Asteroids are metallic, rocky bodies without
atmospheres that orbit the Sun but are too small to be classified
as planets. Known as "minor planets," tens of thousands of asteroids congregate in the so-called main asteroid belt: a vast,
doughnut-shaped ring located between the orbits of Mars and Jupiter from approximately 2 to 4 AU (186 million to 370 million
miles/300 million to 600 million
kilometers). Gaspra and Ida are main belt asteroids.
Asteroids are thought to be primordial material
prevented by Jupiter's strong gravity from accreting into a planet-sized
body when the solar system was born 4.6 billion years ago. It is estimated that the total mass of all asteroids would comprise
a body approximately 930 miles (1,500 kilometers) in diameter -- less than half the size of
Known asteroids range in size from the largest --
Ceres, the first discovered asteroid in 1801 -- at about 600 miles (1,000
kilometers) in diameter down to the size of pebbles. Sixteen asteroids have diameters of 150 miles (240 kilometers) or greater.
The majority of main belt asteroids follow slightly elliptical, stable orbits, revolving in the same direction as the Earth
and taking from three to six years to complete a full circuit of the Sun.
of asteroids has been derived from three main sources: Earth-based remote sensing, data from the Galileo flybys, and laboratory
analysis of meteorites. Asteroids are classified into different types according to their albedo, composition derived from
spectral features in their reflected sunlight, and inferred similarities to known
meteorite types. Albedo refers to an
object's measure of reflectivity, or intrinsic brightness. A white, perfectly reflecting surface has an albedo of 1.0; a black,
perfectly absorbing surface has an albedo of 0.0.
The majority of asteroids fall into the following three categories:
(carbonaceous): Includes more than 75 percent of known asteroids. Very dark with an albedo of 0.03-0.09. Composition
is thought to be similar to the Sun, depleted in hydrogen, helium, and other volatiles. C-type asteroids inhabit the main
belt's outer regions.
S-type (silicaceous): Accounts for about 17 percent of known
asteroids. Relatively bright with an albedo of 0.10-0.22. Composition is metallic iron mixed with iron- and magnesium-silicates.
S-type asteroids dominate the inner asteroid belt.
· M-type (metallic): Includes many
of the rest of the known asteroids. Relatively bright with an albedo of 0.10-0.18. Composition is apparently dominated by
metallic iron. M-type asteroids inhabit the main belt's middle region.
Asteroids with orbits that bring them within 1.3 AU (121 million miles/195 million kilometers) of the Sun are known as Earth-approaching
or near-Earth asteroids (NEAs). It is believed that most NEAs are fragments jarred from the main belt by a combination of
asteroid collisions and the
gravitational influence of Jupiter. Some NEAs may be the nuclei of dead, short-period comets.
The NEA population appears to be representative of most or all asteroid types found in the main belt.
NEAs are grouped into three categories, named for famous members of each: 1221 Amor, 1862 Apollo, and 2062 Aten.
Amors: Asteroids which cross Mars' orbit but do not quite reach the orbit of Earth. Eros -- target of the NEAR mission
-- is a typical Amor.
Apollos: Asteroids which cross Earth's orbit with a period greater
than 1 year. Geographos represents the Apollos.
Atens: Asteroids which cross Earth's
orbit with a
period less than 1 year. Ra-Shalom is a typical Aten.
NEAs are a dynamically
young population whose orbits evolve on 100-million-year time scales because of collisions and gravitational interactions
with the Sun and the terrestrial planets. Approximately 250 NEAs have been found to date, probably only a few percent of their
total population. The largest presently known is 1036 Ganymed, with an approximate diameter of 25.5 miles (41 kilometers).
Estimates suggest at least a thousand NEAs may be large enough -- 0.6 mile (1 kilometer) or more in diameter -- to threaten
Many bodies have struck Earth and the Moon in the past, and one widely accepted theory
blames the impact 65 million years ago of an asteroid or comet at least 6 miles (10 kilometers) in diameter for mass extinctions
among many lifeforms, including the dinosaurs. Other theories suggest that the chemical building blocks of life and much of
Earth's water arrived on asteroids or comets that bombarded
the planet in its youth.
June 30, 1908, a small asteroid 330 feet (100
meters) in diameter exploded over the remote region of Tunguska in Siberia,
devastating more than half a million acres of forest. One of the most recent close calls occurred on March 23, 1989, when
an asteroid 0.25-mile (0.4-kilometer) wide came within 400,000 miles (640,000 kilometers) of Earth. Surprised scientists estimated
that Earth and the asteroid -- weighing 50 million tons and traveling at 46,000 miles/hour (74,000 kilometers/hour) -- had
passed the same point in space just six hours apart.