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Jupiter exploration is big science,and onlythe United States can afford self-containedmissions to the gas giant and its four planet-sized moons.The Galileo spacecraft,whichwas recently flown into Jupiter to prevent it

from contaminating Europas ocean, cost $1.6billion.Despite the failure of its High GainAntenna (HGA), Galileo discovered briny,subsurface oceans on Europa,Ganymede,andCallisto;globally mapped all four Galilean moons;monitored Ios volcanic activity;carried out a7-year study of the Jovian magnetosphere; anddropped an atmospheric probe into Jupitersupper cloud layer.

Of these achievements,the most significantis the indirect detection of a deep subsurfaceliquid-water layer on Europa [Pappalardo etal.,1999; Kivelson et al.,2000].The case for aEuropan ecosystem can be made [e.g.,Marionet al.,2004], although it is important to rememberthe energetic and biogeochemical limits onputative Europan life [e.g.,Soare et al.,2002].Europas low moment of inertia (0.346 0.005MR2) suggests a silicate mantle below theocean,permitting chemical exchanges betweenocean and silicates,as occurs on Earth.Europassurface is geologically young,likely emplaced20180 million years ago.Any recycling of sur-ficial icy crust into the ocean could add oxygen,

sulfur, and organic compounds, either impact-delivered or generated in situ by UV irradiationand the implantation of ionized particles fromJupiters radiation belts.

Because of its astrobiological potential, theSpace Studies Board of the U.S. NationalAcademies has accorded a science priority toEuropan exploration equal to that of Mars.The icy crust (probably ~ 25 km thick,butpossibly much thinner [Nimmo et al.,2003;Greenberg et al.,2000]) bars direct access tothe ocean in the near term,but fresh oceanmaterial may be exposed at localities on thesurface.Beneath a thin,heavily irradiated layer,

biosignatures may be detectable with todaysinstruments.Models of the Europan ocean remain poorly

constrained.Its redox state and temperatureare unknown.Its salt component may bedominated by H2SO4,MgSO4,(Na- K)2SO4,orNa2CO3. Its thickness could be 6 km or 100 km[Kargel et al.,2000].

NASAs current plan for the next phase ofJupiter exploration is an orbiter using nuclear-electric propulsion:preliminary studies envisagea mass of 30 tonnes and a length of 30 m (Fig-ure 1).The Jupiter Icy Moons Orbiter (JIMO) willsuccessively orbit Callisto (for ~ 2 months),Ganymede (~ 4 months),and Europa (12

months),after which time Jovian radiation isexpected to degrade its onboard electronics

[NASA Science Definition Team,2004].JIMO will serve as a test bed for Project

Prometheus,an ambitious U.S. Departmentof Energy (DOE)/NASA program to developadvanced radioisotope power sources andnuclear fission reactors.Compact,high-outputpower is a prerequisite for piloted deep-spacemissions,and might enable lengthy unmannedvoyages to exotic destinations beyond Saturn.Radiosotope thermoelectric generators (RTGs),or even solar panels [Noca et al.,2002],arelikely better suited to a Jupiter orbital tour.

JIMO fits well with NASA Administrator SeanOKeefes declared goal of field-testing the

technologies needed for a revolutionary jumpin spaceflight capabilities.Catastrophic failureof a U

235

fission reactor would cause less envi-ronmental damage than that of a Pu

238

RTG.Fission power would allow JIMO to operateall of its instruments in parallel and to returndata to Earth at unprecedented rates.However,there are serious concerns with an explorationstrategy relying on JIMO:(1) JIMO is not expectedto arrive at Europa before 2025,and the lengthydelays of Galileo and Cassini legitimate doubtabout this schedule; (2) because it must beplaced in a safe orbit before reactor start-up,JIMO requires a heavy-lift launch vehicle thatdoes not yet exist; (3) innovative science

approaches are yoked to the politically sensitiveProject Prometheus, which could be abruptlycancelled;(4) concerns have been raised[Eluszkiewicz, 2004] that JIMOs radar will bescattered at shallow depths by meter-scalecavities in Europas regolith; (5) and mostimportant,JIMO appears to accord less urgencyto an investigation of,Europas biologicalpotential than do NASA s scientific advisors[e.g., COMPLEX,1999] or the taxpaying public.

