PLEASE NOTE:
*
Subject: "Fire from the Sky" comments
To: cambridge-conference@livjm.ac.uk
Date sent: Tue, 25 Mar 1997 03:25:36 -0600 (CST)
From: pib@nwu.edu
On Sunday, March 23, 1997, the Turner Broadcasting System (TBS)
channel
offered two documentaries collectively entitled "Disaster
Sunday." The
first documentary, a National Geographic Explorer program,
discussed
tsunamis and avalanches. The second documentary, entitled
"Fire from the
Sky," discussed the threat from cosmic impacts. Here are my
preliminary
comments after a single viewing of these documentaries.
I enjoyed the segment on tsunamis. I will comment on just a few
items.
There is evidence that prehistoric tsunamis reached heights of
over 300
meters in the Hawaiian islands. The favored explanation is that
giant
landslides in the islands caused these tsunamis. This is
possible, as
landslides have generated giant tsunamis elsewhere in recent
times. (On
July 8, 1958, a landslide in Lituya Bay, Alaska, generated by an
earthquake, caused a tsunami to reach a height of about 525
meters
immediately across the bay.) I suggest that an impact event might
offer
another plausible mechanism for causing tsunamis of this size. I
was
surprised and pleased that Eddie Bernard mentioned
impact-generated
tsunamis.
The oral traditions of native peoples of the Pacific Northwestern
include
stories about giant waves. The Tollua people offered a tale of
two
children whose grandmother urged them to flee the advancing water
by
running to higher ground. When the two children returned to their
village
after the waters receded, nothing was left. Everything -- and
everyone --
was gone. This story certainly sounded like a genuine eyewitness
account
of a destructive tsunami. Geologist Brian Atwater interpreted
possible
traces of such a tsunami near Puget Sound about 300 years ago. He
also
suggested another large tsunami occurred there about 1,000 years
ago.
Perhaps the Tollua tale reflects one of these events.
There is a 10% chance of a major tsunami striking the Northwest
coast of
the United States within the next 50 years. Many communities in
the United
States are not prepared. This contrasts with the situation in
Japan. Many
Japanese towns have spent large amounts of time and money
constructing
giant anti-wave walls and training civil defense teams to deal
with the
aftermath of a tsunami. Japan has felt the wrath of many
tsunamis. One
particularly destructive 30 meter wave at Honshu, Japan in 1896
killed
about 27,000 people.
One of the problems is getting people to take the threat of
tsunamis
seriously. For example, a predicted tsunami in Hawaii in 1994
resulted in
a wave only about an eighth of a meter (six inches) in height. A
new buoy
system tied to satellites is currently being deployed which
should allow for
better predictions of the size of tsunami waves.
Geologist Walter Dudley suggested that a destructive tsunami
occurs in the
Pacific about once in every seven years. It appears we are
overdue for the
next "big one."
The avalanche segment showed that triggering avalanches using
explosives is
a standard practice in many areas. "We're killing avalanches
and saving
people," said one of the interviewees. I was fascinated by
the story of
the man buried alive by an avalanche, who, against all odds, dug
himself out
after many hours. Unfortunately, he was not able to save his
friend who
had also been buried.
"Fire from the Sky" followed. This was not a National
Geographic special.
Overall I found the program enjoyable, but I liked the NBC
National
Geographic special on Gene Shoemaker, and the Discovery Channel
special on
impacts, better. One general peeve I have about all these
programs: they
don't distinguish actual footage from animations. I believe this
confuses
folks who aren't familiar enough with the subject matter -- the
intended
audience, I assume -- to know the difference.
Gene Shoemaker, David Levy, Ed Tagliaferri, Jasper Wolf, and
others started
the program by offering introductory comments outlining what was
to follow.
Next came a dramatization of a possible nuclear strike in the
British
Isles. This turned out to be a major accretion event involving a
sequence
of multimegaton airbursts caused by cometary debris. (Comet
Hale-Bopp was
unfortunately offered as the originating body. This will probably
cause a
deluge of questions to astronomers by those not familiar with the
actual
dynamics of the situation.) The dramatization concluded with the
destruction of an East Coast U. S. city. We saw the blast wave
approach a
commentator who stood outside reporting on the bollide display.
As the
blast wave overwhelmed her when she attempted to flee, the
display blinked
out.
The program stated that the moon bears the scars of some 30,000
impacts.
(I believe this is actually only the number of craters on the
Earth-facing
side of the moon. There are lots more craters on the far side
:-}).
About 180 terrestrial impact craters have been discovered so far.
Possibly
another 2,000 await discovery. Chicxulub was cited as the largest
impact
crater at 300 miles (480 km) in diameter. (I believe current
estimates
place the actual size at about half that.)
Mark Bailey described the effects of the Tunguska blast in
England,
including a night sky so bright that one could read by it. (Those
of us
living in large cities may not find this remarkable because we
are so used
to light pollution, but the bright nights were a novelty in
1908.) Bailey
suggested the explosive yield at Tunguska reached about 30
megatons.
(I assume this represented a compromise between the commonly
cited 15-20
megatons and the 48 megatons suggested by Hills and Goda.) Jasper
Wall
stated the Tunguska event resulted from a meteor exploding about
one kilometer
above the ground. (I believe the usual estimate for the airburst
height is
several kilometers.) The program stated that Kulik launched three
expeditions to study Tunguska. (I assume this refers to the
expeditions of
1927, 1928 and 1929-1930. However, Kulik also returned to
Tunguska in
1937, 1938, and 1939. Sometimes these last three are lumped
together as a
grand fourth expedition. Further planned expeditions did not take
place
because of the war. Kulik died from typhus in a German prisoner
of war camp
after being wounded in action.) Wall also said that if the
Tunguska
impactor had struck three hours later it would have exploded over
Moscow,
killing ten million people. (This seems high to me.)
