Asteroids are relatively small, rocky chunks of matter that shine like small stars, but orbit the Sun like planets. Most asteroids, or minor planets as they are sometimes called, are made of carbon-rich rock, while others (those farthest from the Sun) contain iron, nickel, and a few other elements. One asteroid, Pholus, is coated with a red material that may be organic compounds similar to those that form living material.

Asteroids vary in size from 580 miles (940 kilometers) in diameter (Ceres, the first asteroid discovered) to 33 feet (10 meters) in diameter. Most are so small their size cannot be measured directly. They are generally irregular in shape and vary in brightness as they rotate.

Scientists once thought asteroids were remnants of exploded planets. This theory was soon discarded, however, because asteroids are so small. All the known asteroids combined would form an object much smaller than Earth's moon. Scientists now believe that asteroids are planetesimals—ancient chunks of matter left over from the formation of the solar system that never came together to form a planet, perhaps because of the strong gravitational influence of Jupiter. Asteroid-like objects were among the first structures to form in our solar system some 4.6 billion years ago. For this reason, asteroids can provide valuable information about the beginnings of our solar system.

The discovery of asteroids

For years, astronomers theorized about a hidden planet between Mars and Jupiter. In 1766, German astronomer Johann Titius devised a formula for calculating distances between planets. The formula predicted the existence of a planet between Mars and Jupiter. (Although Titius devised the formula, it did not receive much attention until German astronomer Johann Bode restated it in 1772. By then, Titius had been all but forgotten, and the formula became known as Bode's Law.)

On the night of January 1, 1801, Italian astronomer Giuseppe Piazzi discovered the first asteroid. He observed a starlike body that was not listed in star catalogues. It was moving faster than Mars yet slower than Jupiter, so he deduced it must lie between the two. He named the asteroid Ceres, the Roman goddess of agriculture. Later, Ceres was found to be in the right place to satisfy Bode's Law.

In the mid-1800s, with the improvement of telescopic equipment and techniques, many new asteroids were discovered. There are now more than 5,000 tracked and documented asteroids, an additional 13,000 identified, and an estimated total of 1,000,000. About 100,000 asteroids are bright enough to be photographed from Earth. Only one, Vesta (the fourth one discovered), is bright enough to be seen with the naked eye.

Words to Know

Apollo objects: Group of asteroids that crosses Earth's orbit.

Bode's Law: Simple yet flawed mathematical formula establishing the distances of planets from the Sun.

Kirkwood gaps: Areas separating distinct asteroid belts that lie between the orbits of Mars and Jupiter.

Planetesimals: Ancient chunks of matter that are leftover "building blocks" from the formation of the solar system.

Asteroid belt

Most asteroids lie in a main belt between the orbits of Mars and Jupiter. This belt is divided into smaller belts, separated by distances known as Kirkwood gaps (named for their discoverer, nineteenth-century American astronomer Daniel Kirkwood). Kirkwood gaps are spaces in which the gravitational attraction of Jupiter prevents any object from maintaining an orbit.

Not all asteroids reside in the main belt. Trojan asteroids are located in two clusters, one on either side of Jupiter. This arrangement is a consequence of the gravitational attraction of Jupiter and the Sun. Another class of asteroids crosses the orbits of several planets.

Impacts with Earth

One group of asteroids, called Apollo objects, cross Earth's orbit. These bodies may come relatively close to Earth, and some have even collided with the planet. In 1908, an asteroid about one-tenth of a mile in diameter came through the atmosphere and exploded above central Siberia. The blast produced a mushroom cloud, wiped out a herd of reindeer, scorched and uprooted trees for miles around, and shattered windows 600 miles away. Asteroids ten times the size of this Siberian asteroid are estimated to hit Earth every few hundred thousand years.

Some 65 million years ago an asteroid struck Earth off the northern tip of the Yucatan Peninsula of Mexico. The asteroid, known as Chicxulub (pronounced cheek-soo-LOOB), left a crater under the Gulf of Mexico with an estimated diameter of 110 miles (177 kilometers). Some scientists believe the resulting inferno from the impact incinerated hundreds of thousands of species of plant and animals, and caused the extinction of the dinosaurs.

