From: jscotti@lpl.arizona.edu (Jim Scotti x2717)
Newsgroups: sci.space
Subject: Re: Impact in AD 2000?
Keywords: Asteroids, Impacts
Date: 8 Oct 92 15:39:41 GMT
Organization: Lunar & Planetary Laboratory, Tucson AZ.

In article <9210080128.AA16768@cmr.ncsl.nist.gov> roberts@CMR.NCSL.NIST.GOV (John Roberts) writes:
>
>-From: szabo@techbook.com (Nick Szabo)
>-Subject: Re: Toutatis impact in 2000 AD? (was Re: Help !)
>-Date: 5 Oct 92 06:55:07 GMT
>
>-This translates to (2e1/1e6)*(5e9) or 10,000 dead people, actuarily 
>-speaking, or 0.2 per 100,000 population.  The death rate from
>-airline crashes is 0.04 per 100,000 people.
>
>  {Some stuff deleted....}
>
>I notice you kill just about everybody on the planet, no matter where the
>impact. An impact of a two-mile-diameter asteroid would be pretty severe,
>but I'm not sure it would be quite *that* bad. Isn't that only about a
>tenth of the mass of the alleged "dinosaur killer" asteroid?

We believe that the threshold for devistating global effects by an 
asteroid impact occures with asteroids of about 1/2 to 1 kilometer 
diameter.  The most damaging effects for humans are due to dust and
debris flung into the atmosphere, blocking sunshine and affecting the
global climate.  It is akin to the hypothesized "Nuclear Winter".  By
the time an asteroid is two miles in diamter (about 3 kilometers), you
are well above what we believe would cause such a disaster.  Even the
affects of something as small as about 50 meters has caused noticeable
global affects.  Namely, the Tunguska impact event in 1908 caused 
widespread local devistation and the atmospheric shock was measured
as far away as Great Britain.  Also, the sunsets were enhanced for
several days following the event.  Recently, large volcanic eruptions
have spewed large amounts of dust into the air, causing remarkably
enhanced sunsets for several months as well as perhaps some global
weather changes.  These recent volcanic eruptions are small compared
to the impact of small asteroids larger than a few hundred meters in
diameter.

>The analysis you refer to is apparently just a worst-case scenario - it
>wasn't meant to reflect the actual uncertainty in the trajectory. I believe
>a later post gives the uncertainty as a fraction of an Earth radius.

Yes, this analysis was (more or less) a worst-case scenario.  But this
scenario can happen with much smaller asteroids than you might expect.
Devistating local affects can happen with asteroids (and comets) as small
as 50-100 meters in diameter, and from the recent discovery rates of 
such small objects by Spacewatch, we can estimate the probability of
such an impact.  We think that 100 meter objects (which, by the way,
are at about the size threshold that we expect to survive more often
that not to impact on the surface of Earth rather than break up or
explode higher in the atmosphere) should impact the Earth about once
every century.  I don't think it is a coincidence that the Tunguska
event happened within the last 84 years.  We may have had at least one 
close call since.  A small object (size estimates ranged from 5 meters 
to about 50 meters - I'd guess it was probably about 20 meters diameter) 
skimmed through the atmosphere in 1972 over the western US and Canada, 
missing an impact by a miniscule 10-20 kilometers or so (if it had been 
that much lower, it would have likely been slowed enough by the thicker
atmosphere to have come down instead of bounced back out).

>John Roberts
>roberts@cmr.ncsl.nist.gov

Jim.
---------------------------------------------
Jim Scotti 
{jscotti@lpl.arizona.edu}
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA
---------------------------------------------
Newsgroups: sci.space