Dodging the Asteroids?

Finding asteroids and comets that may someday slam into our planet is the first step. What do we do then? This question is being given a whole lot of attention. In early 1993 NASA and the U.S. Congress received a report of the Near-Earth-Objects Interception Workshop (Spaceguard), the first step toward creating a program for pushing aside approaching asteroids. The report stated that “There is a clear need for continuing national and international scientific investigation and political leadership to establish a successful and broadly acceptable policy.” There are two or three options open to us to avoid being wiped out. The first is to step out of the way. This may not sound very practical, and it isn’t, at least not for a planet-load of people. However, if we plan ahead we could ship a few thousand human beings to other parts of the solar system so that if the earth were to be struck, they, at least, would survive. This would only be a privilege for a few, and getting back to earth after the cataclysm could be a rather large problem in itself. Who will welcome them back upon their return? Where would they land? If we could afford to set up colonies on the moon or Mars, the colonists could wait until after the dust had settled before attempting to return. The problem with this option is that, after a really healthy thwack, the earth’s environment would be so altered that returning human beings might find this to be an alien planet. The second way in which we could avoid getting hit would be to place an object between the onrushing comet or asteroid and ourselves. For such an emergency it might pay to place a few asteroids in geocentric orbit to be maneuvered when we need them. Then we could watch the spectacle as one asteroid slams into another, possibly showering the planet with small bits of debris that might do no more than create a spectacular display of fireballs—if we get it right, of course.

The Aftermath

After all the hoopla associated with Jupiter’s publicity stunt died down, planetary scientists got down to the business of analyzing their data. Simulations of the aftermath of a comet or asteroid impact had been available for years and in July 1994 many of the predictions were confirmed, albeit some more dramatically than expected. The timing of the event was almost as if to remind us to take more seriously what we have been thinking and talking about for some time. Putting aside for a moment the implications for life on earth had something similar happened here, let’s look at some of the things that were learned. Argument continues as to what actually hit Jupiter, a comet or asteroid. When the Space Telescope Science Institute sent out a press release on September 29, 1994, entitled “Hubble Observations Shed New Light on Jupiter Collision,” we were led to expect an answer. The introduction gave us further hope: “Was it a comet or an asteroid?” But the institute didn’t have the answer. Its observations slightly favored a cometary origin, but the asteroid possibility still could not be ruled out. Comets are mostly icy, or so we like to think, and asteroids are mostly rocky or metallic, or so we like to think. When you really get down to it, this business of the difference between comets and asteroids has launched a new cottage industry within astronomical circles. A more recent hint that a comet was involved came from observations made from on board the Kuiper Airborne Observatory, an airplane that carries a beautiful infrared telescope high above most of the water vapor in the atmosphere where it can then see more clearly. Ann Sprague and Donald Huntern from the University of Arizona and their colleagues found evidence for water minutes after two of the fragments smashed into Jupiter. The water signature, a spectral line, indicated it was at a temperature of 500 kelvins (degrees above absolute zero, or about 230 Celcius), much hotter than Jupiter’s usual 200 kelvins (-73 Celcius). Although they could not rule out that the water originated deep in Jupiter’s clouds, the way it came and went over a period of 20 minutes suggested that it was liberated by the impact and was part of a cometlike object.

Reconstructing the Crime

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.

Twentieth-Century Rumblings

During the first century B.C., Lucretius wrote, “Legend tells of one occasion when fire got the upper hand. The victory of fire when the earth felt its withering blast, occurred when the galloping steeds that draw the chariot of the sun swept Phaeton from the true course right out of the zone of the ether and far over all lands.” He knew about comets, which is why he said, “There is no lack of external bodies to rally out of infinite space and blast [the world] with a turbulent tornado or inflict some other mortal disaster.” This awareness made him think that the world was newly made, and perhaps in some sense it is. The wheel has come full circle. We now appreciate that the threat of comets and asteroids is real, although the distinction between comets and asteroids has grown blurred. What is no longer in doubt is that catastrophic impacts have occurred in the past, and that they will happen again. At the same time, the hypothesis that impacts and flood legends are related is beginning to experience a revival. A chink in the dam of prejudice against the idea actually began to appear in the 1940’s when two astronomers, Fletcher Watson and Ralph Baldwin, in separate books considered the implications of the discovery of near-earth asteroids (NEAs) and concluded that impacts were likely every million years or so. They were all but ignored. In 1942 H. H. Nininger, the famous meteorite researcher, gave a talk to the Society for Research on Meteorites entitled “Cataclysm and Evolution.” Because of his highly specialized forum, his remarks also went unheard in the wider astronomical community. He considered the danger following the close encounter with Hermes, the NEA discovered in October 1937 that passed within 670,000 kilometers of our planet, which can be compared with the moon’s distance of 384,000 kilometers. (Oddly, Hermes has never been found again. Its rediscovery is one of the prizes that asteroid hunters strive for.) If, instead, it had “smacked the earth in a single lump,” the consequences would “constitute a catastrophe of a magnitude never yet witnessed by man,” said Nininger.

