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clean up & uniformity
years ago. It is estimated that the total mass of all
years ago. It is estimated that the total mass of all
asteroids would comprise a body approximately 930 miles
asteroids would comprise a body approximately 930 miles
(1,500 kilometers) in diameter -- less than half the size of
(1,500 kilometers) in diameter—less than half the size of
the Moon.
the Moon.
==Sizes==
==Sizes==
Known asteroids range in size from the largest --
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
miles (1,000 kilometers) in diameter down to the size of
pebbles. Sixteen asteroids have diameters of 150 miles (240
pebbles. Sixteen asteroids have diameters of 150 miles (240
==Controversy==
==Controversy==
A long-standing scientific debate exists over whether
A long-standing scientific debate exists over whether
the most common asteroids -- the S-types -- are the source
the most common asteroids—the S-types—are the source
of ordinary chondrites. Spectral evidence so far suggests
of ordinary chondrites. Spectral evidence so far suggests
that the S-type asteroids may be geochemically processed
that the S-type asteroids may be geochemically processed
Aten:
Aten:
*Amors: Asteroids which cross Mars' orbit but do not
*Amors: Asteroids which cross Mars' orbit but do not
quite reach the orbit of Earth. Eros -- target of the NEAR
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
*Apollos: Asteroids which cross Earth's orbit with a
period greater than 1 year. Geographos represents the
period greater than 1 year. Geographos represents the
with an approximate diameter of 25.5 miles (41 kilometers).
with an approximate diameter of 25.5 miles (41 kilometers).
Estimates suggest at least a thousand NEAs may be large
Estimates suggest at least a thousand NEAs may be large
enough—0.6 mile (1 kilometer) or more in diameter—to
threaten Earth.
threaten Earth.
kilometer) wide came within 400,000 miles (640,000
kilometer) wide came within 400,000 miles (640,000
kilometers) of Earth. Surprised scientists estimated that
kilometers) of Earth. Surprised scientists estimated that
Earth and the asteroid -- weighing 50 million tons and
Earth and the asteroid—weighing 50 million tons and
traveling at 46,000 miles/hour (74,000 kilometers/hour) --
traveling at 46,000 miles/hour (74,000 kilometers/hour) --
had passed the same point in space just six hours apart.
had passed the same point in space just six hours apart.
* [http://nssdc.gsfc.nasa.gov/planetary/text/asteroids.txt Asteroids] From the NEAR press kit
* [http://nssdc.gsfc.nasa.gov/planetary/text/asteroids.txt Asteroids] From the NEAR press kit
[[category:astronomy]]
[[Category:Astronomy]]