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This chapter discusses how countless small rocks from space encounter Earth each day, burning up completely in the atmosphere as meteors. Most meteors are tiny, no larger than a grain of sand. These disappear in the upper atmosphere in a matter of seconds. However, the boulder at the center of the 1992 fireball incident in the United States was much larger—too large to burn up entirely—and it plowed straight through the upper atmosphere, continuing toward the ground. As the boulder reached the lower atmosphere, where the air is densest, the wind resistance became strong enough to tear the boulder to pieces. At least one piece of the boulder survived to reach the ground, where it became a meteorite—a lump of rock that literally fell from the sky.

On the morning of June 30, 1908, civilization may have suffered the worst piece of luck in its history. A small cometlike object exploded in the atmosphere above the Tunguska river valley in Siberia. It did little more than scorch and flatten trees for 20 kilometers in all directions and kill a thousand reindeer. However, if that object had struck a heavily populated region, we would not now dwell under any illusion concerning how close to the edge of extinction the human species actually hovers. Because the Tunguska missile missed a populated area, the threat of impact did not really begin to enter the public imagination until after the 1980 announcement of the discovery of the iridium in the K/T boundary layer. Had the Tunguska object struck a large city, a million people or more might have perished, and the phenomenon would have raised everyone’s awareness to the threat of comet impact. Instead, nearly a century later, the threat of comet and asteroid impact is regarded as little more than an interesting anecdote. Very slowly the nature of the threat is being recognized, but only because of the somewhat esoteric discovery that the dinosaurs were wiped out by a major impact 65 million years ago. Such huge collisions are infrequent, perhaps about once every 50 to 100 million years. It is the smaller impacts that pose the greatest danger, and they occur far more frequently. About 800 years ago the South Island of New Zealand suffered widespread fires, which leveled the island and led to the extinction of the Moa bird. Maori legend says that a big explosion in the sky was the cause of the strange fire. Duncan Steel of the University of Adelaide and Peter Snow from Otago in New Zealand have pieced together a fascinating scenario that suggests that the Maori were correct. The fireball created by a comet impact may have ignited the forests of South Island. Near the town of Tapanui in the province of Otago there exists a crater that geologists have been slow to identify as extraterrestrial in origin.

Just what happened to the dinosaurs? In the mind’s eye, travel back to the Cretaceous period, 65 million years ago. First, land in a region of the world that will someday be called Oklahoma. You are in the era of dinosaurs, although there are no longer as many species about, worldwide, as there were ten million or so years before. In all, 23 species roam their individual parts of the planet. It is their lack of spatial diversity that will make them vulnerable to the catastrophe that is about to befall the earth. So imagine you are there, together with triceratops, stegosaurus, velociraptors, and tyrannosaurus rex. Mostly they live off the land, and some of them live off each other. On this day none of the animals on earth can possibly have any awareness that they are about to disappear. Such a luxury will only be granted to a conscious species that has learned to explore the universe. For those who survive the initial impact explosion and its immediate consequences, the coming months will mark a terrible example of one of Cuvier’s “brief periods of terror.” In rapid succession, all life will be subject to a holocaust of staggering proportion, horrendous blast waves, searing winds, showers of molten matter from the sky, earthquakes, a terrible darkness that will cut out sunlight for a year, and freezing weather that will last a decade. The ozone layer will be destroyed, and acid rain will make life intolerable for species that survived the first few months after the impact. You are there and you have been observing an odd phenomenon in the sky. For thousands of years a great comet has loomed, repeatedly lighting up the heavens with its glorious tail and then fading away to reappear a few years later. Long ago it was seen to break into fragments, each of which was a spectacular sight in its own right. Sometimes one of those fragments seemed to loom ever so close to the earth. For thousands of years, spectacular meteor showers have been seen whenever the earth passed through the tail of one of those comets, and sometimes dust drifted down into the atmosphere and disturbed the climate.

