Full Answer According to Hubblesite, a meteor appears as a flash of light in the night sky when a piece of interplanetary debris burns up as it passes through the atmosphere. A meteor is not the debris itself but rather the light that people might see when the debris burns up. The debris is called a meteoroid. Most meteoroids (or meteorites when they land on earth) are thought to be approximately 4.5 billion years old. This is one way to tell that the meteorites come from the solar system because no Earth rocks are this old, according to the Center for Meteorite Studies at ASU. About 3,500 meteorites are believed to make it to the surface of Earth each year. However, only a small number of meteorites are recovered after a fall. A “fall” is defined as a meteorite that was observed falling and then later collected. A “find” is a meteorite that was not observed falling but was collected and identified as a meteorite after careful examination. BBC News - Does gold come from outer space? Does gold come from outer space?

By William Kremer BBC World Service 10 things we didn't know last week The idea that gold came from outer space sounds like science fiction, but it has become well-established - it's pretty much received opinion in the field of earth sciences. How did this bizarre theory take hold, and is it here to stay? For the chieftains of pre-Columbian America, the dazzling yellow stuff they found glinting at the bottom of streams or buried in the rocky ground captured the power of the sun god. They dressed themselves in battle armour wrought from the enchanted metal, believing it would protect them. They were sadly deceived. Gold, an unusually soft metal, wasn't any match for the steel of the Spanish. But the Native Americans may well have been right in believing the element was otherworldly. "Why do you find nuggets of gold on the surface of the Earth?" asks science writer John Emsley. "The answer to that, is that they've arrived here from space in the form of meteorites." This theory has come in the last few decades to be held by the majority of scientists as a way of explaining gold's abundance.

There may only be 1.3 grams of gold per 1,000 tonnes of other material in the Earth's crust (the rocky shell of the planet that is around 25 miles thick) but that's still too much to fit with the standard models of our planet's formation. How telescopes let us see into space After its birth four-and-a-half billion years ago, the surface of the Earth heaved with volcanoes and molten rock. Then, over tens of millions of years, most of the iron sank down through the outer layer, known as the mantle, to the Earth's core. Gold would have mixed with the iron and sunk with it. Matthias Willbold, a geologist at Imperial College London, likens the process to droplets of vinegar collecting at the bottom of a dish of olive oil. "All the gold should be gone," he says. It isn't though. So science has had to come up with an explanation, and the answer currently favoured is - a meteoric shower. "The theory is that after the core formed there was a meteoric shower that struck the Earth," says Willbold.

"These meteorites contained a certain amount of gold and that replenished the Earth's mantle and the continental crust with gold." Willbold says the theory fits with the pattern of meteorite activity as scientists understand it, climaxing with a huge storm that took place more than 3.8 billion years ago, referred to as the "terminal bombardment". The meteorites punched out the craters we see on the moon and came from an asteroid belt that still exists between Earth and Mars. This idea of the gold-laden-meteorite "veneer" was first proposed following the Apollo moon landings of the 1970s. Scientists examining rock samples from the moon's mantle found much less iridium and gold than they did in samples from the surface of the moon or from the earth's crust and mantle. It was proposed that the moon and Earth had been battered by iridium-rich meteorites, known as chondrites, from outer space. While the precious fallout from this meteoric shower lay scattered on the surface of the moon, on Earth the planet's internal activity had churned it into the mantle too. The idea, called the "late veneer hypothesis", has become a fundamental theory in planetary science.

It also helps to explain many other anomalies in the Earth's composition - it is thought that the same meteorites delivered the carbon, nitrogen, water and the amino acids that are vital to all life on the planet. "They are basically the building blocks of Earth," says Willbold. Two years ago, he and a team from the universities of Bristol and Oxford examined some rocks from Greenland which had their origins in a part of the Earth's mantle that was insulated from meteorite activity for a crucial period some 600 million years. The team did not look at the gold content of the 4.4-billion-year-old rocks, but at tungsten. Tungsten has some similarities to gold but exists in different forms or isotopes, and this provides scientists with more historical information. "The tungsten-isotopic composition of these rocks was basically really different from the tungsten-isotopic composition of other rocks," says Willbold. He infers that the Greenland rocks are a remnant of Earth's composition prior to the start of the late veneer meteorite shower, postulated to have taken place between 4.4 and 3.8 billion years ago.

