SSolar System Exploration: A Vision for the Next 100 Years

R. L. McNUTT Jr.

JohNs hopkiNs ApL TechNicAL DigesT, VoLUMe 27, NUMbeR 2 (2006)168

SSolar System Exploration: A Vision for the Next 100 Years

Ralph L. McNutt Jr.

pace travel is multitiered, but the primary challenge is propulsion. Asso-ciated costs continue to stall significant advances for both manned and

unmanned missions. While there continues to be a hope that commercializa-tion will lead to lower launch costs, markets for deep space remain elusive. Hence, initial development beyond Earth orbit will likely remain government sponsored. Against this backdrop, we consider the linkage of scientific goals, current efforts, expectations, cur-rent technical capabilities, and requirements for the detailed exploration of the solar system and consolidation of off-Earth outposts. Over the next century, distances of 50 astronomical units could be reached by human crews, but only if resources are brought to bear by international consortia.

INTRODUCTION

“Where there is no vision the people perish.”1

Proverbs,29:18

Agreatdealofnewinterestinspaceandspacetravelhas arisen with thepublicationof theAldridge Com-missionReport,2efforts todevelopcurrent space tour-ism, and an ongoing series of missions to provide sci-entific exploration (notably of Mercury, Mars, Saturn,andPluto).NASAscienceplanninghastypicallyunder-gonea“roadmapping”activityonceevery3years.ThistypeofplanninghasbeenextendedintheworkoftheAldridgeCommission.

Nonetheless, these exercises typically project onlyinto a “far future” of typically 20 years hence, if that

much,3usually—andrightly—stating thatpolicygoalsand technologies will change so radically on longertimescalesthatfurtherextrapolationmustberelegatedto the realmof sciencefictionor fantasy.However, aswehavenowenteredthesecondcenturyofaeronautics,wehaveabaseline to lookbackon.Wecanseewhatpeoplethoughtmighthappenintheprevious100years,what reallydidhappenandwhy,andperhapsmore tothepoint,atwhatcost.

Especially following the technicalgains in rocketryandatomicpowerbroughtaboutbyWorldWarII,therewasmuchspeculationaboutwhatwecould,andwould,do in space.Historically, space explorationhas alwaysbeenalignedwithanoutward look. In thebeginning,

soLAR sYsTeM eXpLoRATioN: A VisioN

JohNs hopkiNs ApL TechNicAL DigesT, VoLUMe 27, NUMbeR 2 (2006) 169

thevisionwasoneofmannedflightstoEarthorbitandbeyond.Whilethatoutlookgavewaytoroboticprobesgoingtoadestinationfirst,ithasalways,atleasttothemajority of people, been with the idea that humanswouldeventuallyfollow.

The Art of PredictionFuturepredictionscanbe,andmanytimesare,mis-

leading. An example is the prediction from the earlydaysoftheautomobilethatitwouldsoonbesupplantedbypersonalairplanesfor localtravel,ascenarioyettobe borne out. At the same time, such predictions—ifgrounded in limits imposed by physics and chemistryand for which there is a technology path—can workout.Usually,theimplementationisnotaseasyasorigi-nallythoughtbecausethedevilalwaysisinthedetails.AnexampleisVonBraun’s6400-metric-ton(MT)ferryvehiclesforcarrying39.4MTtoa102-kmorbitforbuild-ingMarstransfervehicles.4,5Thepropellantsforthreestageswerenitricacidandhydrazine.

The Space Shuttle, on the other hand, is a singlevehiclewithdetachablesolidrocketboosters(usingsolidpropellant)andathrowawaymaintankcarryingliquidoxygen(LOX)andliquidhydrogen(LH2).Itisa2040-MTvehiclethatcancarry24.4MTtoa204-kmorbit.TheShuttlehasbetterperformancethanVonBraun’svehicles,butwithlesscargobecausearealisticaltitudeishigherthanheenvisioned.VonBraunalsoenvisionedthedrivingcostforhisMarsmissionat$500millionforabout5.3millionMTofpropellantfortheferryshipstotransportthepropellantandsuppliestoEarthorbit;thistranslates to ≈$3.6 billion (in FY1996 dollars) during950flightsby46vesselsin8months(1ferryflightevery10days,6ferriesassumedtobeoutofcommissionatanygiventime).Giventhehistoricalcostsofbuildingandoperating fourSpaceShuttles, theseareobviouslynotthe resilient, cost-effectivevehiclesVonBraunhad inmind.However,hiscalculationsmayyieldinsightintosomeofthetop-levelrequirementsofahumanmissiontoMars thatmay translate intoperformance requiredoftheCrewExplorationVehicle(CEV)ifitistobeuptothattask.

Key Mission ElementsEvery mission has four key elements: the case for

going,themeanstogo,anagreementtogobyallstake-holders,andasourceoffundssufficienttofinancetheexpedition.

ThecasefortheApollomissionswasthegeopoliti-calsituation,whereasthereasonsforgoingforroboticmissions were a combination of supporting Apollogoals (early on), science mixed with geopolitics (theUnited States and Soviet Union were both send-ingmissionstotheMoon,Venus,andMarsearlyon),andscience,withsomeleveloflingeringinternational

competition(MarsExpressandMarsGlobalSurveyor)andinternationalcooperation(Cassini/Huygens).

Themeansforgoing,spacetechnology,hasalonghis-tory6butgrewinlargepartfromtheV-2workofWorldWar II and later ICBM development programs duringtheColdWar.Theendofthatgeopoliticalcompetition,combinedwiththecurrentlackofdeep-spacecommer-cialactivityandlimitedEarth-orbitalsatellitemarkets,hasledtoslowerdevelopmentinmanyareas,especiallywheretheradiationhardnessofelectronicsisanissue.

Stakeholders include advocates of a mission, non-advocate reviewers (whose job is to see that investedmoniesarewellspent),thosewhoareforwardingfunds,and those who can be impacted positively by successand negatively by failure. A well-thought-out strategyincludesall stakeholdersandprovidesboth the frame-workandsubstanceofanagreementtodothemission.

Finally, there must be a source of funds sufficientand available in the needed time-phasing to supporttheendeavor.Suchprogrammaticsupporthastypicallycomefromthegovernmentexclusivelyfornon-commu-nications satellites. New missions usually have signifi-cantnewtechnologiesthatrequireimplementation(andsometimesunanticipateddevelopment)andinvolvethenext step in a non-routine, non-recurring scenario ofexploration.Costingofsuchendeavorsistrickyatbestandrequiressome“adequate”levelofreservesthatcanbe drawn upon to support the resolution of unantici-pated development problems. At the same time, somemembers of the stakeholder community will be press-ingtokeepoutlaysaslowaspossible.Whilesuchten-sionsare“obvious,”spacemissions,aswithother largeengineering endeavors, involve large sums of moneythatarenoteasilyturnedoffmid-projectifthebudgetisexceeded.Overrunsandmissedschedulesare,andhavebeen, tolerated on many individual programs becausetheterminationcostcanbetoohighifsufficientfundshave already been spent. However, such overages canputaprogrammaticendtofollow-onmissions.

System of SystemsCurrently, and for much of the foreseeable future,

deep-space missions (defined as non-Earth orbit), andcertainly those beyond the Earth–Moon system, willbedrivenby,and/orframedintermsof,sciencereturnfromvarioussolarsystembodies.WhilebeginningwiththeMoonandMars, theespousedvision in themainreachesthroughoutthesolarsystem.Thescienceobjec-tivesarethusthegenericdriversthattracedownthroughthehardwarerequirements.Appropriateanalysesshouldthen be able to provide results that answer the posedquestions.Thedemonstrationofsuchclosure(atleastinprinciple)ispartofthestrategythatmustbedevelopedup front (Fig. 1). The global requirements that drivethe mission design, implementing technologies, andespeciallycostincludeanswerstothesequestions:

R. L. McNUTT Jr.


1.Howfararewegoing?Howlongcanwetake?Isthisaflyby,rendezvous,orreturnmission?Thatis,whattotalvelocitychangecapabilityisrequired?

2.Whatarethedatalinkrequirements,bandwidth,andcoveragecontinuity?

3. What is the required operational lifetime—drivenby both point 1 and reserve requirements—basedonautonomy,sparingphilosophy,andconsumables,bothforthemissionandforahumancrew,ifthereisone?