Because radiolytic processing and impactgardening are likely to erase most biosignaturesin the layer of Europa susceptible to remotesensing,a more direct approach is needed ifwe are to constrain our models of the Europanocean habitat.Although some Europa special-ists have expressed guarded support for JIMO,it is widely agreed that a JIMO mission withonly an orbiter would generate limited interestand support from astrobiology researchers.JIMO is baselined to deploy a modest landedpackage, but weight constraints may precludesampling beneath the gardened layer,which isroughly 0.72 m deep [Phillips and Chyba,2004].

Is there an alternative strategy? A 2001 work-shop, sponsored by the Lunar and PlanetaryInstitute,Headquarters Office of Space Science,and the Outer Planets Program Directorate,held at the Lunar and Planetary Institute in

Houston,Texas,produced few practical sugges-tions.One conservative solution would bea Europa-focused orbiter using chemicalpropulsion.Experiences elsewhere are instruc-tive.Although the faster,better,cheaper approachto space exploration has been criticized, ithas achieved some great successes.The JupiterMillennium Mission in 20002001 showed thepotential of synergistic studies using multiplespacecraft.The ambitious goal of sample returnhas unified astrobiologists, geologists,and atmos-pheric scientists.Proposals to study Jupitersgravitational and magnetic fields (Juno) andreturn samples from Europa (Ice Clipper) sug-gest that low-cost ($300$650 million) missionscan now be flown to Jupiter [Drake et al.,2001].

Any Europa landed element should addresstwo disparate goals.The first goal is planetology,and requires only near-surface placement,allowing magnetic field and seismic measure-ments,surface imaging,and radio science.The second goal,astrobiology, requires accessto subsurface material and strict sterility ofthe spacecraft.

As one radical solution,consider the launchof three low-mass,solar-powered spacecrafton direct,ballistic orbits timed for simultaneousarrival around Jupiter in May 2015. One buswould enter Europan orbit,dispensing threelightweight rough-landers [Tamppari et al.,2001].Each rough-lander would carry micro-scopic and far-field imagers, a magnetometer,a seismometer,an enzymatic microarray forchemical assay, and a laser ablation time-of-flight mass spectrometer (TOF-MS),relayingdata to Earth through the bus.

A sacrificial impactor would be flown intothe leading edge of Europa,taking nesteddescent imagery, to guarantee at least one sig-

nal and to calibrate the seismometer net.Thiswas done decades ago for the Moon by crashingdiscarded Apollo-Saturn third stages.The impactcrater and plume would be observed by thethird spacecraft (carrying a smaller,backupimpactor) which would fly through the plumeat < 10 kms

-1

and capture samples on aerogel,tungsten filaments,and sapphire wafers forreturn to Earth [McKay,2002].

Removing any one flight element from thesynergy would damage the missions sciencecapacity,but if desired, the flight elementscould be stretched over successive launchopportunities,creating a sustained Jupiter

System Exploration Program. In a similar way,the stability of the Mars Exploration Program(MEP) has replaced the previous drought-glutpattern,and specializing in areology is now aviable choice for young planetary scientists.

The predicted cost of the hypothetical mis-sion described in the previous paragraph,ascalculated using NASA standard cost estima-tion software,plus margin, is $2 billion, abouta fifth as much as more detailed JIMO estimates,and multiple-spacecraft missions are well suitedto international cost-sharing.

Notably, the 1996 claim of ambiguous evidencefor fossil life in a Martian meteorite led to thecreation of NASAs National Astrobiology

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Institute and the $600 million/yr MEP. Furtherevidence of a viable,present-day habitat onEuropa with a volume as much as double thatof Earths oceans could enormously increasethe resources available to Jovian science.

JIMO faces tremendous challenges,and maybe only a long-term solution for next-generationexploration.The case for science at Europa isvery strong,arguing that a more direct approachto Europa exploration is a gamble worth taking.

Acknowledgments

I am grateful to Torrence V. Johnson,RobertPappalardo,Nick Hoffman,and an anonymousreviewer for comments,and to David J.Stevensonand Andrew Ingersoll for discussions.Opinionsexpressed are the authors own.

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EDWIN S.KITE, Cambridge University, U.K.;E-mail: ek265@cam.ac.uk

Fig.1. Sometime in the 2020s, the Jupiter Icy Moons Orbiter spacecraft approaches Jupiter.