View on the Waterfront

S.S.Lau

5/23/98

      Warren Li saw the movie " Deep Impact" and wrote a review on the Lighter Website, where he asked Larry Wan and/or me to calculate the effects of a billion ton comet sitting on the Atlantic Ocean and how much land it will flood, including height of the tidal wave (movie said 1250 feet high)? Mind you, that comet is 7 miles in diameter and bigger than Mt. Everest.

      Larry is probably busy doing his own things, and I thought I would consider how one would go about calculating something like that (not having seen the movie and do not fully appreciate the special effects that Warren described ).

      It seems to me that one would first estimate the terminal impact velocity of a big object, such as a comet or an asteroid, from which one calculates the kinetic energy, K.E., of the mass upon impact using K.E.=1/2 mv2. The terminal velocity is basically determined by the gravitational force and the drag force as the object falls through the atmosphere of the earth. The first term appears to be easier to calculate, whereas the drag term seems to be a tougher problem. The drag coefficient changes as the falling object evaporates and changes shape ( Bobby would know a lot more about drag than I, he deals with this in aerodynamic problems quite often). Let us say we know the initial shape and the final shape of the falling object and the rate of change of the shape, the terminal velocity can then be estimated. Once determined, the thermal energy released is the algebraic sum of the kinetic energy of the object and the energy of all plastic deformations. The rest is just a matter of mass conservation ( which leads to the area of flooded land) and energy conservation ( which leads to the height of the tidal wave). However, the environmental effects have not been considered here at all. It is these effects that could lead to the extinction of spices, as Prof.Luis Alvarez ( Nobel Laureate in Physics, 1968) theorized in 1980 on the extinction of dinosaurs. In other words, to calculate the global effects of an impacting heavenly body, shall we say, is something one writes scientific papers and books, and a serious problem considered by the best minds of our time. And, of course, a rather nice topic for Hollywood to make movies, utilizing advanced computer graphics technology.

      As I began to consider further, I found that I really did not know the difference between a comet and an asteroid. I always thought a comet was just a body of gas and an asteroids were just rocks. The impact of an asteroid would be causing much more damage than a mass of gas, in which case the gas would become "hot" as it traverses the atmosphere. We all know that all you need to do when one encounters " hot gas" is to cover one's nose and walk away quickly.

      I started trying to found the differences by surfing the Net, and I found a lot of information, much more than I would want to know. Just to remind the Lighters the difference between a comet and an asteroid, I excerpted a few paragraphs from the Net ( an NASA site) as follows :

Asteroids

      An asteroid is sometimes called a minor planet because it orbits the Sun directly. The vast majority are found in a swarm called the asteroid belt, between the orbits of Mars and Jupiter. They are given a number (and sometimes then named) when their orbits are well established. 5,000 such asteroids are now known, and many more have been sighted at least once. One Apollo asteroid, now numbered 4581 and called Asclepius, passed within 800,000 km (500,000 mi) of Earth in 1989--the closest approach of a large asteroid since that of Hermes in 1937. The three largest asteroids, CERES, Pallas, and Vesta, are about 800, 600, and 550 km (488, 379, and 336 mi) wide, respectively.

      Comets are bodies of small mass that travel around the Sun. Most of the mass of a comet is concentrated in its nucleus. The diameter of the nucleus is on the order of a few kilometers. Comets are perhaps best described as dirty snowballs. The nucleus consists of a conglomerate of such compounds as water, carbon dioxide, ammonia, and methane, all frozen and mixed with grit and dust. When the comet approaches the Sun, this frozen matter forms a cloud of gas and grit, called the coma, around the nucleus. This cloud of gas is repelled by the solar wind to produce a visible tail.

It has been estimated that micrometeorites, so small that they drift to the ground without vaporizing, may add 1,000 tons to the mass of the Earth each day. Micrometeorites are not associated with meteor showers, nor are the larger meteorites that form craters. Although 500 large meteorites may fall on the Earth each year, only about 1% of these are recovered. Somewhat more than 120 impact sites have been identified.

An NASA scientist also reviewed the technical aspects of two "IMPACT" movies, and I excerpted a few paragraphs for your reference :

by Dr. David Morrison, (May 5, 1998)

      Two major Hollywood productions dealing with the asteroid and comet impact danger are being released in 1998: Deep Impact on May 8 and Armageddon (Disney Films) on July 1. These films may do more to publicize the impact hazard than all previous media coverage taken together. But are the films technically credible, and what effect will they have on public attitudes toward asteroid and comet impacts?

      Deep Impact (Dreamworks and Paramount Pictures)--- Listed as scientific advisors are Carolyn and Gene Shoemaker, Chris Luchini, Joshua Colwell, Gerry Griffin, and David Walker, and the original idea is from the novel Hammer of God by Arthur C. Clarke. The story line concerns a comet a few miles in diameter that is headed for the Earth. Much of the plot is about what people would consider most important if they knew that they only had a few months to live, reminiscent of the classic science fiction film When Worlds Collide.

      Technically, Deep Impact is reasonably accurate. The idea of a comet being spotted about 2 years before impact is plausible, and the strategy to deflect it with nuclear explosives is also appropriate. The special effects on the surface of the active comet are realistic, as is the tsunami produced when the smaller fragment hits the Atlantic. The film makes no mention of other environmental effects of a 2-km ocean impact, but it correctly anticipates the extremely serious consequences of the larger impact (what they call an ELE or extinction-level event). The idea of a nuclear-powered spacecraft to take astronauts to the comet is fiction, of course, at least in terms of current technology, but the film gets high marks for understanding the nature of the impact threat and for the quality of its special effects imagery.

      Having said a lot on this " View on the waterfront", I’ll stop here, and save the other "asteroid-impact movie" review for the future. I intend to relate the story of Prof. Luis Alvarez’s theory on how dinosaurs became extinct from earth due the impact of a massive asteroid.