NASA artist concept of NEAR at 433 Eros
Asteroids of the Solar System
Asteroids Exploring

Exploring the Asteroid Belt:

What Are These Ancient Rocks?


NASA artist concept of Dawn with Vesta and Ceres.
NASA image of Gaspra 951
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Asteroids are small rocky metallic bodies in orbit around the Sun. Most are in the Asteroid Belt which orbits the Sun at a distance between Mars and Jupiter. They have been found from inside Earth's orbit out to beyond Saturn's orbit.

More than 7,000 asteroids have been discovered. Several hundred more are discovered each year. There are undoubtedly hundreds of thousands more that are too small to be seen from Earth.

Some have orbits that cross Earth's path and some have even hit the Earth in times past. One of the best preserved examples of that is the Barringer Meteor Crater near Winslow, Arizona.

The size of asteroids
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Because they are too small to be considered planets, asteroids are known as minor planets. They range in size from Ceres, which has a diameter of about 620 miles, down to the size of pebbles. In fact, only 16 asteroids have a diameter greater than 150 miles and only 26 asteroids are known to be larger than 125 miles in diameter.

Astronomers tend to know a lot about asteroids larger than 62 miles in diameter, and not very much about smaller ones. Some suggest there could be as many as a million or more asteroids under a mile in diameter. Even so, the total mass of all the asteroids probably is less than that of the Moon.

Biggest. The largest, by far, is 1 Ceres at 580 miles in diameter. It alone contains about 25 percent of the mass of all asteroids combined.

The next largest asteroids are 2 Pallas, 4 Vesta and 10 Hygiea which are between 250 and 325 miles in diameter. All other known asteroids are less than 210 miles across.

What are asteroids made of?
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Asteroids are composed of material left over from the formation of the Solar System billions of years ago. Asteroids may be material that never coalesced into a major planet. On the other hand, they may be the remains of a planet that grew and then was destroyed long ago in a massive collision with something else. If the estimated total mass of all asteroids was gathered into a single object, it would be less than 932 miles across. That's less than half the diameter of our Moon.

How Do We Know? Humans have learned about asteroids by examining pieces of space debris — meteorites — that have dropped through our planet's atmosphere to the surface. We've given those pieces of debris various names: Because asteroids are composed of materials dating back to the earliest times of our Solar System, scientists are interested in their composition. Among the numerous meteorites found on Earth over the years, scientists say 92.8 percent are stone (silicate) and 5.7 percent are iron and nickel. The other 1.5 percent have been a mixture of nickel, iron and silicate. As can be imagined, stone meteorites are hard to spot since they look like terrestrial rocks.

Debates. There are questions about the differences among asteroids, comets and moons. For instance, are any moons around planets actually captured asteroids or even former comets?

Small moons that are more like asteroids than they are like the larger moons of the Solar System include Mars's tiny moons Deimos and Phobos, Jupiter's eight outer moons, Saturn's outermost moon, Phoebe, and some newly discovered moons of Uranus and Neptune.

Looking at asteroids
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ISAS artist concept of Japan's MUSES-C spacecraft landing on asteroid 1998 SF36
NASA concept of
Hubble Space Telescope
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Astronomers have looked at a number of approaching asteroids through telescopes and radar antennas on Earth – notably Toutatis, Castalia, Geographos and Vesta.

As each has approached close to Earth, asteroids Toutatis, Geographos and Castalia were studied using radar signals transmitted from Earth. Vesta was observed by the Hubble Space Telescope, which is in orbit above Earth.

Taking A Closer Look. Before 1991, the only way we could learn about asteroids was though observations from Earth. Then, we started sending probes: Spacecraft from Earth that have flown through the Asteroid Belt have found it mostly empty because, after all, the individual asteroids are separated by great distances. The blizzard of rocks depicted in science fiction movies seems not to be accurate.

