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Re: potentially hazardous objects (PHA's)

At 07:14 AM 2/21/98 -0700, you wrote:

>Second, in you posting, you provide several reasons for a reduction in time
>for both predicting if an asteroid will intersect the earth in orbit (how's
>that for depersonalizing the issue), and in knowing the energy and location
>of the impact. While this information would be interesting in an
>existential sense, is it really important. In other words, what
>would/should/could we do with this information?

        I vaguely remember reading something  somewhere, that if an impact
were to occur that the heads  of state and most   notably the IRS would be
evacuated to a safer location,  then let nature take it's course. But, don't
quote me on that, although that would in all likelyhood be the   way  that
not only   ours  but most   governments would probably  handle the
situation. This being  of  course if the impact were going to occur without
sufficient warning  to evacuate   the populace, or  if say   an  impact  had
occurred in the middle of the ocean. I  have seen the panic when a hurricane
comes ashore and the evacuation is usually OK but somewhat confused, now
just  image a wave that will reach several tens of  miles inland and with
little or no  warning.
>
>How do we apply a time frame to an impending impact. What is  knowing about
>the impact early or late relative to? Is there a plan to deal with the
>impact?

        I believe  these questions were answered above.

 To prevent the impact? Is a certain amount of advanced warning
>needed for something in particular?

        These are  just   my thoughts on this. Comets are perhaps the worst
ones to deal  with. They are outgassing and this produces a jetting  effect
which makes  their path   unpredictable. Now we can predict with some
ammount of certainty the location of a comet for a telescope,  but even
here we encounter some  discrepancies. And the  coma  of a  comet is BIG,
something thatmost amateur astronomers fail to realize. Given this a comet
say like Hale-Bopp is a typical  fuzzball when  first discovered. But, you
have  to put this into perspective, that  tiny fuzz ball when seen up close
is very very large. In the order of tens of thousands of miles  in diameter.
That is why the slight  shift in position is not that  noticable. But, when
it gets say within one or two million miles of the Earth,  the emmense size
becomes very apparent. Hale-Bopp was never closer  than 30 odd million miles
at best, Hyakutake  came  as close  as a little  over 9 million miles. And
by  the way, if you imagine a  sphere of just 0.1 Astronomical Unit, or  9
million miles, there have been 14  comets to venture inside this zone in
recorded history. And Hyakutake doesn't even make  the LIST!!!!
        My point is, that we  would not have time to determine if the
nucleus would impact with  the Earth   until  just a few short weeks at most. 
        Now, most of the  talk is centered on an asteroid collision. Of
these asteroids, basically only (433) Eros is large enough to cause global
extinction. The  rest are in the 5 kilometer or less size range.The IRAS
satellite when it  was in  service discovered many,many objects and should
have detected any of the Earth Approach Objects out there in   the  (433)
Eros size range. And I stress, should have, we  can't be  100% sure nor will
we  ever be. So in effect, if  an impact does occur,  it  will most likely
be from  one of  the lesser varieties. Even an asteroid that is twice the
size  of a football field would certainly  wreck  havoc on the area it
strikes,  and an ocean impact would  be far worse.  !996JA that passed as
close to us  and the moon was just  such a size  range. Jim  Scotti once
told me that given the amount of area in the  sky that he patrols with
Spacewatch and the number  of objects  such as  1996JA  that he and his
colleagues have discovered, there should be by calculation a 200 meter
diameter object passing between   us  and  the  moon  every  twenty four
hours. That's  scary, but I  don't  plan on loosing any sleep over it unless
I plan on looking at it through a telescope. 
        Now, getting back to comets, they pose a slightly different problem.
Being as I  explained earlier that  it  would be difficult if not impossible
to predict an impact until just a few weeks  prior, they DO have the
capability of global  extinction. Comet Hyakutake 's nucleus was only 2
kilometers diameter so  it  did not, but Hale-Bopp had  a  nucleus
calculated at 75 kilometers diameter. And remeber the list I told you about
with  the 14 comets whizzing by at  less than  0.1 AU? Well guess what, good
old faithfull Halley's  Comet makes this list THREE times, and it's nucleus
is about 10 kilometers. I'm not saying that Halley's will ever impact, but
there is one comet that is in the Halley  class that is destined to either
hitting the Earth or being ejected from the solar system due to a very close
encounter with  the Earth. This is our old friend P'Swift-Tuttle. It should
make  a  grande spectacle in about  130 years when  it makes it's next
return. Although  there was some aprehension atone time a few years  back
when   it was recovered that  calculations  revealed  it had a 1 in 400
chance of hitting  theEarthon it's next  return in  2130 AD. But after all
the observations  were in,  it was soon  found not  to be the case. It will
however have it's orbit  shifted ever so slightly that  any future
predictions  become hopeless,and this is where the  uncertainty lies as to
when the event will take   place.
        I could ramble on for hours discussing  asteroids  and comets so  I
had better stop here, I hope this helps.

Jerry
        
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