From: baseball@catch-the-fever.scd.ucar.edu (Gregg Walters)
Newsgroups: soc.religion.christian
Subject: Re: There will be a collision?
Date: 13 Dec 92 05:09:46 GMT
Organization: Scientific Computing Divison/NCAR Boulder, CO

In Article: 17266 of soc.religion.christian,
julian@sfu.ca (Julian Shillcock) wrote >>

>> u presumably know abou tcomet Smith-Tuttle that is scheduled to be in the
>> same place at the same time as the earth in 2126?

NO ONE IS SAYING THAT THE COMET IS "SCHEDULED" TO HIT THE EARTH.  To
do so is to pretend that we can predict the next apparition of this
comet to within 1.5 minutes.  Just 5 months ago Marsden computed a
perihelion time that has already been proven to be off by a day.

Please don't take this as a flame, I just want to clarify the
situation.  Maybe Julian's brief posting belies his full knowledge
and I am overreacting.

>>                                                   It is approx. 5 km
>> in radius and travelling at 60 km/s. But until the orbit is known to
>> greater accuracy the hit probability cannot be calculated. Once oribt
>> tracking takes place its path will be known as certainly as the voyager
>> type spacecraft.

True, but ... several mid-course correcting maneuvers were made along
the orbits of these spacecraft so that they would fall into the gravity
wells of these planets at just the right place and time.

>>                  Although there are some extra problems involved with
>> the comet spewing out matter that can perturb its orbit.

Yes, and they can be significant.  And the comet does not spew matter,
the solar wind "blows" it off the surface of the comet nucleus.

As I write this, I am looking at the January 1993 issue of Sky &
Telescope - the issue with a cover showing an artist's rendering
of the possible encounter under large red letters saying "Target
Earth".  You can probably find a copy in your public library.  The
new editor's cover presentation is a bit sensationalistic by previous
Sky & Telescope standards.

This is easy reading - it is not a professional astronomical journal.
By the way, there is also an article about new ideas related to the
Creation in this issue.

Please read the article on pp.16-19, "Comet Swift-Tuttle: Does It
Threaten Earth?" by Brian Marsden, who runs the International Astronomical
Union's Central Bureau for Astronomical Telegrams in Cambridge, Mass.
In a related article on p.116, another writer says that Marsden, on
October 15, said that "... Swift-Tuttle could strike Earth in 2126."

On p.19, column 1, 5th paragraph Marsden himself states "... the
possibility of a strike by Swift-Tuttle - [is] admittedly remote..."

The flavor of Marsden's entire article is that it has been difficult to
determine the orbit of Swift-Tuttle.  His best guess is that the comet
will reach perihelion on July 11, 2126, passing across the Earth's
orbit well ahead of the Earth (so that, by my rough calculation, it would
miss the Earth by about 25 million miles).  On the remote chance that it
reaches perihelion during one particular 3 minute stretch on July 26,
then it MAY strike the Earth on August 14.

To put that 15 day uncertainty in perspective, back in 1973 (see p.17)
he predicted a 1992 perihelion date of November 25, give or take up
to two months.  This past July he refined the computation to account
for non-gravitational effects (the solar wind "blowing" material off
the object's surface, forming the tail) and predicted December 11.
Subsequent observations of the return put the perihelion time on
December 12, 7:45AM ET.  The Earth moves about 1.6 million miles per
day, or a distance equal to its diameter in about 7 minutes.

Once Swift-Tuttle recedes to about 300 million miles, so that
non-gravitational forces are sufficiently minimized, then observations
should provide good guidance for determining the orbit for the
remainder of its outbound journey and return in 2126.

Y'all relax.  Even after refining the computations, when the comet returns
in 2126, during those few months as it approaches the sun it will again
be subjected to non-gravitational forces that are very hard to predict.
Given present knowledge, I don't see how any one will be able to narrow
its perihelion passage down to those remotely fateful 3 minutes until
just a few weeks before it happens.

Anyone remember how disappointing the appearance of Comet Ikeya-Seki
was back in the early 70s?  Predictions a few weeks before perihelion
were that it would be quite bright - and they missed.

THE SPECULATION OF A HIT IS PREMATURE.  It should not have been made until
after observations of the outbound trajectory.  Of course, from a business
point of view, it may sell a few more magazines.

However, as Marsden suggests, if a refined computation shows an increased
probability that the comet would hit the Earth, then we could launch a
device to intercept and deflect it - about 4 years before perihelion.
Planning such an "ambush" (p.19) would require observations when the
comet is over 4 billion miles away - "inconceivable" at this time.

Gregg            \\                                    //
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