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not　seem　to　have　been　sculpted　by　a　master　craftsman。　Here　too；　there　is　no　hint　of　a　Universe　made　for　us。

*　If　it　had　not；　perhaps　there　would　today　be　another　planet；　a　little　nearer　to　or　farther　from　the　Sun；　on　which　other；　quite　different　beings　would　be　trying　to　reconstruct　their　origins。



THE　DWINDLING　SUPPLY　of　worldlets　is　today　variously　labeled：　asteroids；　ets；　small　moons。　But　these　are　arbitrary　categories—real　worldlets　are　able　to　breach　these　human…made　partitions。　Some　asteroids　（the　word　means　〃starlike；〃　which　they　certainly　are　not）　are　rocky；　others　metallic；　still　others　rich　in　organic　matter。　None　is　bigger　than　1；000　kilometers　across。　They　are　found　mainly　in　a　belt　between　the　orbits　of　Mars　and　Jupiter。　Astronomers　once　thought　the　〃main…belt〃　asteroids　were　the　remains　of　a　demolished　world；　but；　as　I've　been　describing；　another　idea　is　now　more　fashionable：　The　Solar　System　was　once　filled　with　asteroid…like　worlds；　some　of　which　went　into　building　the　planets。　Only　in　the　asteroid　belt；　near　Jupiter；　did　the　gravitational　tides　of　this　most　massive　planet　prevent　the　nearby　debris　from　coalescing　into　a　new　world。　The　asteroids；　instead　of　representing　a　world　that　once　was；　seem　to　be　the　building　blocks　of　a　world　destined　never　to　be。

Down　to　kilometer　size；　there　may　be　several　million　asteroids；　but；　in　the　enormous　volume　of　interplanetary　space；　even　that's　still　far　too　few　to　cause　any　serious　hazard　to　spacecraft　on　their　way　to　the　outer　Solar　System。　The　first　main…belt　asteroids；　Gaspra　and　Ida；　were　photographed；　in　1991　and　1993　respectively；　by　the　Galileo　spacecraft　on　its　tortuous　journey　to　Jupiter。

Main…belt　asteroids　mostly　stay　at　home。　To　investigate　them。　we　must　go　and　visit　them；　as　Galileo　did。　ets；　on　the　other　hand；　sometimes　e　and　visit　us；　as　Halley's　et　did　most　recently　in　1910　and　1986。　ets　are　made　mainly　of　ice；　plus　smaller　amounts　of　rocky　and　organic　material。　When　heated；　the　ice　vaporizes；　forming　the　long　and　lovely　tails　blown　outward　by　the　solar　wind　and　the　pressure　of　sunlight。　After　many　passages　by　the　Sun；　the　ice　is　all　evaporated；　sometimes　leaving　a　dead　rocky　and　organic　world。　Sometimes　the　remaining　particles；　the　ice　that　held　them　together　now　gone；　spread　out　in　the　et's　orbit；　generating　a　debris　trail　around　the　Sun。

Every　time　a　bit　of　etary　fluff　the　size　of　a　grain　of　sand　enters　the　Earth's　atmosphere　at　high　speed；　it　burns　up；　producing　a　momentary　trail　of　light　that　Earthbound　observers　call　a　sporadic　meteor　or　〃shooting　star。〃　Some　disintegrating　ets　have　orbits　that　cross　the　Earth's。　So　every　year；　the　Earth；　on　its　steady　circumnavigation　of　the　Sun；　also　plunges　through　belts　of　orbiting　etary　debris。　We　may　then　witness　a　meteor　shower；　or　even　a　meteor　storm—the　skies　ablaze　with　the　body　parts　of　a　et。　For　example；　the　Perseid　meteors；　seen　on　or　about　August　12　of　each　year；　originate　in　a　dying　et　called　Swift…Tuttle。　But　the　beauty　of　a　meteor　shower　should　not　deceive　us：　There　is　a　continuum　that　connects　these　shimmering　visitors　to　our　night　skies　with　the　destruction　of　worlds。

A　few　asteroids　now　and　then　give　off　little　puffs　of　gas　or　even　form　a　temporary　tail；　suggesting　that　they　are　in　transition　between　etdom　and　asteroidhood。　Some　small　moons　going　around　the　planets　are　probably　captured　asteroids　or　ets；　the　moons　of　Mars　and　the　outer　satellites　of　Jupiter　may　be　in　this　category。

Gravity　smooths　down　everything　that　sticks　out　too　far。　But　only　in　large　bodies　is　the　gravity　enough　to　make　mountains　and　other　projections　collapse　of　their　own　weight；　rounding　the　world。　And；　indeed；　when　we　observe　their　shapes；　almost　always　we　find　that　small　worldlets　are　lumpy；　irregular；　potato…shaped。



THERE　ARE　ASTRONOMERS　whose　idea　of　a　good　time　is　to　stay　up　till　dawn　on　a　cold；　moonless　night　taking　pictures　of　the　sky—the　same　sky　they　photographed　the　year　before　。　。　。　and　the　year　before　that。　If　they　got　it　right　last　time；　you　might　well　ask；　why　are　they　doing　it　again？　The　answer　is：　The　sky　changes。　In　any　given　year　there　might　be　worldlets　wholly　unknown；　never　seen　before；　that　approach　the　Earth　and　are　spied　by　these　dedicated　observers。

