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RE: (meteorobs) Probabilities of REALLY BIG fireballs...

David,
	Thanx for summarising those reports presented to the RASC. I ahd heard some about each of the subjects, but your account provided a greater understanding. Do you know if any printed material is available? I'd love to be able to read them in more detail. 
	By the way, as first posts to the maillist go, that's a doozy! Welcome

Wayne

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From: 	David McCarter[SMTP:mccarterd@claven.fanshawec.ondot ca]
Sent: 	Tuesday, February 10, 1998 8:03 PM
To: 	meteorobs@latrade.com
Subject: 	Re: (meteorobs) Probabilities of REALLY BIG fireballs...

>the latest NEO statistics, who can tell us the likelihood of a kilometer+
>sized dense-material impact within the next few centuries? Can someone
>hazard a guess as to whether we really know the answer??

Hi all. This is my first intentional posting to the list.

At last summer's RASC General Meeting in Kingston Ont. Can. the late Eugine
Shoemaker spoke about past earth cratering rates, and it was his thesis
that there is a period of greater likelyhood of big impacts every 33
million years, which roughly corrisponds to the rate at which the solar
system moves through the galactic plane.

His heart stopping conclusion was that we are actually overdue by a few
years, give or take a million. After he left Kingston, I understand he
spoke on a similar topic to graduates at Acadia University in Nova Scotia,
and then left for Australia to continue his research.

At the RASC London centre 75th anniversary dinner last March Dr. Paul
Chodas, JPL, showed his research using as an example comet Hyautacki to
determine how difficult it really is to fix the probability of a direct hit
upon earth. Had that comet been on a collision course, the numbers six
weeks before impact (at discovery) would have suggested a near miss (as it
did); three weeks before a very slim chance of a hit, maybe 1%; a week
before a 90% chance for a hit somewhere in a very big ellipse. Something
like that anyway. In other words, the observations are usually not nearly
as accurate as required to be definitive, and the further away the event,
the more impossible to determine.

Of course, for all our military bravado, we are not able to mount a big
Saturn/Energia mission in a few days or even weeks and in any event the
nudge we could currently give an approaching asteroid would have to be done
years in advance.

He also concluded that the chaos of orbits in the inner solar system
preclude accurate orbital determination over more than two to five years.

I hope I have not misrepresented either of these two viewpoints to you, but
I think this subject needs far more attention than it is getting. Eugine,
Carolyn and Dave Levy were working from their back yards to further
asteroid research. Hopefully the big military project which is getting
underway will be effective.


David McCarter
Professor Electronics General Servicing
Fanshawe College London, Ontario, Canada
R.A.S.C. London Centre      VE3GSO
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