Asteroids

Black and white image of asteroid Eros.

This composite image shows the comparative sizes of eight asteroids. Up until now, Lutetia, with a diameter of 81 miles (130 kilometers), was the largest asteroid visited by a spacecraft, which occurred during a flyby.  Vesta, which is also considered a protoplanet because it's a large body that almost became a planet, dwarfs all other small bodies in this image, with its diameter sizing up at approximately 330 miles (530 kilometers). Image credit: NASA/JPL-Caltech/JAXA/ESA

Asteroids: 10 Need-To-Know Things

  1. If all of the asteroids were combined into a ball, they would still be much smaller than the Earth's moon. If the sun was as tall as a typical front door, Earth would be the size of a nickel, the moon would be about as big as a green pea and Ceres (the largest asteroid) would be as small as a sesame seed.
  2. Asteroids orbit our sun, a star, in a region of space between the orbits of Mars and Jupiter known as the Asteroid Belt.
  3. One day on asteroid Ida, for example, takes only 4.6 hours (the time it takes for this asteroid to rotate or spin once). Ida makes a complete orbit around the sun (a year in this asteroid's time) in 4.8 Earth years.
  4. Asteroids are solid, rocky and irregular bodies.
  5. Asteroids do not have atmospheres.
  6. More than 150 asteroids are known to have a small companion moon (some have two moons). The first discovery of an asteroid-moon system was of asteroid Ida and its moon Dactyl in 1993.
  7. Asteroids do not have rings.
  8. NASA space missions have flown by and observed asteroids, and one spacecraft (NEAR Shoemaker) even landed on an asteroid (Eros). The Dawn mission is the first mission to orbit (2011) a main belt asteroid (Vesta).
  9. Asteroids cannot support life as we know it.
  10. Fact: Ceres, the first and largest asteroid to be discovered (1801 by Giuseppe Piazzi), encompasses over one-third of the estimated total mass of all the asteroids in the asteroid belt.

Asteroids: Overview: Ancient Space Rubble

Asteroids are rocky, airless worlds that orbit our sun, but are too small to be called planets. Tens of thousands of these minor planets are gathered in the main asteroid belt, a vast doughnut-shaped ring between the orbits of Mars and Jupiter. Asteroids that pass close to Earth are called near-earth objects.


Asteroids: In Depth

Color image of a potato-shaped asteroid.
This image, taken by NASA's Near Earth Asteroid Rendezvous mission in 2000, shows a close-up view of Eros, an asteroid with an orbit that takes it somewhat close to Earth. NASA's Spitzer Space Telescope observed Eros and dozens of other near-Earth asteroids.Image Credit: NASA/JHUAPL
black and white image of spherical asteroid.
A mosaic of the best views of the giant asteroid Vesta.
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Asteroids, sometimes called minor planets, are rocky remnants left over from the early formation of our solar system about 4.6 billion years ago.

Most of this ancient space rubble can be found orbiting the sun between Mars and Jupiter within the main asteroid belt. Asteroids range in size from Vesta - the largest at about 329 miles (530 kilometers) in diameter - to bodies that are less than 33 feet (10 meters) across. . The total mass of all the asteroids combined is less than that of Earth's Moon.

Editor's note: Even with more than one-half million asteroids known (and there are probably many more), they are still much more widely separated than sometimes seen in Hollywood movies: on average, their separation is in excess of 1-3 million km (depending on how one calculates it).

Most asteroids are irregularly shaped, though a few are nearly spherical, and they are often pitted or cratered. As they revolve around the sun in elliptical orbits, the asteroids also rotate, sometimes quite erratically, tumbling as they go. More than 150 asteroids are known to have a small companion moon (some have two moons). There are also binary (double) asteroids, in which two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems.

Color image of an asteroid and its tiny moon.
The Galileo spacecraft found asteroids can have moons.
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The three broad composition classes of asteroids are C-, S-, and M-types. The C-type (chondrite) asteroids are most common, probably consist of clay and silicate rocks, and are dark in appearance. They are among the most ancient objects in the solar system. The S-types ("stony") are made up of silicate materials and nickel-iron. The M-types are metallic (nickel-iron). The asteroids' compositional differences are related to how far from the sun they formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface. Only one such asteroid, Vesta, survives to this day.

