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Re: (meteorobs) Probabilities of REALLY BIG fireballs...



Lew Gramer wrote:

>
>
> But since we're talking about this thread right now, yours is certainly a
> question that deserves an answer. So how about it: is there anyone up on
> the latest NEO statistics, who can tell us the likelihood of a kilometer+
> sized dense-material impact within the next few centuries? Can someone
> hazard a guess as to whether we really know the answer??

An article by Owen Toon appearing in Reviews of Geophysics, February 1997,
has a table outlining impact frequencies. Since tables usually look pretty
ratty in e-mail, I'll try to put in text form what he has in table form. To
wit:

You can expect a 700 meter object carrying about 10,000 Mt of energy to whack
the Earth every 60,000 years. You can expect one of these objects to pass
within the lunar orbit distance (ie, 400,000 km from Earth)  once every 20
years.

You can expect a 1.4km object carrying about 100,000 Mt of energy to whack
the Earth every 300,000 years. You can expect one of these objects to pass
within the lunar orbit distance (ie, 400,000 km from Earth)  once every 90
years.

You can expect a 3 km object carrying about 1,000,000 Mt of energy to whack
the Earth every 2,000,000 years. You can expect one of these objects to pass
within the lunar orbit distance (ie, 400,000 km from Earth)  once every 600
years.

You can expect a 7 km object carrying about 10,000,000 Mt of energy to whack
the Earth every 10,000,000 years. You can expect one of these objects to pass
within the lunar orbit distance (ie, 400,000 km from Earth)  once every 4,000
years.

The article "Environmental perturbations caused by the impacts of asteroids
and comets" lays out what would happen on regional and global scales in the
event of a big whack. Needless to say, the planet gets toasted when slammed
by impactors on the 3km and up scale.

I'm no statistician, so take the following with a bag or two of salt. It's my
understanding that one can say that, on average, a 700 meter object slams
into the Earth every 60,000 years. One cannot say if that will be tomorrow,
or next Wednesday, or sometime in 112577 AD. That's because each impact is
believed to be independent, ie. one has no relation to another. That goes out
the window if you accept a notion dubbed "coherent catastrophism" favored by
a number of British astronomers, in which case you would note that large
impacts may occur in swarms, in which case you'd really be toast. All that
said, I know there are some asteroid watchers who simply feel "in their gut"
that the planet is overdue for a large-sized impact. What can you say?
Gambling is based on statistics, but everybody feels they're going to get
lucky.

Anyway, David Morrision at Ames Research Center has an excellent web page
outling all sorts of NEO business. The URL is http://ccf.arc.nasadot gov/sst/.
Of interest to the group may be a small bit on whether TWA 800 was taken down
by a meteorite.

Richard Wagner
References: