Aug. 17, 1998 Astronomers enjoy a close-up view of near-Earth asteroids
NASA announced the discovery of two new Near Earth Asteroids on Aug. 6, 1998. 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1997XF11 was on a collision course with Earth in 2028. 1997XF11 caused a brief sensation before the collision-alert was retracted based on improved calculations of the asteroid's orbit.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.
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.
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.
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.
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. 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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The above story is reprinted from materials provided by NASA/Marshall Space Flight Center--Space Sciences Laboratory.
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