NASA - National Aeronautics and Space Administration

The asteroid Ida is about 35 miles (55 kilometers) long. It is one of thousands of asteroids in the asteroid belt, a region between the orbits of Mars and Jupiter. Image credit: NASA

An asteroid is any of numerous small planetary bodies that revolve around the sun. Asteroids are also called minor planets or planetoids. Most of them are in the asteroid belt between the orbits of Mars and Jupiter. The belt contains more than 200 asteroids larger than 60 miles (100 kilometers) in diameter. Scientists estimate that there are more than 750,000 asteroids in the belt with diameters larger than 3/5 mile (1 kilometer). There are millions of smaller asteroids. The average temperature of the surface of a typical asteroid is -100 degrees F (-73 degrees C).

Astronomers are not sure how the asteroids originated. According to the leading theory, however, most known asteroids are the shattered remains of a smaller group of larger objects. These objects were left over from the time the planets formed. Elsewhere in the solar system, other such objects gathered together to form the planets and satellites.

Size

Asteroids vary greatly in size. The largest and first known asteroid, Ceres, was discovered in 1801. It is 580 miles (933 kilometers) in diameter. Ceres is believed to contain about 1/3 the total mass of all the asteroids. One of the smallest, discovered in 1991 and named 1991 BA, is only about 20 feet (6 meters) across.

Composition

Studies of an asteroid's reflected light as well as analyses of meteorites have provided information about the composition of asteroids. Astronomers classify asteroids into two broad groups based on their composition. One group of asteroids dominates the outer part of the belt. These asteroids are rich in carbon. Their composition has not changed much since the solar system formed. Asteroids in the second group, which are located in the inner part of the belt, are rich in minerals. These asteroids formed from melted materials.

Measuring asteroids

Until the 1990's, astronomers could determine the size of an asteroid in only three ways. In the first method, they use telescopes to determine the asteroid's distance from the sun, the amount of sunlight it reflects, and the amount of heat it gives off. The amount of sunlight or heat reaching the earth depends on the size of the asteroid and its distance from the sun. Therefore, calculations involving distance and either light or heat yield the size of the asteroid.

In the second method, astronomers use a telescope to measure an asteroid during an occultation, when the asteroid passes in front of a star and is silhouetted against it. The third technique involves the use of radio telescopes to produce images of an asteroid.

In 1991, scientists began to use a fourth method -- close-range observation of asteroids by space probes. That year, the United States space probe Galileo took the first detailed photograph of an asteroid. The asteroid, called Gaspra, was an irregularly shaped object measuring about 12 by 7 1/2 by 7 miles (19 by 12 by 11 kilometers).

Craters cover the surface of the asteroid Eros. The asteroid is about 21 miles (33 kilometers) long, about 1 1/2 times the length of Manhattan Island. Image credit: NASA

In 1996, the U.S. National Aeronautics and Space Administration (NASA) launched the Near Earth Asteroid Rendezvous (NEAR) probe. The probe flew within 753 miles (1,216 kilometers) of the asteroid Mathilde in 1997. The next year, NEAR flew past the asteroid Eros at a distance of 2,378 miles (3,829 kilometers). NEAR went into orbit around Eros in February 2000. In March 2000, the probe was renamed Near Earth Asteroid Rendezvous-Shoemaker (NEAR Shoemaker) in honor of American astronomer Eugene Shoemaker. In February 2001, NEAR Shoemaker became the first spacecraft to land on an asteroid.

In October 1998, NASA launched a probe called Deep Space 1. The probe flew within only about 16 miles (26 kilometers) of the asteroid Braille in July 1999.

Orbits

Most asteroids follow elliptical (oval-shaped) orbits in the asteroid belt. Groups of asteroids that follow the same orbit are called Hirayama families, named after Kiyotsugu Hirayama, the Japanese astronomer who first discovered them.

Many asteroids follow orbits outside the belt. For example, a number of asteroids called Trojans follow the same orbit as does Jupiter. Three groups of asteroids -- Atens, Amors, and Apollos -- orbit in the inner solar system and are known as near-Earth asteroids. Some near-Earth asteroids cross the path of Mars, while others cross Earth's orbit.

Asteroid collisions

The Chicxulub Basin along the northern coast of Mexico's Yucatan Peninsula formed when an asteroid hit the earth about 65 million years ago. Debris from the impact may have led to the extinction of the dinosaurs. Image credit: World Book map

Many scientists believe that a near-Earth asteroid collided with Earth about 65 million years ago, triggering widespread environmental changes that led to the extinction of the dinosaurs. The asteroid created a huge circular depression called the Chicxulub (CHEEK shoo loob) Basin centered in Mexico's Yucatan Peninsula. The diameter of the basin is about 190 miles (300 kilometers).

In 1908, an object exploded about 6 miles (10 kilometers) above the Tunguska River area of Siberia. The object may have been a comet's nucleus or a large meteorite -- sometimes referred to as a small asteroid. Debris from the explosion flattened forests and burned an area about 50 miles (80 kilometers) across.

The gravitational pull of Jupiter and other large planets causes asteroid orbits to change very slowly. Orbital changes lead to collisions that create smaller asteroids and fragments, increasing the chance of more collisions. Some small fragments reach Earth's surface as meteorites.

Contributor: Marian E. Rudnyk, B.S., Planetary Photogeologist/Astronomer; Consultant, Jet Propulsion Laboratory, National Aeronautics and Space Administration.

How to cite this article: To cite this article, World Book recommends the following format: Rudnyk, Marian E. "Asteroid." World Book Online Reference Center. 2005. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar034580.

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