home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Space & Astronomy
/
Space and Astronomy (October 1993).iso
/
mac
/
TEXT_ZIP
/
jplnews
/
1404B.ZIP
/
1404B.PR
Wrap
Text File
|
1993-05-03
|
7KB
|
137 lines
PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Contact: Diane Ainsworth
GENERAL BACKGROUND RELEASE November 14, 1991
While scientific knowledge of asteroids has increased
significantly in the last decade, Galileo's recent flight past
the main-belt asteroid, Gaspra, may yield new information about
planet formation in the early solar system.
"These small rocky bodies represent matter in a pristine
form," said Dr. Alan Harris of JPL's Geology and Planetology
Section. "Asteroids may be able to tell us more about the early
solar system than planets, because they have evolved far less.
Their chemical and physical structures are nearly the same as
they were during the formation of the solar system."
Asteroids, named for the Greek word meaning "starlike," are
the remnants of planetary materials that failed to accrete during
the formation of the solar system about 4.6 billion years ago.
More than 90 percent of all known asteroids have orbits in the
vast region of space between Mars and Jupiter, called the main
asteroid belt.
"The gravitational disturbances surrounding Jupiter
prevented any materials from forming, or coalescing, in that
region," Harris said. "So far, we would estimate there are about
10,000 asteroids 10 kilometers or larger in the main belt. About
1,000 are the size of Gaspra or larger."
The main-belt asteroids orbit between Mars and Jupiter at an
average inclination of about 10 degrees from the ecliptic plane
1
in which the planets of the solar system orbit. The main
asteroid belt stretches from its inner edge of 2.2
astronomical units (AUs) to about 3.3 AUs, or 200 to 300 million
miles from the sun. Planetary scientists have been able to
calculate and catalog the orbital paths of about 5,000 of the
asteroids.
Some asteroids have very different orbits, however. The
Trojan asteroids, for instance, orbit far beyond the main
asteroid belt, circling the sun in step behind and ahead of the
giant planet Jupiter, at the same distance of 5.2 AUs that
Jupiter orbits the sun. Others follow elliptic paths that cross
the orbits of Mars and Earth. A few even come closer to the sun
than Mercury.
Asteroids that cross Earth's orbit are called "near-Earth
asteroids." The most common subclasses are the Apollo, Amor and
Aten groups.
"Among the near-Earth asteroids, the largest we have
discovered is about 10 kilometers (6 miles) across," Harris said,
"but the more common asteroids are 1 kilometer or larger. We
estimate there are a couple thousand that size, of which only 200
have been discovered."
While a chance collision with an asteroid that crosses
Earth's orbit is possible, Harris said the probability is low.
"On average, the chance of a collision with a single
asteroid will be about once every 100 million years," he said.
"Since there are about 2,000 of them 1 kilometer or larger in
size, the Earth will be struck by one such asteroid about every
50,000 years."
The Diablo Canyon crater near Flagstaff, Ariz., is an
example of a collision with a meteorite that was less than 1
kilometer in diameter.
"But every 100 million years or so, the Earth is struck by
one of the largest near-Earth asteroids (10 kilometers or more
across), causing massive biological extinctions," Harris added.
"One such event is thought to have caused the extinction of the
dinosaurs 65 million years ago."
Asteroids vary not only in size but in shape. Most are
irregularly shaped, with a long and short axis, and may look
something like a potato. The largest known asteroid, Ceres, has
a diameter of 570 miles. Gaspra is a small, main-belt asteroid,
about the size of the Martian moon Deimos, with an average
diameter of eight miles. Ground-based observations indicate
Gaspra is quite irregular in shape, perhaps twice as long as it
is high.
Asteroids also vary in composition and are grouped
accordingly in taxonomic classes, Harris said. Their colors and
albedos -- the amount of sunlight reflected off the asteroid --
are measured to determine the object's composition and
classification. The variety of colors and spectra suggest
different surface compositions.
"The most common asteroids are extremely dark and thought to
be composed of minerals analogous to a class of meteorites called
carbonaceous chondrites," Harris said. They are known as the C-
asteroids, where "C" implies carbonaceous. C-class asteroids
dominate the outer part of the main asteroid belt.
S-asteroids, such as Gaspra, are the next most populous
group. The "S" refers to silicaceous. In these asteroids, the
dark carbon compounds are missing, so the objects have moderate
albedos -- typically about 16 percent -- and reddish hues. The
compositions of these asteroids are thought to be combinations of
iron and magnesium-bearing silicates, and may also contain some
metallic nickel-iron.
Scientists want to know whether the S-asteroids, like the
C-asteroids, are linked chemically to the ordinary chondrite
meteorites, the most common class of meteorites. They hope
Galileo's close-range spectral scans of Gaspra will tell them
more about the asteroid's composition, as well as its crater
density and surface structure.
"The third type of asteroid family is the M-class, for
metallic asteroids," Harris said. "These asteroids also have
moderate albedos, but without the spectral signatures of silicate
compounds."
Distribution patterns of the asteroids show a clear
progression with solar distance.
"The S-asteroids are closer to Earth than the M-asteroids
and C-asteroids," Harris said. S's are found primarily in the
inner regions of the asteroid belt, while M's and C's seem to be
distributed in the middle and outer belt.
Some asteroids may also be the cores of extinct comets.
Scientists believe that between 10 to 50 percent of the near-
Earth asteroids may be comet cores.
"The only clear distinction between a comet and an asteroid
lies in whether the object has a visible atmosphere," said Ray
Newburn of JPL's Atmospheric and Cometary Sciences Section.
"If the object begins to produce copious gases as it nears
the sun, it's considered a comet," he said. "Comets contain
water ice and other volatiles that vaporize when heated by the
sun. Generally, they begin to produce an extensive atmosphere,
called the coma, and a tail of escaping gas as they warm up.
"By contrast, asteroids are inactive bodies," Newburn said.
"Some could be the cores of extinct comets."
The Oct. 29, 1991 Gaspra flyby was part of NASA's Galileo
mission to Jupiter. The mission is managed by the Jet Propulsion
Laboratory for NASA's Office of Space Science and Applications.
######
#1404
11/14/91 dea JPL-PIO