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Advances in Space Research 60 (2017) 1080–1100

FIRE - Flyby of Io with Repeat Encounters: A conceptual design fora New Frontiers mission to Io

Terry-Ann Suer a,⇑, Sebastiano Padovan b, Jennifer L. Whitten c, Ross W.K. Potter d,Svetlana Shkolyar e, Morgan Cable f, Catherine Walker f, Jamey Szalay g, Charles Parker h,John Cumbers i, Diana Gentry j, Tanya Harrison k, Shantanu Naidu f, Harold J. Trammell l,

Jason Reimuller m, Charles J. Budney f, Leslie L. Lowes f

a Institut de Mineralogie, de Physique des Materiaux, et de Cosmochimie (IMPMC) Sorbonne Universites - UPMC, Univ Paris 06, FrancebGerman Aerospace Center (DLR), Department of Planetary Physics, Rutherfordstraße 2, Berlin 12489, Germany

cCenter for Earth and Planetary Studies, Smithsonian Institution, MRC 315, PO Box 37012, Washington, DC 20013-7012, United StatesdDepartment of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, United States

eGeophysical Laboratory, Carnegie Institution for Science, Jocelyn St NW, Washington, DC 20015, USAf Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, United States

gSouthwest Research Institute, San Antonio, TX, United Statesh John Hopkins Applied Physics Lab, Laurel, MD 20723, United States

iSynBioBeta LLC, Mountain View, CA 94040, United StatesjNASA Ames, Moffett Field, CA, United States

kSchool of Earth and Space Exploration, Arizona State University, AR, United StateslUniversity of Houston, Houston, TX, United States

m Integrated Space Flight, Boulder, CO, United States

Received 5 February 2017; received in revised form 12 May 2017; accepted 15 May 2017Available online 6 June 2017

Abstract

A conceptual design is presented for a low complexity, heritage-based flyby mission to Io, Jupiter’s innermost Galilean satellite andthe most volcanically active body in the Solar System. The design addresses the 2011 Decadal Survey’s recommendation for a NewFrontiers class mission to Io and is based upon the result of the June 2012 NASA-JPL Planetary Science Summer School. A sciencepayload is proposed to investigate the link between the structure of Io’s interior, its volcanic activity, its surface composition, and itstectonics. A study of Io’s atmospheric processes and Io’s role in the Jovian magnetosphere is also planned. The instrument suite includesa visible/near-IR imager, a magnetic field and plasma suite, a dust analyzer, and a gimbaled high gain antenna to perform radio science.Payload activity and spacecraft operations would be powered by three Advanced Stirling Radioisotope Generators (ASRG). The pri-mary mission includes 10 flybys with close-encounter altitudes as low as 100 km. The mission risks are mitigated by ensuring that relevantcomponents are radiation tolerant and by using redundancy and flight-proven parts in the design. The spacecraft would be launched onan Atlas V rocket with a delta-v of 1.3 km/s. Three gravity assists (Venus, Earth, Earth) would be used to reach the Jupiter system in a6-year cruise. The resulting concept demonstrates the rich scientific return of a flyby mission to Io.� 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Io; Volcanism; Radiation; Spacecraft

http://dx.doi.org/10.1016/j.asr.2017.05.019

0273-1177/� 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: [email protected] (T.-A. Suer).


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Fig. 1. The logo for the FIRE mission.

T.-A. Suer et al. / Advances in Space Research 60 (2017) 1080–1100 1081

1. Introduction

The NASA-JPL Planetary Science Summer School(PSSS) allows its participants to take part in the designof a Phase A spacecraft mission. The authors of this workparticipated in the first edition of the 2012 PSSS. Stimu-lated by the richness of the phenomena that characterizeIo, the team selected an Io flyby mission. Under the guid-ance of Charles Budney and of members of JPL’s TeamX (http://jplteamx.jpl.nasa.gov), the authors formulatedscience and mission objectives and designed a Phase A mis-sion concept using concurrent system design methodology(e.g., McGuire et al., 2012) and JPL design principles(e.g., JPL-D-17868, 2004–2008). To cover the missiondesign process through all stages and increase the valueof the output of the exercise, instruments (or combinationsof instruments) and architectures which had not previouslybeen proposed together for Io missions were given prefer-ence where appropriate.

