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Cosmic Near Misses Hit The Scientific Bullseye


       Cosmic near misses hit the scientific bullseye
       August 7, 1998
       A NASA/Marshall Space Flight Center Space Sciences news release
       http://science.msfc.nasa.gov/newhome/headlines/ast07aug98_1.htm 

       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.

       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       collision-alert was retracted based on improved calculations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.

       Using 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. Learn more
       about asteroid radar research. 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.

       The 70-meter (230-foot) diameter antenna in Goldstone, Calif.
       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. 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.

       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. A computer model of asteroid
       Toutatis based on radar data obtained with the Goldstone
       radio telescope. more info..

       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.

       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.

       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.

       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.

       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.

       More on the way 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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