Jupiter's massive gravity and occasional close encounters with Mars or another object changed the asteroids' orbits, knocking them out of the main belt and hurling them into space in both directions towards or away from the sun, across the orbits of the planets. Stray asteroids and asteroid fragments have slammed into Earth and the other planets in the past, playing a major role in altering the geological history of the planets and in the evolution of life on Earth.

Scientists continuously monitor Earth-crossing asteroids, whose paths intersect Earth's orbit, and near-Earth asteroids that approach Earth's orbital distance to within about 45 million kilometers (28 million miles) and may pose an impact danger. Radar is a valuable tool in detecting and monitoring potential impact hazards. By reflecting transmitted signals off objects, images and other information can be derived from the echoes. Scientists can learn a great deal about an asteroid's orbit, rotation, size, shape, and metal concentration.

Illustration that shows the locations of many asteroids.
A snapshot of near-Earth asteroids.
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Several missions have flown by and observed asteroids. The Galileo spacecraft flew by asteroids Gaspra in 1991 and Ida in 1993; the Near-Earth Asteroid Rendezvous (NEAR-Shoemaker) mission studied asteroids Mathilde and Eros; and the Rosetta mission encountered Steins in 2008 and Lutetia in 2010. Deep Space 1 and Stardust both had close encounters with asteroids.

In 2005, the Japanese spacecraft Hayabusa landed on the near-Earth asteroid Itokawa and attempted to collect samples. On June 3, 2010, Hayabusa successfully returned to Earth a small amount of asteroid dust now being studied by scientists.

NASA's Dawn spacecraft, launched in 2007, orbited and explored asteroid Vesta for over a year. Once it left in September 2012, it headed towards dwarf planet Ceres, with a planned arrival of 2015. Vesta and Ceres are two of the largest surviving protoplanet bodies that almost became planets. By studying them with the same complement of instruments on board the same spacecraft, scientists will be able to compare and contrast the different evolutionary path each object took to help understand the early solar system overall.


Asteroid Classifications

Main asteroid belt: The majority of known asteroids orbit within the asteroid belt between Mars and Jupiter, generally with not very elongated orbits. The belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of smaller ones. Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today.

Trojans: These asteroids share an orbit with a larger planet, but do not collide with it because they gather around two special places in the orbit (called the L4 and L5 Lagrangian points). There, the gravitational pull from the sun and the planet are balanced by a trojan's tendency to otherwise fly out of the orbit. The Jupiter trojans form the most significant population of trojan asteroids. It is thought that they are as numerous as the asteroids in the asteroid belt. There are Mars and Neptune trojans, and NASA announced the discovery of an Earth trojan in 2011.

Near-Earth asteroids: These objects have orbits that pass close by that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. As of June 19, 2013, 10,003 near-Earth asteroids are known and the number over 1 kilometer in diameter is thought to be 861, with 1,409 classified as potentially hazardous asteroids - those that could pose a threat to Earth.


How Asteroids Get Their Names

The International Astronomical Union's Committee on Small Body Nomenclature is a little less strict when it comes to naming asteroids than other IAU naming committees. So out there orbiting the sun we have giant space rocks named for Mr. Spock (a cat named for the character of "Star Trek" fame), rock musician Frank Zappa, regular guys like Phil Davis, and more somber tributes such as the seven asteroids named for the crew of the Space Shuttle Columbia killed in 2003. Asteroids are also named for places and a variety of other things. (The IAU discourages naming asteroids for pets, so Mr. Spock stands alone).

Asteroids are also given a number, for example (99942) Apophis. The Harvard Smithsonian Center for Astrophysics keeps a fairly current list of asteroid names.