Io has been observed by Pioneer 10 and 11 (e.g.,Anderson et al., 1974), Voyager 1 and 2 (e.g., Smithet al., 1979b,a), Galileo (e.g., McEwen et al., 1998; Lopeset al., 2004), Cassini (Belluci, 2004; Geissler, 2004) andNew Horizons (e.g., Spencer et al., 2007), and telescopessuch as Hubble (e.g., Spencer, 1997) and Keck (e.g., dePater et al., 2014a,b). These observations have improvedour understanding of Io, but unresolved questions remainabout its interior structure, tidal energy dissipation, vol-canic processes, bulk composition, and atmospheric prop-erties. Acknowledging the importance of improvingscientific understanding of Io, the 2009 New FrontiersAnnouncement of Opportunity (AO) (NationalAeronautics and Space Administration (NASA), 2009) des-ignated an Io observer as one of the potential missions thatcould be considered for flight in this decade. The 2011Planetary Science Decadal Survey, Visions and Voyages(National Research Council, 2011) re-categorized an Iomission to the New Frontiers # 5 mission opportunity,which will not occur for this decade. Though an Io missionis not among those included in the most recent New Fron-tiers AO (National Aeronautics and Space Administration(NASA), 2016), Io remains of great interest to the plane-tary science community.

Io’s most distinct attribute is the prevalence of volcanicfeatures on its surface: vents, calderas, paterae and plumes(Spencer, 1997; McEwen et al., 1998; Lopes et al., 2004;Geissler and McMillan, 2008). This plethora of volcanismis caused by the relationship between tidally driven distor-tion and Io’s internal heat production (e.g., Ojakangas andStevenson, 1986; Segatz et al., 1988; Moore, 2003; Tyleret al., 2015). The orbital periods of the three innermostGalilean satellites - Io, Europa, and Ganymede - are inthe ratio 1:2:4, respectively (e.g., Yoder, 1979). Thisdynamical configuration, known as a Laplace resonance,maintains the orbital eccentricities of the satellites atforced, non-zero values. Peale et al. (1979) hypothesizedthat the tidal dissipation associated with the forced eccen-

tricity of Io would induce large amounts of melting in itsinterior. This hypothesis was confirmed by Voyager 1observations (Smith et al., 1979b) that revealed a satellitewhose surface was dominated by volcanic processes.

Io, a body whose energy transport is largely controlledby volcanism, is an analog for the early phases in the evo-lution of terrestrial bodies, for example the Archean Earth(e.g., Moore and Webb, 2013; Kankanamge and Moore,2016). In addition to powering Io’s volcanic activity, theprocess of tidal dissipation also sustains global liquid wateroceans on Europa (e.g., Pappalardo et al., 1999) and likelyon Ganymede (Kivelson et al., 2002; Saur et al., 2015). Byproviding the energy necessary to sustain liquid water inthe outer Solar System (e.g., Cassen et al., 1979) at shallowdepths (at least in the case of Europa, Schmidt et al., 2011),the process of tidal dissipation thus expands the concept ofthe habitable zone beyond its classical definition (Kastinget al., 1993; Kopparapu et al., 2013). The significance ofstudying Io is thus heightened by its role as a window intothe evolution of terrestrial bodies and as a laboratory tofurther investigate tidal dissipation.

The mission concept that we formulated, Flyby of Iowith Repeat Encounters (FIRE) (See logo in Fig. 1), wouldprovide the measurements necessary to addressfundamental questions about Io within the constraints ofthe 2009 New Frontiers Announcement of Opportunity(AO) (National Aeronautics and Space Administration(NASA), 2009) and within the framework of the questionsposed by the Decadal Survey. Ten flybys are planned forthe primary mission; however, major science objectivescould be addressed by the science payload within only fiveorbits (Table 1). Details of the proposed science goals arepresented in Section 2; the system design rationale andthe science payload are presented in Section 3; the instru-ment suite, the flight profile and the flight system aredescribed in Sections 4–6. As a mission to Io will face theharsh conditions of the inner Jovian magnetosphere, Sec-tion 7 assesses the risks associated with the mission whileSection 8 discusses the schedule and the costs. Section 9summarizes the design of FIRE and its relevance in thescope of the NASA’s scientific priorities.

http://jplteamx.jpl.nasa.gov

Table 1Summary of the proposed threshold science goals possible in 5 flybys with the science payload on FIRE.

