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This chapter illustrates how the solar system has a decidedly two-dimensional aspect to it. The orbits of the eight major planets all lie in almost the same plane, deviating by no more than seven degrees. Bodies in the asteroid belt and the Kuiper belt stray a little further afield, but these belts are arranged like flattened donuts, aligned with the same plane as the planets. Immanuel Kant and Pierre-Simon de Laplace noted the planar nature of the solar system and used this as the basis for their nebular theories in which the solar system grew out of a flattened disk of matter. Young stars like those in the constellation Orion are often surrounded by disk-shaped clouds of gas and dust. Astronomers quickly dubbed these “protoplanetary” disks, assuming that they will someday form planetary systems.

Discovery of the iridium layer in the K/T boundary clay was the first clue that pointed to a cosmic impact as the trigger of the mass extinction that wiped out the dinosaurs and their associates. But what hit the earth? Was it an asteroid or a comet? To answer, we need to know something about their differences. Unfortunately, the distinction is very blurred. Comets are thought to be huge icy objects, probably with cores made of a mix of water ice and silicates (sandy material), pristine examples of the type of material out of which the solar system was formed. Some of them are hundreds of kilometers in size and they may have been built in the envelopes of gas and dust that surround cool, supergiant stars at the end of their lives. Part of the doubt about distinctions comes from trying to decide what a comet would be like after the ice evaporates. Would it then be like an asteroid? Around the end of the nineteenth century the British astronomer Sir Richard Gregory pictured comets as made up of a cloud of meteorites. He thought that when such an object was first pulled into the solar system from interstellar space it began to glow because of internal heat created as particles began to jostle one another. As the object drew closer to the sun a tail was supposed to be formed as the particles between the meteorites bumped into one another and began to escape. He did consider the potential risk to earth if it were to run into the head of a comet made up of lots of meteorites. The picture he painted was based on what an earlier astronomer, Sir Simon Newcomb, had written about this possibility. Newcomb admitted that, although there were more likely ways to die than as a result of comet collision, such a fate was real. Should such a collision, occur, Gregory conjured up a picture of what might happen. On the one hand, if the comet head was made up of dust, the earth’s inhabitants would experience nothing more than a stunning display of shooting stars. But if the comet head was made of cannonball sized objects the consequences would be dire.

Cosmology is a subject that borders on and sometimes merges with philosophy and religion. Since antiquity, the deep mysteries of the universe have intrigued mankind. Who are we? Where do we come from? What are we made of? Is the development of advanced intelligence capable of comprehending the grand design of the cosmos, the ultimate purpose of the universe? Is there life elsewhere in the universe? Is ours the only advanced intelligence or the most advanced intelligence in the universe? These questions have motivated great thinkers to pursue what Einstein called "the highest wisdom and the most radiant beauty." In the fourth century B.C., the essence of the cosmological question was formulated by the philosopher Chuang Tzu:… If there was a beginning, then there was a time before that beginning. And a time before the time which was before the time of that beginning. If there is existence, there must have been non-existence. And if there was a time when nothing existed, then there must be a time before that—when even nothing did not exist. Suddenly, when nothing came into existence, could one really say whether it belonged to the category of existence or of nonexistence? Even the very words I have just uttered, I cannot say whether they have really been uttered or not. There is nothing under the canopy of heaven greater than the tip of an autumn hair. A vast mountain is a small thing. Neither is there any age greater than that of a child cut off in infancy. P'eng Tsu [a Chinese Methuselah] himself died young. The universe and I came into being together; and I, and everything therein, are one. … Fortunately, our subject matter, solar system chemistry, is less esoteric than the questions asked by Chuang Tzu. A schematic diagram showing the principal pathways by which our solar system is formed is given in figure 4.1. The great triumphs of modern science have been summarized in this figure as fundamental contributions to the five "origins": (a) origin of the universe, (b) origin of the elements, (c) origin of the solar system, (d) origin of life, and (e) origin of advanced intelligence.

As we apprehend the likelihood of an almost inconceivable cosmic impact occurring again at some time in the future, it is worth considering how we got to be here in the first place. The quest for an explanation of our origins is, of course, as old as the ability of humans to conceptualize questions and consider answers. Our species has probably been able to do that for hundreds of thousands of years, since well before evidence of its ability to comprehend was etched in cave paintings, perhaps back in an age when stone tools began to be patiently chipped out of flint rock. But when questions about origins were first hesitatingly formulated, answers could only be invented. There was no way any human beings could have known back then what we know now about the nature of the universe and its contents. Our collective ability to understand the world in which we live received an enormous impetus starting about 400 years ago when the scientific method for approaching reality was first practiced. That was when it was discovered that through experiment and observation, and above all through measurement, it became possible to unravel the secrets of the universe. That was when Galileo first pointed a telescope at the heavens, William Gilbert experimented with natural magnets, and Johannes Kepler discovered the laws of planetary motion. Since then, our species has gathered a stunning new perspective on the nature of this universe and its origins, a perspective that has relegated to the back burner of human thought most of the fantasies that have so long held sway over the human mind. As a result of the high technology that has emerged during this century, scientists have learned to probe into the depths of matter and into the farthest reaches of space. In the course of this exploration, astronomers, in particular, have learned that the universe has its roots in awesome violence and that the birth of the earth and moon were accompanied by what, from our perspective, would be considered catastrophic events. Were anything remotely similar to occur today, all life on earth would be instantly terminated.

There is no doubt that the earth continues to be struck by objects from space. Most of the impactors are very tiny, such as those that produce common meteor trails, and major collisions no longer happen very often. But if a large object, a half kilometer across say, were to strike our planet, the consequences would be devastating. In 1989 an asteroid large enough to bring civilization to the brink of total destruction missed earth by 6 hours and this close encounter in itself should be enough to give us food for thought. There will be other close shaves in the years to come, but no one can predict just when or how close. Only time will tell. Fortunately there are several dedicated groups of astronomers around the world searching for near-earth asteroids (NEAs) in order to catalog their existence and figure orbits lest any should be on a collision course. As a result of their efforts, crucial data are being obtained that will allow the probability of impact to be more accurately estimated, even if only in a statistical sense. The best anyone can do, or will ever be able to do, is to offer odds on the chance of collisions. Odds on comet impact, in the form of estimates of the period between such events, have been published for two centuries. Each generation no doubt felt that the latest estimates were superior to those that went before. For example, in 1861 James Watson, in A Popular Treatise on Comets, said that “it has been found by actual calculation, from the theory of probabilities, that if the nucleus of a comet having a diameter equal to only one fourth part of that of the earth…the probability of receiving a shock from it, is only one in two hundred and eighty-one millions.” This estimate was also quoted by Thomas Dick in 1840 who, in turn, credited it to Francois Arago for calculating this around 1800.