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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 m