Asteroid

I

Introduction

Asteroid, small rocky or metallic body that orbits the Sun. Hundreds of thousands of asteroids exist in the solar system. Asteroids range in size from a few meters to over 500 km (300 mi) wide. They are generally irregular in shape and often have surfaces covered with craters. Like icy comets, asteroids are primitive objects left over from the time when the planets formed, making them of special interest to astronomers and planetary scientists.

Most asteroids are found between the orbits of the planets Mars and Jupiter in a wide region called the asteroid belt. Scientists think Jupiter’s gravity prevented rocky objects in this part of the solar system from forming into a large planet. The giant planet Jupiter’s gravity also helped throw objects out of the asteroid belt. The hundreds of thousands of asteroids now in the asteroid belt represent only a small fraction of the original population.

Thousands of asteroids have orbits that lie outside the asteroid belt. Some of these asteroids have paths that cross the orbit of Earth. Many scientists think that an asteroid that hit Earth 65 million years ago caused the extinction of the dinosaurs. Because asteroids can pose a danger to people and other life on Earth, astronomers track asteroids that come near our planet. Space scientists are also studying ways to deflect or destroy an asteroid that might strike Earth in the future.

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II

Asteroids Compared to Other Solar System Objects

Asteroids are sometimes called planetoids or minor planets. Most rotate on their axes every 5 to 20 hours. Some smaller asteroids may orbit larger asteroids as satellites, forming binary pairs. Asteroids differ from true planets mainly by their much smaller size—only about 200 asteroids have diameters of more than 97 km (60 mi). Planets are defined as bodies that have settled into a rounded shape because of the inward pull of their own gravitation. Asteroids are not thought to have enough mass to settle into rounded shapes in the same way as planets. With their low masses, asteroids also do not have atmospheres.

In 2006 the International Astronomical Union (IAU) designated a new category of solar system objects called dwarf planets. The dividing line between asteroids and dwarf planets is still being clarified. Dwarf planets have rounded shapes but are not massive enough to clear other bodies from around their orbits. Major or “classical” planets had enough mass to clear their neighborhoods of small bodies, either by pulling such objects in as the planets formed or by throwing the small bodies into distant orbits or out of the solar system. Dwarf planets orbit the Sun in regions of the solar system that contain swarms of small bodies such as the asteroid belt or the Kuiper Belt. The largest object in the asteroid belt, 1 Ceres, is now classified as a dwarf planet because of its rounded shape. Future research may show that additional large asteroids also qualify as dwarf planets because their shape was rounded by their own gravitation.

Because of their small size and primitive composition, asteroids share many basic properties with comets. Comets typically have much more elliptical orbits than asteroids and actively shed gas and dust. Although asteroids are mainly rocky, some may also contain water-ice material and so are not clearly distinct from objects that can become comets if heated by enough sunlight. It is also possible that some objects that are considered asteroids are remains of dead comets that have lost their gas and dust. In 2006 astronomers announced finding a number of icy comet-like objects orbiting in the main asteroid belt, suggesting that asteroids and comets can occur together.

The term “asteroid” is sometimes extended to the small icy bodies found in the outer system beyond Jupiter. Unlike rocky asteroids, these more distant bodies are mainly made of ice, along with dust and rock. Closely related to comets, these solar system bodies are now generally called Kuiper Belt Objects (KBOs) or Trans-Neptunian Objects (TNOs). Most of these objects orbit the Sun beyond Neptune in a disk called the Kuiper Belt. A special population of these icy objects called centaurs have left the Kuiper Belt and have unstable orbits that lie between Neptune and Jupiter. Chiron, the first centaur discovered, was originally classified as an asteroid. Like asteroids and dwarf planets, KBOs and centaurs are listed in the official catalog of minor planets.

III

Origin of Asteroids

Asteroids are ancient objects. According to current theories, the Sun formed about 4.6 billion years ago and was surrounded by a disk of dust, ice, and gas. Planets are thought to have formed within this disk by the clumping together (accretion) of particles of dust and ice into larger and larger objects, producing small bodies called planetesimals. Gravitation drew groups of planetesimals together to form protoplanets, which in turn built up full-size planets. Asteroids and comets are left over from the planetesimal-forming stage and can preserve clues about the earliest material that formed the solar system.

The asteroid belt likely represents a region in which a large rocky planet could not form because the gravitation of the giant planet Jupiter disrupted the accretion process. Scientists estimate that the ancient asteroid belt may have held enough material to equal twice the mass of Earth. However, gravitational effects of Jupiter and orbiting protoplanets in the early solar system cleared out most of the objects in the asteroid belt. The planetestimals were either thrown out of the solar system or were sent crashing into planets and moons, or into the Sun. The mass of all the material now left in the asteroid belt is estimated to be only about 0.001 the mass of Earth. If all the objects in the asteroid belt were combined into a single body, it would be much smaller than Earth’s Moon.

The early asteroid belt evidently contained a number of objects large enough to heat up inside from radioactive minerals and form metallic cores and lava. Mutual collisions of asteroids later broke up or reshaped these larger bodies, accounting for unusual types of metallic and igneous asteroids and meteorites.

Collisions that broke up larger asteroids may also explain asteroid families—groups of asteroids that have the same chemical makeup and have closely related orbits within the asteroid belt. The 4 Vesta and the 8 Flora families are among the best known of such asteroid families. In some cases asteroid debris has reformed into new asteroids that appear to be piles of rubble loosely held together by gravity. Debris from collisions in the asteroid belt also includes dust particles and small rocky meteoroids that can burn up as meteors in Earth’s atmosphere or fall as meteorites if they reach the ground.

IV

Composition of Asteroids

With the exception of a few that have been traced to the Moon and Mars, most of the meteorites recovered on Earth are thought to be asteroid fragments. Remote observations of asteroids by telescopic spectroscopy and radar support this hypothesis. They reveal that asteroids, like meteorites, can be classified into a few distinct types.

Three-quarters of the asteroids visible from Earth belong to the C type, which appear to be related to a class of rare stony meteorites known as carbonaceous chondrites. These meteorites contain tiny spheres of melted material called chondrules and are considered the oldest materials in the solar system, with a composition reflecting that of the primitive solar nebula. Thus, unlike the Earth and the Moon, they have never either melted or been reheated since they first formed. About 5 percent of meteorites found in Earth are carbonaceous chondrites, which contain organic carbon compounds. These meteorites are extremely dark in color, probably because of their hydrocarbon content, and show evidence of chemical reactions with water. In addition to a major group of asteroids, objects with a carbonaceous chondrite composition include the dwarf planet 1 Ceres and the moons of Mars.

Asteroids of the S type, related to the stony iron meteorites, make up about 15 percent of the total population of asteroids that can be seen from Earth. Much rarer are the M-type objects, corresponding in composition to the meteorites known as “irons.” These objects are made up of an iron-nickel alloy and may represent the cores of bodies that were large enough to differentiate into layers and to melt deep inside. Their rocky outer layers may have been removed by impacts with other asteroids .

A very few asteroids, notably 4 Vesta, are probably related to the rarest meteorite class of all: the achondrites. These asteroids appear to have an igneous surface composition like that of many lunar and terrestrial lava flows. Thus, astronomers are reasonably certain that Vesta was, at some time in its history, at least partly melted. Scientists are puzzled that some of the asteroids have been melted but others have not. One possible explanation is that the early solar system contained certain concentrated, highly radioactive isotopes that might have generated enough heat to melt the asteroids.