Asteroids: Significant Dates

  • 1801: Giuseppe Piazzi discovers the first and largest asteroid, Ceres, orbiting between Mars and Jupiter.
  • 1898: Gustav Witt discovers Eros, one of the largest near-Earth asteroids.
  • 1991-1994: The Galileo spacecraft takes the first close-up images of an asteroid (Gaspra) and discovers the first moon (later named Dactyl) orbiting an asteroid (Ida).
  • 1997-2000 : The NEAR Shoemaker spacecraft flies by Mathilde and orbits and lands on Eros.
  • 1998: NASA establishes the Near Earth Object Program Office to detect, track and characterize potentially hazardous asteroids and comets that could approach Earth.
  • 2006: Japan's Hayabusa becomes the first spacecraft to land on and take off from an asteroid. It also returned the first asteroid samples to Earth four years later.
  • 2006: Ceres attains a new classification -- dwarf planet -- but retains its distinction as the largest known asteroid.
  • 2007: The Dawn spacecraft is launched on its journey to the asteroid belt to study Vesta and Ceres.
  • 2008: The European spacecraft Rosetta, on its way to study a comet in 2014, flies by and photographs asteroid Steins, a type of asteroid composed of silicates and basalts.
  • 2010: Japan's Hayabusa returns its asteroid sample to Earth.
  • 2010: Rosetta flies by asteroid Lutetia, revealing a primitive survivor from the violent birth of our solar system.
  • 2011-2012: Dawn studies Vesta. Dawn is the first spacecraft to orbit a main-belt asteroid and continues on to dwarf planet Ceres in 2015.

Asteroids: Trivia

Queen of the Asteroids
With a diameter of about 600 miles (966 kilometers), Ceres is the undisputed ruler of the asteroid belt. The dwarf planet accounts for a quarter of all the mass of all the known asteroids in the main asteroid belt.

First Moon
While studying asteroid 243 Ida in 1993, Jupiter-bound Galileo spotted a surprise - a tiny moon. Scientists named their discovery - the first known moon orbiting an asteroid - Dactyl.

Cosmic Small Change
If you could lump together the more than 500,0000 asteroids in orbit between Jupiter and Saturn, their total mass wouldn't even equal that of Earth's moon.

Space in Space
The asteroid belt is not as tightly packed as we see in the movies. Asteroids are actually far apart - on aver more than 1.2 million miles (2 million kilometers).

Cosmic Crash
In 2010, astronomers using the Hubble Space Telescope spotted what appears to be the first collision between two asteroids. Such a smash-up had never been seen before.

Tiny Moons
More than 150 of the asteroids are known to have a small companion moon. Some even have two moons.

Bright Flash
About once a year an car-sized asteroid enters Earth's atmosphere, creates an impressive fireball and disintegrates before ever reaching the surface.

Touchdown
After completing it's mission, NASA's NEAR-Shoemaker spacecraft descended to surface of asteroid Eros - the first soft landing on an asteroid.

Dino-Killer
It is possible that a gigantic asteroid impacted the Earth and led the extinction of the dinosaurs.

Impact
About 20,000 to 50,000 years ago, a small asteroid about 80 feet (24 meters) in diameter smacked into what is now Arizona and formed Meteor crater. Measuring about 4,000 feet in diameter (1.2 kilometers), Barringer Crater is the best preserved impact crater on Earth.

Two Times One Equals ... Binary
We've found double systems of stars, planets and even asteroids. Binary asteroids are two rocky bodies of roughly equal size orbiting each other. We've even found triple asteroid systems.

 



Comets

This four-image mosaic comprises images taken from the Rosetta spacecraft at a distance of 28.0 km from the centre of Comet 67P/Churyumov-Gerasimenko on 31 January.
This four-image mosaic comprises images taken from the Rosetta spacecraft at a distance of 28.0 km from the centre of Comet 67P/Churyumov-Gerasimenko on 31 January. Credit: ESA/Rosetta/NAVCAM

