From: tholen@galileo.ifa.hawaii.edu (Dave Tholen)
Subject: Re: comet approaching?
Organization: Institute for Astronomy, Hawaii
Date: Thu, 16 Apr 1992 07:53:57 GMT

rjp1@cbnewsc.cb.att.com writes:

> I've heard stories about an approaching comet or meteor that is
> to pass quite close to the Earth, sometime in December 1992.

The object that will be passing close to the Earth this December is an
asteroid, known as 4179 Toutatis.  It is estimated to be approximately
6 km in diameter, and it will pass about 0.024 astronomical units from
the Earth.  In 2004, it will make an even closer pass, at 0.011 AU.

> I have also heard that a previous meteor passed close the earth last
> December as well, but not as close as was predicted.  Perhaps the
> story on the up and coming one for 1992 is nothing more than a bit
> of leftover wishful thinking from the last (not so) near miss?

You're probably referring to the passage of 1991 VG, another asteroid,
but only of about 7 m diameter.  There is some speculation, and some
supporting, but not convincing, data, that 1991 VG is a manmade object,
perhaps the booster rocket from some spacecraft.  Some of these spent
boosters were injected into heliocentric orbits.  It wouldn't be proper
to say that its pass was not as close as predicted, because it was a
brand new discovery, and its close approach wasn't predicted at all.

> A couple of days ago I saw a short blurb on CNN about scientists
> taking seriously, the notion of putting together some sort of anti-
> meteor defense system so if there was a meteor on collision course,
> it could be blasted or moved out of harm's way.  Just how serious
> is this kind of reasoning.  Is there precedence??

Precedence for what, changing the orbit of an asteroid?  It's been talked
about before (the "reasoning"), but it's never been done (the "blasting").
Yes, it is being discussed seriously, but mainly by the weapons specialists.
The asteroid specialists favor undertaking a survey to find as complete a
sample of these objects as possible, to first determine whether any are on
a collision course, before embarking on the development of a very expensive
defensive system.  The search would be similar in principle to that which
is currently done for Earth-orbiting debris; if we know what is up there,
we can alter the orbit of the Space Shuttle to avoid any potential collisions.

Are these asteroid scientists hopelessly paranoid, as suggested by some
pundits?  Consider the fact that the Earth is constantly running into
debris (ever see a meteor?).  Bright meteors are less frequent than faint
meteors, and collisions with even larger objects are similarly less
frequent, but they do happen.  Consider the 1978 South Atlantic blast
that was originally believed to be an Israeli-South African nuclear test,
or the passage of the spectacular fireball over the Rockies in 1972, or
the 1908 Tunguska event, or... or... or...  Evidence currently favors the
theory that the extinction of the dinosaurs was caused by the impact of
a large (about 10 km) asteroid 65 million years ago.  These larger impacts
are extremely rare, but when they happen, they are immensely destructive.
Should something be done about the situation?  Well, the astronomers have
done their job by calculating probabilities to the best of our limited
knowledge.  Now it's up to politicians to decide priorities.
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