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elated　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—an　explosion　that　would　work　a　global　catastrophe；　killing　（unless　unprecedented　precautions　were　taken）　a　significant　fraction　of　the　human　species。　A　million　megatons　of　TNT　is　100　times　the　explosive　yield　of　all　the　nuclear　weapons　on　the　planet；　if　simultaneously　blown　up。　Dwarfing　even　this；　in　a　hundred　million　years　or　so；　you　can　bet　on　something　like　the　Cretaceous…Tertiary　event；　the　impact　of　a　world　10　kilometers　across　or　bigger。　The　destructive　energy　latent　in　a　large　near…Earth　asteroid　dwarfs　anything　else　the　human　species　can　get　its　hands　on。

As　first　shown　by　the　American　planetary　scientist　Christopher　Chyba　and　his　colleagues；　little　asteroids　or　ets；　a　few　tens　of　meters　across；　break　and　burn　up　on　entering　our　atmosphere。　They　arrive　paratively　often　but　do　no　significant　harm。　Some　idea　of　how　frequently　they　enter　the　Earth's　atmosphere　has　been　revealed　by　declassified　Department　of　Defense　data　obtained　from　special　satellites　monitoring　the　Earth　for　clandestine　nuclear　explosions。　There　seem　to　have　been　hundreds　of　small　worldlets　（and　at　least　one　larger　body）　impacting　in　the　last　20　years。　They　did　no　harm。　But；　we　need　to　be　very　sure　we　can　distinguish　a　small　colliding　et　or　asteroid　from　an　atmospheric　nuclear　explosion。

Civilization…threatening　impacts　require　bodies　several　hundred　meters　across；　or　more。　（A　meter　is　about　a　yard；　100　meters　is　roughly　the　length　of　a　football　field。）　They　arrive　something　like　once　every　200；000　years。　Our　civilization　is　only　about　10；000　years　old；　so　we　should　have　no　institutional　memory　of　the　last　such　impact。　Nor　do　we。

et　Shoemaker…Levy　9；　in　its　succession　of　fiery　explosions　on　Jupiter　in　July　1994；　reminds　us　that　such　impacts　really　do　occur　in　our　time—and　that　the　impact　of　a　body　a　few　kilometers　across　can　spread　debris　over　an　area　as　big　as　the　Earth。　It　was　a　kind　of　portent。

In　the　very　week　of　the　Shoemaker…Levy　impact；　the　Science　and　Space　mittee　of　the　U。S。　House　of　Representatives　drafted　legislation　that　requires　NASA　〃in　coordination　with　the　Department　of　Defense　and　the　space　agencies　of　other　countries〃　to　identify　and　determine　the　orbital　characteristics　of　all　Earth…approaching　〃ets　and　asteroids　that　are　greater　than　1　kilometer　in　diameter。〃　The　work　is　to　be　pleted　by　the　year　2005。　Such　a　search　program　had　been　advocated　by　many　planetary　scientists。　But　it　took　the　death　throes　of　a　et　to　move　it　toward　practical　implementation。

Spread　out　over　the　waiting　time；　the　dangers　of　asteroid　collision　do　not　seem　very　worrisome。　But　if　a　big　impact　happens；　it　would　be　an　unprecedented　human　catastrophe。　There's　something　like　one　chance　in　two　thousand　that　such　a　collision　will　happen　in　the　lifetime　of　a　newborn　baby。　Most　of　us　would　not　fly　in　an　airplane　if　the　chance　of　crashing　were　one　in　two　thousand。　（In　fact　for　mercial　flights　the　chance　is　one　in　two　million。　Even　so；　many　people　consider　this　large　enough　to　worry　about；　or　even　to　take　out　insurance　for。）　When　our　lives　are　at　stake；　we　often　change　our　behavior　to　arrange　more　favorable　odds。　Those　who　don't　tend　to　be　no　longer　with　us。

Perhaps　we　should　practice　getting　to　these　worldlets　and　diverting　their　orbits；　should　the　hour　of　need　ever　arise。　Melville　notwithstanding；　some　of　the　chips　of　creation　are　still　left；　and　improvements　evidently　need　to　be　made。　Along　parallel　and　only　weakly　interacting　tracks；　the　planetary　science　munity　and　the　U。S。　and　Russian　nuclear　weapons　laboratories；　aware　of　the　foregoing　scenarios；　have　been　pursuing　these　questions：　how　to　monitor　all　sizable　near…Earth　interplanetary　objects；　how　to　characterize　their　physical　and　chemical　nature；　how　to　predict　which　ones　may　be　on　a　future　collision　trajectory　with　Earth；　and；　finally；　how　to　prevent　a　collision　from　happening。

The　Russian　spaceflight　pioneer　Konstantin　Tsiolkovsky　argued　a　century　ago　that　there　must　be　bodies　intermediate　ill　size　between　the　observed　large　asteroids　and　those　asteroidal　fragments；　the　meteorites；　that　occasionally　fall　to　Earth。　He　wrote　about　living　on　small　asteroids　in　interplanetary　space。　He　did　not　have　military　applications　in　mind。　In　the　early　1980s；　though；　some　in　the　U。S。　weapons　establishment　argued　that　the　Soviets　might　use　near…Earth　asteroids　as　first…strike　weapons；　the　alleged　plan　was　called　〃Ivan's　Hammer。〃　Countermeasures　were　needed。　But；　at　the　same　time；　it　was　suggested；　maybe　it　wasn't　a　bad　idea　for　the　United　States　to　learn　how　to　use　small　worlds　as　weapons　of　its　own。　The　Defense　Department's　Ballistic　Missile　Defense　Organization；　the　successor　to　the　Star　Wars　office　of　the　1980s；　launched　an　innovative　spacecraft　called　Clementine　to　orbit　the　Moon　and　fly　by　the　near…Earth　asteroid　Geographos。　（After　pleting　a　remarkable　reconnaissance　of　the　Moon　in　May　1994；　the　spacecraft　failed　before　it　could　reach　Geographos。）