Exploring the two largest asteroids
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NASA artist concept of Dawn with Vesta and Ceres.
NASA artist concept of
Dawn with Vesta and Ceres
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A NASA deep space probe named Dawn will be launched in 2006 on a path that will carry it into orbit around the two largest asteroids in the Solar System.

Dawn will fly for nine years on its journey to orbit the two most massive asteroids known, Vesta and Ceres. Those two "baby planets," located in the main Asteroid Belt between Mars and Jupiter, are very different from each other, yet both contain tantalizing clues about the formation of the Solar System. Using the same set of instruments to observe these two bodies, Dawn will improve human understanding of how planets formed during the earliest epoch of the Solar System.

Exploring asteroids NEAR Earth
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NASA artist concept of NEAR at 433 Eros
NASA concept of
NEAR at 433 Eros
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Near-Earth asteroids seem to have broken away from the main Asteroid Belt which orbits the Sun at a distance between Mars and Jupiter. They fly into the inner Solar System, within 121 million miles of the Sun. Aside from the Moon. they're our closest neighbors in the Solar System.

The NEAR-Shoemaker spacecraft was launched on Feb. 17, 1996, as the first in NASA's Discovery series of low-cost science missions. It became the first spacecraft to orbit an asteroid in February 2000 and then the first to land on an asteroid in February 2001. NEAR is short for Near Earth Asteroid Rendezvous.

The car-sized spacecraft snapped 160,000 pictures while orbiting asteroid 433 Eros. After a year in orbit around 433 Eros, NEAR-Shoemaker made an historic controlled descent and touch down on the surface of the asteroid Eros in February 2001. As the first human probe to land on an asteroid, it transmitted 69 close-up images of the surface during its final approach.

Will an asteroid hit Earth?
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NASA artist concept of large rocky body smashing into Earth
NASA concept of asteroid hitting Earth

Maybe in the distant future, but not anytime soon.

NASA researchers have found several near-Earth asteroids (NEA) larger than 1 kilometer (about 0.6 miles) in diameter. Those discoveries suggest there might be from 500 to 1,000 similar NEAs orbiting the Sun and near us in the Solar System.

None of the NEAs seen to date will crash into Earth in the near future. In addition, NEAR and other spacecraft have made important contributions. If people ever were to decide to try to deflect an asteroid, data from the various asteroid probes could provide the necessary knowledge about composition, density, surface strength and other technical aspects of asteroids.

For instance, NEAR-Shoemaker data showed that Eros is a consolidated object. While its surface may be strewn with rubble, its body is not a loosely bound pile of rubble composed of numerous smaller pieces.

Earth had a close call in 2002. In one of the nearest passes ever recorded, a rock the size of a football field that passed extraordinarily close to our planet on June 14 went undetected until June 17. The asteroid — speeding along at 6.2 miles per second — missed us by only 75,000 miles. That's only one-third the distance to the Moon.

Given the identity 2002 MN — the rock is small as asteroids go and would not have caused global damage if it had collided with Earth, according to the Near Earth Objects (NEO) Information Center in Leicester, England. Its destructive force probably would have been comparable to the object that exploded over Siberia back in 1908, flattening 77 square miles of trees. A body capable of global damage probably would have to bigger than 0.6 miles (1 km) in diameter.

Asteroid 2002 MN was found by astronomers at the Lincoln Near Earth Asteroid Research (LINEAR) project in Socorro, New Mexico. The closest near miss in recent times was asteroid 1994XL1 that buzzed the planet within 65,000 miles in 1994.

As predicted, Asteroid Toutatis Missed Earth
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Toutatis mysteries explored by NASA radar
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A giant dumbbell-shaped space rock, broader than the city of Passaic, New Jersey, did not strike Earth as it passed by on September 29, 2004 – and it will not strike our planet in our lifetimes, according to NASA.

The giant asteroid we call Toutatis passed us by at a distance of less than 994,000 miles. That is only about four times the distance from the Earth to the Moon.