Comets: 10 Need-To-Know Things

  1. If the sun were as tall as a typical front door, Earth would be the size of a nickel, dwarf planet Pluto would be the size of a head of a pin and the largest Kuiper Belt comet (about 100 km across, which is about one twentieth the size of Pluto) would only be about the size of a piece of dust.
  2. Short-period comets (comets that orbit the sun in less than 200 years) reside in the icy region known as the Kuiper Belt beyond the orbit of Neptune from about 30 to 55 AU. (When comet Halley is at its farthest distance from the sun, it is 5.3 billion km away (3.3 billion miles) or 35 AU.) Long-period comets (comets with long, unpredictable orbits) originate in the far-off reaches of the Oort Cloud, which is five thousand to 100 thousand AUs from the sun.
  3. One day on comet Halley varies between 2.2 to 7.4 Earth days (the time it takes for comet Halley to rotate or spin once). Comet Halley makes a complete orbit around the sun (a year in this comet's time) in 76 Earth years.
  4. Comets are cosmic snowballs of frozen gases, rock and dust.
  5. A comet warms up as it nears the sun and develops an atmosphere, or coma. The coma may be hundreds of thousands of kilometers in diameter.
  6. Comets do not have moons.
  7. Comets do not have rings.
  8. Several missions have visited impacted and even collected samples from comets. Two recent missions are Stardust-NExT and Deep Impact EPOXI.
  9. Comets may not be able to support life themselves, but they may have brought water and organic compounds -- the building blocks of life -- through collisions with the Earth and other bodies in our solar system.
  10. Fact: Comet Halley makes an appearance in the Bayeux tapestry from the year 1066, which chronicles the overthrow of King Harold by William the Conqueror at the Battle of Hastings.

Comets: Overview

Comets are cosmic snowballs of frozen gases, rock and dust roughly the size of a small town. When a comet's orbit brings it close to the sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the sun for millions of kilometers.


Comets: In Depth

Black and white image showing bursts of particles from a comet nucleus.
A surprising view of active jets on the unusual comet Hartley 2.
Color image of comet and massive tail over the ocean.
Comet McNaught setting over the Pacific Ocean.
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In the distant past, people were both awed and alarmed by comets, perceiving them as long-haired stars that appeared in the sky unannounced and unpredictably. Chinese astronomers kept extensive records for centuries, including illustrations of characteristic types of comet tails, times of cometary appearances and disappearances, and celestial positions. These historic comet annals have proven to be a valuable resource for later astronomers.

We now know that comets are leftovers from the dawn of our solar system around 4.6 billion years ago, and consist mostly of ice coated with dark organic material. They have been referred to as "dirty snowballs." They may yield important clues about the formation of our solar system. Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system.

Black and white image of a comet with surface details labeled.
Details of comet Tempel 1.
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As theorized by astronomer Gerard Kuiper in 1951, a disc-like belt of icy bodies exists beyond Neptune, where a population of dark comets orbits the sun in the realm of Pluto. These icy objects, occasionally pushed by gravity into orbits bringing them closer to the sun, become the so-called short-period comets. Taking less than 200 years to orbit the sun, in many cases their appearance is predictable because they have passed by before. Less predictable are long-period comets, many of which arrive from a region called the Oort Cloud about 100,000 astronomical units (that is, 100,000 times the distance between Earth and the Sun) from the Sun. These Oort Cloud comets can take as long as 30 million years to complete one trip around the Sun.

Each comet has a tiny frozen part, called a nucleus, often no larger than a few kilometers across. The nucleus contains icy chunks, frozen gases with bits of embedded dust. A comet warms up as it nears the Sun and develops an atmosphere, or coma. The sun's heat causes the comet's ices to change to gases so the coma gets larger. The coma may extend hundreds of thousands of kilometers. The pressure of sunlight and high-speed solar particles (solar wind) can blow the coma dust and gas away from the Sun, sometimes forming a long, bright tail. Comets actually have two tails - a dust tail and an ion (gas) tail.

Most comets travel a safe distance from the sun - comet Halley comes no closer than 89 million kilometers (55 million miles). However, some comets, called sungrazers, crash straight into the Sun or get so close that they break up and evaporate.

Color image showing gas and material erupting from comet nucleus.
Comet Halley in 1986.
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Scientists have long wanted to study comets in some detail, tantalized by the few 1986 images of comet Halley's nucleus. NASA's Deep Space 1 spacecraft flew by comet Borrelly in 2001 and photographed its nucleus, which is about 8 kilometers (5 miles) long.