THE PAST

“When change is absolute there remains no being to improve and no direction is set for possible improvement: and when experience is not retained, as among savages, infancy is perpetual. Those who cannot remember the past are condemned to repeat it.”7

G.Santayana

BeginningsTheconquest of the solar systemhasbeen a favor-

ite theme of science fiction writers for well over 100years.ModerninterestinMarswaskindledbyPercivalLowell’ssuggestionofintelligentbeingsonthatplanet,a theme taken up in the early writings of Edgar RiceBurroughs8 and others. Contemporaneous technicalworkbyspacepioneersTsiolkovskii,Oberth,andGod-dard laid the foundations for the technical realizationof travel enabledoffEarth.Realizationof space travelhasbeenmoredifficult.Aswithmanylarge-scaletech-nicalinnovations,nationalinterestsprovidedthemainmeansofdevelopment, in this caseWorldWar II andtheensuingColdWar.ThefirstactivitysupportedthedevelopmentoftheV-2,andthesecondenhancedandconsolidated those technical efforts, first in thedevel-opmentofICBMsandlater inthedevelopmentofthelaunchvehiclescurrentlyusedformanned,commercial,andscientificexploration.

Immediately following WorldWar II, Clarke9 and others10–12discussed the application of fissionpowertorocketstoenablenewcapa-bilities.Clarkenotes:

Beforetheyear2000mostofthemajorbodiesintheSolarsystemwillprob-ably have been reached, but it willtakecenturiestoexaminethemallinanydetail.Thosewhoseemtothinkthat the Moon is the goal of inter-planetary travel should rememberthat theSolar systemcontains eightother planets, at least thirty moonsandsomethousandsofasteroids.Thetotalareaofthemajorbodiesisabout250 times that of the Earth, thoughthefourgiantplanetsprobablydonotpossessstablesurfacesonwhichland-

Figure 1. Answering solar system science questions is a “system-of-systems” problem.

ingscouldbemade.Nonetheless,thatstillleavesanareatentimes as great as all the continents of the Earth—withoutcounting the asteroids, which comprise a sort of irregularinfiniteseriesIdonotproposetotrytosum.

Clarke9alsoaddsthatthepracticalusesfor(manned)spacestationsinEarthorbitwouldbeasmeteorologicalobservationstationsandas relays forworldwidetelevi-sionbroadcasting.

The Launch RecordBeginningwithSputnikIinOctober1957andgoing

throughcalendaryear(CY)1998,therewere3973suc-cessful space launches, the majority conducted by theUnitedStates(1161)andtheformerSovietUnionandRussia(2573).However,mostofthesemissionshavebeentoEarthorbit.Evenextendingthedatabaseoflunarandplanetary missions through CY2004, there have onlybeen139,ofwhich112weresuccessful.Thus,lessthan3% of all space missions launched through 2004 hadgonebeyondEarthorbit(Fig.2).13,14

Interplanetary Robotic ProbesDiscussion of Earth-orbiting satellites accelerated

during the planning for the International GeophysicalYear (IGY)4 and culminated with the Sputnik launchof October 1957. The problematic U.S. Vanguard proj-ectwasmovedaside,andthesuccessfulU.S.ExplorerIsatellite was placed into orbit in January 1958. NASAwas established in 1958 to coordinate all non-military,scientificspaceactivities.Acombinationofscientificandnationalgoalsledtoanaccelerationofsatellitelaunches,withanearlyinterestinlunarandlaterVenusandMarsprobes.Trends are shown inFig. 2.Historically (in theUnited States at least), a focus on the Moon (Ranger,Surveyor, Lunar Orbiter) and early interplanetaryprobes (Pioneer5–9missions tomeasure interplanetaryradiation) supported the manned lunar landing effortannouncedbyPresidentKennedyinMay1961.Boththe



UnitedStatesandtheSovietUnionsentscientificprobestoVenusandMars.

The Outer PlanetsFollowingtheApolloProgramsuccess,NASAfocus

turned toward theouter solar system.Pioneers 10 and11 were the first probes to reach Jupiter and Saturn,respectively,andshowedthatspacecraftcouldtraversetheasteroidbeltwithaminimumnumberofdusthits.Voyager1and2spacecraftthatweredescopedfromtheoriginalGrandTourmissionconcept15followedupwithmoreintensivedetailedstudiesoftheJupiterandSaturnsystems.Voyager2followedtheoriginalGrandTourtra-jectory,providingthefirstin-depthstudiesoftheUranusandNeptunesystems.AlthoughthePioneershavefallensilent,thetwoVoyagerscontinuetobroadcastbacktoEarthdataontheinterplanetarymediumanditsinter-actionwiththeverylocalinterstellarmedium.

Voyagerresultsmotivatedadesireformorein-depthstudyoftheoutersolarsystem.AdetailedstudyoftheJovian systemwasprovidedby the recently completedGalileomission. Such studies of theSaturnian systemare continuing with the Cassini/Huygens mission, ajointeffortbyNASAandtheEuropeanSpaceAgency(ESA),thatreachedtheSaturnsysteminJuly2004.

Venus Venusexplorationcanbesplit intoorbitalmissions

thatprobedthespaceenvironmentoftheplanetusing

radar to penetrate the permanentcloud deck and landers that gavebrief spot measurements of condi-tionsatthesurface.Fromthemid-1960stothemid-1980s, theSovietUnion built up a capability culmi-natinginphotographsandcomposi-tiondatafromthesurfaceofVenus.

Flybys of Venus by the U.S.probes Mariner 2, 5, and 10 mea-suredtheglobaltemperatureoftheplanet,showedthelackofaglobalmagneticfield, andprovided infor-mationontheinteractionofVenuswiththesolarwindandtheglobalcloudcirculationpattern. (Mariner2alsoprovidedconclusiveevidencefor thesolarwind,andMariner10went on to provide the first—andonly to date—observations of theplanetMercury.)

Orbital studies by Pioneer 12(alsoPioneerVenusOrbiter[PVO]),atmosphere studies by Pioneer 13(and Pioneer Venus Multiprobes),andradarstudiesbyPVOandlater

Figure 2. Historical launches from 1957 to 1998. Successful interplanetary launches through 2004 have been added from NASA websites. All launches, including the percent-age that were beyond Earth orbit, continued to grow until about 1965 when the number of U.S. launches began to taper off. The downturn in USSR/CIS (Commonwealth of Inde-pendent States) launches coincided with the end of the Cold War era. Lunar and interplan-etary launches include those of all countries. Following a peak in the mid-1960s, launches have tapered off to 10% or less of the United States–only launches.

the Magellan spacecraft in the 1990s have completedVenus investigations to date. Venus Express, an ESAmissionbasedonthesuccessfulMarsExpressspacecraft,isnowoperationalinorbitaboutVenus.

MarsWhileMarsfirstevokedpublicinterestinsolarsystem

travel,initialresultsprovedtobediscouraging.Mariner4’sflybyprovided21picturesshowingacrateredterrainandverylowatmosphericpressuremeasurements.MoredatawereaccumulatedbytheU.S.Mariner6,7,and9spacecraftandbytheSovietMars2–7probes.

The United States made an all-out effort to deter-minewhetherlifeexistedonMarswiththeViking1and2missionsthatconsistedoforbiters,whichmappedtheplanetanditssatellitesPhobosandDeimos,andland-ers.Thelanderscarriedsophisticatedatmosphericandchemical instruments formeasuringatmosphericprop-ertiesanddetectinglifeandwerepoweredbyradioiso-topethermoelectricgenerators.WhilethechangingoftheMartianseasonswasstudiedbythelanders,thelife-findingexperimentsprovideddisappointingandcontra-dictoryresults,nowthoughtduetothehighlyionizingconditions in the top layer of Martian regolith at thelandingsites.

Ahiatus in roboticMarsexplorationensued for20years, including the failed Soviet Phobos missions of1988 and the failed NASA Mars Observer mission of1992. However, Mars exploration is back as a main-stay of world scientific investigation. Starting with

R. L. McNUTT Jr.


the NASA Mars Global Surveyororbiter (operational through late2006 in extended mission), suc-cessful missions include NASA’sMarsPathfinder rovermission(thesecond of the agency’s Discoveryline of missions) and Mars Odys-sey orbiter (operating), ESA’s MarsExpress Orbiter (operating), andNASA’s Spirit and Opportunityrovers (operating in extended mis-sions).ProblemshavecontinuedasevidencedbythefailuresofNozomi(JapaneseSpaceAgency);MarsCli-mateObserver,MarsPolarLander,andDeepSpace2(allNASA);andBeagle2(ESA/UKlander).

