Asteroids
are rocky fragments left over from the formation of
the solar system about 4.6 billion years ago. Most
of these fragments of ancient space rubble - sometimes
referred to by scientists as minor planets - can be
found orbiting the Sun in a
belt between Mars and Jupiter.
This region in our solar system, called the Asteroid
Belt or Main Belt, probably contains millions of asteroids
ranging widely in size from Ceres, which at 940 km
in diameter is about one-quarter the diameter of our
Moon, to bodies that are less than 1 km across. There
are more than 20,000 numbered asteroids.
As asteroids revolve around the Sun
in elliptical orbits, giant Jupiter's gravity and
occasional close encounters with Mars or with another
asteroid change the asteroids' orbits, knocking them
out of the Main Belt and hurling them into space across
the orbits of the planets. For example, Mars moons
Phobos and Deimos may be captured asteroids. Scientists
believe that stray asteroids or fragments of asteroids
have slammed into Earth in the past, playing a major
role both in altering the geological history of our
planet and in the evolution of life on it. The extinction
of the dinosaurs 65 million years ago has been linked
to a devastating impact near the Yucatan peninsula
in Mexico.
Asteroids were first observed with telescopes
in the early 1800s, and in 1802, the astronomer William
Herschel first used the word "asteroid," which means
"starlike" in Greek, to describe these celestial bodies.
Most of what we have learned about asteroids in the
past 200 years has been derived from telescopic observations.
Ground-based telescopes are used to watch asteroids
that orbit close to Earth, not only to detect new
ones or keep track of them, but also to watch for
any asteroids that might collide with Earth in the
future. Scientists define near-Earth asteroids (NEAs)
as those whose orbits never take them farther than
about 195 million kilometers from the Sun.
In the last few decades,
astronomers have used instruments called spectroscopes
to determine the chemical and mineral composition of
asteroids by analyzing the light reflected off their
surfaces. Scientists also examine meteorites - the remains
of comets or
asteroids that can be found on Earth - for clues to
the origin of these bodies. About three-quarters of
asteroids are extremely dark and are similar to carbon-rich
meteorites called carbonaceous chondrites (C-type).
About one-sixth of asteroids are reddish, stony-iron
bodies (S-type).
In
1997, instruments on the Hubble Space Telescope mapped
Vesta, one of the largest asteroids, and found an enormous
crater formed a billion years ago. Interestingly, Vesta
is an uncommon asteroid type, yet meteorites hav-ing
the same composition have been found on Earth. Could
these be rem-nants from the collision that created Vesta's
giant crater?
NASA's Galileo spacecraft
was the first to observe an asteroid close-up, fly-ing
by main-belt asteroids Gaspra and Ida in 1991 and 1993,
respectively. Gaspra and Ida proved to be irregularly
shaped objects, rather like potatoes, riddled with craters
and fractures, 19 km long and 52 km long respectively.
Galileo also discovered that Ida has its own moon, Dactyl,
a tiny body in orbit around the asteroid that may be
a fragment from past collisions.
NASA's Near-Earth Asteroid
Rendezvous (NEAR) mission was the first dedicated scientific
mission to an asteroid. The NEAR Shoemaker spacecraft
caught up with asteroid Eros in February 2000 and orbited
the small body for a year, studying its surface, orbit,
mass, composition, and magnetic field. In February 2001,
mission controllers guided the spacecraft to the first-ever
landing on an asteroid.
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Source: NASA
Last Updated : 08.20.2003 |
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