More from BBC World Service Willbold's influential study, published in Nature in September 2011, provides the most compelling evidence yet for the late veneer hypothesis. This hypothesis seems the best explanation for the unusual tungsten-isotopic profile of Willbold's Greenland rocks, just as it seemed to explain the different quantities of gold and iridium in the mantles of the Earth and the Moon in the 1970s. But the hypothesis has been challenged. Last year, Mathieu Touboul and a team from the University of Maryland examined some different rocks, this time from Russia and significantly younger than those in the Greenland study - a mere 2.8 billion years old. These younger rocks had their full complement of elements known as siderophiles - the iron-loving group of metals that includes gold - but in terms of tungsten isotopes, the rocks turned out to be very similar to Willbold's. And yet they date from after the time proposed for the late veneer bombardment. "We reach a different conclusion about what is generating these tungsten anomalies inside the rocks," says Touboul. He thinks differences in the Earth's mantle might have caused tungsten isotopes to develop in different ways.

Touboul though still believes the late veneer hypothesis is right - he just doesn't think that tungsten isotope measurements provide a demonstration of it. Other scientists think it's time for a major rethink. BBC Space: Explore the Solar System "I used to accept the late veneer hypothesis back when we had so little data that it seemed to be a sensible interpretation, but I think it's past its prime now," says Munir Humayun of Florida State University. "It seemed so elegant, but there were so many gaps in the data. We presumed a lot and knew very little back then." Humayun says the original 1970s studies on moon and Earth rocks produced imprecise results, at variance with more sophisticated follow-up studies from the 1990s. One of these studies, from the University of Maryland, found less resemblance than expected between the Earth's rocks and chondrites - the iridium-rich meteorites. "This is where the late veneer failed in my opinion," says Humayun. "The answer came back that none of the known meteorite types were anything like the veneer." Scientists also began to find metals like gold much deeper in the Earth's mantle than they had anticipated.

This could be explicable if the Earth underwent a much bigger meteoric barrage than originally supposed, and at an earlier point in time. But the way Humayun sees it, the late veneer hypothesis stopped answering old questions - and started posing new ones. He is one of a small group of scientists who subscribe to an alternative theory. Their proposition is that all the gold in the Earth's crust - or the overwhelming majority of it - was here on Earth all along. Most of it certainly alloyed with iron and migrated to the Earth's core, but a significant proportion - perhaps 0.2% - dissolved into a 700km deep magma "ocean" within the Earth's outer mantle. Continue reading the main story “Start Quote Why the analytical community likes the idea (of a late veneer) so much is something that continues to trouble me” End Quote Munir Humayun, Florida State University Later, the gold was brought back up to the crust by volcanic action. This is the stuff we wear round our necks and on our fingers today. This theory requires gold and other siderophile elements to be more soluble than has previously been thought, otherwise insufficient quantities would have dissolved in the magma.

Experiments by two scientists at Nasa - Kevin Righter and Lisa Danielson - indicate that gold's solubility in mantle rocks does increase with high pressures and temperatures. However, it has not yet been possible to measure in a lab the solubility of all the highly siderophile elements over the full range of temperatures and pressures of the Earth's mantle, so for now this proposed explanation for the abundance of gold also remains no more than a hypothesis. But it is attracting interest and was bashed against the late veneer theory at length in a session last month at geochemistry's annual international symposium - the Goldschmidt Conference in Florence. Matthias Willbold, who attended the session, says the consensus in the room was that the late veneer hypothesis was still the best explanation for the unusual tungsten-isotopic profile of his Greenland rocks. He adds that, unlike Humayun, most scientists believe that chondritic meteorites are a "match" for concentrations of metals in the Earth's mantle and crust. But he says he accepts that the case for the late veneer hypothesis is not exactly sewn-up. "You can never be absolutely sure," he says.