OthersOther probes have visited Mer-

curyandothersmallbodies inthesolar system, and there are plansfor future visits as well. As noted,Mariner 10 has provided the only

population grew from ≈3.8 to ≈280 million, yielding aGDP per capita increase of a factor of ≈30 over ≈200years. The economy exhibits an e-folding time of ≈30yearsthathasyettoshowsignsofdecelerating.Althoughcorresponding figures for developed and developingcountrieswerenotasreadilyavailableforthisstudy,thecontinued economic capacity for human explorationappearstobepresent.

Manned Planetary Mission StudiesNohumanexcursionshavegonebeyondEarthorbit

except to the Moon with the Apollo missions. TheseexpeditionsincludedthelunarflybysofApollo8,10,and13andthelandingsofApollo11,12,and14–17(Apollo7and9werecheckoutmissionsinEarthorbit).

Das Marsprojekt The first detailed study of manned planetary mis-

sionrequirementswascarriedoutbyVonBraun5foramanned Mars expedition (in the appendix to a novelthat was never published4). The plan provided for ten4000-tonships,7withacrewof10each,and3cargoships.The950flightsofthree-stageferryrocketswouldbe required to assemble the flotilla in low-Earth orbit(LEO), each ferry using 5583 tons of nitric acid andalcohol (later modified to hydrazine) to put 40 tonsintoorbit (theSpaceShuttlecapability toLEO is≈20tons).Whileacknowledgingthepossibilityofadvancesleading to nuclear rockets, Von Braun’s scheme wasbased on nearer-term chemical technology, in which

Figure 3. U.S. economic and population trends, 1789–2002.16 The GDP in fixed-year dol-lars shows an e-folding time of ≈28 years.

close-up look at Mercury to date. The MErcury Sur-face,SpaceENvironment,GEochemistry,andRanging(MESSENGER)mission isnowenroute to the inner-mostplanet,andESAisproceedingwithdevelopmentof the multiple-spacecraft BepiColombo mission toMercury.Multiplespacecrafthavemadecloseflybysofavarietyofcomets,includingtheDeepImpactmissionthatreleasedanimpactprobethathitTempel1.Variousspacecrafthavealsoflowncloselybyasteroids,includingtheNearEarthAsteroidRendezvous(NEAR)missionto 433 Eros, which concluded with the first successfullandingonanasteroid,andtheJapaneseHayabusamis-siontoasteroid25143Itokawa,whichistoreturnwithasamplein2010.NewmissionsincludetheDawnmissiontothelargeasteroidsCeresandVesta.

Capacity for Future GrowthWhether financed by governments or by private

interests, space exploration is expensive. Unless anduntilcommercialadvantagescanbeidentifiedfordeep-space exploration, missions beyond Earth, and almostcertainlybeyond theEarth–Moon system,will remainfinanced by individual governments or internationalgovernmentconsortia.

Thecapacityforsuchgrowthisthereforetiedtoeco-nomic activity and levels capable of supporting suchcontinued expansion. Figure 3 shows records of U.S.economicindicatorsfrom1789through2002.16Duringthattime,thisreconstructionoffinancialactivityshowsa gross domestic product (GDP) in fixed-year (1996)dollars increasing from ≈$4 to ≈$9400 billion as the



casetherewasaneedof5.3milliontonsofpropellanttoassemble(andfuel)theMarsships.About10%ofanequivalentquantityofhigh-octaneaviationgasolinewasburnedduringthe6-monthoperationoftheBerlinAir-liftin1948–1949.TheestimatedtotalcostforlaunchingthemissionintoEarthorbitwas$500million(nowover$3billionasnotedearlier).

In discussing the many Mars mission plans, Por-tree4 notes that Von Braun’s thinking was influencedby the then-recent Antarctic Operation Highjump, aU.S.navalexpeditionin1946–1947consistingof4000men, 13 ships, and 23 aircraft. In the late 1940s VonBraunexplicitlystatedthatsuchamissionwouldtendtoresembleMagellan’sexpeditionaroundtheworld;theMars mission would need to be self-reliant and wouldcostasmuchasa“smallwar.”

These factors resulted in a variety of NASA stud-ies (Early Manned Planetary-Interplanetary RoundtripExpeditions or EMPIRE17,18) and, after President Ken-nedy’s1961speech,a focuson spacecraftarchitecturalelementsforreachingtheMoonbytheendofthe1960s.ThegoalwasMars,butmanyoftheEMPIREconceptsincluded a manned Venus flyby on the return leg tominimize propellant requirements. Manned flyby-onlymissionswerealsoadvocatedtodropprobes,basedonthe (then)notion that roboticprobeswere toounreli-abletodropprobessuccessfullytotheMartiansurface(anotionborneoutbythetrackrecordofroboticprobesuptothattime).

Magellan’s VoyageRecall that while this first circumnavigation of the

worldisconsideredtobeoneofthedefiningmomentsof exploration, it was entirely motivated by the com-bined geopolitical and economic goals of determiningthelocationoftheSpiceIslandsandbreakingthePor-tuguesemonopolysolidifiedonlysome10yearsearlier.Theexpeditionbeganon10August1519asfive shipswithacombinedcrewof234leftSevilleforSanLucardeBarrameda;5weekslater,on20September1519,theysailed(Table1).

The Magellan Voyage experienced two mutiniesearlyon,onebeforeandoneafter reaching theSouthAmericancoast.On28November1520,threeremain-ingshipsreachedthePacificOceanthroughtheStraitsofMagellan.FollowingarrivalattheSpiceIslandson6November1521with115crewleft,Magellanwaskilledby localsbefore theremnantof theexpedition left forhome. On 6 May 1522, Victoria rounded the Cape ofGoodHopeunder thecommandof JuanSebastiandeElcano. Twenty of the crew died of starvation beforereaching theCapeVerde Islandsandanother13wereabandoned there on 9 July (Cape Verde was a Portu-gueseholding).EighteenmenreturnedontheVictoriatoSevillein1522;4oftheoriginal55ontheTrinidad

reachedSpainin1525.Thevoyagehadcovered14,460leagues(69,000km)

TheVictoria reachedSpain1dayearlydespitemain-taining a careful log; it had gained a day by travelingwest.Despitethelossoffourofthefiveships,thereturnof 26 tons of cloves along with nutmeg, mace, cinna-mon,andsandalwoodcoveredtheexpeditioncost.

The cargoes were sufficiently valuable per weightthatvoyagesofPortuguese,Dutch,British,andAmeri-cantraderscontinued.Fortunesandempiresweremadeandlost,butthestakeholdersofthenextthreecentu-riessawthepotentialgainsasoutweighingtheriskandupfrontcosts.19,20

“Small Wars”WithrespecttoVonBraun’sassertionthatthecostof

aMarsmissionwouldbecomparabletothatofasmallwar, acomparisonof thefinancialcostsofmajorU.S.warsallowssuchanassertiontobeputintoperspective(Table2).21

Von Braun’s initial assertion of the cost of a MarsexpeditionliesbetweenthefinancialoutlaysoftheU.S.governmentfortheMexicanWar(1846–1848)andtheSpanish-American War of 1898. The effects of theseeventscanbeseenmoreclearlyinFig.4.22,23SpikesinthefederalanddefensebudgetsindicatethetwoWorldWars; smaller localized increases indicate the Koreanand Vietnam conflicts. The large relative peak in theNASA budget is from the Apollo program (and theotherprogramssupportingthemannedlunarmissions).

Downsizing the ArchitectureOn11July1962,NASAannouncedthelunarorbit

rendezvous approach for gettingmen to the surfaceof

Table 1. Magellan’s expedition.

Ship Tonnage Crew Disposition

Trinidad 110 55 CapturedbythePor-tuguesewhiletryingtoreturnviathePacific

San Antonio 120 60 Turnedbackduringpas-sagethroughtheStraits

Conception 90 45 Abandonedinfall1521intheSpiceIslands(Moluccas)

Victoria 85 42 ReachedSpainon6September1522

Santiago 75 32 WreckedonArgentinecoastduringsouthernwinter

Totals 480 234

R. L. McNUTT Jr.


theMoon.Thishadtheriskofrendezvousanddockingin lunarorbitbut required the lowest lunar spacecraftweight,andonlyoneSaturnlaunchvehiclewasneededperflight.Hence,thiswasthecheapest, fastestwaytoget totheMoon. Iteliminatedtheneed for the largerNovalaunchvehicle,foranyEarth-orbitalassembly,andforaman-ratednuclearrocket.TotheextentthatanysuchcomponentswouldbeneededforahumanmissiontoMars, theirdevelopmentwouldhave tocomeanewfromaMars-specificprogram.

