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    Re: Not exactly Navigation, but close...
    From: Frank Reed
    Date: 2008 Apr 23, 23:48 -0400

    Alex wrote:
    "Shortly speaking, scientists DO monitor these bodies. The bodies that
    are on the "possible collision course". They are able to detect any such
    body many years in advance. If a really dangerous one is detected this will
    be known, and we DO have means to deflect it. The main problem is how much
    in advance we know. Modern technology (and moderate expenses) permit to
    detect really dangerous bodies about 20 years in advance."
    
    There's no 'a priori' guarantee that such objects will be detected twenty
    years in advance. The big unknown is long-period comets which are frequently
    discovered as little as one year before they reach the vicinity of the
    Earth's orbit. It's not clear what fraction of the impact risk comes from
    long-period comets. It may be as little as 5%. It might be as much as 30%.
    
    And you wrote:
    "There are technologies with permit us to deal with this danger in 20 years
    period (including research and development)."
    
    Yes. And one of the best solutions, which many people find surprising, is
    "paint it white". Dust a dangerous asteroid with white powder and it will
    reflect much more light increasing the sunlight pressure on it and
    deflecting the asteroid over the course of a few years.
    
    And you wrote:
    "Anyway, my personal conclusion from this lecture was that "there is nothing
    to worry about" in comparison to the REAL dangers facing us. Roughly
    speaking, the risk of a serious collision (in terms of the number of victims
    and the chance that this happens) is comparable to the risk of a major
    earthquake). All this is negligible in comparison with the real dangers we
    face:-("
    
    You should look into it yourself. The risk is certainly not negligible --but
    it has, of course, been exaggerated frequently in the media.
    
    The really nice thing about this particular source of mass destruction is
    that the risk can be SIGNIFICANTLY reduced, and indeed has already been
    significantly reduced (in the past dozen years or so), by spending a
    relatively small amount of money. All we have to do is compile an almanac
     --an almanac of all of the exceedingly faint asteroids that happen to be in
    orbits near the Earth's orbit.
    
    Twenty years ago, the risk of an asteroid impact had to be treated as a
    purely statistical risk, very similar to the risk of an earthquake. But
    asteroids are different. We can predict their motion once they are
    catalogued. It's as good as having a train schedule. During the 1990s and
    continuing today, the United States, in particular, has spent a little money
    funding these cataloguing projects. The number of known asteroids with
    well-determined orbits has exploded in recent years (see attached graphic).
    The statistical risk has thus been reduced by a factor of five or better.
    
    There are still uncertainties regarding impact risk. There's the long-period
    comet problem that I mentioned above. And even among the ordinary asteroids,
    no one is even considering the enormous task of tracking "city-buster"
    objects (like the one believed to be responsible for the Tunguska event in
    1908). There are far too many. In the case of those objects, we simply have
    to count on the fact that the population of the Earth, as well as our
    valuable assets, are highly concentrated geographically so, even though we
    may get one impact every century or two, the expected casualties and costs
    are "acceptable".
    
    Attachment shows the count of "numbered asteroids" versus time. The numbered
    asteroids are objects with well-determined orbits. The number of asteroids
    with partially determined orbits is around a million. The majority have been
    discovered in the past ten years. Almost all of these were discovered by
    projects intended to reduce the impact risk.
    
     -FER
    
    
    
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