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From: kgd@inf.rl.ac.uk (Keith Dancey)
Newsgroups: sci.astro,sci.space,sci.space.shuttle
Subject: Re: asteroid almost hits earth
Message-ID: <6101@nfs4.rl.ac.uk>
Date: 12 May 89 09:08:30 GMT
References: <256@ringwood.Morgan.COM> <3200009@hpindda.HP.COM> <4566@tekigm2.MEN.TEK.COM> <2635@ssc-vax.UUCP> <103026@sun.Eng.Sun.COM>
Reply-To: kgd@inf.rl.ac.uk (Keith Dancey)
Organization: Rutherford Appleton Laboratory, Informatics Department, U.K.
Lines: 34

In article <103026@sun.Eng.Sun.COM> mae@sun.UUCP (Mike Ekberg, Sun {GPD-LEGO}) writes:

>
>But even greater menace lurks in the darkness of space.  Scientists have
>speculated that objects as large as several miles across have crashed into the
>Earth, spewing millions of tons of debris into the atmosphere, blotting out the
>Sun for months or years, and causing mass extinctions of life -- including,
>many believe, the dinosaurs.  Of the known larger Earth-crossers, none seem to
>pose a threat in the near future.  But, says Shoemaker, "until we have tracked
>all of them, something could sneak up on us."
>

My understanding is that the demise of the dinosaurs extended over a period
of order of magnitude of a thousand years.  Certainly long enough to place
doubt upon the viability of a single catastrophy such as the one mentioned.

If the palaeontological evidence is not contradicted (and I have understood
it correctly) then a *series* of such catastrophic strikes would be required.
 
That is not say a single catastrophy is ruled out, but it looks as though
its effects must be longer-lived than a few years.
 
As for tracking "all" large, earth-crossing asteriods, there is
a danger of conveying the unspoken idea that the number may be fixed and
finite for all time.  I think such a concept is also under question.
There is just the possibility that these bodies are disturbed out of
an otherwise harmless state by the dynamics of the galaxy.  This means
their numbers may be added to at any time unexpectedly.
 
-- 
Keith Dancey,                               UUCP:   ..!mcvax!ukc!rlinf!kgd
Rutherford Appleton Laboratory,
Chilton, Didcot, Oxon, England  OX11 0QX             
Tel: (0235) 21900   ext 6756                JANET:       K.DANCEY@uk.ac.rl.inf