Pib's Comments on "Fire from the Sky"

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.)

(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:


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. Land-based shock waves differ somewhat from tsunamis.)

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?

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Last modified by pib on July 6, 2003.