NASA's Stardust mission successfully flew within 236 kilometers (147 miles) of the nucleus of Comet Wild 2 in January 2004, collecting cometary particles and interstellar dust for a sample return to Earth in 2006. The photographs taken during this close flyby of a comet nucleus show jets of dust and a rugged, textured surface. Analysis of the Stardust samples suggests that comets may be more complex than originally thought. Minerals formed near the Sun or other stars were found in the samples, suggesting that materials from the inner regions of the solar system traveled to the outer regions where comets formed.

Another NASA mission, Deep Impact, consisted of a flyby spacecraft and an impactor. In July 2005, the impactor was released into the path of the nucleus of comet Tempel 1 in a planned collision, which vaporized the impactor and ejected massive amounts of fine, powdery material from beneath the comet's surface. En route to impact, the impactor camera imaged the comet in increasing detail. Two cameras and a spectrometer on the flyby spacecraft recorded the dramatic excavation that helped determine the interior composition and structure of the nucleus.

After their successful primary missions, the Deep Impact spacecraft and the Stardust spacecraft were still healthy and were retargeted for additional cometary flybys. Deep Impact's mission, EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), comprised two projects: the Deep Impact Extended Investigation (DIXI), which encountered comet Hartley 2 in November 2010, and the Extrasolar Planet Observation and Characterization (EPOCh) investigation, which searched for Earth-size planets around other stars on route to Hartley 2. NASA returned to comet Tempel 1 in 2011, when the Stardust New Exploration of Tempel 1 (NExT) mission observed changes in the nucleus since Deep Impact's 2005 encounter.


How Comets Get Their Names

Comet naming can be complicated. Comets are generally named for their discoverer -- either a person or a spacecraft. This International Astronomical Union guideline was developed only in the last century. For example, comet Shoemaker-Levy 9 was so named because it was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. Since spacecraft are very effective at spotting comets many comets have LINEAR, SOHO or WISE in their names.


Comets: Significant Dates

  • 1070-1080: The comet later designated Halley's Comet is pictured in the Bayeux Tapestry, which chronicles the Battle of Hastings of 1066.
  • 1449-1450: Astronomers make one of the first known efforts to record the paths of comets across the night sky.
  • 1705: Edmond Halley determines that the comets of 1531, 1607 and 1682 are the same comet and predicts its return in 1758. The comet arrives on schedule and is later named Halley's Comet.
  • 1986: An international fleet of five spacecraft converges on comet Halley as it makes its regular (about every 76 years) pass through the inner solar system.
  • 1994: In the first observed planetary impact by a comet, awed scientists watch as fragments of comet Shoemaker-Levy 9 smash into Jupiter's atmosphere.
  • 2001: Deep Space 1 flies by and photographs comet Borrelly.
  • 2004: NASA's Stardust spacecraft collects dust samples from comet Wild 2 and images the nucleus.
  • 2005: The Deep Impact impactor collides with comet Tempel 1 to reveal the interior of the nucleus.
  • 2006: The Stardust sample return capsule lands in Utah carrying cometary particles and interstellar dust.
  • 2009: Scientists announce that the amino acid glycine, a building block of life, was collected by the Stardust spacecraft from comet Wild 2.
  • 2010: The Deep Impact spacecraft studies its second cometary target, Hartley 2, a small, hyperactive comet.
  • 2011: The Stardust spacecraft encounters Tempel 1 and captures views of the Deep Impact impact site, the opposite side of the nucleus, and evolution on the comet's surface.

Comets: Trivia

Survivor
Sungrazer comets are comets that come so close to the sun that they vaporize and disappear -- never to be seen again. However, in December of 2011 one comet (Lovejoy) was the first spacecraft-viewed comet to graze the sun and survive to tell the tale.

Hang On
If you weigh 45 kg (100 lb.) on Earth, you'd weigh only about 0.005 kg (0.01 lb.) on a comet. At that weight, you could easily jump right off into space (although we don't recommend it ). If you rode on a comet close to the sun, you'd probably get blown off into space on a jet of dust and gas.

Darkest
A comet's nucleus is very dark -- as dark as coal or asphalt, which are some of the darkest materials on Earth. This makes a comet's nucleus one of the darkest things in our solar system.