Work continued on Mars mission concepts basedonApollohardwareand,inparticular,multipleSaturnVlaunchvehicles formovingthehardware intoorbit.WiththeMariner4flybyofMarsandthemeasurements

ofatmosphericpressure,allnotionsofaerodynamicland-ingssuchasusedintheVonBraunschemevanished,asdidallnotionsofthepossiblepresenceofadvancedlifeonMars.4

In January1966 theNationalAcademyofSciencesSpace Studies Board issued its report, Space Research: Directions for the Future,targeting“thenearplanets”forscientificfocusfollowingthemannedlunarlanding.Asa result, the significant roboticmission,Voyager—firstproposedattheJetPropulsionLaboratoryin1960asafollow-ontotheMarinerflybymissionsanddesignedtousetheSaturnVlaunchvehicle—wasreassessed.Thenew atmospheric data drove a redesign and increasedprogramcosts,butlauncheswerescheduledfor1971and1973.4

Within2years,theMarsVoyagermissionswerecan-celledandreplacedwiththeVikingorbitersandlandersthatsubsequentlyflewtoMars.By1973,thepost-ApolloprogramformannedexplorationbeyondEarthorbithadcollapsed, along with the nuclear rocket developmentprogram.PlansforanyfutureusesoftheSaturnVwereshelved,anddevelopmentoftheSpaceShuttlebeganasanewinitiative.

Over the next 30 years, plans for manned Marsmissions continued to be studied, notably includingthe National Commission on Space Report of 1986,with priorities reset by the Ride report following thelossoftheSpaceShuttleChallenger;theSpaceExplo-ration Initiative of 1989; and the Design ReferenceMission(DRM)of1997.4Almostallconceivablearchi-tectures were advocated in these and other studies,which required masses in LEO (sometimes referredto as “initial mass in low-Earth orbit,” or IMLEO) ofhundreds of tons and estimated costs ranging from

Table 2. U.S. war costs in fixed 1990 dollars.

ConflictCost

(billions$)

CostpercapitaRevolutionaryWar 1.2 342.86Warof1812 0.7 92.11MexicanWar 1.1 52.13CivilWar 44.4 1,294.46Spanish-AmericanWar 6.3 84.45WorldWarI 196.5 1,911.48WorldWarII 2,091.3 15,665.17Korea 263.9 1,739.62Vietnam 346.7 1,692.04GulfWar 61.1 235.00

Figure 4. Comparison of fixed-year (1996) costs per capita. National Advisory Committee on Aeronautics (NACA) and NASA budgets on a per capita basis are combined. The NACA budget is for the years 1915–1959; NASA was formed in 1959. All budgets are based on actual figures from 1901 or later until 2002, and projections are from 2003–2008.

tens to hundreds of billions ofdollars.4

In each case the combined costof infrastructure and implementa-tionwasseenasprohibitive,andtheprogramswerenotstarted.Whetherthe new exploration initiative24will fare any better remains to beseen.2,25

While there have been somereportsonwhatamannedmissiontodestinationsfartherfromEarththanMars would require—notably veryadvancedpropulsionsystems26—thegoalofMarshasbeenthefocusofall“near-term” thinking.Herewe takeamoreglobalviewofthedriversforsolarsystemexploration,therequire-mentsthatfollow,andhowtheover-all task can be divided into coher-ent synergistic robotic and humancomponents.



An Antarctic Paradigm?

Discovery and ExplorationWhile the drivers behind Magellan’s voyage do not

appeartobestrictlyapplicabletoMars,theexplorationofAntarcticaandestablishmentofscientificbasestheremayhavemorerelevance.TheAntarcticcontinentwasdiscoveredonlyrelativelyrecentlyin1820.Initialexpe-ditionswereallprivatelyfunded,includingAmundsen’ssuccessfultraversetotheSouthPolein1911andByrd’sestablishment of his Little America bases up throughLittleAmericaIIIduringhis1939–1941expedition.27,28

HighjumpEven with a growing commercial whaling and fish-

ingcompetitionofftheshoresofAntarctica,themodernhistoryofthecontinentbeganforpoliticalreasonsfol-lowingtheendofWorldWarII.TheUnitedStatescom-mencedoperationsofTaskForce68in1946–1947.ThecontextwasthedisputedclaimsofArgentinaandBrit-ain during World War II and growing interests in thepossibleexploitationofnatural resources following thewarbyavarietyofgovernments.

With then-Admiral Byrd in command, OperationHighjumpestablishedtheLittleAmericaIVbaseinvolv-ing13ships,33aircraft,and4700men.Largerthanallpreviousexpeditionscombined,theoperationssurveyed≈350,000mi2ofthecontinentandphotographed≈60%ofitscoast.

Building—a rough estimate of resources transportedintoandoutofthecontinent.Lookingatlargemassesthat have been transported by air, the Berlin Airlifttransported2.1millionMT(slightly less thanhalf themassoftheGreatPyramidofGiza)over14monthswith≈280,000airplaneflights.Thiswasamajorundertaking,butthepointisthatitwasnotimpossible.27

WithalloftheconstraintsofcostsandthetermsoftheAntarcticTreaty,in situresourceutilizationhasnotbeenaviable approach forprovidingneeded resourcesforthebases,e.g.,fuelfortransportandelectricalpower.Evenattheserelativelylargeamountsofcargothatmustbe shipped in, the “break-even” point for using localresourcesinAntarcticahasnotbeenreached.

Extreme Engineering Limits?Before contemplating large masses of materials in

spaceforeventualuse,e.g.,fordeep-spacehumanexplo-ration and our establishment of permanently crewedbases, it is worth inquiring about the scale of largeengineeringprojects.Togiveanideaofscale,Table3listsprojectsanditemsthathavebeenbuiltandTable4 listsothers thathavebeencontemplated.The larg-estdeep-spacevehiclebuiltandflown is the≈30-MTApollosystem,lessthan1/1000themassofVonBraun’soriginal Mars expedition concept. In these exampleshumancapabilitiesandimaginationspansome9ordersofmagnitudefor“extremeengineering.”

STRATEGY AND REQUIREMENTS

“If you can fill the unforgiving minute / With sixty seconds worth of distance run, / Yours is the Earth and everything that’s in it… .”49

R.Kipling

The Previous NASA Strategic PlanThe NASA Strategic Plan of 200350 posed three

compellingquestionsthatdriveexploration:(1)Howdidwegethere?(2)Wherearewegoing?(3)Arewealone?Asanactionablegoal,thesequestionsbroadlymapintoMissionII:ToexploretheUniverseandsearchforlife,and,inturn,toGoal5:ExplorethesolarsystemandtheUniversebeyond,understandtheoriginandevolutionoflife,andsearchforevidenceoflifeelsewhere.

This approach, largely endorsed by the scientificcommunityintherecentdecadalstudyonsolarsystemexploration goals, focuses on understanding the originof theconditions for lifeonEarthandpossibleoriginsoflifeelsewhereinthesolarsystem.Emphasishasbeenon sites with water and suspected prebiotic chemistry,namely,Mars,Europa,andTitan.

Foralloftheseprimarytargets,aswellasothertargetsofexploration,theissueisalwaysoneoftimelyreturnofconclusivescientificdata.Forroboticmissions,“timely”

Deep FreezeHighjumpwasfollowedbyOperationDeepFreezeIin

1955,layingmuchofthegroundworkforU.S.participa-tionintheIGY.Thateffortinvolved12countriesestab-lishing some 40 scientific bases, and led to the devel-opmentoftheAntarctictreatygoverningtreatmentofthecontinentand itsusesasamultinational scientificexploration site.Thisworkcontinues to thisday.TheIGYmarkedthestartofthefirstpermanentbasesonthecontinent—some137yearsafteritsdiscovery.

LogisticsU.S.costsforsustainingtheAntarcticresearchbases,

infrastructure,andfacilitieshavegrownfrom≈$150mil-lion/yearatthetimeoftheIGYuptoacurrentspendinglevelof≈$200million/year.28

To support Antarctic operations, both people andcargomustbemovedinandout.Inthe1990–1991DeepFreezeoperation,≈7200MTofcargoweretransported(ship and air, intercontinental and intracontinental)and6200personnelweretransportedbyair(interconti-nentalandintracontinental).29

If we extrapolate over a 46-year period, the totalmass is ≈330,000 tons—the mass of the Empire State

R. L. McNUTT Jr.


“The Vision for Space Exploration”On 14 January 2004, President

Bush espoused a new vision forNASA,24whichhasfourpoints:

1. Implement a sustained andaffordable human and roboticprogram to explore the solarsystemandbeyond

2. Extend human presence acrossthe solar system, startingwithahuman return to the Moon bytheyear2020,inpreparationforhumanexplorationofMars andotherdestinations

3. Develop the innovative tech-nologies, knowledge, and infra-structurestoexploreandsupportdecisionsabout thedestinationsforhumanexploration

4. Promoteinternationalandcom-mercial participation in explo-ration to furtherU.S. scientific,security,andeconomicinterests

If the logical extensionofpoint2 is tobeconsidered seriously,andif all stakeholders, both those ofthenewvisionandthoseoftheoldplan, are to come together to fur-therspaceexploration,thenacom-prehensiveplanfortheexplorationof the solar system—or at least aframeworkforone—isneeded.

