Asteroids: Formation, Discovery and ExplorationAsteroids – Overview
Asteroids are small, airless rocky worlds revolving around the sun that are too small to be called planets. They are also known as planetoids or minor planets. In total, the mass of all the asteroids is less than that of Earth's moon. But despite their size, asteroids can be dangerous. Many have hit Earth in the past, and more will crash into our planet in the future. That's one reason scientists study asteroids and are eager to learn more about their numbers, orbits and physical characteristics. If an asteroid is headed our way, we want to know that.
Most asteroids lie in a vast ring between the orbits of Mars and Jupiter. This main belt holds more than 200 asteroids larger than 60 miles (100 kilometers) in diameter. Scientists estimate the asteroid belt also contains more than 750,000 asteroids larger than three-fifths of a mile (1 kilometer) in diameter and millions of smaller ones. Not everything in the main belt is an asteroid — for instance, comets have recently been discovered there, and Ceres, once thought of only as an asteroid, is now also considered a dwarf planet.
Many asteroids lie outside the main belt. For instance, a number of asteroids called Trojans lie along Jupiter's orbital path. Three groups — Atens, Amors, and Apollos — known as near-Earth asteroids orbit in the inner solar system and sometimes cross the path of Mars and Earth.
Many asteroids lie outside the main belt. For instance, a number of asteroids called Trojans lie along Jupiter's orbital path. Three groups — Atens, Amors, and Apollos — known as near-Earth asteroids orbit in the inner solar system and sometimes cross the path of Mars and Earth.
Formation
Asteroids are leftovers from the formation of our solar system about 4.6 billion years ago. Early on, the birth of Jupiter prevented any planetary bodies from forming in the gap between Mars and Jupiter, causing the small objects that were there to collide with each other and fragment into the asteroids seen today.
Physical Characteristics
Asteroids can reach as large as Ceres, which is 940 kilometers (about 583 miles) across and is also considered a dwarf planet. On the other hand, one of the smallest, discovered in 1991 and named 1991 BA, is only about 20 feet (6 meters) across.
Nearly all asteroids are irregularly shaped, although a few are nearly spherical, such as Ceres. They are often pitted or cratered — for instance, Vesta has a giant crater some 285 miles (460 km) in diameter.
As asteroids revolve around the Sun in elliptical orbits, they rotate, sometimes tumbling quite erratically. More than 150 asteroids are also known to have a small companion moon, with some having two moons. Binary or double asteroids also exist, in which two asteroids of roughly equal size orbit each other, and triple asteroid systems are known as well. Many asteroids seemingly have been captured by a planet's gravity and become moons — likely candidates include among Mars' moons Phobos and Deimos and most of the distant outer moons of Jupiter, Saturn, Uranus and Neptune.
The average temperature of the surface of a typical asteroid is minus 100 degrees F (minus 73 degrees C). Asteroids have stayed mostly unchanged for billions of years — as such, research into them could reveal a great deal about the early solar system.
Classification
In addition to classifications of asteroids based on their orbits, most asteroids fall into three classes based on composition. The C-type or carbonaceous are greyish in color and are the most common, including more than 75 percent of known asteroids. They probably consist of clay and stony silicate rocks, and inhabit the main belt's outer regions. The S-type or silicaceous asteroids are greenish to reddish in color, account for about 17 percent of known asteroids, and dominate the inner asteroid belt. They appear to be made of silicate materials and nickel-iron. The M-type or metallic asteroids are reddish in color, make up most of the rest of the asteroids, and dwell in the middle region of the main belt. They seem to be made up of nickle-iron. There are many other rare types based on composition as well — for instance, V-type asteroids typified by Vesta have a basaltic, volcanic crust.
Earth Impacts
Ever since Earth formed about 4.5 billion years ago, asteroids and comets have routinely slammed into the planet. The most dangerous asteroids are extremely rare, according to NASA.
An asteroid capable of global disaster would have to be more than a quarter-mile wide. Researchers have estimated that such an impact would raise enough dust into the atmosphere to effectively create a "nuclear winter," severely disrupting agriculture around the world. Asteroids that large strike Earth only once every 1,000 centuries on average, NASA officials say.
Smaller asteroids that are believed to strike Earth every 1,000 to 10,000 years could destroy a city or cause devastating tsunamis.
Dozens of asteroids have been classified as "potentially hazardous" by the scientists who track them. Some of these, whose orbits come close enough to Earth, could potentially be perturbed in the distant future and sent on a collision course with our planet. Scientists point out that if an asteroid is found to be on a collision course with Earth 30 or 40 years down the road, there is time to react. Though the technology would have to be developed, possibilities include exploding the object or diverting it.
For every known asteroid, however, there are many that have not been spotted, and shorter reaction times could prove more threatening.
