From: pbrown@uwovax.uwo.ca
Subject: IS SWIFT-TUTTLE THE REAL THREAT?
Organization: University of Western Ont, London
Date: Fri, 6 Nov 1992 13:27:34 GMT




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The Comet Swift-Tuttle saga : an addendum  - Duncan Steel, 1992 November 6th


One thing that no-one has picked up on so far, it seems, is that it is more
likely that an asteroid or comet big enough to wipe out mankind will hit
the Earth before P/ST may perchance clobber us on 2126 August 14th than
the probability of that comet itself doing the job.  That was a mouthful,
but think about it: what I am saying is that even IF P/ST does hit the
Earth on that date there is a chance that we may have already been hit by
some other as-yet undiscovered object.

A very crude estimate of the chance of P/ST hitting, a priori (that is at
this stage without the observations to be made over the next decade) is
about 1 in 10,000.  That is, the perihelion date is uncertain by about
25 days, and if it does come back so as to pass its node on 2126 August 14th
then since it takes about 3.5 minutes to cross the Earth's path and
24 hours = 400 x 3.5 minutes, more-or-less, the probability is about 1 in
25 x 400 = 10,000.

However, if there are about 3,000 Earth-crossing asteroids larger than 1 km
in size (that is about the size limit for a global catastrophe, depending upon
mass and impact speed) and these have a mean collision probability with our
planet of about 6 x 10^(-9) per year (see Steel & Baggaley, MNRAS, 212, 817,
1985) then one expects one impact by such an object every ~50,000 years.
[See also the discussion in the Spaceguard Survey report, ed. D. Morrison].
That figure may be a little on the high (frequent) side, but I would plump
for one per 100,000 years as being in the right ball park.

This means that in the next century there is about a 1 in ~1,000 chance of
a 1 km asteroid dropping in to play (havoc).  This is 10 times our current
estimate that mankind will be stuffed by P/ST in 2126.  Note that no comets
(either SP or LP) have been included in my sum, and these may comprise
about 25% of the total risk (i.e. asteroids 75% of the hazard).

Now, people get all upset about such calculations for asteroids and comets
and say that the public should not be alarmed.  In fact there are some of
us around who think that there is every reason to be alarmed: not panic-
stricken, just alarmed.  It's about time we took out some insurance against
this hazard.  Yes I do have a vested interest in that this is my area of
work; but I have that vested interest since I have directed myself this way
since I did the sums referred to above over 8 years ago and found that the
results were worrying.

In order to get politicians on the track nowadays, everything has to be done
in terms of economical arguments.  To that extent I commend to you for
perusal the "Mr Statistics" column in FORTUNE magazine for 1992 June 1st.
There it is shown that the annual expectancy of economic loss to the US
alone from asteroid impacts is of order $750 million; and that figure is
derived using a far-too-low probability of impact by a mankind-killing
object of one in a million per year, whereas in fact the estimate of that
probability should be more like 1 in 100,000.  Note that this is the "US
alone" figure (i.e. sod everyone else in the world), and it considers only
the infrequent huge impacts with global effects.  One could add on to that
annual expectancy the much more frequent smaller impactors to get a rather
higher sum.  For example, a Tunguska over Tokyo would bankrupt not only
the US but also the rest of the western world.  Most certainly the
Australian economy could not survive the withdrawal of all Japanese
investment here which would follow from a levelling of Tokyo.

Yes I know that it all sounds very melodramatic and over-stated; but
just look at the numbers.  I am not suggesting that $750 million p.a.
should be spent on looking for these damned things to see whether there are
any due to hit us in the next century; but NOT to spend the $300 million
over 20 years required for Spaceguard would be crazy, and false economy.
Nor am I suggesting that the US should pick up the tab: an international
program is needed.  I pisses me off to hell that my own countries 
(Australia and the UK) are doing absolutely nothing, indeed with our
own search program here, the only one covering the southern sky, being
terminated in a few weeks.

Take this seriously, guys, and lobby.  NASA's Spaceguard program must be
implemented, and the time to strike is now.


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