Key issues are the sustainabil-ity and public engagement of theeffort over a space of decades,especially where this is an act ofpublicpolicyandnotawarforsur-vival. Underlying these issues areassumptions thatwillmotivate theexpenditure of funds required forthisactivity.

Access to Space“Once you get to earth orbit, you’re half-way to anywhere in the solar system.”51

R.A.Heinlein

Ifthenewinitiativeistobesus-tainedaswastheAntarcticprogram

Table 3. Extreme engineering: What we have built.

Item Type Mass(tons)

GreatWallofChina30 Structure 730,000,000(est.)a

HooverDam31 Structure 6,600,000GoldenGateBridge32 Bridge 811,500Jahre Viking33 Ship 647,955b

EmpireStateBuilding34 Building 365,000Nimitz-classaircraftcarriers35 Ship 102,000Ohio-classsubmarines36 Ship 16,600EiffelTower37 Structure 10,000SaturnV38 Rocket 3,038.5c

An-225Cossack39 Cargoplane 600StatueofLiberty40 Structure 225Apollo1738 Crewedspacecraft 30.34d

Cassini41 Roboticspacecraft 5.65e

aMassoftheGreatWallisestimatedfromadensityof2.0gcm–3,alengthof7300km,anaverageheightof10m,andanaveragewidthof5m.

bJahre Vikingisthelargestultra-largecrudecarrier(tanker)currentlyinmaritimeservice.cSaturnVmassatlaunchisthemassasfullyfueledbeforelaunch.dApollo17massisthemassinjectedfromEarthtotheMoon.eCassinimassisthetotalmasslaunchedtoSaturnbeforeanytrajectorycorrectionmaneuversorinjectionintoSaturnorbit.

Table 4. Extreme engineering: What we have thought about building.

Item Type Mass(tons)

Interstellarphotonrocket42 Crewedspacecraft 2,301,000,000a

L5spacehabitat(10,000pop.)43 Structure 10,618,000Das Marsprojektpropellant5 Propellant 5,303,850Das Marsprojektvehicles5 Crewedspacecraft 37,200ROOSTbpost-NovaLV44 Launchvehicle 16,980SaturnV-Dcconcept45 Launchvehicle 9,882.1Jupiter/Saturnfusionship26 Crewedspacecraft 1,690CompletedInternationalSpace Station46

Structure

453.6

TAU-probed(interstellar)47 Roboticspacecraft 61.5RISEprobee(interstellar)48 Roboticspacecraft 1.1aThephotonrocketwasbasedonaround-tripat1gaccelerationtoAlphaCentauriusingtotalmatterannihilationforthrust.

bROOST=ReusableOneStageOrbitalSpaceTruck.cSaturnV-DconceptwasfromaMarshallSpaceFlightCenterstudyof1968.dTAU=ThousandAstronomicalUnitsmission.eRISE=RealisticInterstellarProbe.

canbe from1 year for reachingMarsorbit, to almost10yearsfortheHuygensprobeintoTitan’satmosphere,to even multiple decades for the Voyager spacecrafttraveling to the interstellarmedium.Scientists canbe

patient; “reasonable” timescales are set by the designlifetimeforspacecraftcomponentsandbudgets,formis-sionoperations,andforthescienceteamsastheyawaitthedata.



andnotlikeashort-termprogramsuchasApollo—andifhumansaretobeamajorplayerindeepspace—thenonethingisclear:wemustbeabletomovealotofmassreliably, efficiently, and cheaply from the Earth’s sur-face into orbit. Once free of Earth’s gravity, appropri-atevehiclescanbedeployed(e.g.,Apollo)orbuilt(e.g.,VonBraun’sMarsvehicles),butwewillneedcapabili-ties well past those of the Space Shuttle without thehighcosts.

Costsofaccesstospacecanalreadybedecreasedifsufficientup-frontordersareplaced.Ifthearchitecturerequires 20,000 MT in LEO, an order for 1000 heavyliftlaunchvehicles(HLLVs)withacapabilityof20MTtoLEOwillcomeinata lowerunitcost thanwillanorderfor10.

IfpayloadstoLEOcomeinunitsof1000MT,thenanewclassoflaunchvehicleshouldbeconsidered.BeforetheLunarOrbitRendezvoustechniquewasadoptedforApollo, “million pound to LEO” vehicles (genericallycalledNova)werestudiedintheearly1960s.44,52Ifweare serious about takinghumans toMars andbeyond,thisoldideashouldbeconsideredintheguiseofanewclassoflaunchvehicle.

Extremelyheavyliftlaunchvehicles(EHLLVs)couldextendtheNovaconcept:take1–5millionlb(454–2270MT)toLEO.Suchvehicleswouldbeverylargeandhenceinherentlyexpensive,soreusabilitywouldberequiredtobeeconomical,anideastudiedin,e.g.,ReusableOrbitalModule-BoosterandUtilityShuttle(ROMBUS).

Developmentcostswouldalsobeverylarge,butsuchvehicles could solve the access-to-space problem fortransporting infrastructure to the Moon and beyond.Recalling the Antarctic example, suppose that ≈7,200MTofcargowas required tobe shipped toMarseachyear.Totransport600MTofcargotoMarsorbit,VonBraunneeded37,000MTinLEO.Getting7,200MTofsupplies(whatevertheyare)toMarsorbit(andstillnottothesurface),wouldlikelyneed≈300,000MTinLEO.Witha2,000-MTtoLEOEHLLV,thetransportcouldbedonewith150flightsinaboutayearatthreeflightsa week. Such an undertaking would be neither cheapnor easy, but it could be done. Both in leaving Earthand traveling to other destinations, propulsion is thekeyissue.

“Difficult” Robotic MissionsThesamemissionsthatwere“difficult”intheNASA

1976OutlookforSpaceStudy53remaindifficulttoday,primarily,butnotexclusively,becauseofthepropulsionrequirements.AsamplereturnmissionfromMarsisjustdoablewith current technology, providingone iswill-ingtopaytheprice.SamplereturnsfromMercuryandVenusneedhigh thrust todealwithdescent (atMer-cury)andascent(atboth).Inaddition,round-triptravelto Mercury is prohibitively long if chemical systemsareused.54

Titan orbiters, Uranus and Neptune system probes(similartoGalileoatJupiterandCassiniatSaturn),havealso“beenonthebooks”foralmost30years.ThesehavebeenjoinedbynewermissionconceptssuchasEuropalandersandsubmarinesandIoorbitersthatalsolieatorbeyondcurrent(chemical)propulsivecapabilities.

Thepropulsivedifficultiesaregeneric.Togototheouter solar system to any body and into orbit with acapable science payload requires much higher specificimpulsetobeimplemented,butcanalsobelowthrust(butnottoolow).Tolandonanysolid—andairless—bodyfromMercurytoPlutorequireshighthrustatthetargetaswell,andhencemoremass.Theproblemisfur-therexacerbatedifasampleistobereturnedtoEarth.

Dependingonthebody,potentialdestinationscanbearrangedintofourgroupsasafunctionoftheirescapevelocity, indicative of the amount of high-thrust pro-pulsivecapacity required (objects inTable5are listedin order of increasing distance from the Sun and inincreasingdistancefromthecentralplanetarybodyineachsystem).

NASA’s Project Prometheus was a first step towardtryingtosolvethesetransportationdifficulties.Thefirstplannedmission,theJupiterIcyMoonsOrbiter(JIMO),55wastouseanuclearelectricpropulsionsystempoweredbyafissionreactortoaddresssciencegoalsintheJoviansystemthatcannotbeaddressedotherwisewithasinglemission. Science goals include the deduction of inter-nal structure and the possible presence of liquid waterbeneath the ice crusts of these worlds. A June 2003NASAworkshopidentified8scienceareas,33objectives,and 102 investigations requiring 188 different types ofmeasurements.Themainquestioniscost:reactor1cer-tification!1spacecraft1instruments/science.