When an asteroid, or a part of it, crashes into Earth, it's called a meteorite. Here are typical compositions:
Iron Meteorites:
* Iron 91%
* Nickel 8.5%
* Cobalt 0.6%
Stony Meteorites:
* Oxygen 36%
* Iron 26%
* Silicon 18%
* Magnesium 14%
* Aluminum 1.5%
* Nickel 1.4%
* Calcium 1.3%
Discovery
In 1801, while making a star map, Italian priest and astronomer Giuseppe Piazzi accidentally discovered the first and largest asteroid, Ceres, orbiting between Mars and Jupiter. Ceres accounts for a quarter of all the mass of all the thousands of known asteroids in or near the main asteroid belt.
Naming
Since the International Astronomical Union is less strict on how asteroids are named when compared to other bodies, there are asteroids named after Mr. Spock of "Star Trek" and rock musician Frank Zappa as well as more solemn tributes, such as the seven asteroids named for the crew of the Space Shuttle Columbia killed in 2003. Naming asteroids after pets is no longer allowed.
Asteroids are also given numbers — for example, 99942 Apophis.
Exploration
The first spacecraft to take close-up images of asteroids was NASA's Galileo in 1991, which also discovered the first moon to orbit an asteroid in 1994.
In 2001, after NASA's NEAR spacecraft intensely studied the near-earth asteroid Eros for more than a year from orbit, mission controllers decided to try and land the spacecraft. Although it wasn't designed for landing, NEAR successfully touched down, setting the record as the first to successfully land on an asteroid.
In 2006, Japan's Hayabusa became the first spacecraft to land on and take off from an asteroid. It returned to Earth in June 2010, and the samples it recovered are currently under study.
NASA's Dawn mission, launched in 2007, began exploring Vesta in 2011 and is slated to explore Ceres in 2015 and will be the first spacecraft to visit either body.
In 2012, a company called Planetary Resources, Inc. announced plans to eventually send a mission to a space rock to extract water and mine the asteroid for precious metals.
Asteroids are leftovers from the formation of our solar system about 4.6 billion years ago. Early on, the birth of Jupiter prevented any planetary bodies from forming in the gap between Mars and Jupiter, causing the small objects that were there to collide with each other and fragment into the asteroids seen today.
Physical Characteristics
Asteroids can reach as large as Ceres, which is 940 kilometers (about 583 miles) across and is also considered a dwarf planet. On the other hand, one of the smallest, discovered in 1991 and named 1991 BA, is only about 20 feet (6 meters) across.
Nearly all asteroids are irregularly shaped, although a few are nearly spherical, such as Ceres. They are often pitted or cratered — for instance, Vesta has a giant crater some 285 miles (460 km) in diameter.
As asteroids revolve around the Sun in elliptical orbits, they rotate, sometimes tumbling quite erratically. More than 150 asteroids are also known to have a small companion moon, with some having two moons. Binary or double asteroids also exist, in which two asteroids of roughly equal size orbit each other, and triple asteroid systems are known as well. Many asteroids seemingly have been captured by a planet's gravity and become moons — likely candidates include among Mars' moons Phobos and Deimos and most of the distant outer moons of Jupiter, Saturn, Uranus and Neptune.
The average temperature of the surface of a typical asteroid is minus 100 degrees F (minus 73 degrees C). Asteroids have stayed mostly unchanged for billions of years — as such, research into them could reveal a great deal about the early solar system.
Classification
In addition to classifications of asteroids based on their orbits, most asteroids fall into three classes based on composition. The C-type or carbonaceous are greyish in color and are the most common, including more than 75 percent of known asteroids. They probably consist of clay and stony silicate rocks, and inhabit the main belt's outer regions. The S-type or silicaceous asteroids are greenish to reddish in color, account for about 17 percent of known asteroids, and dominate the inner asteroid belt. They appear to be made of silicate materials and nickel-iron. The M-type or metallic asteroids are reddish in color, make up most of the rest of the asteroids, and dwell in the middle region of the main belt. They seem to be made up of nickle-iron. There are many other rare types based on composition as well — for instance, V-type asteroids typified by Vesta have a basaltic, volcanic crust.
Earth Impacts
Ever since Earth formed about 4.5 billion years ago, asteroids and comets have routinely slammed into the planet. The most dangerous asteroids are extremely rare, according to NASA.
An asteroid capable of global disaster would have to be more than a quarter-mile wide. Researchers have estimated that such an impact would raise enough dust into the atmosphere to effectively create a "nuclear winter," severely disrupting agriculture around the world. Asteroids that large strike Earth only once every 1,000 centuries on average, NASA officials say.
Smaller asteroids that are believed to strike Earth every 1,000 to 10,000 years could destroy a city or cause devastating tsunamis.
