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Asteroids What are Asteroids?Asteroids are metallic, rocky bodies that have no atmosphere and are too small to be considered planets. Tens of thousands of asteroids can be found orbiting the sun in the Asteroid Belt located between the elliptical orbits of Mars and Jupiter. The Asteroid Belt is approximately 2-4 AU from the sun. Where do Asteroids Come From?Asteroids are thought to be left over material from the formation of the solar system nearly 4.6 billion years ago. It has been suggested that the material was prevented from developing into a planet in part by the gravitational effects of Jupiter. If all the asteroids were combined into one body, it would have a diameter of about 930 miles, which would be about half the size of the moon. Asteroids vary greatly in size. The largest known asteroid is named Ceres (discovered in 1801), and it is about 600 miles in diameter. The smallest of asteroids are pebble-sized. The average period of asteroid orbits vary from 3-6 years, but there are exceptions. Classification
Asteroids are classified according to albedo (reflectivity), composition
(derived from spectral features), and inferred similarities to meteorite
types. Albedo is also known as intrinsic brightness. A white body with
perfect reflection has an albedo of 1.0, and a black body with perfect
absorption has an albedo of 0.0. There are three main categories of
asteroids:
M-type (metallic): The remaining asteroids are M-type, or metallic asteroids. As the name implies, they are composed mostly of metallic iron. These asteroids are also relatively bright, ranging from 0.10 to 0.18 albedo. Metallic asteroids are found in the middle portions of the asteroid belt. Near-Earth Asteroids (NEAs)Asteroids that have orbits within 1.3 AU from the sun are known as Earth approaching or near-Earth asteroids (NEAs). These asteroids are probably fragments of once larger asteroids that were jarred loose from the main belt due to a combination of collisions and the gravitational influence of Jupiter. Some NEAs are suspected to be the nuclei of dead, short-period comets. There have been about 250 NEAs found to date, but there are probably many more. The largest discovered NEA is called 1036 Ganymed, which has a diameter of about 25.5 miles. Evidence suggests that at least 1000 NEAs are large enough to threaten the Earth. Classifying NEAs
Like main belt asteroids, there are three types of NEAs. The first group
are called Amors NEAs. These are asteroids that cross the orbit of Mars,
but they do not quite reach the Earth's orbit. Apollos NEAs do cross the
Earth's orbit, and they have an orbital period of greater that 1 year.
Finally, Atens NEAs also cross the Earth's orbit, and they have a period
of less than 1 year.
References: Images and textual references from www.NASA.gov |
C-type (carbonaceous): This category includes 75% of known
asteroids. These asteroids are relatively dark having albedo readings
that range from 0.03 to 0.09. These asteroids inhabit the outer regions
of the main belt and are similar in compositon to the sun - depleted in
hydrogen, helium, and other volatile materials. The image on the right is
the C-type asteroid named Mathilde.
S-type (silicaceous): About 17% of known asteroids are S-type
asteroids. They are relatively bright (0.10-0.22 albedo), and they are
composed mostly of metallic iron mixed with iron and magnesium silicates.
S-type asteroids can generally be found in the inner part of the main
belt. The image at the right includes two close fly-by views of Eros, an
S-type asteroid.