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Asteroids - Meteorites
Visitors from the solar system
This chapter covers all of the solar system that is not the planets or the Sun:
meteorites, asteroids, and comets. Some group them all together as ``solar
system fluff'' because the objects are much smaller than planets and most
The ``fluff'' may be small in size but certainly not in importance. We get clues
of the origin of the solar system from these small objects.
There are millions of
small rocks that
orbit the Sun, most
of them between the
orbits of Mars and
Jupiter. About one
million of them are
larger than 1
kilometer across.
This diagram shows the positions of all numbered asteroids and
comets within the orbit of Jupiter as of July 1, 2002. Asteroids
are yellow dots; comets are white, sunward-pointing arrowheads.
Those smaller than about
300 kilometers across have
irregular shapes because
their internal gravity is not
strong enough to compress
the rock into a spherical
The largest asteroid is Ceres
with a diameter of 1000
kilometers. Pallas and Vesta
have diameters of about 500
kilometers and about 15
others have diameters larger
than 250 kilometers.
The combined mass of all of the asteroids is less than the
Moon's mass. Very likely the asteroids are pieces that
would have formed a planet if Jupiter's strong gravity had
not stirred up the material between Mars and Jupiter.
Unlike the movie The Empire Strikes Back, where the spacecrafts
flying through an asteroid belt could not avoid crashing into them,
real asteroids are at least tens of thousands of kilometers apart
from each other. Several spacecraft sent to the outer planets
have traveled through the asteroid belt with no problems.
Three basic types of asteroids:
C = Carbonaceous
S = Silicates
M = Metals (iron and nickel)
Made of silicate materials with a lot of carbon compounds so they
appear very dark. They reflect only 3 to 4% of the sunlight hitting
them. You can tell what they are made of by analyzing the spectra
of sunlight reflecting off of them. This reflectance spectra shows
that they are primitive, unchanged since they first solidified about
4.6 billion years ago.
The asteroid called Mathilde, recently explored by the NEAR
spacecraft is an example of this type
They are made of silicate materials without the dark carbon
compounds so they appear brighter than the C types. They
reflect about 15 to 20% of the sunlight hitting them. Most of
them appear to be primitive and they make up a smaller fraction
of the asteroids than the C types. Gaspra and Ida, explored by
the Galileo spacecraft on its way to Jupiter, are examples of
this type.
They are made of metals like iron and nickel. These
rare type of asteroids are brighter than the S and C
types. We think they are the remains of the cores of
differentiated objects. Large objects were hot enough
in the early solar system so that they were liquid. This
allowed the dense materials like iron and nickel to sink
to the center while the lighter material like ordinary
silicate rock floated up to the top. Smaller objects
cooled off quicker than larger objects, so they
underwent less differentiation.
When asteroids collide with one another,
they can chip off pieces from each other.
Some of those pieces, called meteoroids, can
get close to the Earth and be pulled toward
the Earth by its gravity.
Using imagery collected from NASA
shuttle missions, scientists have been
studying a large, multi-ringed crater
centered near the town of Puerto
Chicxulub on the Yucatan peninsula. It is
believed that the crater was formed by
an asteroid or comet which slammed into
the Earth more than 65 million years ago.
It is this impact crater that has been
linked to a major biological catastrophe
where more than 50 percent of the
Earth's species, including the dinosaurs,
became extinct.
A view of what a large
asteroid might look like as
it hits the Earth.
Missions to Asteroids and Comets
1. Deep Impact - NASA Flyby of Comet P/Tempel 1 (2005)
2. Rosetta - ESA Mission to Rendezvous with Comet ChuryumovGerasimenko (2004)
3. Hayabusa (Muses-C) - ISAS (Japan) Sample Return Mission to
Asteroid 25143
4. Contour - NASA Mission to fly by three comet nuclei (2002)
5. Stardust - NASA Discovery Mission to Comet P/Wild 2 (1999)
6. Deep Space 1 - NASA Flyby Mission to asteroid 1992 KD (1998)
7. Galileo - NASA Mission to Jupiter via asteroids Gaspra and Ida
8. Giotto - ESA mission to Comets Halley and Grigg-Skjellerup
9. ICE (ISEE-3) - NASA Mission to Comet Giacobini-Zinner
10. NEAR - NASA Near-Earth Asteroid Rendezvous with 433 Eros
11. Sakigake - Japanese ISAS mission to Comet Halley
12. Suisei - Japanese ISAS mission to Comet Halley
13. Vega 1 - Soviet mission to Venus and Comet Halley
14. Vega 2 - Soviet mission to Venus and Comet Halley
NEAR Shoemaker
The Near Earth Asteroid Rendezvous Shoemaker (NEAR Shoemaker), renamed in
honor of Gene Shoemaker, was designed to
study the near Earth asteroid Eros from
close orbit over a period of a year. The
mission was the first-ever to orbit an
asteroid and to touch down on the surface
of an asteroid.
The ultimate goal of the mission was to study the near
Earth asteroid 433 Eros from orbit for approximately
one year. Eros is an S-class asteroid approximately 13 x
13 x 33 km in size, the second largest near-Earth
asteroid. The mission ended with a touchdown in the
"saddle" region of Eros on 12 February 2001.
NEAR Descent Image
NEAR Shoemaker took this
image of asteroid 433 Eros
from a range of 3,773 feet
Launched in January 2005, NASA's Deep Impact spacecraft will
deploy an impactor that will essentially be "run over" by the
nucleus of comet Tempel 1 on, July 4.
Before, during and after the demise of this 820-pound impactor,
a "flyby" spacecraft will be watching the 4-mile wide comet
nucleus from nearby, collecting pictures and data of the event.
The impactor's impact with comet Tempel 1 is expected to form a
football-field-sized crater, seven stories deep. Ice and dust
debris will be ejected from the crater.
This glimpse beneath the surface of a comet, where material and
debris from the formation of the solar system remain relatively
unchanged, will answer basic questions about the formation of the
solar system.
Your opinion matters!
Should the government, prepare a meteorite
defense plan for earth?
What are your greatest global concerns?