Table 5. Escape velocities for some objects.

Group ObjectEscapespeed

(km/s)

1 Earth 11.18Venus 10.36

2 Mars 5.02Mercury 4.25

3 Moon 2.38Io 2.56

Europa 2.02Ganymede 2.74

Callisto 2.444 Rhea 0.643 Titan 2.644 Titania 0.77

Triton 1.45Pluto ≈1.20

R. L. McNUTT Jr.


The Role of Humans (Do We Need Astronauts?)Robotic missions can be summarized as follows:

Launchaprobetoatargetandtheprobereturnsknowl-edge.Theproblemisthatwithoutreturnofallthedata,therearealwayslingeringdoubtsabouthowtheknowl-edgeistobevalidated(“compression”leavesdoubtsofdatafidelity).Returnofalldatarequireslargebandwidthwhich, in turn, drives receiver and transmitter sizesand transmitter power. Thus data return drives powerandhencemass,whichinturndrivespropulsion.Evenwithallthedatareturned,doubtcanstilllingeraboutwhetherwereally—andcorrectly—thoughtthroughallofthecontingenciesandpossibilitiesaheadoftime.

Havinghumansintheloopcanmakealargediffer-ence.Anobvious example is thehand-selectedMoonrocksreturnedbyastronaut/geologistSchmidtonApollo17versusthe≈100gof lunarmaterialreturnedbytheroboticLuna24probe.However, if onealso considersthecostdifferentialof the twomissions,one immedi-ately reaches two conclusions: (1) sending astronautsbeyondEarthorbitisreallyexpensive,and(2)onegetswhatonepaysfor.Sothereisaroleforhumans,butitwillnotcomecheaply.Whileeventualhumanmissionsto Mars are an assumption of the exploration vision,it is relevant to ask whether it is desirable—or evendoable—totakehumanmissionsbeyondMars, fartherintothesolarsystem.26

The RequirementsSample return missions, or human missions (which

also involve a return), have significantly harder pro-pulsive requirements than those of a simple, fast flybymission.Suchmissionsmuststopatthetarget,reversedirection,andagain stopatEarth. If the top speed inbothdirections is thesameasthatofa fastflyby(andassumingthetimetoaccelerateissmallcomparedtothetransittime),thetriptimeisdoublethatofafastflyby,butthepropulsionrequirementisquadrupled.Ifwetake≈50astronomicalunits(AU)asthecharacteristicoutersize of the solar system (the heliocentric distance toPlutofromtheSunnearitsaphelionintheyear2113aswellasthedistancetomostofthedistantKuiperBeltobjectsseentodate),thentherequiredmissiontimesetstherequiredpropulsivecapabilityforaccessingthefar-thestpartofthesolarsystem.

Asanexample,consideraround-triptimeof4years.Withahumancrew,suchamaximummissionduration(longerthanMagellan’svoyage!)wouldstillbelongandperhapsunacceptablefrombothpsychologicalaswellasradiationexposureaspects. Inaddition,withoutatrue100%enclosedecosystem,themassoffood,oxygen,andwaterforthecrewincreaseswithmissiontime.Tocon-tinuewiththeexample,totraversethesolarsystemin4years(round-trip)requiresa2-yearvoyagetoPlutoandthesamedurationback.Inturn,thisconstraintrequiresaccelerating to ≈25 AU/year and then decelerating by

thesameamountatthedestination.Thereturnvoyagewouldrequirethesameperformance,foratotalonboardcapability of 4 3 25 AU/year = 100 AU/year or ≈500km/s of delta-V. The corresponding specific impulse is≈51,000sandsignificantlyexceedsevennuclearthermalrocketcapabilities.Withoutpracticalcontrolledfusion,suchavoyagecouldonlybecontemplatedwithanuclearelectric propulsion (NEP) system of a very advanceddesign.

Thisconceptualapproachcanprovideaquickbutreliable estimate of the implied propulsion systemrequirements. Backing requirements off to shorterdistances or longer times yields less required propul-sioncapability.A sample returnmission toPlutonearaphelionthatrequired40yearstobringasamplebackwould require a total delta-V capability of “only” 4 3(50AU/20years)=10AU/yearor≈50km/s.Similarly,acrewedmissiontotheJupitersystemat5AUwitharound-trip time of 4 years would also require a delta-V capability of 4 3 (5 AU/2 years) = 10 AU/year or≈50km/s,assumingthat theaccelerationtime is shortcompared to the cruise time. For example, a constantaccelerationof0.001gwouldbuildup to12.5km/s inabout 15 days. Reaching 125 km/s would require 150daysatthesameacceleration.

The primary difference between crewed and roboticmissionsismass,theotherdrivingrequirementforhumanmissions. Even in comparing a sample return missionwith the same timescale for mission completion, massand itsassociatedcost remains theprincipaldriver.Forexample,theSovietlunarsamplereturnmissions—Luna16,20,and24—hadmassesof5600,5600,and4800kg,respectively.Incontrast,theApollospacecraftattrans-lunarinjectionhadacapabilityforamassof≈48,000kg,about10 timesasmuch for the sample returnmissions.The Viking 1 and 2 spacecraft had masses of 3399 kg.ThiscanbecontrastedwithNASA’screwedDRMMarsmissionof1997that requiredan initial303tons.4ThiswasascrubbedversionoftheDRMof1993thattooksixastronautstoMarsfora2.5-yearmission,includinga600-day stayand requiring≈380 tons(including thepropel-lant,butwithsomeoffsetfromthemanufactureofsomefuelstockonMars).Inanycase,about100timesthemassinitiallyinLEOisrequired(foraMarsmissionatleast)for a crewed round-trip mission versus a robotic lander(the twin Spirit and Opportunity rovers had masses of≈830kg).Thesenumbersarealsosignificantlylessthanthe 40,000 tons in orbit required for the initial VonBraunconcept.

Power ConsumptionTheimpliedmassandrequireddelta-Vtranslateinto

substantial in-space power requirements for a space-craft.Again,somesimpleestimatescanprovideinsightinto thecapability andcharacteristics for the requiredpowerplant.



Thehighspecificimpulserequired(tokeepthepropel-lantmasstractable)alreadyrulesouttheuseofchemicalorevennuclearfissionthermalrocketengines.Supposethetotaldelta-Vcapabilityis50km/swithatotal“burntime”of43150days,i.e.,≈1.6years,ataconstantmassflow rate.Suppose also that the initial propellantmassfractionis60%,translatingintoamassratioof2.5andarequiredexhaustspeedof≈55km/s(specificimpulseof≈5600s).Fora3000-kgspacecraft,theinitialpropellantloadwouldbe1800kg. If this amountwere tobepro-cessedover600days,therequiredmassflowratewouldbe3kg/dayor≈35mg/s.The impliedexhaustpower is51.7kW,andacceleration is in thetens tohundredsofmicrogees. For a prime power conversion efficiency of20% using an NEP system, the reactor thermal powerwouldneedtobe≈260kW.TheseparametersareclosetospecificationsforthereactorconceptsstudiedfortheJIMOspacecraftunderProjectPrometheus.

Theneedfora“mass-efficient”powersourceisexac-erbatedwhenahumancrewisconsidered.Foracrewedexpedition of ≈300 MT, and the same scalings, thepower consumption required is increased by the in-creasedmassflowrate,a factorof100to≈5.2MWe(megawatts electric), with a thermal reactor poweroutputof≈26MW,consistentwithStuhlinger’sNEPsystem designs56,57 of the 1960s. For either of thesecases,anefficientvehicleshouldhaveapowerplantspecificmassaof≈t/v2

exhaustasderivedbyStuhlinger,wherevexhaustisjusttheexhaustspeedofthepropel-lant and t is the acceleration time. In these cases,a = ≈17.5kg/kWe, implying apower supplyof≈905kg out of 1200 kg dry and 90 MT out of 120 MT,

A PLAN FOR THE FUTURE“If you can keep your head when all about you / Are losing theirs and blaming it on you.”49

R.Kipling

“The greatest gain from space travel consists in the extension of our knowledge. In a hundred years this newly won knowledge will pay huge and unexpected dividends.”59

W.VonBraun

Todiscussfuturepossibilities,onehastomakemanytrialswiththeknowledgethatmost,ifnotallofthem,willmissthemark.WherewecurrentlyareincludesarobustroboticprogramatMars,anewprobetoMercury,an ongoing program to reach Pluto robotically, and astalledhumanprogram.FutureplansshowpromiseforenhancingroboticmissionswithNEPandavisionfortakingthehumanracebeyondEarthorbitandstayingtherethistime.Thepasthasshownusthattherobotswill come first. If we combine this principle with thegoalofexpandingthehumanpresencethroughoutthesolarsystemduringthenext100years,thenastructurefor doing so can be imagined that starts with samplereturnsandendswithpermanentbasesandexpansionintotheoutersolarsystem(Table6).Underlyingallofthese possibilities is the assumption that easy accessto space for large masses is available to develop self-sustaininginfrastructureoffEarth.

