Meteoroids, Meteors, Meteorites . . . What’s the Difference?
Meteoroids are small particles — often no bigger than a grain of sand — that orbit our Sun. When meteoroids enter Earth’s atmosphere, they produce brilliant streaks of light that can be seen in our sky. These brief streaks of light — and the particles that are moving through our atmosphere — are meteors. Meteorites are rocks from space that actually have landed on Earth’s — or another planet’s — surface.
How Are Asteroids and Comets Related to Meteorites?
Asteroids are rocky bodies, less than 1000 kilometers across, that orbit our Sun. Asteroids occur in the asteroid belt between Mars and Jupiter. Comets are masses of ice and dust, less than 10 kilometers across, that usually stay in the cold outer reaches of our solar system. Meteoroids are small pieces of asteroids or comets.
Where Do Meteorites Come From?
Most meteorites appear to come from asteroids. This is based on a comparison of the composition of meteorites with our understanding of the composition of asteroids, based on remote sensing. It also is based on a comparison of the orbits of asteroids and the orbits of meteoroids, calculated from photographs of the meteoroids as they approached Earth. A few meteorites are from the Moon and Mars. These are pieces of the planets that were broken off and knocked into orbit when asteroids struck the planets. Meteorites from the Moon are similar to the samples collected by the Apollo astronauts. The Mars meteorites include sealed pockets of gas that scientists discovered contain the same gases as occur in the atmosphere of Mars.
Comets as Meteoroid Sources
Rarely, meteorites may also come from comets. Comets have been called “dirty snowballs” because their nucleus — their solid core — consists mostly of ice with a bit of dust, rock particles, and a little organic material mixed in. Most comets are found at the outer edge of the solar system — beyond the orbit of Pluto — in a region called the Kuiper belt. Some comets reside even farther away in a large spherical cloud around our solar system called the Oort cloud. Comets are so far away from the Sun that they remain frozen; they are important relics from the earliest times of our solar system. Some comets do orbit our Sun in periodic, elliptical paths. Comets are nearly invisible except when they get close to the Sun. Heat from the Sun vaporizes the ice on the comet's surface causing gas and dust to flow away and form the cloud of the coma. The solar wind — the flow of particles out from the Sun — sweeps the coma away into a long tail. The tail always points away from the Sun because of the solar wind, no matter what direction the comet is moving in its orbit. The tail actually has twin pieces, a gas tail and a dust tail, that can extend for millions of kilometers from the comet nucleus as it travels around the Sun. As the comet gets very close to the Sun, small pieces of dust, rock grains, and ice are left behind as a trail of meteoroids.
Why Do We Have Meteor Showers?
Meteor showers occur when Earth passes through the trail of dust and gas left by a comet. The particles enter Earth’s atmosphere and most burn up in a lively light show — a meteor shower. Some meteor showers, like the Perseids and the Leonids, occur annually when Earth’s orbit takes our planet through the debris path left along the comet’s orbit. For upcoming meteor showers and viewing suggestions, explore Sky and Telescope’s Meteor Showers page.
What are Meteorites Made Of?
Scientists classify meteorites into three groups: stony meteorites, iron meteorites, and stony iron meteorites.
Stony meteorites make up about 95% of the meteorites reaching Earth. Stony meteorites include chondrites and achondrites. Chondrites contain small spheres of silicate minerals called chondrules. There also are carbonaceous chondrites — stony meteorites that contain water and organic (carbon) molecules such as simple amino acids. Achondrites are also stony meteorites, but they do not have chondrules and they have undergone heating and change. Achondrites include meteorites from our Moon and Mars.
Iron meteorites make up about 5% of the meteorites found on Earth. These have high amounts of iron and nickel. Iron meteorites are very heavy!
Stony-iron meteorites are in between the other two types of meteorites. These are rare — only about 1% of the meteorite finds on Earth are stony iron meteorites.
What Do Meteorites Tell Us?
Meteorites provide us with information about the processes and materials in our early solar system. The early solar system did not consist of a sun and planets. It was a spinning cloud of dust and hydrogen gas that was hotter in the center and cooler toward the edges. As the gas and dust began to come together, chondrules — tiny spheres of minerals containing silica — condensed. These tiny spheres and dust gradually grew as other particles collided with them and became attached — a process called accretion. Some of the particles grew to the point that they were large enough to gravitationally attract other particles, and they accreted all the material in their path as they orbited the young Sun — some of these became our planets. Other particles remained small, space rocks left behind after the planets formed. Accretion is a hot process; when a particle slams into another particle, its motion is converted to heat. The planets and some of the space rocks became so hot that they began to change, in some cases melting. Melting allowed the bodies to differentiate, with the heavier metals of iron and nickel sinking into a central core, and the lighter materials making a mantle and outer crust.
Chondrites are meteorites that contain chondrules. Most chondrites were heated and changed early in their formation. However, some chondrites have not changed since they formed. These chondrites provide scientists with essentially unaltered samples of our early solar system. They also help us determine the age of our solar system; chondrites are between 4.5 and 4.56 billion years old.
Carbonaceous chondrites are also very old samples of our solar system. They contain water in some of their minerals and organic compounds. Carbonaceous chondrites provide scientists with more complete samples of the chemical composition of our early solar system.