The program stated that Tunguska-size events occur about once a
century on
average. (This is the upper end of commonly cited frequencies.
Other
estimates range down to about once a millennium.)
Two decades after Tunguska, in 1930, three small asteroids
exploded over
Brazil. The blasts destroyed about 800 square miles (1280 sq km)
of
jungle. Mark Bailey suggested these asteroids together totalled
an
explosive yield of about 50 to 100 kilotons, much lower than
Tunguska.
Randall Carlson addressed a topic of personal interest to me as a
life-long
Chicagoan. He suggested a cometary impact triggered the Great
Chicago
Fire of 1871 and the simultaneous fires in Wisconsin and
Michigan. The
fire at Peshtigo may have been the worst ever in U. S. history in
terms of
loss of life. The program stated that some scientists had
suggested that
all these fires were ignited by a cometary impact, specifically a
fragment
of Biela's comet. As far as I know, this hypothesis actually
originated
with the granddaddy of American catastrophism, the inimitable
Ignatius
Donnelly. Chicago writer Mel Waskin elaborated Donnelly's idea in
his book
_Mrs. O'Leary's Comet!_. (Some of the eyewitness accounts, from
Peshtigo
in particular, suggest an airburst origin for the fires. However,
I would
assign a much lower probability to the impact hypothesis than
Carlson and
the program did.)
(If I may get a plug in here, those interested in learning more
about the
Great Chicago Fire should peruse the Chicago Historical Society
web site
exhibit curated by Carl Smith, professor of English here at
Northwestern:
http://www.chicagohs.org/fire/index.html
Carl mentions the impact hypothesis very briefly :-}.)
Carlson stated that no one was killed at Tunguska. (This is
commonly repeated
but probably wrong. Two men are reported to have died at
Tunguska: Vasiliy
son of Okhchen from wounds sustained after being hurled against a
tree by the
blast, and the aged hunter Lyuburman of Shanyagir from shock.)
Ray Newburn provided a short summary of what comets are like.
Asteroids
were described as pieces of a planet that never formed. Bill
Bottke
related that approximately 1,500 to 2,000 asteroids 1 km or
larger in
diameter lie in earth-crossing orbits. Some of these will surely
strike
the Earth eventually. Carl Hergenrother noted that we have only
located
about 300 to 400 of these objects.
Bottke offered that an asteroid the size of a house passes
between the
Earth and Moon every day. An asteroid the size of a football
field passes
between the Earth and the Moon once a month. Carl Hergenrother
described a
close approach from one of these: asteroid 1996JA1 missed Earth
by about
280,000 miles (448,000 kilometers), or about seven hours.
It was nice to see Thomas Bopp. The general public seems unaware
of the role
amateur astronomers play in discovering comets.
Ed Tagliaferri discussed his role in getting satellite tracking
information
about airbursts declassified. He stated that there were about 250
such
airbursts recorded over a ten year period, averaging about one
every two
weeks. One of these exploded with the force of 50-70 kilotons of
TNT over
Micronesia in 1994. The danger that such an airburst might be
mistaken for
a nuclear attack was reiterated. Tagliaferri mentioned that
President
Clinton is rumored to have been awakened when the Micronesia
airburst
occurred. The military feared it might have been a nuclear blast.
Ray Newburn discussed impact-generated tsunamis (hey, two
programs in one
night with this information!). A one kilometer asteroid striking
the ocean
would raise a tsunami hundreds of feet high at the coast. The
destructive
wave might continue hundreds of miles inland. A similar size
impact on the
ground would raise a "dirt" wave which would circle the
Earth at 500 mph.
(This didn't make sense to me. Time for me to pull out my copy of
Melosh,
I guess :-}).
Gene Shoemaker and David Levy described the consequences of a
large impact:
darkness, winter, world-wide forest fires, earthquakes and
volcanoes
triggered by tectonic slip, massive acid rain, and an extended
Greenhouse
effect lasting possibly hundreds of years.
David Levy pointed out that the impact of Shoemaker-Levy 9 on
Jupiter
helped removed some of the "giggle factor" preventing
studies of the impact
threat from being taken seriously by governments. However, it is
still not
being taken seriously enough. Levy further noted that the chance
of dying
by impact is about the same as dying in a plane crash. We spend a
lot of
money trying to minimize the risk of death in a plane crash. Why
are we
not willing to spend a comparable amount to minimize the risk of
death from
impact?
There was a little bit of discussion about methods for diverting
incoming
objects. Bill Bottke described the mass driver as one non-nuclear
method
for altering the course of an asteroid.
The program concluded by noting that over 99% of all species are
now
extinct, many possibly as the result of impact events. We are the
first
species with the capability to prevent our own destruction from
impact
events. Yet, funding for NEO search operations continues to be
cut
world-wide. Bill Bottke offered that the yearly cost of such an
operation
amounts to the salaries of a few star professional athletes. Does
it make
sense not to find the funds to protect ourselves and our
descendants --
indeed the whole of life on earth -- from the impact danger?
-- Phil "Pib" Burns
Northwestern University, Evanston, IL. USA
pib@nwu.edu
http://pibweb.it.nwu.edu/~pib/