NEAR EARTH OBJECTS
What are NEOs? Where do they come from? Do they pose any real threat to
Earth? Can they provide viable space resources? All of these questions are now under
investigation by planetary scientist. There are two highly recognized research programs
that I will discuss with you. The Spaceguard program is sponsored and run by NASA Ames
Space Science Division: Asteroid and Comet Impact Hazard. Also under the direction of
Dr. Tom Gehrels the University of Arizona has the Spacewatch program.
NEOs can be either asteroid or comets. Ninety percent of the information that I came
across discussed asteroids. Therefore, I will concentrate on asteroids alone. I'm not
fully knowledgeable on the subject but I did learn a great deal.
What are NEOs? The "Webster's New World Dictionary" states, "Any of the small planets
between Mars and Jupiter". The "Funk and Wagnalls Encyclopedia from Infopedia" states,
"One of the many small or minor planets that are members of the solar system and that
move in elliptical orbits primarily between the orbits of Mars and Jupiter".
Where dot they come from? The NEOs are small objects (<7 miles) with a range of
compositions spanning all common asteroid types. They are derived from a mixture of
main-belt collisional fragments and burned-out short-period comets. According to Dr. Tom
Gehrels of the University of Arizona Spacewatch program, "The total number of NEOs over
100 meters is estimated to be about 100,000, with 150 or so currently known".
Do they pose any real threat to Earth? The Earth orbits the Sun in sort of a cosmic
shooting gallery, subject to impacts from asteroids. It is only fairly recent that we
have come to appreciate that these impacts by asteroids pose a significant hazard to life
and property. Although the annual probability of the Earth being struck by a large
asteroid is extremely small, the consequences of such a collision are so devastating that
it is prudent to assess the nature of the threat and prepare to deal with it.
Studies have shown that the risk from a cosmic impact increases with the size of the
projectile. The greatest risk is associated with objects large enough to perturb the
Earth's climate on a global scale by injecting large quantities of dust into the
stratosphere. Such an event could depress temperatures around the globe, leading to
massive loss of food crops and possible breakdown of society. Global catastrophes are
qualitatively different from other more common hazards that we face (except nuclear war),
because of their potential effect on the entire planet and its population.
Various studies have suggested that the minimum mass impacting body to produce such
global consequences is several ten of billions of tons, resulting in a groundburst
explosion with energy in the vicinity if a million megatons of TNT. The diameter for
Earth-crossing asteroids are between 1 3/5 and 3 1/4 miles. Smaller objects (down to 32
feet in diameter) can cause severe local damage but pose no global threat.
According to Spaceguard, "Of approximately 200 Earth-crossing asteroids, fewer that 200
have actually been discovered. At present no asteroid is known to be on a collision
course with the Earth. David Morrison of the NASA Spaceguard Research Center states,
"The chances of a collision within the next century with an object 1 3/5 mile in diameter
or more are very small (less than 1 in a 100). But, such a collision is possible and
could happen at any time. If we did have sufficient warning, however, the incoming
object could be deflected or destroyed". Cosmic impacts are the only known natural
disaster that could be avoided entirely by the appropriate application of space
technology.
The Spacewatch telescope located on Kitt Peak is used to survey for moving objects,
including asteroids whose orbits approach or cross the orbit of the Earth. Among these
are asteroids that may someday be used as sources of raw materials. Spacewatch uses a
Charg-Coupled Device or (CCD) and an automated computer program to discover NEOs.
`The Spacewatch Observatory has already detected one of the smallest asteroids known,
and also the one that passed very close to Earth, the Apollo asteroid 1991 BA. The
semi-automatic Spacewatch system at the University of Arizona has considerably increased
the discovery rate, and will have profound consequences on the utility of NEOs as
near-Earth space resources.
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