| August 7, 1998:
Yesterday NASA announced the discovery of two new Near Earth Asteroids.
They were found in observations made with JPL's Near-Earth
Asteroid Tracking (NEAT) system.
Projected orbits that show that neither of the objects pose an immediate threat to Earth,
although one of the two could pass as close as 5 million kilometers
(about 3 million miles) --
about 12 times the distance between Earth and the Moon.
That's a near miss in cosmic terms, but NASA scientists note that there's no
significant probability of a collision with Earth, at least in the near future.
|
A RADAR image of Asteroid Toutatis, which passed within 2.3 million miles of Earth in 1992.
Credits.
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Asteroid and comet collisions have become a popular topic
thanks to the recent movies "Armageddon" and "Deep Impact",
and the
premature announcement earlier
this year that asteroid 1997XF11 was on a
collision course with Earth in 2028.
1997XF11 caused a brief sensation before the
collision-alert was retracted based
on improved calculations of the asteroid's
orbit. Often lost in the
hoopla over the potential for catastrophe is the potential
for scientific
discovery represented by such near-Earth objects.
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In this story:
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Space Radar
| When an asteroid or comet passes near the Earth,
it's an opportunity for astronomers to make close-up observations to
find out what these objects are really like. One of the most
exciting new methods for studying asteroids is "radar astronomy." Astronomers
can use large radio telescopes to bounce radio signals off nearby asteroids.
From the echos they can make three-dimensional maps of
the asteroid, measure its rotation, estimate the surface composition, and
pinpoint its orbital elements. We need to
know all these
things to have a realistic hope of deflecting an asteroid, should one
ever zero in on Earth. There's also discussion in scientific circles
of mining asteroids for their
resources. The more we know about these
objects, the easier that will be.
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The Arecibo Observatory, located in Puerto Rico, is
operated by the National Astronomy and Ionosphere Center.
The giant 305 meter dish is the world's most powerful RADAR. It
can transmit pulses of radio energy with up to 500 mega-watts of power.
The Arecibo radar has been used to map the surfaces of Venus and Mars, and
to study the properties of near-Earth asteroids.
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The 70-meter (230-foot) diameter antenna in Goldstone California is
part of NASA's Deep Space Network (DSN).
It is the largest of the DSN antennas, and is capable
of
tracking a spacecraft travelling more than 16
billion kilometers (10 billion miles) from Earth.
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The two radars most commonly used for asteroid and comet studies are the
Arecibo radio telescope in Puerto, Rico, and the Goldstone Solar System Radar
(part of the Deep Space Network) in the Mojave desert. The Arecibo
is
a behemoth -- it measures 305 meters in diameter and fills a karst valley
in central Puerto Rico. Although the radar is large and powerful, it
is only partially steerable and cannot view asteroids unless they pass
almost directly overhead. For this reason the 70m Goldstone antenna, although smaller,
has an advantage in some cases. It can be steered to view the entire northern sky.
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All together over 40 Near Earth Asteroids have been
detected by either Arecibo or Goldstone. In some cases dramatic 3D maps have been obtained.
In 1992 Asteroid Toutatis passed within 2.3 million miles of Earth.
These four radar images of Toutatis show shallow craters,
linear ridges and a deep topographic "neck" whose geologic origin is not known.
It may have been sculpted by impacts into a single body,
or this asteroid might actually consist of two
separate objects that came together in a gentle collision.
Toutatis is about 4.6 kilometers (3 miles) long and the
resolution of the computer image is about 84 meters.
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A computer model of asteroid Toutatis based on radar data
obtained with the Goldstone radio telescope.
more info..
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According to scientists at JPL, Toutatis has one of the strangest rotation states yet observed
in the solar system. Instead of the spinning about a single axis
as do the
planets and the vast majority of asteroids, it
"tumbles" somewhat like a football after an errant
pass. Its rotation is the result of two different
types of motion with periods of 5.4 and 7.3 Earth
days that combine in such way that Toutatis's orientation
with respect to the solar system never repeats.
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Asteroid Mining
The possibility of mining asteroids for their natural resources
has been suggested for two reasons: (1) extracted minerals might be
returned to Earth or (2) in situ materials could be used to build
space stations or used as fuel for exploration. Returning pieces of
an asteroid to Earth will be expensive, but it might be worth it.
Planetary astronomers believe the average asteroid should have
relatively high abundances of the rare platinum and platinum-group
precious metals as well as gold.
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It is more likely that asteroid mining would be used to support
space exploration, i.e., space stations or even a lunar base.
The most useful material
for these applications would likely be water, extracted
from near-earth asteroids that
are either C-type (carbonaceous) asteroids
or extinct comet nuclei.
Together these make up half or more of the near-earth
asteroid population. Water would be used to make hydrogen and
oxygen rocket propellants, and
water and oxygen would be useful for life support
in space habitats.
Metals like iron and nickel
might also be mined as raw material for the
construction of structures in space.
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Above is an artist's concept of an early Moon base
under development. Such
a base could rely on raw materials mined
from near-Earth asteroids and from the moon.
(painting by Pat Rawlings of SAIC for NASA/Lewis)
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Solar powered processing plants may be common elements
of future asteroid mining operations, but it's necessary to first
stop or slow the asteroid's rapid spin so that the
solar panels will face the sun. (Artist's
concept of solar panels on the moon by Pat Rawlings of SAIC for NASA/Lewis)
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Mining an asteroid almost certainly requires that we be able to land on it.
Landing on a tumbling asteroid like Toutatis
would be difficult, if not impossible.
Some mining studies call for stopping the rotation
in order to attach the solar-powered processing
equipment to the asteroid. The solar panels would always face the sun.
This might be done by anchoring
a cable, wrapping it around the
asteroid, and using a rocket-powered
"space jeep" to slow down and stop its rotation.
But, for a 100 meter diameter asteroid rotating
4 times per day, about 29 tons of fuel would be needed.
Toutatis has about 300,000 times more rotational energy (on two spin axes),
so de-spinning
it would probably be impractical. Miners would need to find
a smaller asteroid with less complicated spin.
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More on the way
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Astronomers are now waiting expectantly for next
close approach of the infamous
Near-Earth asteroid 1997XF11.
In
October
2002 XF11 will pass about 9.5 million km from Earth. It will then be an excellent target for detailed radar
observations, and in
2028 it may even be bright enough to be seen without
telescopic aid. In the meantime, scientists will continue to make radar
observations of
near-earth asteroids whenever possible. As the pace of asteroid
discoveries continues to increase, observing opportunities
should be numerous.
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News from JPL:
These Goldstone radar echoes from 1998KY26 were obtained on June 6,
1998, five days after the announcement of its discovery by the
Spacewatch telescope. Credits
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