posted on May, 24 2004 @ 04:07 PM
"What is a NEO?
Near-Earth-Objects (NEOs) are small bodies in the solar system (asteroids and short-period comets) with orbits that regularly bring them close to the
Earth and which, therefore, are capable someday of striking our planet. Sometimes the term NEO is also used loosely to include all comets (not just
short-period ones) that cross the Earth’s orbit. Those NEOs with orbits that actually intersect the Earth’s orbit are called Earth-Crossing-Objects
What size NEOs are dangerous?
The Earth’s atmosphere protects us from most NEOs smaller than a modest office building (50 m diameter, or impact energy of about 5 megatons). From
this size up to about 1 km diameter, an impacting NEO can do tremendous damage on a local scale. Above an energy of a million megatons (diameter about
2 km), an impact will produce severe environmental damage on a global scale. The probable consequence would be an "impact winter" with loss of crops
worldwide and subsequent starvation and disease. Still larger impacts can cause mass extinctions, like the one that ended the age of the dinosaurs 65
million years ago (15 km diameter and about 100 million megatons).
How many NEOs exist?
There are many more small NEOs than large ones. Astronomers estimate that there are approximately 1000 Near Earth Asteroids (NEAs) larger than 1 km in
diameter, and perhaps a million larger than 50 m in diameter (the threshold for penetration through the Earth's atmosphere). The largest NEAs are
less than 25 km in diameter. There are probably many more comets than NEAs, but they spend almost all of their lifetimes at great distances from the
Sun and Earth, so that they contribute only about 10% to the census of objects that strike the Earth.
Who is searching for NEOs?
Several teams of astronomers worldwide are surveying the sky with electronic cameras to find NEOs, but the total effort involves fewer than 100
people. The most productive NEO survey is the LINEAR search program of the MIT Lincoln Lab, carried out in New Mexico with US Air Force and NASA
support. The LINEAR team, which operates two search telescopes with one-meter aperture, discovered more NEOs in 1999 and 2000 than all other searches
combined. Other active survey groups include the NEAT search program in Hawaii, carried out jointly by the NASA Jet Propulsion Lab and the US Air
Force; the Spacewatch survey at the University of Arizona, funded by NASA and a variety of private grants, the LONEOS survey at Lowell Observatory in
Flagstaff Arizona, supported by NASA grants, and the Catalina Sky Survey in Tucson Arizona, also supported by NASA. Other searches in the US, France,
Japan and China also contribute to discovery of NEOs, while additional astronomers (many of them amateur astronomers) follow up the discoveries with
Are any NEOs predicted to hit the Earth?
As of the end of 2001, astronomers had discovered more than half of the larger Near Earth Asteroids (diameter greater than 1 km). None of the known
asteroids is a threat, but we have no way of predicting the next impact from an unknown object.
What is the risk of impacts?
We don’t know when the next NEO impact will take place, but we can calculate the odds. Statistically, the greatest danger is from an NEO with about 1
million megatons energy (roughly 2 km in diameter). On average, one of these collides with the Earth once or twice per million years, producing a
global catastrophe that would kill a substantial (but unknown) fraction of the Earth’s human population. Reduced to personal terms, this means that
you have about one chance in 20,000 of dying as a result of a collision. Such statistics are interesting, but they don’t tell you, of course, when the
next catastrophic impact will take place—next year or a million years from now.
How much warning will we have?
With at least half of even the larger NEOs remaining undiscovered, the most likely warning today would be zero -- the first indication of a collision
would be the flash of light and the shaking of the ground as it hit. In contrast, if the current surveys actually discover a NEO on a collision
course, we would expect many decades of warning. Any NEO that is going to hit the Earth will swing near our planet many times before it hits, and it
should be discovered by comprehensive sky searches. This is the purpose of the Spaceguard Survey. In almost all cases, we will either have a long lead
time or none at all.
Is there a problem with blind spots in the Spaceguard Survey?
Some press reports express concern that an asteroid could hit the Earth coming out of a "blind spot", such as the daylight sky or high southern
latitudes where no Spaceguard telescopes are looking. Some worry that if an asteroid is found after its closest approach to Earth, this is an
indication that the system is not working. These concerns seem to be based on the misconception that we are trying to detect asteroids as they
approach the Earth on their final plunge toward impact. In fact, any such last-minute warning system is impractical as well as unproductive. In this
survey, it makes no difference if a NEA is discovered on approach or departure from the vicinity of the Earth. The important thing is that it is
discovered and its orbit determined. The only effect of blind spots, whether they be due to sunlight or moonlight or bad weather or lack of a southern
hemisphere survey telescope, is to slow down the completion of the NEA catalog. Objects in blind spots will be picked up later, usually within a few
years, in a more favorable geometry.
How can we protect ourselves?
NEO impacts are the only major natural hazard that we can effectively protect ourselves against, by deflecting (or destroying) the NEO before it hits
the Earth. The first step in any program of planetary defense is to find the NEOs; we can’t protect against something we don’t know exists. We also
need a long warning time, at least a decade, to send spacecraft to intercept the object and deflect it. Many defensive schemes have been studied in a
preliminary way, but none in detail. In the absence of active defense, warning of the time and place of an impact would at least allow us to store
food and supplies and to evacuate regions near ground zero where damage would be the greatest."