"But the beauty of our model at the moment is that all the numbers match up very well." His isotope measurements indicate that about 0.5% of the Earth's mantle mass fell in the form of meteorites (that's 20 billion billion tonnes, if you were wondering). This figure matches geologists' current best guess, based on the overall concentrations of precious metals in the Earth's mantle and crust. Willbold describes this match as a "smoking gun". For now it remains unproven why there is just so much gold on Earth But Humayun says that the extent to which geochemists believe it depends on their precise field of study. Analytical geochemists - the group of researchers that measures trace elements in rocks - have come to see their research as crucial to understanding the emergence of life on Earth. Humayun says that experimental geochemists - the group of scientists attempting to recreate the conditions of the mantle in the lab - are more open-minded. "It's about how you make your money! If you're an experimentalist, then you're eating the late veneer guys' lunch by doing these experiments.

"Why the analytical community likes the idea (of a late veneer) so much is something that continues to trouble me. It's because of this relevance they have tied in to the origin of life. There's a lot riding on it!" The Why Factor is broadcast on BBC World Service on Fridays at 18:30 GMT. Listen to the gold episode via iPlayer or The Why Factor download . You can follow the Magazine on Twitter and on Facebook More on This Story Campaign posters warn women not to inflict smoke on their unborn children. Related Stories 08 SEPTEMBER 2011, SCIENCE & ENVIRONMENT Related Internet links Frequently Asked Questions + View the NASA Portal NOTICE: JPL's Center for NEO Studies (CNEOS), which operates neo.jpl.nasa.gov, will substantially upgrade the site in early 2017, giving it a new look-and-feel, improved navigation and added content. Scripts which extract data from HTML on the current site will have to be revised to use the related API on the new site. The new APIs are now functional, and specifics on them are now available at . Frequently Asked Questions What Is A Near-Earth Object (NEO)?

Near-Earth Objects (NEOs) are comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth's neighborhood. Composed mostly of water ice with embedded dust particles, comets originally formed in the cold outer planetary system while most of the rocky asteroids formed in the warmer inner solar system between the orbits of Mars and Jupiter. What Is The Purpose Of The Near-Earth Object Program? The purpose of the Near-Earth Object Program is to coordinate NASA-sponsored efforts to detect, track and characterize potentially hazardous asteroids and comets that could approach the Earth. With over 90% of the near-Earth objects larger than one kilometer already discovered, the NEO Program is now focusing on finding 90% of the NEO population larger than 140 meters. In addition to managing the detection and cataloging of Near-Earth objects, the NEO Program office will be responsible for facilitating communications between the astronomical community and the public should any potentially hazardous objects be discovered. What Are Asteroids And Comets? Asteroids and comets are believed to be ancient remnants of the earliest years of the formation of our solar system more than four billion years ago.

From the beginning of life on Earth to the recent spectacular impact of Comet Shoemaker-Levy 9 with Jupiter, these so-called "small bodies" play a key role in many of the fundamental processes that have shaped the planetary neighborhood in which we live. Comets are bodies of ice, rock, and organic compounds that can be several miles in diameter. Comets are thought to originate from a region beyond the orbits of the outermost planets. Scientists believe that gravitational perturbations periodically jar comets out of this population, setting these "dirty snowballs" on orbital courses that bring them closer to the Sun. Some, called long-period comets, are in elliptical orbits of the Sun that take them far out beyond the planets and back. Others, called short-period comets, travel in shorter orbits nearer the Sun. When comets venture into the more intense sunlight of the inner solar system, the ices in the comet nucleus begin to vaporize and fall away. The evolved gas forms a tenuous atmosphere around the nucleus called a coma, while the dust previously in the nucleus forms a tail that can be thousands of miles long and sometimes can be seen from Earth.

While striking the early Earth billions of years ago, comets are thought to have created major changes to Earth's early oceans, atmosphere, and climate, and may have delivered the first carbon-based molecules to our planet, triggering the process of the origins of life. Most asteroids are made of rock, but some are composed of metal, mostly nickel and iron. They range in size from small boulders to objects that are hundreds of miles in diameter. A small portion of the asteroid population may be burned-out comets whose ices have evaporated away and been blown off into space. Almost all asteroids are part of the Main Asteroid Belt, with orbits in the vast region of space between Mars and Jupiter. Some asteroids pass very close to Earth's orbit around the Sun. Scientists have found evidence that asteroids have hit our planet in the past. Usually, asteroids and smaller debris called meteoroids are too small to survive the passage through Earth's atmosphere. When these burn up on their descent, they leave a beautiful trail of light known as a meteor or "shooting star." Larger asteroids occasionally crash into Earth, however, and create craters, such as Arizona's kilometer-wide Meteor Crater near Flagstaff.

