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RE: (meteorobs) David McCarter's Posting on Probabilities of REALLY BIG fireballs.

Victor
	Please thank Paul for sending in the rather detailed explanation to a very appreciative and interested audience! And to Dave for at least remembering just enough to get in trouble :-)

Wayne

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From: 	Victor D. Noto[SMTP:vnn2@phoenixat.com]
Sent: 	Friday, February 13, 1998 10:43 PM
To: 	meteorobs@latrade.com
Subject: 	(meteorobs) David McCarter's Posting on Probabilities of REALLY BIG fireballs.

Hi David:
I have been busy tracking down some of the statements you mentioned in your
first listing.
I have ordered the tape of Gene's last lecture and I got in touch with Dr.
Paul Chodas.

He wrote back the following corrections to your comments in the Posting to
me (Victor Noto) and Dr. Steve Ostro
Email from Paul follows:
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Hi Victor and Steve:

Thanks for bringing McCarter's posting to my attention.  He has indeed 
misrepresented some of my conclusions, so I'd like to set the record 
straight.

Mr. McCarter has misunderstood my conclusions regarding orbits in the inner

solar system: it is not true that "the chaos of orbits in the inner solar 
system preclude (sic) accurate orbital determination over more than two to 
five years".  In fact, the situation is much more favorable for asteroids 
and short period comets than it is for long period comets like Hyakutake, 
simply because we are able to track these objects over an extended period 
of time, and therefore determine more precise orbits.  My research has 
shown that once we have observed an object over one complete orbit or so 
(over a period of two to five years, say), our orbit solutions become 
precise enough to be able to say whether or not the object will hit the 
Earth during the next several decades, or even the next century.  In other 
words, once we have observed an asteroid over at least a couple 
oppositions, our long-range predictions become much more reliable.

The situation is quite different for inbound objects discovered only a 
fraction of an orbit before impacting.  In these cases, the warning time 
cannot be much more than a few months: our orbit estimate would simply not 
be accurate enough to project ahead in time by more than this amount.

Of course, getting some radar measurements on this fractional-orbit inbound

object would would certainly help improve our orbital solution, but we 
would still have the problem of waiting until the object came close enough 
to be 'in range' for radar. The warning time might then lengthen by a month

or two.  Radar also has the advantage that we could observe the object 
while it was in the daytime sky, when it would be otherwise unobservable.

I would also like to correct what Mr. McCarter said about my case study of 
a simulated impacting comet on a Hyakutake-like orbit.  I kept the 
observational circumstances as close as possible to what actually happened 
with Hyakutake, and only changed the comet's trajectory.  In particular, 
the comet was discovered about 60 days before impact (not six weeks).  The 
probability of impact was 4% at Impact-40 days, 19% at Imp-30 days, rose 
rapidly past 50% at Imp-25 days, and was essentially 100% for the last 20 
days.  The precise timeline would depend on how many observations were made

when, and if and when radar observations were made, but my general 
conclusion was that we could not make a reliable hit-or-miss prediction 
until about a month before impact.  

Mr. McCarter certainly got the right message when he states "the 
observations are usually not nearly as accurate as required to be 
definitive, and the further away the event, the more impossible to 
determine".  But this applies much more to comets and asteroids discovered 
only a fraction of an orbit before impact.  The situation is more hopeful 
for multi-opposition asteroids, where we may well have decades of warning.

Paul Chodas

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Victor Noto - Kissimmee, Florida USA
vnn2@phoenixat.com
http://www.phoenixat.com/~vnn2/BIGROCK.htm
Website theme quote:
"Life really is a Rock and 
the Big Rock giveth and taketh away all life!!" 
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