Small Town / Big State
The size of a comet is no small matter: Did you know that the average size of a comet is as big as an entire town? Now, imagine a comet as an average town and the Kuiper Belt (where comets reside and come from) as a state, but this state would be gigantic. The Kuiper Belt expands from about 30 to 55 AU, which is about 3.7 billion km (2.3 billion miles) long. The circumference of the entire Earth is only about 40 thousand km (25 thousand miles).

Killer Comet?
Were the dinosaurs wiped out by a comet impact 65 million years ago? It's possible. In the earliest days of the solar system, comets regularly bombarded the planets.

One Tough Robot
No one expected Europe's Giotto spacecraft to survive the beating it took from dust and rocks when it made a close pass by comet Halley in 1986. Damaged, but far from dead, Giotto flew on to study a second comet.

Swell Comet
When they are far from the sun, most comets are insignificant specks less than 10 km (6 miles) across. But when a comet gets close to the sun, the cloud of gases surrounding it can swell larger than the size of Jupiter -- more than 10 times the diameter of Earth. Comets also sprout beautiful tails that stretch for millions of kilometers away from the sun.

Thar She Blows
Comet Hale-Bopp spewed out about 250 tons of gas and dust per second -- more than 50 times what most other comets churn out. It made for a spectacular show as it passed through the inner solar system in 1997.

Don't Wait Up
Comet Hale-Bopp won't return to our skies for about 2,740 years -- and that's not even close to the longest orbit comets make around our sun. Some far travelers take millions of years to make one orbit. Others, such as comet Encke, run by the sun every few years.

Comet Collision
Creating space fireworks just in time for the Fourth of July (2005), Deep Impact flew by and sent an impactor into the path of the nucleus of comet Tempel 1. Deep Impact was the first mission to collide with and eject material from a comet.

 



Meteors & Meteorites

Color image of meteorite on Mars.
NASA's Opportunity rover found this meteorite on Mars. It is about the size of a basketball.

Meteors & Meteorites: 10 Need-To-Know Things

  1. Meteorites may vary in size from tiny grains to large boulders. One of the largest meteorite found on Earth is the Hoba meteorite from southwest Africa, which weighs roughly 54,000 kg (119,000 pounds).
  2. Meteor showers are usually named after a star or constellation which is close to the radiant (the position from which the meteor appears to come).
  3. Meteors and meteorites begin as meteoroids, which are little chunks of rock and debris in space.
  4. Most meteorites are either iron, stony or stony-iron.
  5. Meteorites may look very much like Earth rocks, or they may have a burned appearance. Some may have depressioned (thumbprint-like), roughened or smooth exteriors.
  6. Many of the meteor showers are associated with comets. The Leonids are associated with comet Tempel-Tuttle; Aquarids and Orionids with comet Halley, and the Taurids with comet Encke.
  7. When comets come around the sun, they leave a dusty trail. Every year the Earth passes through the comet trails, which allows the debris to enter our atmosphere where it burns up and creates fiery and colorful streaks (meteors) in the sky.
  8. Leonid MAC (an airborne mission that took flight during the years 1998 - 2002) studied the interaction of meteoroids with the Earth's atmosphere.
  9. Meteoroids, meteors and meteorites cannot support life. However, they may have provided the Earth with a source of amino acids: the building blocks of life.
  10. Fact: Meteoroids become meteors -- or shooting stars -- when they interact with a planet's atmosphere and cause a streak of light in the sky. Debris that makes it to the surface of a planet from meteoroids are called meteorites.

Meteors & Meteorites: Overview

Little chunks of rock and debris in space are called meteoroids. They become meteors -- or shooting stars -- when they fall through a planet's atmosphere; leaving a bright trail as they are heated to incandescence by the friction of the atmosphere. Pieces that survive the journey and hit the ground are called meteorites.


Meteors & Meteorites: In Depth

Color image of meteor over Earth.
An astronaut image of a Persied meteor over Earth.

Shooting stars, or meteors, are bits of interplanetary material falling through Earth's atmosphere and heated to incandescence by friction. These objects are called meteoroids as they are hurtling through space, becoming meteors for the few seconds they streak across the sky and create glowing trails.

