For many scientists—but especially for Carolyn and Eugene Shoemaker and David Levy—there was something poignant about the etary fragments； one after the other； making their death plunges into Jupiter。 They had lived with this et； in a manner of speaking； for 16 months； watched it split； the pieces； enshrouded by clouds of dust； playing hide…and…seek and spreading out in their orbits。 In a limited way； each fragment had its own personality。 Now they're all gone； ablated into molecules and atoms in the upper atmosphere of the Solar System's largest planet。 In a way； we almost mourn them。 But we're learning from their fiery deaths。 It is perhaps some reassurance to know that there are a hundred trillion more of them in the Sun's vast treasure…house of worlds。
THERE ARE ABOUT 200 known asteroids whose paths take them near the Earth。 They are called； appropriately enough； 〃near…Earth〃 asteroids。 Their detailed appearance （like that of their main…belt cousins） immediately implies that they are the products of a violent collisional history。 Many of them may be the shards and remnants of once…larger worldlets。
With a few exceptions； the near…Earth asteroids are only a few kilometers across or smaller； and take one to a few years to make a circuit around the Sun。 About 20 percent of them； sooner or later； are bound to hit the Earth—with devastating consequences。 （But in astronomy； 〃sooner or later〃 can enpass billions of years。） Cicero's assurance that 〃nothing of chance or hazard〃 is to be found in an absolutely ordered and regular heaven is a profound misperception。 Even today； as et Shoemaker…Levy 9's encounter with Jupiter reminds us； there is routine interplanetary violence； although not on the scale that marked the early history of the Solar System。
Like main…belt asteroids； many near…Earth asteroids are rocky。 A few are mainly metal； and it has been suggested that enormous rewards might attend moving such an asteroid into orbit around the Earth； and then systematically mining it—a mountain of high…grade ore a few hundred miles overhead。 The value of platinum…group metals alone in a single such world has been estimated as many trillions of dollars—although the unit price would plummet spectacularly if such materials became widely available。 Methods of extracting metals and minerals from appropriate asteroids are being studied； for example by John Lewis； a planetary scientist at the University of Arizona。
Some near…Earth asteroids are rich in organic matter； apparently preserved from the very earliest Solar System。 Some have been found； by Steven Ostro of the Jet Propulsion Laboratory； to be double； two bodies in contact。 Perhaps a larger world has broken in two as it passed through the strong gravitational tides of a planet like Jupiter； more interesting is the possibility that two worlds on similar orbits made a gentle overtaking collision and stuck。 This process may have been key to the building of planets and the Earth。 At least one asteroid （Ida； as viewed by Galileo） has its own small moon。 We might guess that two asteroids in contact and two asteroids orbiting one another have related origins。
Sometimes； we hear about an asteroid making a 〃near miss。〃 （Why do we call it a 〃near miss〃？ A 〃near hit〃 is what we really mean。） But then we read a little more carefully； and it turns out that its closest approach to the Earth was several hundreds of thousands or millions of kilometers。 That doesn't count—that's too far away； farther even than the Moon。 If we had an inventory of all the near…Earth asteroids； including those considerably smaller than a kilometer across； we could project their orbits into the future and predict which ones are potentially dangerous。 There are an estimated 2；000 of them bigger than a kilometer across； of which we have actually observed only a few percent。 There are maybe 200；000 bigger than 100 meters in diameter。
The near…Earth asteroids have evocative mythological names： Orpheus； Hathor； Icarus； Adonis； Apollo； Cerberus； Khufu； Amor； Tantalus； Aten； Midas； Ra…Shalom； Phaethon； Toutatis； Quetzalcoatl。 There are a few of special exploratory potential—for example； Nereus。 In general； it's much easier to get onto and off of near…Earth asteroids than the Moon。 Nereus； a tiny world about a kilometer across； is one of the easiest。* It would be real exploration of a truly new world。
* Asteroid 1991JW has an orbit very much like the Earth's and is even easier to get to than 4660 Nereus。 But its orbit seems too similar to the Earth's for it to be a natural object。 Perhaps it's some lost upper stage of the Saturn V Apollo Moon rocket。
