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nAsA、lAunChED thE First spACE mission to pluto yEstErDAy As A powErFul roCkEt hurlED thE nEw horizons spACECrAFt on A ninE-yEAr, thrEE-Billion-milE journEy to thE EDgE oF thE solAr systEm As it soArED towArD A 2007 mEEting with jupitEr, whosE powErFul grAvitAtionAl FiElD will shoot it on its wAy to pluto. mission mAnAgErs sAiD rADio CommuniCAtions ConFirmED thAt thE 1,054-pounD CrAFt wAs in gooD hEAlth. thE $700 million mission BEgAn whEn A loCkhEED mArtinAtlAs 5 roCkEt rosE From A lAunChing pAD At thECApECAnAvErAlAir ForCE stAtion in FloriDA At 2 p.m., Almost An hour lAtEr thAn plAnnED BECAusE oF low ClouDs thAt oBsCurED A ClEAr viEw oF thE Flight pAth By trACking CAmErAs. lEss thAn An hour lAtEr, All thrEE stAgEs oF thE BoostEr roCkEt workED As plAnnED, AnD thE spACECrAFt sEpArAtED From thEm AnD sprintED AwAy towArD DEEp spACE. thE roBot ship spED AwAy At ABout 36,000 milEs pEr hour, thE FAstEst Flight oF Any spACECrAFt sEnt FromEArth. Allowing it to pAss thE moon in ABout ninE hours. "this is A historiC DAy," sAiDAlAn stEm oF thE southwEst rEsEArCh institutE inBoulDEr,Colo, thE mission’s prinCipAl sCiEntist AnD tEAm lEADEr. spEAking At A nEws ConFErEnCE At thE kEnnEDy spACECEntEr in FloriDA、Dr. stErn sAiD thE timing AssurED thAt thE nEw horizons woulD ArrivE For its ClosEst ApproACh to pluto on july 14, 2015—thE 50th AnnivErsAry oF thE First FlyBy oF mArs By thE mArinEr 4. thE mission thAt BEgAn thE ExplorAtion oF thE plAnEts. thE nEw horizons is powErED By A smAll plutonium-FirED ElECtriC gEnErAtor. its instrumEnts inCluDE thrEE CAmErAs, For visiBlE-light, inFrArED AnD ultrAviolEt imAgEs, AnD thrEE spECtromEtErs to stuDy thE Composition AnD tEmpErAturEs oF pluto’s thin AtmosphErE AnD surFACE FEAturEs. it Also CArriEs A univErsity oFColorADo Dust CountEr, thE First ExpErimEnt to Fly on A plAnEtAry mission thAt is EntirEly DEsignED AnD opErAtED By stuDEnts. this is thE only ExpErimEnt thAt will not hiBErnAtE During thE mission. yEstErDAy’s liFtoFF Also pAiD rEgArD to pluto’s DisCovErEr, thE AstronomErClyDE w. tomBAugh. who in 1930 BECAmE thE onlyAmEriCAn to FinD A plAnEt in thE solAr systEm.(hE DiED At 90. in 1997.) his wiDow, pAtriCiA tomBAugh. 93. AnD othEr FAmily mEmBErs wErE prEsEnt At thE CApE, AnD somE oF his rEmAins wErE Among thE CommEmorAtivE itEms ABoArD thE spACECrAFt. "somE oFClyDE’s AshEs ArE on thEir wAy to pluto toDAy,"Dr. stEm sAiD、 thE nEw horizons is to rEACh jupitEr’s grAvitAtionAl FiElD in 13 months. thE trip to pluto will tAkE Eight morE yEArs, most oF whiCh thE CrAFt will spEnD in ElECtroniC "hiBErnAtion" to sAvE powEr AnD wEAr on thE EquipmEnt nEEDED For its sEvEn ExpErimEnts. in ADDition to thE two-hour DElAy, thE lAunChing wAs postponED twiCE in two DAys—on tuEsDAy By strong winDs At thE CApE AnD on wEDnEsDAy By A storm thAt CAusED A powEr; FAilurE At thE spACECrAFt’s Control CEntEr At thE johns hopkins univErsityAppliED physiCs lABorAtory in lAurEl. mD、mission plAnnErs hAD until FEB、14 to lAunCh thE mission this yEAr, But only until thE EnD oF this month to usE thE grAvity Boost From jupitEr, whiCh will shortEn thE trip to pluto By FivE yEArs.

in thE opEning pArAgrAph, thE Author introDuCEs his topiC By______.A．DEsCriBing A situAtion
B．justiFying An Assumption
C．mAking A CompArison
D．prEsEnting A phEnomEnon