p and down。 Smiles filled the room。 They broke out the champagne。 Here was a group of young American scientists—about a third of them； including the team leader； Heidi Hammel； women—and you could imagine youngsters all over the world thinking that it might be fun to be a scientist； that this might be a good daytime job； or even a means to spiritual fulfillment。
For many of the fragments； observers somewhere on Earth noticed the fireball rise so quickly and so high that it could be seen even though the impact site below it was still in Jovian darkness。 Plumes ascended and then flattened into pancake…like forms。 Spreading out from the point of impact we could see sound and gravity waves； and a patch of discoloration that for the largest fragments became as big as the Earth。
Slamming into Jupiter at 60 kilometers a second （130；000 miles an hour）； the large fragments converted their kinetic energy partly into shock waves； partly into heat。 The temperature in the fireball was estimated at thousands of degrees。 Some of the fireballs and plumes were far brighter than all the rest of Jupiter put together。
What is the cause of the dark stains left after the impact？ It might be stuff from the deep clouds of Jupiter…from the region to which ground…based observers cannot ordinarily see…that welled up and spread out。 However； the fragments do not seem to have penetrated to such depths。 Or the molecules responsible for the stains might have been in the etary fragments in the first place。 We know from the Vega 1 and 2 Soviet missions and the Giotto mission of the European Space Agency—both to Halley's et—that ets may be as much as a quarter posed of plex organic molecules。 They are the reason that the nucleus of Halley's et is pitch black。 If some of the etary organics survived the impact events； they may have been responsible for the stain。 Or； finally； the stain may be due to organic matter not delivered by the impacting etary fragments； but synthesized by their shock waves from the atmosphere of Jupiter。
Impact of the fragments of et Shoemaker…Levy 9 with Jupiter was witnessed on seven continents。 Even amateur astronomers with small telescopes could see the plumes and the subsequent discoloration of the Jovian clouds。 Just as sporting events are covered at all angles by television cameras on the field and from a dirigible high overhead； six NASA spacecraft deployed throughout the Solar System； with different observational specialties； recorded this new wonder—the Hubble Space Telescope； the International Ultraviolet Explorer； and the Extreme Ultraviolet Explorer all in Earth orbit； Ulysses； taking time out from it investigation of the South Pole of the Sun； Galileo； on the way to its own rendezvous with Jupiter； and Voyager 2； far beyond Neptune on its way to the stars。 As the data are accumulated and analyzed； our knowledge of ets； of Jupiter； and of the violent collisions of worlds should all be substantially improved。
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 dio