In　principle；　you　could　use　big　rocket　engines；　or　projectile　impact；　or　equip　tile　asteroid　with　giant　reflective　panels　and　shove　it　with　sunlight　or　powerful　Earth…based　lasers。　But　with　technology　that　exists　right　now；　there　are　only　two　ways。　First；　one　or　more　high…yield　nuclear　weapons　might　blast　the　asteroid　or　et　into　fragments　that　would　disintegrate　and　atomize　on　entering　the　Earth's　atmosphere。　If　the　offending　worldlet　is　only　weakly　held　together；　perhaps　only　hundreds　of　megatons　would　suffice。　Since　there　is　no　theoretical　upper　limit　to　the　explosive　yield　of　a　thermonuclear　weapon；　there　seem　to　be　those　in　the　weapons　laboratories　who　consider　making　bigger　bombs　not　only　as　a　stirring　challenge；　but　also　as　a　way　to　mute　pesky　environmentalists　by　securing　a　seat　for　nuclear　weapons　on　the　save…the…Earth　bandwagon。

Another　approach　under　more　serious　discussion　is　less　dramatic　but　still　an　effective　way　of　maintaining　the　weapons　establishment—a　plan　to　alter　the　orbit　of　any　errant　worldlet　by　exploding　nuclear　weapons　nearby。　The　explosions　（generally　near　the　asteroid's　closest　point　to　the　Sun）　are　arranged　to　deflect　it　away　from　the　Earth。*　A　flurry　of　low…yield　nuclear　weapons；　each　giving　a　little　push　in　the　desired　direction；　is　enough　to　deflect　a　medium…sized　asteroid　with　only　a　few　weeks'　warning。　The　method　also　offers；　it　is　hoped；　a　way　to　deal　with　a　suddenly　detected　long…period　et　on　imminent　collision　trajectory　with　the　Earth：　The　et　would　be　intercepted　with　a　small　asteroid。　（Needless　to　say；　this　game　of　celestial　billiards　is　even　more　difficult　and　uncertain—and　therefore　even　less　practical　in　the　near　future—than　the　herding　of　an　asteroid　on　a　known；　well…behaved　orbit　with　months　or　years　at　our　disposal。）

*　The　Outer　Space　Treaty；　adhered　to　both　by　the　United　States　and　Russia；　prohibits　weapons　of　mass　destruction　in　〃outer　space。〃　Asteroid　deflection　technology　constitutes　just　such　a　weapon—indeed；　the　most　powerful　weapon　of　mass　destruction　ever　devised。　Those　interested　in　developing　asteroid　deflection　technology　will　want　to　have　the　treaty　revised。　But　even　with　no　revision；　were　a　large　asteroid　to　be　discovered　on　impact　trajectory　with　the　Earth；　presumably　no　one's　hand　would　be　stayed　by　the　niceties　of　international　diplomacy。　There　is　a　danger；　though；　that　relaxing　prohibitions　on　such　weapons　in　space　might　make　us　less　attentive　。bout　the　positioning　of　warheads　for　offensive　purposes　in　space。

We　don't　know　what　a　standoff　nuclear　explosion　would　do　to　an　asteroid。　The　answer　may　vary　from　asteroid　to　asteroid。　Some　small　worlds　might　be　strongly　held　together；　others　might　be　little　more　than　self…gravitating　gravel　heaps。　If　an　explosion　breaks；　let's　say；　a　10kilometer　asteroid　up　into　hundreds　of　1…kilometer　fragments；　the　likelihood　that　at　least　one　of　them　impacts　the　Earth　is　probably　increased；　and　the　apocalyptic　character　of　the　consequences　may　not　be　much　reduced。　On　the　other　hand；　if　the　explosion　disrupts　the　asteroid　into　a　swarm　of　objects　a　hundred　meters　in　diameter　or　smaller；　all　of　them　might　ablate　away　like　giant　meteors　on　entering　the　Earth's　atmosphere。　In　this　case　little　impact　damage　would　be　caused。　Even　if　the　asteroid　were　wholly　pulverized　into　fine　powder；　though；　the　resulting　high…altitude　dust　layer　might　be　so　opaque　as　to　block　the　sunlight　and　change　the　climate。　We　do　not　yet　know。

A　vision　of　dozens　or　hundreds　of　nuclear…armed　missiles　on　ready　standby　to　deal　with　threatening　asteroids　or　ets　has　been　offered。　However　premature　in　this　particular　application；　it　seems　very　familiar；　only　the　enemy　has　been　changed。　It　also　seems　very　dangerous。

The　problem；　Steven　Ostro　of　JPL　and　I　have　suggested；　is　that　if　you　can　reliably　deflect　a　threatening　worldlet　so　it　does　not　collide　with　the　Earth；　you　can　also　reliably　deflect　a　harmless　worldlet　so　it　does　collide　with　the　Earth。　Suppose　you　had　a　full　inventory；　with　orbits；　of　the　estimated　300；000　near…Earth　asteroids　larger　than　100　meters—each　of　them　large　enough；　on　impacting　the　Earth；　to　have　serious　consequences。　Then；　it　turns　out；　you　also　have　a　list　of　huge　numbers　of　inoffensive　asteroids　whose　orbits　could　be　altered　with　nuclear　warheads　so　they　quickl