Some humans （all from the former Soviet Union） have already been in space for periods longer than the entire roundtrip time to Nereus。 The rocket technology to get there already exists。 It's a much smaller step than going to Mars or even； in several respects； than returning to the Moon。 If something went wrong； though； we would be unable to run home to safety in only a few days。 In this respect； its level of difficulty lies somewhere between a voyage to Mars and one to the Moon。
Of many possible future missions to Nereus； there's one that takes 10 months to get there from Earth； spends 30 days there； and then requires only 3 weeks to return to home。 We could visit Nereus with robots； or—if we're up to it—with humans。 We could examine this little world's shape； constitution； interior； past history； organic chemistry； cosmic evolution； and possible tie to ets。 We could bring samples back for examination at leisure in Earthbound laboratories。 We could investigate whether there really are mercially valuable resources—metals or minerals—there。 If we are ever going to send humans to Mars； near…Earth asteroids provide a convenient and appropriate intermediate goal—to test out the equipment and exploratory protocols while studying an almost wholly unknown little world。 Here's a way to get our feet wet again when we're ready to reenter the cosmic ocean。
CHAPTER 18 THE MARSH OF CAMARINA
＇I＇t's too late to make any improvements now。 The universe is finished；
the copestone is on； and the chips were carted off a million years ago。
—HERMAN MELVILLE； MOBY DICK； CHAPTER 2 （1851）
Camarina was a city in southern Sicily； founded by colonists from Syracuse in 598 B。C。 A generation or two later； it was threatened by a pestilence—festering； some said； in the adjacent marsh。 （While the germ theory of disease was certainly not widely accepted in the ancient world； there were hints…for example； Marcus Varro in the first century B。 C。 advised explicitly against building cities near swamps 〃because there are bred certain minute creatures which cannot be seen by the eyes； which float in the air and enter the body through the mouth and nose and there cause serious disease。〃） The danger to Camarina was great。 Plans were drawn to drain the marsh。 When the oracle was consulted； though； it forbade such a course of action； counseling patience instead。 But lives were at stake； the oracle was ignored； and the marsh was drained。 The pestilence was promptly halted。 Too late； it was recognized that the marsh had protected the city from its enemies—among whom there had now to be counted their cousins the Syracusans。 As in America 2；300 years later； the colonists had quarreled with the mother country。 In 552 B。C。； a Syracusan force crossed over the dry land where the marsh had been； slaughtered every man； woman； and child； and razed the city。 The marsh of Camarina became proverbial for eliminating a danger in such a way as to usher in another； much worse。
THE CRETACEOUS…TERTIARY COLLISION （or collisions—there may have been more than one） illuminates the peril from asteroids and ets。 In sequence； a world…immolating fire burned vegetation to a crisp all over the planet； a stratospheric dust cloud so darkened the sky that surviving plants had trouble making a living from photosynthesis； there were worldwide freezing temperatures； torrential rains of caustic acids； massive depletion of the ozone layer； and； to top it off； after the Earth healed itself from these assaults； a prolonged greenhouse warming （because the main impact seems to have volatilized a deep layer of sedimentary carbonates； pouring huge amounts of carbon dioxide into the air）。 It was not a single catastrophe； but a parade of them； a concatenation of terrors。 Organisms weakened by one disaster were finished off by the next。 It is quite uncertain whether our civilization would survive even a considerably less energetic collision。
Since there are many more small asteroids than large ones； run…of…the…mill collisions with the Earth will be made by the little guys。 But the longer you're prepared to wait； the more devastating the impact you can expect。 On average； once every few hundred years the Earth is hit by an object about 70 meters in diameter； the resulting energy released is equivalent to the largest nuclear weapons explosion ever detonated。 Every 10；000 years； we're hit by a 200…meter object that might induce serious regional climatic effects。 Every million years； an impact by a body over 2 kilometers in diameter occurs； equivalent to nearly a million megatons of TNT�
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