The last time the 2.9-mile-wide Toutatis came so close was back in the 12th century in the year 1353. The 2004 pass was the closest Toutatis will come for another 500 years.

Officially known as Number 4179, the asteroid Toutatis has concerned us since 1989 when it first was spotted by French astronomers and named for a Celtic god. Today, it is listed officially as a potentially hazardous asteroid.

How big is it? Toutatis is a small body about 2.9 miles long and 1.5 miles wide and 1.2 miles deep.

The asteroid's odd shape and peculiar rotation makes it one of the strangest bodies in the Solar System. In pictures, it looks like two rocks connected by a narrow neck. Its configuration probably is the result of violent collisions in the past.

Toutatis flies around the Sun every four years. Its path takes it out to the Asteroid Belt between Mars and Jupiter and then back inside the orbit of Earth around the Sun.

The plane of Toutatis' orbit is closer to the plane of the Earth's orbit than any known Earth-orbit-crossing asteroid.

If it ever struck Earth, Toutatis would devastate the planet. However, it won't be that close again until 2562 and it won't hit Earth then, NASA says. Of course, there are statistical margins of error in calculating the orbits of distant bodies.

Astronomers know about Toutatis because they bounced radar signals off of it during previous flybys of Earth. In addition to its interesting shape, Toutatis rotates oddly.

While most asteroids, and all of the planets, spin around one axis, Toutatis wobbles like a careless football pass. A good football passer throws a perfect spiral.

What makes this asteroid different? Instead of one north pole, it has two. This asteroid wanders around in two rotation cycles of 5.4 and 7.3 Earth days.

If you were able to look up at the sky from the pockmarked surface of Toutatis, the stars never would seem to follow the same path twice as they crisscrossed the sky.

By the way, astronomers aren't sure what it would be like to stand on the asteroid because they don't know if Toutatis is covered with a thick layer of loose dirt like the Moon or a harder surface.

There are other asteroids that don't rotate on just one axis, and there are many odd-shaped bodies. In fact, hundreds of kilometer-wide and larger asteroids have been seen near Earth in the last decade.

Watchers on Earth as Toutatis passes by will be able to see it tumbling through space. Binoculars or a small telescope will be needed on a clear dark night. The asteroid will be seen as a point of light, like a star. Surface details won't be see because the asteroid will be too far away.

Should we send a probe to land on Toutatis when it returns four or eight years from now?
Japanese probe to return asteroid dust
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ISAS artist concept of Japan's MUSES-C spacecraft at asteroid 1998 SF36
JAXA artist concept of Japanese probe Falcon at asteroid Itokawa
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Learn more about Falcon »
A Japanese interplanetary probe named Falcon arrived September 12, 2005, at the small near-Earth asteroid known as 1998 SF36 some 186 million miles across the Solar System to pick up a sample of the dust from that small body and return it to Earth for analysis.

Originally, Falcon was known as MUSES-C, short for Mu Space Engineering Spacecraft - C. The spacecraft was a project of Japan's Institute of Space and Astronautical Science (ISAS). After it was launched, the spacecraft was renamed Hayabusa, which means Falcon in Japanese.

JAXA renamed the asteroid Itokawa after Hideo Itokawa, an early leader of the Japanese space program. The asteroid is 2,300 feet long, 1,000 ft. wide, and oval shaped somewhat like a football.

As Falcon loitered near the asteroid, it observed the rocky surface with visible-light and infrared-light cameras.

The probe is to leave the asteroid for a return trip to Earth with the dust sample carried in a capsule inside Falcon. When the spacecraft arrives back in Earth orbit, the sample capsule will be ejected to re-enter the atmosphere. It will parachute to land in the desert near the southern Australia town of Woomera in July 2007.

If successful, Falcon will be the first probe to make a round trip to an asteroid. NASA's Eros probe collected data for two weeks from the surface of the asteroid Eros in 2001, but did not return to Earth with samples. NASA's Stardust probe is on its way to visit Comet Wild-2 in January 2004 and return samples to Earth. That package will parachute into Utah in January 2006.