Dozens of asteroids have been classified as "potentially hazardous" by the scientists who track them. Some of these, whose orbits come close enough to Earth, could potentially be perturbed in the distant future and sent on a collision course with our planet. Scientists point out that if an asteroid is found to be on a collision course with Earth 30 or 40 years down the road, there is time to react. Though the technology would have to be developed, possibilities include exploding the object or diverting it.
For every known asteroid, however, there are many that have not been spotted, and shorter reaction times could prove more threatening.
When an asteroid, or a part of it, crashes into Earth, it's called a meteorite. Here are typical compositions:
Iron Meteorites:
* Iron 91%
* Nickel 8.5%
* Cobalt 0.6%
Stony Meteorites:
* Oxygen 36%
* Iron 26%
* Silicon 18%
* Magnesium 14%
* Aluminum 1.5%
* Nickel 1.4%
* Calcium 1.3%
Discovery
In 1801, while making a star map, Italian priest and astronomer Giuseppe Piazzi accidentally discovered the first and largest asteroid, Ceres, orbiting between Mars and Jupiter. Ceres accounts for a quarter of all the mass of all the thousands of known asteroids in or near the main asteroid belt.
Naming
Since the International Astronomical Union is less strict on how asteroids are named when compared to other bodies, there are asteroids named after Mr. Spock of "Star Trek" and rock musician Frank Zappa as well as more solemn tributes, such as the seven asteroids named for the crew of the Space Shuttle Columbia killed in 2003. Naming asteroids after pets is no longer allowed.
Asteroids are also given numbers — for example, 99942 Apophis.
Exploration
The first spacecraft to take close-up images of asteroids was NASA's Galileo in 1991, which also discovered the first moon to orbit an asteroid in 1994.
In 2001, after NASA's NEAR spacecraft intensely studied the near-earth asteroid Eros for more than a year from orbit, mission controllers decided to try and land the spacecraft. Although it wasn't designed for landing, NEAR successfully touched down, setting the record as the first to successfully land on an asteroid.
In 2006, Japan's Hayabusa became the first spacecraft to land on and take off from an asteroid. It returned to Earth in June 2010, and the samples it recovered are currently under study.
NASA's Dawn mission, launched in 2007, began exploring Vesta in 2011 and is slated to explore Ceres in 2015 and will be the first spacecraft to visit either body.
In 2012, a company called Planetary Resources, Inc. announced plans to eventually send a mission to a space rock to extract water and mine the asteroid for precious metals.
1st Asteroid Samples Reveal Surprising Look at Space Rock Crashes
The first dust grains ever retrieved from the surface of an asteroid now confirm that these minor planets are constantly shaped by a continuous barrage of high-speed microscopic impacts, scientists find.
The Japanese asteroid probe Hayabusasucceeded in returning more than 1,500 grains of dustfrom the asteroid 25143 Itokawa when it parachuted into the Australian outback in June 2010. Already, the samples from this 1,800 foot-long (550 meter) rubble pile have helped solve the longstanding mystery of where most meteorites striking our planet come from.
To uncover still more details about asteroids, scientists analyzed the size, mineralogy, shape and geochemistry of five dust grains recovered by Hayabusa. The smallest of these was just 40 microns, or millionths of a meter, in diameter — less than half the width of a human hair — and were cut into pieces using focused beams of electrically charged ions for analysis under microscopes.
Shine a Light
Hayabusa spacecraft streaked across the sky through the clouds as it re-entered Earth's atmosphere over the Woomera Test Range in Australia. In Kingoonya, the re-entry was visible to the human eye for only 15 seconds.
So Lonely
Hayabusa's sample return capsule lies in the Australian wilderness
I Fall to Pieces
This still from a NASA video shows the Hayabusa spacecraft as it burned up over Australia during re-entry on June 13, 2010 to cap a 7-year mission to the asteroid Itokawa. Hayabusa ejected a sample return capsule (bright dot at lower right) before burning up. It landed in the Australian outback and has been recovered.
NASA, SETI Use Airship to Hunt Meteorites From Big Fireball
In what must be a meteorite-hunting first, a team of scientists took a zeppelin out Thursday (May 3) to search for fragments from a rare daytime fireball that exploded over California last month.
The huge airship Eureka took off from McClellan Park airfield in Sacramento around 12:45 p.m. PDT (3:45 p.m. EDT; 1945 GMT) carrying six researchers from NASA and the Search for Extreterrestrial Intelligence (SETI) Institute.
As the zeppelin cruised over the foothills of California's Sierra Nevada mountains at an altitude of about 1,000 feet (300 meters), the scientists scanned the ground for any signs of recent impact craters. Their aim: to find fragments of the minivan-size space rock that slammed into Earth's atmosphere on April 22.
The huge airship Eureka took off from McClellan Park airfield in Sacramento around 12:45 p.m. PDT (3:45 p.m. EDT; 1945 GMT) carrying six researchers from NASA and the Search for Extreterrestrial Intelligence (SETI) Institute.
