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Asteroids
Asteroid
951 Gaspra (top) compared with Deimos (lower left) and Phobos
(lower right), the moons of Mars. The three bodies are shown at
the same scale and nearly the same lighting conditions. Gaspra is
about 17 kilometers (10 miles) long. credit:NASA
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.
credit:
NASA
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 the Moon.
This
artist's impression shows how the families of asteroids follow the
orbits of their prototypes. Amors cross Mars’ orbit but do not
cross Earth’s. It is the Apollo asteroids that cross Earth’s
orbit. There are also the Atens. These asteroids spend most of
their time inside Earth’s orbit, crossing it briefly before
disappearing back into the glare of the Sun.Credits: ESA
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.
Comparative
sizes of asteroids.
Credits: courtesy Observatoire de Paris
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.
The
asteroid Ida and its moon, Dactyl
credit:NASA
Our understanding 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:
- C-type (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.
The relationship between
asteroids and meteorites remains a puzzle. The most common
meteorites, known as ordinary chondrites, are composed of small
grains of rock and appear to be relatively unchanged since the
solar system formed. Stony-iron meteorites, on the other hand,
appear to be remnants of larger bodies that were once melted so
that the heavier metals and lighter rocks separated into different
layers.
A long-standing scientific
debate exists over whether the most common asteroids -- the
S-types -- are the source of ordinary chondrites. Spectral
evidence so far suggests that the S-type asteroids may be
geochemically processed bodies akin to the stony-irons. If S-types
are unrelated to ordinary chondrites, then another parent source
must be found. If the two are related, then scientists need an
explanation for why they aren't spectrally similar.
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 Earth.
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.
On 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.
Trees
felled by the Tunguska blast
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.
In 2002 June, the small
100-meter asteroid 2002 MN was discovered only after it whizzed by
the Earth, passing well within the orbit of the Moon.
On 12 May 2007 there were 859
known Potentially Hazardous Asteroids. Many asteroids remain
undiscovered.
NASA's Jet
Propulsion Laboratory is introducing a new Web site that will
provide a centralized resource for information on near-Earth
objects - those asteroids and comets that can approach Earth.
Click
on the logo below to visit the site-
Credit: NASA, Observatoire de Paris,
European Space Agency
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