No probe has brought back extraterrestrial samples since the American and Russian Moon programs of the late 1960s and early 1970s.

Asteroid named for New Mexican
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Warren Offutt of Cloudcroft, New Mexico, has gained considerable renown as an amateur astronomer. In fact, his name has been given to an asteroid 350-million miles away from Earth.

Offutt retired from engineering and moved with his wife, Beverly, from Illinois to New Mexico, where they set set up a research observatory in the Sacramento Mountains of south-central New Mexico, specializing in precision astrometry of faint Solar System objects. Their astronomical outpost – the W & B Observatory – is on top of a mountain 8,300 feet above sea level.

He discovered Minor Planet 7639 in 1985. Then, just before his 70th birthday in 1998, Offutt learned that the International Astronomical Union (IAU) had named Minor Planet 7639 in honor of his scientific contributions. From now on the asteroid will be known as Minor Planet Offutt (7639).

Estimated to be several miles in diameter, Minor Planet Offutt orbits the sun in the Asteroid Belt between Mars and Jupiter.

In 1997, Offutt used his home observatory to help with three more major discoveries. Among them, he and his wife confirmed observations of a newly discovered moon of Uranus.

Offutt also has another hobby. He is an amateur radio operator, with the government-issued callsign AF9Q. The American Radio Relay League member was first licensed in 1943. Beverly Offutt is a radio ham with callsign N9JVN.

Asteroid named for Mister Rogers
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Fred Rogers
Fred Rogers

Asteroid No. 26858 has been named "Misterrogers" in honor of the late Fred Rogers who created and hosted the public television program Mister Rogers' Neighborhood.

Asteroid Misterrogers was discovered in 1993 by E.F. Helin at the Palomar Observatory in California. It is some 218 million miles from the Sun between the orbits of Mars and Jupiter. By comparison, Earth is some 93 million miles from the Sun. While goes around the Sun in one year, the asteroid takes about 3.5 years to complete one orbit around the Sun.

Fred Rogers died February 27, 2003, at age 74.

The International Astronomical Union names minor bodies of the Solar System like asteroids and comets, and features of those bodies such as craters on the Moon. Such honors for individuals are based on merits judged by astronomers.

The Henry Buhl Jr. Planetarium & Observatory of the Carnegie Science Center at Pittsburgh, Pennsylvania, worked with Fred Rogers' TV production company, Family Communications Inc., on production of a show for preschoolers — The Sky Above Mister Rogers' Neighborhood — which is seen at planetariums across the United States.

Asteroid named for journalist Johnny Horne
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Johnny Horne
Johnny Horne

Minor planet 11132 has been named Asteroid Horne in honor of Johnny Horne, photo editor for the Fayetteville Observer, a 75,000 circulation daily newspaper in southeastern North Carolina where Horne has worked for three decades.

The International Astronomical Union (IAU) recognized Horne with the naming in January 2004 for his work with public outreach in astronomy.

Horne has been an amateur astronomer since age 10.

Since 1989, he has written a monthly astronomy column, Backyard Universe, for The Observer.

Horne is a contributing editor for Sky and Telescope magazine and has served as a study leader for S&T astronomical expeditions to Mexico, Africa, the Caribbean and Iceland.

He photographed Halley┬╣s Comet from the Australian Outback in 1986 and his astronomical photographs have appeared in magazines and newspapers worldwide. He regularly reviews amateur astronomy products for Sky and Telescope┬╣s test reports.

During 2002, Horne produced a collection of his astronomical photographs over 25 years. That Backyard Universe Gallery collection was displayed at the Morehead Planetarium and Science Center in Chapel Hill, North Carolina, 40 years after a class visit there had triggered Horne's lifelong interest in astronomy.

Learn more about asteroids
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Dawn: Falcon: NEAR: Asteroids: Warren Offutt: NASA Research:

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