Another impact site off the coast of the Yucatan Peninsula in Mexico, which is buried by ocean sediments today, is believed to be a record of the event that led to the extinction of the dinosaurs 65 million years ago. Fortunately for us, these big asteroid impacts are rare. A smaller rocky meteoroid or comet less than 100 yards in diameter is believed to have entered the atmosphere over the Tunguska region of Siberia in 1908. The resulting shockwave knocked down trees for hundreds of square miles. What Are The Differences Between An Asteroid, Comet, Meteoroid, Meteor and Meteorite? In space, a large rocky body in orbit about the Sun is referred to as an asteroid or minor planet whereas much smaller particles in orbit about the Sun are referred to as meteoroids. Once a meteoroid enters the Earth's atmosphere and vaporizes, it becomes a meteor (i.e., shooting star). If a small asteroid or large meteoroid survives its fiery passage through the Earth's atmosphere and lands upon the Earth's surface, it is then called a meteorite. Cometary debris is the source of most small meteoroid particles.

Many comets generate meteoroid streams when their icy cometary nuclei pass near the Sun and release the dust particles that were once embedded in the cometary ices. These meteoroid particles then follow in the wake of the parent comet. Collisions between asteroids in space create smaller asteroidal fragments and these fragments are the sources of most meteorites that have struck the Earth's surface. Because they are readily available for study, many meteorites have already been subjected to detailed chemical and physical analyses in laboratories. If particular asteroids can be identified as the sources for some of the well-studied meteorites, a detailed knowledge of the meteorite's composition and structure will provide important information on the chemical mixture and conditions from which the parent asteroid formed 4.6 billion years ago. Summary Table Why Study Asteroids? The scientific interest in asteroids is due largely to their status as the remnant debris from the inner solar system formation process. Because some of these objects can collide with the Earth, asteroids are also important for having significantly modified the Earth's biosphere in the past. They will continue to do so in the future.

In addition, asteroids offer a source of volatiles and an extraordinarily rich supply of minerals that can be exploited for the exploration and colonization of our solar system in the twenty-first century. Asteroids represent the bits and pieces left over from the process that formed the inner planets, including Earth. Asteroids are also the sources of most meteorites that have struck the Earth's surface and many of these meteorites have already been subjected to detailed chemical and physical analyses. If certain asteroids can be identified as the sources for some of the well-studied meteorites, the detailed knowledge of the meteorite's composition and structure will provide important information on the chemical mixture, and conditions from which the Earth formed 4.6 billion years ago. During the early solar system, the carbon-based molecules and volatile materials that served as the building blocks of life may have been brought to the Earth via asteroid and comet impacts. Thus the study of asteroids is not only important for studying the primordial chemical mixture from which the Earth formed, these objects may hold the key as to how the building blocks of life were delivered to the early Earth. Every day, the Earth is bombarded with more than 100 tons of dust and sand-size particles.

Many of the incoming particles are so small that they are destroyed in the Earth's atmosphere before they reach the ground. These particles are often seen as meteors or shooting stars. The vast majority of all interplanetary material that reaches the Earth's surface originates as the collision fragments of asteroids that have run into one another some eons ago. With an average interval of about 10,000 years, rocky or iron asteroids larger than about 100 meters would be expected to reach the Earth's surface and cause local disasters or produce the tidal waves that can inundate low lying coastal areas. On an average of every several hundred thousand years or so, asteroids larger than a mile could cause global disasters. In this case, the impact debris would spread throughout the Earth's atmosphere so that plant life would suffer from acid rain, partial blocking of sunlight, and from the firestorms resulting from heated impact debris raining back down upon the Earth's surface. The probability of an asteroid striking the Earth and causing serious damage is very remote but the devastating consequences of such an impact suggests we should closely study different types of asteroids to understand their compositions, structures, sizes, and future trajectories.