Our instinct for survival drives us to learn as much as possible about what goes on around us. The better we understand nature, the better we will be able to predict its vagaries so as to avoid life-threatening situations. Unfortunately, nature is seldom so kind as to arrange for disasters to occur like clockwork, yet that does not dampen our enthusiasm when even a hint of periodicity in a complex phenomenon is spotted. This helps account for the furor that was created when a few paleontologists claimed that mass extinctions of species seemed to recur in a regular manner. A cycle, a periodicity, had been found! That implied that perhaps they might be able to predict nature’s next move. This is how I interpret the extraordinary public interest that was generated by the claims made around 1984 that the mass extinction phenomenon showed a roughly 30-million-year period (others said it was 26 million years). Almost immediately, several books appeared on the subject as well as many, many articles in the popular press and in science magazines. This activity marked the short life of the Death Star fiasco. Given our instinctual urge to look for order in the chaos of existence, the identification of a periodicity in mass-extinction events was a great discovery, if real. What was not highlighted by those who climbed aboard the bandwagon, however, was that the last peak in the pattern occurred about 13 million years ago. If impact-related mass extinction events were produced every 30 million years, there obviously was no cause for concern that we would be hit by a 10-kilometer object in the next 17 million years. Phew! I think that the suggestion that mass extinctions occurred on a regular cycle caused as much interest as it did because we all want to believe that there is no immediate danger to us. The Death Star fiasco began when David Raup and John Sepkowski of the University of Chicago published a report claiming that mass extinction events recurred about every 26 million years. They were followed by Michael Rampino and Richard Stothers of the Goddard Institute for Space Studies in New York who claimed that the period was more like 30 million years, at least during the last 250 million years.

Just as everyone offering odds was beginning to feel secure that comet impact is not an immediate threat to life on earth, I heard several planetary scientists state confidently that there was not even a chance of our seeing a comet-planet collision in our lifetimes. Since then Jupiter suffered the humiliation of being lashed by no less than 21 comet fragments, most of them sizable objects in their own right. The saga began on March 24, 1993, when Carolyn Shoemaker discovered a “squashed comet” in a photograph that had just been taken by the team of which she was a member. She and her husband, Eugene, together with David Levy were using the 18-inch Schmidt telescope on Palomar Mountain near San Diego to hunt for near-earth asteroids and none of them had ever seen a squashed comet before. What happened to set the scene for the discovery of the squashed comet is now apocryphal. The previous night had been perfect for the search, but someone had apparently exposed a box of film to daylight so that the exposures turned out totally black. The box of film was set aside, which was no immediate loss because on the fateful night the skies were mostly cloudy, which made comet hunting very difficult. Apparently David Levy, not wont to waste any opportunity to take more pictures, suggested that they go ahead and use some of the ruined sheets of film just in case some were not totally useless. At a cost of $4 per sheet, the film was usually very carefully used, given that they took photographs every ten minutes or so, all night long. Normally they would not have taken data that night, but why not go ahead and fire off a few exposures with the bad film. between and through the clouds. As luck would have it, the sheets of film deeper into the box were only light-damaged around their edges, so they managed to get some nice photographs. It was Carolyn Shoemaker’s task to place a pair of developed pictures taken 45 minutes apart of the same area of sky into a stereo microscope.

How does the plasma sheet respond to the complex pattern of waves coming over the poles from bursty magnetopause reconnection events, or to the vortices and other irregular perturbations coming around the flanks of the magnetosphere in the low-latitude boundary layer? It is probably too much to expect that the complex input from the dayside will sort itself out into a steady flow on the nightside, but there has been a seductive hope that, on a statistical basis, the observations of the plasma sheet could be rationalized using steady convection thinking. This hope depends on the belief that the average magnetic field configuration in the plasma sheet actually is compatible with steady convection. The first doubts on this score were raised by Erickson and Wolf (1980), and were subsequently elaborated by Tsyganenko (1982), Birn and Schindler (1983), and Liu and Hill (1985); the“plasma sheet pressure paradox” they posed is the subject of Section 9.2. Theoretical arguments are one thing, measurements are another; the truly important issue is whether the real plasma sheet manifests steady flow. Several groups have searched large data sets to see whether the statistically averaged flow in the central plasma sheet resembles the flow predicted by the steady convection model. This effort has led to a growing but still incomplete comprehension of the statistical properties of plasma sheet transport. Results obtained using ensembles of data acquired by ISEE 1 and AMPTE/IRM will be reviewed in Section 9.3. The unusual distribution of bulk flow velocities suggests that the plasma sheet flow is bimodal, alternating between a predominant irregular low-speed state and an infrequently occurring state of high-speed earthward flow. In search of steady plasma sheet flow, one could also look into substormfree periods of stable solar wind properties. One of the best such studies, in which great care was taken to find periods of exceptionally stable solar wind and geomagnetic conditions, is reviewed in Section 9.4. Even this study found highly irregular and bursty flow.