In the meantime, we need to keep exploring withadiversityofmissions to enableall levelsof scientificenquiry.Plutowillbeataphelionabout2113;wemustdecide whether we will be there as well. It really isuptous.

Table 6. A future exploration plan: Selected milestones (notional).

Year Goal

2011 JunolaunchtoJupiter2015 InterstellarPrecursorMissionlaunch(“InterstellarProbe”)2016 EuropaGeophysicalExplorerlaunch2020 HumanlaunchtoL2toserviceJamesWebbSpaceTelescope2023 MarsSampleReturn(robotic)missionlaunch2025 Crewed expedition to a near-Earth object to demonstrate hazard

mitigation2025 LaunchofarobotictourtotheNeptunesystem2030 Permanentlystaffedlunarbase2030 HumanmissiontoMars2035 Launchofroboticsample-returnmissionstotheoutersolarsystem2050 HumanmissiontoCallisto(Jupitersystem)2075 HumanmissiontoEnceladus(Saturnsystem)2080 PermanentMarsbaseestablishment2090 HumanmissiontoTriton(Neptunesystem)

respectively.58For a far-ranging mission with the

samepropellantburntime(300daysoutofacruiselegof2years)andaninitialmassof300tonsbutwithatotaldelta-Vof500km/s,theexhaustspeedwouldneed to increaseby a factor of≈10 forefficientpropellantuse.But inkeepingthepowerplantworkingefficiently,thepowerplantspecificmasswouldneedtodecreasebyafactorof100forthesame“burn” time. “Low-thrust” missionsthroughoutthesolarsystemarecertainlypossible for sufficiently long durations,butthesimultaneousimplementationofshortmissiontimesisproblematicwithcurrentlyforeseeabletechnology.

HumanmissionstotheJoviansystemmaynotbetotallycrazy,butarecertainlynoteasy,andarefarmoredifficultthanahumanmissiontoMars.Humanmis-sions to more distant destinations willbe even more difficult—and certainlymoremassive.

R. L. McNUTT Jr.


CONCLUSION AND PERSPECTIVE

“And pluck till time and times are done / The silver apples of the moon, / The golden apples of the sun.”60

W.B.Yeats

Althoughcrewedmissionsbeyond themainaster-oid belt may remain problematic (if even desirable)throughout this century, major scientific questionsremainthatcanonlybeansweredbygoingthere,asitu-ationthathasnotchangedduringthelast30years.6,54EnigmaticEuropamaycontainevenmoreclues thanMars about the evolutionof life in theUniverse, yetactuallyremotelyprobingbeneaththatmoon’sicycrustwilllikelybeasdifficultaslandingafieldgeologistontheMartiansurface.Theentire solar systemmustbeconsideredaspartoftheultimateVisionifitistotakefireandgoforward.

Anewperspective is required.Herewe are talkingaboutaprogramthatwilllastfordecadesandwill—atsomepoint—require largecapitalpiecesofequipment.Again, the analogy of permanent science stationsin Antarctica comes to mind. For the solar system tobecome our extended home, the effort involved mustbecome“obvious”tothepublicandpoliticiansalikeasalong-termgoalinthenationalinterest.Onlyinthiswaycanprogressbemaintainedtowardtheconsolidationofthesolarsystemduringthenextcentury.

Tocontinuemovingoutward,significantnewdevel-opmentswillberequired,allenabledbyandrequiringnew advances in propulsion. Top-level elements andrequirements include communications infrastructure,modular hardware that is evolvable and scalable (notunlikeAirbusorBoeingairliners),aCEVformultipleuses, and minimum buys of large numbers of evolved,expendable LVs to keep unit costs—and the cost ofaccesstospace—down.Inaddition,thereisaneedforcargo-capable HLLVs (or larger EHLLVs) to push pastApollo-liketasksontheMoonandpreparethewayforMars. There will be roles for the L2 Lagrange point,bothasaway-stationforoursolarsystemandasaloca-tionforobservatoriesofotherstarsystems.AutomatedCEVs wed to advanced Prometheus vehicles for outersolar system exploration and sample returns will alsohavetheirplace.

Thecurrentgoalistoanswerfundamentalquestionsaboutouroriginsandtheoriginsoflifeinthissystem,buttherewillalwaysbeaneedforaclear,yetevolving,conceptofwherewecanultimatelygo—ifwewantto.

ACKNOWLEDGMENTS: portions of this work have been sup-ported under the NAsA institute for Advanced concepts. The author thanks s. A. McNutt, c. J. McNutt, and three other referees for a close reading of this manuscript. The original version of this article was presented as iAc-04-iAA.3.8.1.02 at the 55th international Astronautical con-gress held in Vancouver, canada, on 4–8 oct 2004. The

views expressed are entirely those of the author and not necessarily endorsed by the sponsor or ApL.

REFERENCES 1Proverbs,Ch.29,v.18,The Holy Bible, Authorised King James Version,

Collins’Clear-TypePress,NewYork(1952). 2Aldridge,E.C.Jr.,Fiorina,C.S.,Jackson,M.P.,Leshin,L.A.,Lyles,

L.L.,etal.,A Journey to Inspire, Innovate, and Discover: Report of the President’s Commission on Implementation of United States Space Explo-ration Policy,U.S.GovernmentPrintingOffice,Washington,DC(Jun2004).

3Belton,M.J.S.,etal.,New Frontiers in the Solar System: An Integrated Exploration Strategy,TheNationalAcademiesPress,Washington,DC(2003).

4Portree, D. S. F., Humans to Mars: Fifty Years of Mission Planning, 1950–2000, Monographs in Aerospace History #21, SP-2001-4521,His-toryDivision,OfficeofPolicyandPlans,NASAHeadquarters,Wash-ington,DC(Feb2001).

5VonBraun,W.,The Mars Project(Englishtrans.),TheUniv.ofIlli-noisPress,Urbana(1953).

6Gruntman,M.,Blazing the Trail: The Early History of Spacecraft and Rocketry,American InstituteofAeronauticsandAstronautics, Inc.,Reston,VA(2004).

7Santayana,G.,“FluxandConstancyinHumanNature,”Chap.10,inThe Life of Reason,PrometheusBooks,Amherst,NY,p.82(1998).

8Burroughs,E.R.,A Princess of Mars,A.C.McClurgandCo.,Chicago,IL(1917).

9Clarke,A.C.,“TheChallengeoftheSpaceship,”J. Brit. Int. Soc. 5,66–81,1946.

10Shepherd,L.R.,andCleaver,A.V.,“TheAtomicRocket-1,”J. Brit. Int. Soc. 7,185–194(1948).



13Thompson,T.D.(ed.),TRW Space Log 1996,RedondoBeach,CA(1997).

14Thompson,T.D.(ed.),TRW Space Log 1997–1998,RedondoBeach,CA(1999).

15Rubashkin,D., “WhoKilled theGrandTour?ACaseStudy in thePolitics of FundingExpensiveSpaceScience,” J. Brit. Int. Soc. 50, 177–184(1997).

16Johnston,L.,andWilliamson,S.H.,“TheAnnualRealandNominalGDP for the United States, 1789–Present,” Economic History Ser-vices;http://www.eh.net/(Mar2003).

17Ordway, F. I. III, Sharpe, M. R., and Wakeford, R. C., “EMPIRE:EarlyMannedPlanetary-InterplanetaryRoundtripExpeditions,PartI:AeroneutronicandGeneralDynamicsStudies,”J. Brit. Int. Soc. 46,179–190(1993).

18Ordway,F.I.III,Sharpe,M.R.,andWakeford,R.C.,“EMPIRE:EarlyManned Planetary-Interplanetary Roundtrip Expeditions, Part II:LockheedMissilesandSpaceStudies,”J. Brit. Int. Soc. 47,181–190(1994).

19Pigafetta,A.,Magellan’s Voyage: A Narrative Account of the First Cir-cumnavigation,R.A.Skelton(ed.andtranslator),DoverPublications,Inc.(1994);(TranslationofPrimo viaggio interno al globo erraqueo).

20“FerdinandMagellan,”fromWikipedia;http://en.wikipedia.org/wiki/Ferdinand_Magellan(2004).