Instrument Requirement Function Science objective

FLARE 0.1 mm/s over60 s period

Radio science;Stability and accuracy

Io’s interior structure;Presence of a global magma ocean

and/or liquid coreMAGMAS 0.5 nT

8 vectors/s1500–6000 nT20 degrees

Sensitivity and sampling rate;Dynamic range of magnetometer;

Angular resolution

Constrain magma ocean;Constrain dipole magnetic field

VOLCANO(visible)

Global coverageat 500 m/pixel

Imaging resolutioncoverage

Tidal dissipation mechanisms;Eruption mechanisms;Mountain formation

VOLCANO(IR)

Global IR coverageat 1 km/pixelbetween 1–5 m

Imaging resolution coverage andspectral range

Distribution of heat;Heat flux temporal variability;

Dissipation mechanism;Crust and mantle composition

VOLCANO(visible)

Local visiblecoverage at

100 m/pixel of50 km features

Imaging resolutioncoverage

Eruption mechanisms;Tectonic processes

VOLCANO(IR)

Local IR coverageat 200 m/pixel of50 km features

between 1 and 5 lm

Imaging resolutioncoverage andspectral range

Eruption mechanism;Tectonics;

Crust and mantle compositions

CALDERA M/dM = 10 at 1 amu1 nm

50 nm–5 m

Atomic mass resolution:Grain size resolution:

Grain size range

Eruption mechanisms;Volatiles and silicatespresent on Io’s surface;Magma composition;Mass loss mechanisms

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2. FIRE science goals

Summarized here are the science objectives that a mis-sion to Io should accomplish, according to the DecadalSurvey (National Research Council, 2011). These questionsare separated into three domains of relevance: (1) Interiorstructure and mechanism of tidal heating; (2) Tectonicand volcanic processes; (3) Plumes, atmospheric and mag-netospheric dynamics. This subdivision is meant only tomake clear the connection between the scientific questionsand the approach taken to address them in the design ofFIRE. The relationship between the science questions,the measurement objectives and mission functional require-ments are made explicit in the Science Traceability Matrix(STM, Table 2).

2.1. Interior structure and mechanism of tidal heating

In the absence of a seismological experiment, the accu-rate measurement of the gravitational field and of its timevariations provides the best approach to investigate theinterior structure of Io. In principle, the gravitational fielddepends on the bulk composition of the satellite and on theradial distribution, physical and thermal state of materialsin the interiors. Anderson et al. (1996) used measurementsof Io’s mass, radius, and moment of inertia from the Gali-leo spacecraft to investigate its interior structure. Theydetermined that Io is a differentiated body with a metallic

core and a silicate mantle. However, the large uncertaintiesin these measurements (e.g., leading to core mass uncer-tainty of 10%), and the lack of a measurement of the tidaldistortion of Io hindered additional insights into the inte-rior structure of the satellite (Anderson et al., 1996). Thegravity science experiment planned for FIRE would be ableto improve on these measurements (Section 4.1.)

Electromagnetic data collected by the Galileo spacecraftgave further insights into the interior structure of Io.Khurana et al. (2011) analyzed the electromagnetic induc-tion signal at Io and inferred the presence of a global sub-surface molten layer (with a melt fraction in excess of 20%)of at least 50 km in thickness. The presence of this hot layerwould prevent core cooling and could explain the absence(at the �100 nT level) of an internally generated magneticfield at Io (Khurana et al., 2011). Several FIRE flybyswould have the relevant geometry for obtaining thesemeasurements.

Io’s surface volcanic activity is a manifestation of theunderlying internal structure of the satellite and its tidaldissipation mechanism. Tidal dissipation models of Io(e.g., Segatz et al., 1988; Tyler et al., 2015) show that thedistribution of surface heat flow would change in the pres-ence of a fluid or partially molten asthenosphere. Currentmaps of Io’s volcanic heat flow do not allow strong conclu-sions to be drawn about the interior structure responsiblefor the observed heat output (Hamilton et al., 2013;Davies et al., 2015). Further measurements of heat flow

Table 2The proposed science traceability matrix for the FIRE mission showing the expected science that would be accomplished by the payload. Abbreviations (NIR = near-infrared, FOV = field of view,VIS = visible, res = resolution, EPD = energetic particle detector).