The asteroids that are potentially the most hazardous because they can closely approach the Earth are also the objects that could be most easily exploited for raw materials. These raw materials could be used in developing the space structures and in generating the rocket fuel that will be required to explore and colonize our solar system in the twenty-first century. By closely investigating the compositions of asteroids, intelligent choices can be made as to which ones offer the richest supplies of raw materials. It has been estimated that the mineral wealth resident in the belt of asteroids between the orbits of Mars and Jupiter would be equivalent to about 100 billion dollars for every person on Earth today. Why Study Comets? Life on Earth began at the end of a period called the late heavy bombardment, some 3.8 billion years ago. Before this time, the influx of interplanetary debris that formed the Earth was so strong that the proto-Earth was far too hot for life to have formed. Under this heavy bombardment of asteroids and comets, the early Earth's oceans vaporized and the fragile carbon-based molecules, upon which life is based, could not have survived.

The earliest known fossils on Earth date from 3.5 billion years ago and there is evidence that biological activity took place even earlier - just at the end of the period of late heavy bombardment. So the window when life began was very short. As soon as life could have formed on our planet, it did. But if life formed so quickly on Earth and there was little in the way of water and carbon-based molecules on the Earth's surface, then how were these building blocks of life delivered to the Earth's surface so quickly? The answer may involve the collision of comets with the Earth, since comets contain abundant supplies of both water and carbon-based molecules. As the primitive, leftover building blocks of the outer solar system formation process, comets offer clues to the chemical mixture from which the giant planets formed some 4.6 billion years ago. If we wish to know the composition of the primordial mixture from which the major planets formed, then we must determine the chemical constituents of the leftover debris from this formation process - the comets. Comets are composed of significant fractions of water ice, dust, and carbon-based compounds.

Since their orbital paths often cross that of the Earth, cometary collisions with the Earth have occurred in the past and additional collisions are forthcoming. It is not a question of whether a comet will strike the Earth, it is a question of when the next one will hit. It now seems likely that a comet or asteroid struck near the Yucatan peninsula in Mexico some 65 million years ago and caused a massive extinction of more than 75% of the Earth's living organisms, including the dinosaurs. Comets have this strange duality whereby they first brought the building blocks of life to Earth some 3.8 billion years ago and subsequent cometary collisions may have wiped out many of the developing life forms, allowing only the most adaptable species to evolve further. Indeed, we may owe our preeminence at the top of Earth's food chain to cometary collisions. A catastrophic cometary collision with the Earth is only likely to happen at several million year intervals on average, so we need not be overly concerned with a threat of this type. However, it is prudent to mount efforts to discover and study these objects, to characterize their sizes, compositions and structures and to keep an eye upon their future trajectories.

As with asteroids, comets are both a potential threat and a potential resource for the colonization of the solar system in the twenty first century. Whereas asteroids are rich in the mineral raw materials required to build structures in space, the comets are rich resources for the water and carbon-based molecules necessary to sustain life. In addition, an abundant supply of cometary water ice can provide copious quantities of liquid hydrogen and oxygen, the two primary ingredients in rocket fuel. One day soon, comets may serve as fueling stations for interplanetary spacecraft. What Are Atiras, Atens, Apollos and Amors? Atiras, Atens, Apollos and Amors are subgroups of Near-Earth asteroids, and are categorized by their orbits. In terms of orbital elements, NEOs are asteroids and comets with perihelion distance q less than 1.3 AU. The vast majority of NEOs are asteroids, referred to as Near-Earth Asteroids (NEAs). NEAs are further divided into the following groups according to their perihelion distance (q), aphelion distance (Q) and their semi-major axes (a): Group What Is A Potentially Hazardous Asteroid (PHA)?

Potentially Hazardous Asteroids (PHAs) are currently defined based on parameters that measure the asteroid's potential to make threatening close approaches to the Earth. Specifically, all asteroids with a minimum orbit intersection distance ( MOID ) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or less are considered PHAs. In other words, asteroids that can't get any closer to the Earth (i.e. MOID) than 0.05 AU (roughly 7,480,000 km or 4,650,000 mi) or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%) are not considered PHAs. This ``potential'' to make close Earth approaches does not mean a PHA will impact the Earth. It only means there is a possibility for such a threat. By monitoring these PHAs and updating their orbits as new observations become available, we can better predict the close-approach statistics and thus their Earth-impact threat. Comet And Asteroid Mission Summary Table NEAR

Question: According to NASA where do the majority of meteorites found on Earth originate from?
Answer: Asteroids