The Nineteenth-Century Perspective

If comets and asteroids have a habit of wandering dangerously close to the earth, why wasn’t the danger recognized a long time ago? It was. In fact, before the beginning of the twentieth century the threat of comets was taken for granted (asteroids had not yet entered the picture). Most astronomers in the nineteenth century accepted that the danger of collision was so obvious that it hardly warranted argument. How they elaborated on the danger varied from the understated, as in the case of Sir John Herschel who in 1835 said that the experience of passing through a comet’s tail might not be “unattended by danger,” to the dramatic, as we shall see. In 1840, Thomas Dick, a well-known popularizer of astronomy, wrote a wonderful book entitled The Sidereal Heavens. In it he reviewed all that was known about the heavens, and did so from a theologian’s perspective. This meant that he repeatedly reminded his readers that the splendor of the night skies was largely the responsibility of the “Divine.” But then, if the existence of planets, comets, nebulae, stars, the sun and moon could be attributed to God, this raised a difficult issue for Dick. If comets were also part of God’s plan, why did the threat of impact exist? Surely God would never allow his creation to be destroyed. Dick did not shy from his predicament and began to search for an answer by conceding that little was known about the nature and origin of comets. At the time it was thought that the head of a comet probably consisted of “something analogous to globular masses of vapor, slightly condensed towards the center, and shining either by inherent light or by the reflected rays of the sun.” The reason he could not be sure as to why the head glowed was because the means to study the properties of light had not yet been invented. That required the development of the spectroscope decades later, a device that breaks light into its various colors, which, when examined closely, can reveal the chemical signature of the object from which the light arrives.

The Saga of the Chicxulub Crater

When the Alvarez team announced to the world that the K/T boundary clay contained a excess of iridium they suggested that it could only be explained if a comet or asteroid had slammed into the earth 65 million years ago. The iridium was deposited when a cloud of debris created by the vaporization of the object upon impact girdled the earth and fell back to form the so-called fireball layer. Most earth scientists were skeptical when they first heard about this. If an object 10 kilometers across had collided with enough force to trigger a global environmental catastrophe that precipitated the extinction of more than half of the species alive at the time, where was the crater? It didn’t take crater experts long to figure that the scar left by such an impact should be huge hole in the ground about 180 kilometers across and a tenth as deep. If it existed, it shouldn’t be hard to find, unless it was under the ocean somewhere, or covered in vast amounts of sediment. It turns out that when the search for the crater began there were several people, perhaps dozens, who already knew where it was. However, they either didn’t know that the search was on, or weren’t allowed to reveal what they knew. The saga of the discovery of the K/T impact crater beneath the north coast of the Yucatan Peninsula of Mexico began many decades before the discovery of iridium in the K/T boundary layer. The saga goes all the way back to 1947 when a gravity survey was started in the Yucatan by the Mexican national oil company, PEMEX. Surface gravity measurements allow geophysicists to detect the structure of rock formations deep beneath the earth’s surface. The study of gravity maps of a region then helps the scientists to figure out where oil might be found; at least that is the goal. The Yucatan survey turned up some intriguing data, including hints of a circular feature some 1,000 meters deep. In the early 1950s test wells were drilled, but no oil was found.

The Search

The earth orbits the sun in a veritable swarm of asteroids that have the nasty habit of occasionally slamming into the planet. To add to the potential danger, comets sometimes wander into the vicinity of the sun, break up, change course, and hang about posing a threat for up to tens of thousands of years. We don’t really want to be hit by any of these, so what can we do to avoid the blows? Most important, we have to keep our collective, astronomical eyes open and try to spot the dangerous ones before they get here. With almost religious fervor some planetary scientists have been seeking near-earth asteroids in recent years to determine how many may be out there that might yet pose a threat to our planet. These scientists have met on many different occasions in the past five years to discuss search strategies and what to do next. The first major meeting of this type, open to more than those in the inner circle that grew from the discovery of the iridium layer in the K/T clays, was held at San Juan Capistrano in 1991. It was billed as the First International Symposium on Near-Earth Asteroids and brought together interested scientists from all over the world. Eugene Shoemaker was there and he urged caution about describing what a civilization-destroying asteroid is. After all, two out of three will hit water, he said, ignoring the point raised in chapter 12, that a water impact is almost certain to produce a greater catastrophe than a land strike, especially if several fragments should be involved. He estimated a civilization-threatening impact once every few million years. Tom Gehrels of Spacewatch at the University of Arizona did not take well to this caution and pointed out that “Humanity is ill-advised to say the probabilities are so low as to ignore them. I will not take that point of view.” As I listened to the talks and arguments, a sobering thought crossed my mind. Resting upon the efforts of this group of searchers might be the future of humankind. All of them realized that estimating probabilities of that magnitude was nit-picking.

Craters and Tsunamis

Until the lunar explorations began in earnest in the 1960s, the Barringer crater in Arizona was believed to be one of the few, if not the only, impact crater on earth. Before the moon landings, many scientists thought that lunar craters were volcanic in origin and that the moon might be covered in a layer of volcanic dust meters thick so that astronauts would sink up to their eyeballs when disembarking from their space capsules. A pleasant sense of relief greeted the news that the first unmanned lunar spacecraft did not disappear into the dust. For a century or more it was doubted that lunar craters were produced by impacts because it was assumed that such craters would seldom be circular. It seemed obvious that circular craters could only be produced by objects falling straight down, a rare situation, since meteorites are likely to approach from random directions, especially on the moon where there is no atmosphere to slow them down before impact. W. M. Smart in 1928 stated this explicitly: “Objections to lunar craters being caused by meteors is that the craters are round and there is no a priori reason why meteors should fall vertically and in no other direction.” He also shuddered at the notion that the impactors would have to be as large as asteroids to create the lunar basins. At about the same time, Thomas Chamberlin ruled out impacts on the moon because there was no evidence for an appropriate population of objects anywhere in the solar system that could have made the craters That was in 1928 when near-earth asteroids had not yet been found, and when little was known about the history of the moon or the formation of the solar system. Richard A. Proctor in 1896, however, had concluded that because so many meteors continued to fall to earth that the planet and the solar system were still forming. To him, this made more sense than to blame the formation of the planets on “the creative fiats of the Almighty.” There is merit to his point of view, because today’s bombardment merely represents a faint, ongoing manifestation of the process of accretion that assembled the planets in the first place.

Death Star or Coherent Catastrophism?

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.

Something about Comets

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.