Woman holding meteorite in sterile facility.
A scientist work in the Meteorite Processing Laboratory.
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Scientists estimate that 44 tonnes (44,000 kilograms, about 48.5 tons) of meteoritic material falls on the Earth each day. Several meteors per hour can usually be seen on any given night. Sometimes the number increases dramatically - these events are termed meteor showers. Some occur annually or at regular intervals as the Earth passes through the trail of dusty debris left by a comet. Meteor showers are usually named after a star or constellation that is close to where the meteors appear in the sky. Perhaps the most famous are the Perseids, which peak around 12 August every year. Every Perseid meteor is a tiny piece of the comet Swift-Tuttle, which swings by the Sun every 135 years. Other meteor showers and their associated comets are the Leonids (Tempel-Tuttle), the Aquarids and Orionids (Halley), and the Taurids (Encke). Most comet dust in meteor showers burns up in the atmosphere before reaching the ground; some dust is captured by high-altitude aircraft and analyzed in NASA laboratories.

Color image showing Mercury in front of the sun.
Barringer is one of the best-preserved impact craters on Earth.
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Chunks of rock and metal from asteroids and other planetary bodies that survive their journey through the atmosphere and fall to the ground are called meteorites. Most meteorites found on Earth are pebble to fist size, but some are larger than a building Early Earth experienced many large meteorite impacts that caused extensive destruction.

One of the most intact impact craters is the Barringer Meteorite Crater in Arizona, about 1 kilometer (0.6 mile) across, formed by the impact of a piece of iron-nickel metal approximately 50 meters (164 feet) in diameter. It is only 50,000 years old and so well preserved that it has been used to study impact processes. Since this feature was recognized as an impact crater in the 1920s, about 170 impact craters have been identified on Earth.

A very large asteroid impact 65 million years ago, which created the 300-kilometer-wide (180-mile-wide) Chicxulub crater on the Yucatan Peninsula, is thought to have contributed to the extinction of about 75 percent of marine and land animals on Earth at the time, including the dinosaurs.

Color image showing Mercury in front of the sun.
An iron meteorite on Mars.
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Well-documented stories of meteorite-caused injury or death are rare. In the first known case of an extraterrestrial object to have injured a human being in the U.S., Ann Hodges of Sylacauga, Alabama, was severely bruised by a 3.6-kilogram (8-pound) stony meteorite that crashed through her roof in November 1954.

Meteorites may resemble Earth rocks, but they usually have a burned exterior. This fusion crust is formed as the meteorite is melted by friction as it passes through the atmosphere. There are three major types of meteorites: the "irons," the "stones," and the stony-irons. Although the majority of meteorites that fall to Earth are stony, more of the meteorites that are discovered long after they fall are irons - these heavy objects are easier to distinguish from Earth rocks than stony meteorites. Meteorites also fall on other solar system bodies. Mars Exploration Rover Opportunity found the first meteorite of any type on another planet when it discovered an iron-nickel meteorite about the size of a basketball on Mars in 2005, and then found a much larger and heavier iron-nickel meteorite in 2009 in the same region. In all, Opportunity has discovered six meteorites during its travels on Mars.

Color image of streaks of light against the night sky.
A burst of Perseids meteors.
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More than 50,000 meteorites have been found on Earth. Of these, 99.8 percent come from asteroids. Evidence for an asteroid origin includes orbits calculated from photographic observations of meteorite falls projected back to the asteroid belt; spectra of several classes of meteorites match those of some asteroid classes; and they are very old, 4.5 to 4.6 billion years. However, we can only match one group of meteorites to a specific asteroid - the eucrite, diogenite, and howardite igneous meteorites come from the third-largest asteroid, Vesta. Asteroids and the meteorites that fall to Earth are not pieces of a planet that broke apart, but instead are the original diverse materials from which the planets formed. The study of meteorites tells us much about the earliest conditions and processes during the formation and earliest history of the solar system, such as the age and composition of solids, the nature of the organic matter, the temperatures achieved at the surface and interiors of asteroids, and the degree to which materials were shocked by impacts.