21Nofi, A., Statistical Summary: America’s Major Wars; http://www.cwc.lsu.edu/cwc/other/stats/warcost.htm(2001).

22Historical Tables: Budget of the United States Government, Fiscal Year 2004, U.S. Government Printing Office, Washington, DC; http://www.whitehouse.gov/omb/budget(2003).

23AerospaceFactsandFigures2002/2003andAerospaceFactsandFig-ures1998/1999:http://www.aia-aerospace.org/stats/facts_figures/facts_figures.cfm(2004);“1983/84AerospaceFactsandFigures,”Aviation Week and Space Technology,Washington,DC(1983).

24NASA,The Vision for Space Exploration,NP-2004-01-334-HQ,NASAHeadquarters,Washington,DC(Feb2004).

25Arthur,D.,Ramsay,A.,andRoy,R.S.,A Budget Analysis of NASA’s New Vision for Space Exploration,CongressionalBudgetOffice,Wash-ington,DC(Sep2004).



RalphL.McNuttJr.

The AuthorRalph L. McNutt Jr.isaphysicistandamemberofthePrincipalProfessionalStaffofAPL.HereceivedhisB.S.inphysicsatTexasA&MUniversityin1975andhisPh.D.inphysicsattheMassachusettsInstituteofTechnologyin1980.Dr.McNutthas

beenatAPLsince1992andbeforethatheldpositionsatVisidyne,Inc.,M.I.T.,andSandiaNationalLaboratoriesinAlbuquerque,NewMexico.HeistheProjectScientistandCo-InvestigatoronNASA’sMESSENGERmission toMercury,Principal Investigatoron thePEPSSI investigationon theNewHorizonsmissiontoPluto,Co-InvestigatorfortheVoyagerPLSandLECPinstruments,andamemberof theIonNeutralMassSpectrometerTeamontheCassiniOrbiterspacecraft.HehasheldvariousNASAgrantsandhasservedonanumberofNASAreviewandplanningpanelsandtheScienceandTechnologyDefinitionTeamsforSolarProbeandInterstellarProbe.HeisaCorrespondingMemberoftheInternationalAcademyofAstronautics;aFellowofTheBritishInterplanetarySociety;amemberoftheAmericanAstronomicalSocietyanditsDivisionofPlanetarySciences;andamemberoftheAmericanGeophysicalUnion,SigmaXi,ThePlanetarySociety,andtheAmericanInstituteofAero-nauticsandAstronautics.Dr.McNutthaspublishedmorethan100scienceandengineeringpapersandgivenover150professionalandpopulartalks.Hise-mailaddressisralph.mcnutt@jhuapl.edu.

26Williams,C.H.,Dudzinski,L.A.,Borowski,S.K.,andJuhasz,A.J.,“Realizing‘2001:ASpaceOdyssey’:PilotedSphericalTorusNuclearFusion Propulsion,” AIAA 2001-3805, 37th AIAA/ASME/SAE/ASEE Joint Propulsion Conf. and Exhibit, Salt Lake City, UT (8–11Jul2001).

27McNutt,R.L. Jr.,Gunn,S.V.,Sackheim,R.L.,Siniavskiy,V.V.,Mulas,M.,andPalaszewski,B.A.,“PropulsionforMannedMarsMis-sions:Roundtable3,”inProc. 10th Int. Workshop on Combustion and Propulsion,Lerici,Italy(2005).

28The United States in Antarctica, Report of the U.S. Antarctic Program External Panel, National Research Council, Washington DC (Apr1997).

29Commander, U.S. Naval Support Force, Antarctica, Final Report of Operation Deep Freeze ’91 (1990-91), Department of the Navy (31May1991).

30Construction of the Great Wall; http://www.travelchinaguide.com/china_great_wall/construction/index.htm(10Sep2004).

31Great American Structures; http://greatstructures.info/great_main.htm.

32Golden Gate Bridge, Research Library, Construction Data;http://www.goldengatebridge.org/research/factsGGBDesign.html(2004).

33TheTinyTitanic:SeaGoingShipSizes;http://www.rpsoft2000.com/shipsize.htm.

34TheEmpireStateBuildingFacts;http://www.newyorktansportation.com/info/empirefact2.htm.

35The Website for Defense Industries—Navy, Nimitz-class NuclearPowered Aircraft Carriers, USA—Specifications; http://www.naval-technology.com/projects/nimitz/specs.html.

36TheWebsiteforDefenseIndustries—Navy,SSBNOhio-classBallis-ticMissileSubmarine—Specifications;http://www.naval-technology.com/projects/ohio/specs.html.

37The Official Site of the Eiffel Tower: A Few Statistics; http://www.tour-eiffe.fr/teiffel/uk/documentation/structure/page/chiffres.html.

38Wade,M.,”SaturnV,”inEncyclopedia Astronautica;http://www.astro-nautix.com/lvs/saturnv.htm(28Aug2004).

39Global Aircraft: An-225 Cossack; http://www.globalaircraft.org/planes/an-225_cossack.pl.

40Statue of Liberty Facts; http://www.endex.com/gf/buildings/liberty/libertyfacts.htm.

41NSSDC Master Catalog Spacecraft, Cassini; http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1997-061A(7Sep2004).

42McNutt,R.L.Jr.,“InterstellarTravel:IsItFeasible?,” ScienceFair,FortWorth,TX,availablefromtheauthor(1970).

43Johnson,R.D.,andHolbrow,C.(eds.),Space Settlements: A Design Study, SP-413,ScientificandTechnicalInformationOffice,NASA,Washington,DC(1977).

44Final Report—Phase I and II Post-Nova Launch Vehicle Study, SM-42969, Missile and Space Systems Division, Douglas Aircraft Co.,Inc.,SantaMonica,CA(Apr1963).

45Wade, M., “Saturn V-D,” in Encyclopedia Astronautica; http://www.astronautix.com/lvs/saturnvd.htm(9Aug2003).

46ISS-Mir Size Comparison; http://www.hq.nasa.gov/osf/funstuff/sta-tionoverview/sld006.htm(2004).

47Nock,K.T.,“TAU—AMissiontoaThousandAstronomicalUnits,”AIAA-87-1049, 19th AIAA/DGLR/JSASS Int. Electric Propulsion Conf., ColoradoSprings,CO(11–13May1987).

48McNutt,R.L.Jr.,Andrews,G.B.,Gold,R.E.,Bokulic,R.S.,Boone,B.G.,etal.,“ARealisticInterstellarExplorer,”Adv. Space Res. 34,192–197(2004).

49Kipling, R., “If-,” in Rewards and Fairies (first published in 1910),HouseofStratus,Thirsk,UK(2001).

50National Aeronautics and Space Administration 2003 Strategic Plan,NP-2003-01-298-HQ,NASAHeadquarters,Washington,DC(2003).

51Heinlein,R.A.,GreatAviationQuotes:SpaceFlight;websiteofD.English,http://www.skygod.com/quotes/space.htm.

52Koelle,H.H.,Nova and Beyond: A Review of Heavy Lift Launch Vehicle Concepts in the Post-Saturn Class,TechnicalUniversityBerlin,Aero-spaceInstitute(2001).

53McNutt, R. L. Jr., “Space Exploration in the 21st Century,” inProc. 10th Int. Workshop on Combustion and Propulsion, Lerici, Italy(2005).

54Hearth,D.P.(StudyDirector),Allen,J.P.,Baker, J.L.,Bannister,T. C., Billingham, J., et al., Outlook for Space: Report to the NASA Administrator by the Outlook for Space Study Group, SP-386, Sci-entific and Technical Information Office, NASA, Washington,DC(1976).

55Project Prometheus: The Nuclear Systems Program, JIMO—JupiterIcyMoonsOrbiter;http://ossim.hq.nasa.gov/jimo/.

56Stuhlinger, E., Ion Propulsion for Space Flight, McGraw-Hill BookCompany,NewYork(1964).

57Stuhlinger,E.,“ElectricSpacePropulsionSystems,”Space Sci. Rev.7,795–847(1967).

58Shepherd,L.R.,“PerformanceCriteriaofNuclearSpacePropulsionSystems,” J. Brit. Int. Soc. 52,328–335(1999).

59Von Braun, W., Great Aviation Quotes: Space Flight; website ofD.English,http://www.skygod.com/quotes/space.html.

60Yeats,W.B.,The Song of Wandering Aengus, The Collected Poems of W. B. Yeats,MacmillanPublishingCo.,Inc.,NewYork(1979).

Documents

SSolar System Exploration: A Vision for the Next 100 Years