Achondrites, iron meteorites, and stony iron meteorites have different compositions. These come from bodies — planets and asteroids — in our solar system that were heated and altered, and in some cases melted. The iron meteorites come from the metallic cores of asteroids. Achondrites may be from the crust. Stony meteorites are from the mantle, between the iron core and the crust. All of these meteorites provide information about the composition of the bodies in our solar system, and about the processes that have shaped it. The “differentiated” meteorites often have ages of about 4.4 to 4.5 billion years, which tells scientists that differentiation of the asteroids took place early in the history of our solar system.
Some of the achondrites come from the Moon and Mars and some of these are much younger. These are basalts — dark fine-grained volcanic rocks — and they help us understand that there were volcanos erupting on these bodies, as well as give us a timeframe for the eruptions. We know, for example, that in the last 180 million years, volcanos were erupting on Mars.
What Happens to a Meteoroid On Its Way to Earth?
Not much when it is in space. When the meteoroid enters Earth’s atmosphere, things begin to heat up! Actually, it is the air in front of the meteoroid that heats up. The particle is traveling at speeds between 20 and 30 kilometers per second. It compresses the air in front, causing the air to get hot. The air is so hot it begins to glow — creating a meteor - the streak of light observed from Earth. The intense heat also melts the outside of the meteoroid. The trip through Earth’s atmosphere is fast enough that the inside of a meteoroid often is not heated at all. However, for most rocks from space, even the short trip is sufficient to melt away much of it; a meter-sized meteoroid can be reduced to the size of a baseball. Small meteoroids are vaporized completely. The atmosphere becomes thicker as the meteoroid gets closer to Earth’s surface, causing the rock to slow and cool. The outer melted part of the meteoroid solidifies, leaving a fusion crust — a thin dark glassy rind. Some meteoroids break up just before they reach Earth’s surface, creating a fireball accompanied by an explosion that can be heard kilometers away.
The impact from a large meteoroid striking the surface may leave a crater — a circular depression. Large meteoroids leave craters about 10 times their size, although the size depends on how fast the meteoroid is moving, its angle of approach, and other factors. Meteor Crater was formed about 50,000 years ago when the 30-meter-wide Canyon Diablo meteorite struck the ground, creating a kilometer-wide depression in Arizona.
Large impacts are rare now, but were much more common during the early history of our solar system when the space debris was being swept up. The surfaces of Mercury, the Moon, and Mars are covered with impact craters, most of which scientists believe formed during the first half billion years of solar system formation. Earth also has several impact craters on its surface, some quite large. One of the most famous — and destructive — impacts believed to have occurred took place about 65 million years ago. A meteroid, 10–16 kilometers in diameter, struck Earth near what is now the Yucatán Peninsula of Mexico. This impact is thought to have triggered global fires and tsunamis and created a cloud of dust and water vapor that enveloped the Earth in a matter of days, resulting in fluctuating global climate changes. The extreme environmental shifts are believed to have caused a mass extinction of 75% of Earth’s species, including the dinosaurs.
Where Do We Find Meteorites?
Meteorites are fairly indiscriminate about where they land. They fall everywhere on Earth. Finding them is the challenge! A little more than two-thirds of Earth is covered by water; locating a meteorite on the deep sea floor is difficult, to say the least. Meteorites also fall in unpopulated regions and places that are difficult to reach. There are a few places where scientists concentrate their efforts because the meteorites are easier to find. Desert areas are not covered by vegetation, and meteorites differ from the background. Many meteorite expeditions in the deserts of Africa and Australia have increased the collections under study. There is one desert that has provided the most meteorites — the polar desert of Antarctica! There are several reasons why Antarctica is such a spectacular collecting place. The first is that the dark meteorites are easy to see against the white ice! In addition, meteorites do not break down as quickly in the frozen, dry atmosphere. The movement of the ice covering Antarctica also helps in the search for meteorites. Meteorites that land on the surface of the ice sheet are carried along by the ice flow. There are locations where mountains act as a barrier to ice movement. The ice flows up along this barrier and is sublimated — evaporated — by the fast dry winds of Antarctica. The meteorites do not evaporate — they are left behind. This process of ice flow and sublimation has continued for thousands of years, concentrating the meteorites into distinct patches. Collecting expeditions in Antarctica have nearly doubled the number of meteorite finds in the world.
Should I Lose Sleep Worrying that I May Be Hit by A Meteorite?
Uh, no. To date, no person has been killed by being hit by a meteroid (or at least no one claims to have been!). There are, however, a few cases of cars and houses being hit, and a few near misses. In 1954 a meteroid struck a home in Alabama, passed through the roof, and bounced into the living room, hitting and bruising the occupant, who was napping on the couch. In 1992 a meteroid passed through the trunk of a parked car in New York, coming to rest under the car. There are many more unsubstantiated cases!
An interesting tidbit:
How Are Meteorites Named?
Well, not by their parents. Most meteorites are named after the nearest town — like the Noblesville meteorite of Noblesville, Indiana. If there is not a town close by, they may be named after a geographic feature such as a river or mountain. In places where lots of meteorites are found, like a desert (including Antarctica!), the meteorites are usually given a random number in the field and then later, once they are described, the number is replaced by a final “name.” The name includes a geographic designation, the year of the find, and a sample number. For example, ALH 84001 was collected during the 1984 (84) collecting season in Antarctica near the Allan Hills (ALH). It was the first described in the laboratory for that season (001).
Thanks to Dr. Kevin Righter, Planetary Scientist, Astromaterials Research and Exploration Science Program,
NASA Johnson Space Center, for reviewing the content material.
January 9, 2007