The remaining 0.2 percent of meteorites is split roughly equally between meteorites from Mars and the moon. The over 60 known martian meteorites were blasted off Mars by meteoroid impacts. All are igneous rocks crystallized from magma. The rocks are very much like Earth rocks with some distinctive com-positions that indicate martian origin. The nearly 80 lunar meteorites are similar in mineralogy and composition to Apollo mission moon rocks, but distinct enough to show that they have come from other parts of the moon. Studies of lunar and martian meteorites complement studies of Apollo Moon rocks and the robotic exploration of Mars.


Meteors & Meteorites: Significant Dates

  • 4.55 billion years ago: Formation age of most meteorites, taken to be the age of the solar system.
  • 65 million years ago: Chicxulub impact leads to the death of 75 percent of the animals on Earth, including the dinosaurs.
  • 50,000 years: Age of Barringer Meteorite Crater in Arizona.
  • 1478 BCE: First recorded observation of meteors.
  • 1794: Ernst Friedrich Chladni publishes the first book on meteorites, in which he proposes that they have an extraterrestrial origin.
  • 1908: (Tunguska), 1947 (Sikote Alin), 1969 (Allende and Murchison), 1976 (Jilin) - Important 20th-century meteorite falls.
  • 1969: Discovery of meteorites in a small area of Antarctica leads to annual expeditions by U.S. and Japanese teams.
  • 1982-1983: Meteorites from the moon and Mars are identified in Antarctic collections.
  • 1996: A team of NASA scientists suggests that martian meteorite ALH84001 may contain evidence of microfossils from Mars, a still-controversial claim.
  • 2005: NASA's Mars Exploration Rover Opportunity finds a basketball-size iron-nickel meteorite on Mars.
  • 2009 : Opportunity finds another iron-nickel meteorite on Mars.

Major Meteor Streams Peak Night
(may vary by +/- 1 day)
Time to Watch*
(24 hour clock)
Maximum Rate**
(per hour)
Parent Body (Asteroid or Comet)
Quadrantids January 3-4 23:00 to dawn 60-200 (196256) 2003 EH1
Lyrids April 21-22 21:30 to dawn 10-15 typical Comet C/1861 G1
Eta Aquarids May 5-6 01:30 to dawn 40-85 Comet 1P/Halley
Delta Aquarids July 27-28 21:30 to dawn 15-20 Unknown sungrazing comet
Perseids August 11-12 dusk to dawn 60-100 Comet 109P/Swift-Tuttle
Orionids October 20-21 22:00 to dawn 25 Comet 1P/Halley
Leonids November 17-18 23:30 to dawn 10-15 Comet 55P/Tempel-Tuttle
Geminids December 13-14 19:00 to dawn 60-120 (3200) Phaethon
* For observers in the northern hemisphere.
** Under perfect conditions

(Primary source material from: Dr. Harry B. Herzer, III, NASA Aerospace Education Services Project, Oklahoma State University, Stillwater, Oklahoma.)


Meteors & Meteorites: Trivia

Hoba
The largest individual meteorite found is the Hoba meteorite in southwest Africa, which has a mass of about 54,000 kg and mostly consists of iron. (Picture is of a smaller iron meteorite.)

Transformers
How is it that the same object can have three names? Through a change in appearance or attributes, that's how. Meteoroids become meteors -- or shooting stars -- when they interact with a planet's atmosphere and cause a streak of light in the sky. Debris that makes it to the surface of a planet from meteoroids are called meteorites.

Bits and Pieces
Scientists estimate that 1,000 tons to more than 10,000 tons of meteoritic material falls on the Earth each day. However, most of this material is very tiny -- in the form of micrometeoroids or dust-like grains a few micrometers in size. These grains pose no threat to life on Earth.

It Fell From the Sky
There are only two known instances of someone getting hurt from a meteorite. In one instance, a woman was severely bruised by an eight pound stony meteorite when it crashed through her roof in 1954.

Life
Meteoroids, meteors and meteorites cannot support life on their own. However, these chunks of space debris may have brought the building blocks of life (amino acids) to Earth.