About 250 million years ago, an asteroid 4 to 8 miles (6.4 to 12.8 kilometers) across slammed into Earth with the force of more than a million earthquakes, setting off what scientists theorize was the most catastrophic of mass extinctions. The impact carved out a crater some 75 miles (121 kilometers) wide and caused huge volcanic eruptions that buried much of the planet in lava. It blasted millions of tons of rock and dust into the sky, blotting out the Sun's rays. It also brought about changes in sea levels and climatic shifts. These changes killed 90 percent of the marine species and 70 percent of the backboned land animals that lived during that geologic period. Scientists discovered evidence of this devastating extinction by digging deep into Earth's core. Trapped with soccer-ballshaped carbon molecules called buckminsterfullerenes or buckyballs, scientists found a mix of helium and argon gases similar to that found in certain stars but unlike anything that could form naturally on Earth.

Scientists now believe a flurry of asteroids barraged Earth and the Moon some 4 billion years ago, roughly the same time life was forming on the planet. In a study released at the end of 2000, scientists reported that a rain of asteroids lasting from 20 million to 200 million years melted rocks, blasted out craters, and reshaped the surface of both the Moon and Earth. On the Moon, the bombardment produced the great basins that are clearly visible from Earth. The asteroids that slammed into Earth, ten times larger than the Chicxulub asteroid, blasted craters rim to rim the size of present-day continents, vaporized the oceans, and filled the atmosphere with life-choking fog. Some scientists theorize that the impacts may have affected the evolution of life, forcing it to begin anew, or may have brought minerals, water, or even the building blocks of life to the planet.

Orbiting, then landing on, an asteroid

In April 2000, after traveling some 2 billion miles (3.2 billion kilometers) since it left Earth on February 17, 1996, the unmanned spacecraft NEAR (Near Earth Asteroid Rendezvous) Shoemaker began a circular orbit around the asteroid Eros. It was the first time a spacecraft had orbited an asteroid. During its one-year mission around Eros, the 1,100-pound (500-kilogram) spacecraft settled into an orbit that at one point was as close 3 miles (5 kilometers) above the potato-shaped asteroid. Eros, named after the Greek god of physical love, is one of the larger asteroids in the solar system, measuring about 21 miles (34 kilometers) long and 8 miles (13 kilometers) thick. It is called a near-Earth asteroid because its orbit crosses that of Earth and poses a potential collision danger. Scientists estimate it is 4.5 billion years old, almost unchanged since the beginning of the solar system.

Loaded with six instruments, NEAR Shoemaker took measurements to determine the mass, density, chemical composition, and other geological characteristics of the asteroid. It also beamed back to Earth some 160,000 images of Eros. On February 12, 2001, NEAR Shoemaker used the last of its fuel in an attempt to land on the surface of Eros. The craft had not been designed with landing gear, and mission scientists had given it a 1 percent chance of survival. Bumping into the asteroid at a mere 4 miles (6.4 kilometers) per hour, however, the hardy spacecraft survived, becoming the first spacecraft to land on an asteroid. On its way down to the surface, NEAR Shoemaker continued to transmit pictures back to Earth. Once on the surface, it continued to collect invaluable data about the oddly shaped asteroid. Even though scientists will probably study the data for years, they did learn early on that the asteroid does not tumble wildly through space. Instead, it rotates around one axis, much like a planet.

An end to its successful five-year, $225 million mission came on February 28, 2001, when scientists put NEAR Shoemaker into a planned hibernation. Scientists did not believe the spacecraft would survive the frigid darkness of winter on Eros, when temperatures would plummet to −238°F (−150°C). There is a very slight chance it might come alive again sometime in 2002 when full sunlight returns to that part of the asteroid.

[ See also Meteors and meteorites ]

Also read article about Asteroid from Wikipedia

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