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Re: potentially hazardous objects (PHA's)

At 04:02 PM 2/20/98 -0500, you wrote:
>Victor,
>
>As it is, the best we could do right now is identify a hazard and watch it
>come in. Even with a couple years notice there isn't much we could do to
>destroy or deflect a large object. We're facing the same situation as the
>neighborhood group that wants a new stop sign installed by their community:
>Nothing will happen until someone gets killed. Let's hope the wake up call is
>a 20 or 30 meter object in the desert and not a 20 or 30 kilometer object in
>the ocean.
>
>Gene
>

I must  agree with  Gene, in  as much as the objects  in question do need to
be identified, it is pretty much  watch  and see  if it  hits game right
now. I might add that to  our knowledge at the present time the largest of
the Earth Approach Asteroids is (433) Eros, the remainder are mostly in the
2 kilometer and  smaller size. We learned much from the impact of
P/Shoemaker-Levy 9 and JPL correctly predicted the energy released prior to
impact (most underestimated). Most of these objects would not cause the
extinction of humanity, although it would wreck  havoc on it. The velocity
is simply  not there for the energy to be released such as was the case with
S-L9 (impact  velocity if I recall about 60 kps). 
        This is not true of the long period comets however. . I call your
attention  to  an article in the February, 1997  issue of Sky & Telescope.
Paul W. Chodas of Jet Propulsion Lab did a study on  finding out how  much
warning was needed especially from comets. In it he hypothetically placed
comets Hyakutake and Hale-Bopp on  a collision course with the  Earth. 
        Hyakutake was one of the largest comets in apparent size in  recent
years, yet it was discovered only 60 days prior to it's encounter  with the
Earth. In Chodas' simulation Hyakutake would have come at us for four weeks
before astronomers could  even calculate  at least a  50% chance of
collision. The impact becomes  certain only  22 days before it strikes. 
        Hale-Bopp doesn't fare much better. Discovered  21   months prior to
encounter,we would not know of a 50% chance until 200 days in advance. But,
what Chodas learned is that it drops to 50 days when outgassing in
considered, which is  what  comets  do best. This so-called jetting  action
can affect a comets course and until we get  better and more precise star
locations we cannot predict  with 100%  accuracy even the asteroids impact. 
        A lot of the very close approach asteroids were discovered only
after  they made their close encounter with the Earth. This is not to say we
must not keep on the lookout and try and find each  and  every one of them.
I for one study the Earth  Approach Objects, yet at the same time I realize
that we currently do not  have the luxery of being able to  determine an
impact with the  amount of time it would require  to  do something about  it.

Jerry
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