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heCassini-HuygensMission:LiftingtheVeilofSecrecy

Itisdifficulttosaywhatisimpossible,forthedreamofyesterdayisthehopeoftodayandrealityoftomorrow.-RobertGoddard

Introduction

OnJuly1st,2004,theinternationalCassini-HuygensMissionspacecraftenteredintoorbitaroundtheplaSaturn.TheCassinispacecraftwillremaininSaturn'sorbitforfouryearstoprobeitsrings,itsatmosphere,anditsmoons,amongotherthings.OnChristmas2004,theHuygensprobe,attachedtotheCassinispacecraft,willdisengagefromCassiniandisscheduledtolandonoraboutJanuary14,2005,onSaturn'slargestmoon,Titan.Ifsuccessful,thiswouldmarkoneofmankind'sgreatestachievements.Titan,thesecondlargestmooninthesolarsystem,isofgreatinteresttoscientistsbecauseitistheonlymoonknowntohavecloudsandamysteriouspla-likeatmosphere.Titan'satmosphereappearsasanopaqueorangehaze,obscuringitssurfacefromview,andveilingitssecrets.TheCassini-HuygensMissionseekstoliftthatveilofsecrecyduringitsfour-yeartourofSaturn,itsmoons,anditsmagosphere.ScientistshopetogainabetterunderstandingofTitan'ssurface,atmosphere,andchemicalposition,toperhapsshedsomelightaboutwhatprimordialEarthmighthavebeenlikebillionsofyearsago.1

I.ThePlaSaturnanditsNaturalSatellites

InRomanmythology,Saturnisthegodofagriculture.TheassociatedGreekgod,Cronus,wasthesonofUranusandGaiaandthefatherofZeus(Jupiter).SaturnistherootoftheEnglishword"Saturday"(i.e.,Saturn'sday).

TheplaSaturnhasbeenknownsinceprehistorictimes.TheItalianastronomer,physicistandmathematicianGalileoGalilei(1564-1642)wasthefirsttoobserveitwithatelescopein1610,henoteditsoddappearancebutwasconfusedbyit.ThisconfusioncontinuedthroughearlyobservationsofSaturn,whichwereplicatedbythefactthattheEarthpassesthroughtheplaneofSaturn'sringseveryfewyearsasSaturnmovesinitsorbit.AlowresolutionimageofSaturnthereforechangesdrastically.Itwasnotuntil1659thattheDutchastronomerChristiaanHuygens(1629-1695)correctlyinferredthegeometryoftherings.Saturn'sringsremaineduniqueintheknownsolarsystemuntil1977whenveryfaintringswerediscoveredaroundtheplaUranus(andshortlythereafteraroundtheplasJupiterandNeptune).

SaturnisthesixthplafromtheSunandisthesecondlargestinthesolarsystem

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,withanequatorialdiameterof119,300kilometers(74,130miles).SaturnwasfirstvisitedbythePioneer11spacecraftin1979,muchofwhatisknownabouttheplaisduetotheVoyager1andVoyager2spacecraftexplorationsin1980-81.Visiblyflattenedatthepolesasresultoftheveryfastrotationonitsaxis,Saturnhasaday10hours,39minuteslong,andtheplatakes29.5EarthyearstorevolveabouttheSun.Theatmosphereisprimarilyposedofhydrogen(about97percent)withsmallamountsofhelium(about3percent)andmethane.BecauseSaturnistheonlyplalessdensethanwater(about30percentless),ifalargeenoughbodyofwatercouldbefound,Saturnwouldfloatinit.Thepla'shazyyellowhueismarkedbybroadatmosphericbandingsimilarto,butfainterthan,thatfoundonJupiter.

ThewindblowsathighspeedsonSaturn,mostlyinaneasterlydirection.Neartheequator,itreachesvelocitiesof500meterspersecond(1100milesanhour).Thestrongestwindsarefoundneartheequator,andthevelocityfallsoffuniformlyathigherlatitudes.Atlatitudesgreaterthan35degrees,windsalternateeastandwestaslatitudeincreases.

Saturn'sringsystemmakestheplaoneofthemostbeautifulobjectsinthesolarsystem.Theringsaredividedintosevenmajordivisionswithalphabeticdesignatorsintheorderofdiscovery.FromtheinnermostringtotheoutermostringthedesignatorsareD,C,B,A,F,GandE.Eachmajordivisionisfurthersubdividedintothousandsofindividualringlets.TheFandGringsareverythinanddifficulttoseewhiletheA,B,andCringsarebroadandquitevisible.Theringsystemhasvariousgaps,themostnotableofwhichistheCassiniDivision,whichseparatestheAandBrings.TheFrench-ItalianastronomerJeanDominique(GiovanniDomenico)Cassini(1625-1712)discoveredthisdivisionin1675.BetweentheAandFringsliestheKeeler(Encke)gap,whichincludesthebrightAandBringsandafainterCring.TheEncke(pronouncedEN-kee)Division,whichsplitstheARing,isnamedafterJohannEncke,whoiscreditedwithdiscoveringitin1837,butmostlikelyneversawit.TheVoyagerpicturesshowthefourD,E,F,andGfaintrings.Spaceprobeshaveshownthatthemainringsarereallymadeupofalargenumberofnarrowringlets.Theoriginoftheringsisobscure,itisthoughtthattheymayhavebeenformedfromlargermoonsthatwereshatteredbyimpactsofets,andmeteoroids.Theringpositionisnotdefinitivelyknown,buttheringsdoshowasignificantamountofwater.Theymaybeposedoficebergsand/orsnowballswhosesizesmayrangefromafewcentimeterstoafewmeters.Muchoftheelaboratestructureofsomeoftheringsisduetothegravitationaleffectsofnearbynaturalsatellites(moons).

Radialspoke-likefeaturesinthebroadB-ringwerealsofoundbytheVoyagerspacecraft.Thefeaturesarebelievedtobeposedoffinedust-sizeparticles.Thespokeswereobservedtoformanddissipateinthetime-lapseimagestakenbytheVoyagers.Whileelectrostaticchargingmaycreatespokesbylevitatingdustparticlesabovethering,theexactcauseoftheformationofthespokesisnotwellunderstood.2

Toanearthling,Saturn'sphysicalcharacteristicsaremind-bogglinglylarge.SaturnsmeandistancefromtheSunis1,429,400,000kmor9.54AstronomicalUnits(AU)(AnAUisthemeandistancefromtheEarthtotheSun).Saturn'sdiameteris120,536kmatitsequatoranditsmassis5.68x1026kg,or9.5181Earthmasses.Asofnow,Saturnhasatotalof31namedmoons(ornaturalsatellites).The31standlatestnamedmoon(S/2003S1)wasfoundonFebruary5,2003,previously,twoasyetunnamedmoonswerediscoveredin2004.Saturn'sdensityis0.69gramspercubiccentimeter,anditsmeanorbitalvelocity(themeanspeedwithwhichittravelsaroundtheSun)is9.67kilometerspersecond(9.67km/s),whichisaround6miles/s.ThetiltangleofSaturn'saxisofrotationis25.33deg.Saturn'satmosphericpressureis1.4bars,itsmeancloudtemperatureis-125degreesC(-125C).ItslargestmoonTitan,witharadiusof2575.5+/-2km,wasdiscoveredbyChristiaanHuygensin1655.Itsfournextlargestmoons,indescendingorder,areRhea(764+/-4kmradius),Iapetus(730kmradius)Dione(559+

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