On　March　25；　1993；　a　group　of　asteroid　and　et　hunters；　looking　at　the　photographic　harvest　from　an　intermittently　cloudy　night　at　Mount　Palomar　in　California；　discovered　a　faint　elongated　smudge　on　their　films。　It　was　near　a　very　bright　object　in　the　sky；　the　planet　Jupiter。　Carolyn　and　Eugene　Shoemaker　and　David　Levy　then　asked　other　observers　to　take　a　look。　The　smudge　turned　out　to　be　something　astonishing：　some　twenty　small；　bright　objects　orbiting　Jupiter；　one　behind　the　other；　like　pearls　on　a　string。　Collectively　they　are　called　et　Shoemaker…Levy　9　（this　is　the　ninth　time　that　these　collaborators　have　together　discovered　a　periodic　et）。

But　calling　these　objects　a　et　is　confusing。　There　was　a　horde　of　them；　probably　the　fragmented　remains　of　a　single；　hitherto　undiscovered；　et。　It　silently　orbited　the　Sun　for　4　billion　years　before　passing　too　close　to　Jupiter　and　being　captured；　a　few　decades　ago；　by　the　gravity　of　the　Solar　System's　largest　planet。　On　July　7；　1992；　it　was　torn　apart　by　Jupiter's　gravitational　tides。

You　can　recognize　that　the　inner　part　of　such　a　et　would　be　pulled　toward　Jupiter　a　little　more　strongly　than　the　outer　part；　because　the　inner　part　is　closer　to　Jupiter　than　the　outer　part。　The　difference　in　pull　is　certainly　small。　Our　feet　are　a　little　closer　to　the　center　of　the　Earth　than　our　heads；　but　we　are　not　in　consequence　torn　to　pieces　by　the　Earth's　gravity。　For　such　tidal　disruption　to　have　occurred；　the　original　et　must　have　been　held　together　very　weakly。　Before　fragmentation；　it　was；　we　think；　a　loosely　consolidated　mass　of　ice；　rock；　and　organic　matter；　maybe　10　kilometers　（about　6　miles）　across。

The　orbit　of　this　disrupted　et　was　then　determined　to　high　precision。　Between　July　16　and　22；　1994；　all　the　etary　fragments；　one　after　another；　collided　with　Jupiter。　The　biggest　pieces　seem　to　have　been　a　few　kilometers　across。　Their　impacts　with　Jupiter　were　spectacular。

No　one　knew　beforehand　what　these　multiple　impacts　into　the　atmosphere　and　clouds　of　Jupiter　would　do。　Perhaps　the　etary　fragments；　surrounded　by　halos　of　dust；　were　much　smaller　than　they　seemed。　Or　perhaps　they　were　not　coherent　bodies　at　all；　but　loosely　consolidated—something　like　a　heap　of　gravel　with　all　the　particles　traveling　through　space　together；　in　nearly　identical　orbits。　If　either　of　these　possibilities　were　true　Jupiter　might　swallow　the　ets　without　a　trace。　Other　astronomers　thought　there　would　at　least　be　bright　fireballs　and　giant　plumes　as　the　etary　fragments　plunged　into　the　atmosphere。　Still　others　suggested　that　the　dense　cloud　of　fine　particles　acpanying　the　fragments　of　et　Shoemaker…Levy　9　into　Jupiter　would　disrupt　the　magnetosphere　of　Jupiter　or　form　a　new　ring。

A　et　this　size　should　impact　Jupiter；　it　is　calculated；　only　once　every　thousand　years。　It's　the　astronomical　event　not　of　one　lifetime；　but　of　a　dozen。　Nothing　on　this　scale　has　occurred　since　the　invention　of　the　telescope。　So　in　mid　July　1994；　in　a　beautifully　coordinated　international　scientific　effort；　telescopes　all　over　the　Earth　and　in　space　turned　towards　Jupiter。

Astronomers　had　over　a　year　to　prepare。　The　trajectories　of　the　fragments　in　their　orbits　around　Jupiter　were　estimated。　It　was　discovered　that　they　would　all　hit　Jupiter。　Predictions　of　the　timing　were　refined。　Disappointingly；　the　calculations　revealed　that　all　impacts　would　occur　on　the　night　side　of　Jupiter；　the　side　invisible　from　the　Earth　（although　accessible　to　the　Galileo　and　Voyager　spacecraft　in　the　outer　Solar　System）。　But；　happily；　all　impacts　would　occur　only　a　few　minutes　before　the　Jovian　dawn；　before　the　impact　site　would　be　carried　by　Jupiter's　rotation　into　the　line　of　sight　from　Earth。

The　appointed　moment　for　the　impact　of　the　first　piece；　Fragment　A；　came　and　went。　There　were　no　reports　from　ground…based　telescopes。　Planetary　scientists　stared　with　increasing　gloom　at　a　television　monitor　displaying　the　data　transmitted　to　the　Space　Telescope　Science　Institute　in　Baltimore　from　the　Hubble　Space　Telescope。　There　was　nothing　anomalous　Shuttle　astronauts　took　time　off　from　the　reproduction　of　fruit　flies；　fish；　and　newts　to　look　at　Jupiter　through　binoculars。　They　reported　seeing　nothing。　The　impact　of　the　millennium　was　beginning　to　look　very　much　like　a　fizzle。

Then　there　was　a　report　from　a　ground…based　optical　telescope　in　La　Palma　in　the　Canary　Islands；　followed　by　announcements　from　a　radiotelescope　in　Japan；　from　the　European　Southern　Observatory　in　Chile；　and　from　a　University　of　Chicago　instrument　in　the　frigid　wastelands　of　the　South　Pole。　In　Baltimore　the　young　scientists　crowding　around　the　TV　monitor—themselves　monitored　by　the　cameras　of　CNN—began　to　see　something；　and　in　exactly　the　right　place　on　Jupiter。　You　could　witness　consternation　turn　into　puzzlement；　and　then　exultation。　They　cheered；　they　screamed；　they　jumped　up　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　〃