As the zeppelin cruised over the foothills of California's Sierra Nevada mountains at an altitude of about 1,000 feet (300 meters), the scientists scanned the ground for any signs of recent impact craters. Their aim: to find fragments of the minivan-size space rock that slammed into Earth's atmosphere on April 22.
Researchers are eager to get their hands on these meteorites, because they think the parent asteroid was a rare and primitive type called a carbonaceous chondrite. Such space rocks are typically packed full of organic molecules — carbon-based compounds that are the building blocks of life as we know it.
"It's a type of meteorite you'd like to have as an origin-of-life researcher," said Peter Jenniskens of the SETI Institute and NASA's Ames Research Center in Moffett Field, Calif., one of the six scientists who went up in the Eureka Thursday. [Fallen Stars: A Gallery of Famous Meteorites]
"It's a type of meteorite you'd like to have as an origin-of-life researcher," said Peter Jenniskens of the SETI Institute and NASA's Ames Research Center in Moffett Field, Calif., one of the six scientists who went up in the Eureka Thursday. [Fallen Stars: A Gallery of Famous Meteorites]
A five-hour ride
The helium-filled Eureka is one of just two zeppelins flying today, according to Airship Ventures, the company that owns and operates it. The 246-foot (75-m) Eureka is slightly longer than a standard Boeing 747 jet.
Zeppelins are different than blimps, though both are classified as airships. Zeppelins sport a light but rigid internal frame, while blimps do not. Blimps' shapes are maintained solely by the pressure of their internal lifting gas.
Jenniskens said conditions were great during the team's five-hour ride, with cloudy skies keeping crater-obscuring shadows at a minimum. The scientists spotted 12 possible impact features from the April 22 fall, though they're not sure yet if any of them are the real deal. The next step is to come back and check out the candidates on foot.
Several fragments from the April 22 fireball have been found near the California towns of Coloma and Lotus, in historic Gold Rush country. But Jenniskens and his colleagues want bigger chunks, and more of them.
"Very few pieces have been found so far," Jenniskens told SPACE.com. "I'm hoping more will be found."
The clock is ticking, he added, because Earth's weather and organisms are already degrading the meteorites from their pristine in-space condition.
Amateur space-rock hunters wanted
Even if the zeppelin expedition doesn't lead directly to any meteorite discoveries, it may still have an impact. Jenniskens hopes the publicity the airship trip is generating inspires amateurs to do a little space-rock hunting of their own.
A lot of boots on the ground are needed, because the fall area measures roughly 25 miles long by 6 miles wide (40 kilometers by 10 kilometers), Jenniskens said.
If you do want to try your luck, be on the lookout for rocks with a so-called fusion crust. This outer layer — caused by the extreme heat meteorites experience on their journey through Earth's atmosphere — should have a sort of velvety sheen, Jenniskens said.
Zeppelins are different than blimps, though both are classified as airships. Zeppelins sport a light but rigid internal frame, while blimps do not. Blimps' shapes are maintained solely by the pressure of their internal lifting gas.
Jenniskens said conditions were great during the team's five-hour ride, with cloudy skies keeping crater-obscuring shadows at a minimum. The scientists spotted 12 possible impact features from the April 22 fall, though they're not sure yet if any of them are the real deal. The next step is to come back and check out the candidates on foot.
Several fragments from the April 22 fireball have been found near the California towns of Coloma and Lotus, in historic Gold Rush country. But Jenniskens and his colleagues want bigger chunks, and more of them.
"Very few pieces have been found so far," Jenniskens told SPACE.com. "I'm hoping more will be found."
The clock is ticking, he added, because Earth's weather and organisms are already degrading the meteorites from their pristine in-space condition.
Amateur space-rock hunters wanted
Even if the zeppelin expedition doesn't lead directly to any meteorite discoveries, it may still have an impact. Jenniskens hopes the publicity the airship trip is generating inspires amateurs to do a little space-rock hunting of their own.
A lot of boots on the ground are needed, because the fall area measures roughly 25 miles long by 6 miles wide (40 kilometers by 10 kilometers), Jenniskens said.
If you do want to try your luck, be on the lookout for rocks with a so-called fusion crust. This outer layer — caused by the extreme heat meteorites experience on their journey through Earth's atmosphere — should have a sort of velvety sheen, Jenniskens said.
If you do find something intriguing, don't pick it up with your bare hands, he added. Wrap it in aluminum foil and send him a picture, along with information about where and when you found the rock. Jenniskens is also interested in viewing any video footage that may exist of the April 22 fireball, which could help researchers pin down just where the asteroid came from in space.
Jenniskens can be reached at petrus.m.jenniskens@nasa.gov.
Jenniskens can be reached at petrus.m.jenniskens@nasa.gov.