Science goals Science objectives Scientific measurement requirements Instrument Instrument functionalrequirements

Projectedperformance

Science return Mission functionalrequirements (toplevel)

Observables Physicalparameters

Function Requirement

Characterize Io’stidal heating

Io’s tidal heating magnitude Spacecraftvelocity

Io’s gravity field Radio Science Pointingaccuracy

Derived0.1 mm/sover 60 s


Gravity field map 0.0015 rad pointingaccuracy

Temperature ofsurfacehotspots

Surface IR spectra Camera (NIR) Spectralrange, res.& FOV

Global at10 km/pixel,1–5 lm

Global at720 m/pixel,1–5 lm

Global temperaturemap

Spatial distribution of Io’stidal heating

Spacecraftvelocity

Io’s gravity field Radio science Pointingaccuracy



Gravity field map






Temporal variability ofIo’stidal heating

Spacecraftvelocity

Io’s gravity fieldover time

Radio science Pointingaccuracy



Gravity field map






4 flybys

Identify Io’s tidal heatingdissipation mechanisms

Hotspots onsurface

Global surfaceimages

Camera (VIS) Image res.& FOV

Globalcoverage at500 m/pixel

Global at360 m/pixel

Visible surfaceimage map

0.0015 rad pointingaccuracy, 4 arcsec/spointing stability


Global infraredspectra

Camera (NIR) Spectralrange, res.& FOV




Determine Io’sinteriorstructure

Establish limits on the depthand thickness of Io’sputative magma ocean

Magneticinductionsignal

Magnetic field Magnetometer Axes 3 3 Magnetic inductionsignal

0.0015 rad pointingaccuracySensitivity 0.5 nT 0.1 nT

Samplingrate

8 vectors/s 8 vectors/s

Dynamicrange

1500–60,000 nT

1500–51,300 nT

Plasma inducedcurrents

EPD Angularres.

20, 120 � 12FOV

20, 120 � 12FOV

Determine Io’s inertia Spacecraft’svelocity

Io’s gravity field Radio science Pointingaccuracy



Gravity field map 0.0015 rad antennapointing accuracy

Determine theexistence ofIo’s magneticfield

Determine the existence ofIo’s magnetic field

Magneticinductionsignal

Magnetic field Magnetometer Axes 3 3 Magnetic inductionsignal

0.0015 rad pointingknowledgeSensitivity 0.5 nT 0.1 nT

Samplingrate

8 vectors/s 8 vectors/s

Dynamicrange

1500–60,000 nT

1500–51,300 nT

Plasma inducedcurrents

EPD Angularres.

20, 120 � 12FOV

20, 120 � 12FOV

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Table 2 (continued)

Science goals Science objectives Scientific measurement requirements Instrument Instrument functionalrequirements

Projectedperformance

Science return Mission functionalrequirements (toplevel)

Observables Physicalparameters

Function Requirement

Identify themechanismdriving Io’svolcanism

Characterize the eruptionmechanisms on Io

Distribution ofvolcanicfeatures

Feature surfaceimages


100 m/pixel,50 km2 FOV

100 m/pixel,50 km2 FOV

Surface featuremap


Composition ofvolcanicfeatures

Feature infraredspectra

Camera (NIR) Spectralrange,res., FOV

200 m/pixel,1–5 lm,50 km2 FOV

200 m/pixel,1–5 lm,100 km2FOV

IR imagesofvolcanic features

Plumelocation/frequency

Global surfaceimages




Maps of plumes

Plumecomposition

Size andcomposition ofplume dustparticulates

Dust analyzer Atomicmass res.

MdM ¼ 10 at1 amu

MdM ¼ 10–50 < 190 amu

Erupted plumeparticlecharacteristicdataset

Instrument velocity2–40 km/s

Particlesize res

1 nm 10 nm

Particlesize range

50 nm–5 lm 20 nm–20 lm

Investigate Io’stectonics

Characterize the formationof mountains on Io

Characteristicsof tectonicfeatures

Topography oftectonic featuresvia stereo imaging


100 m/pixel,200 km2

FOV

100 m/pixel,100 km2

FOV

High-res. visualimages of tectonicfeatures


Composition oftectonic surfacefeatures

Infrared spectra oftectonic surfacefeatures


500 m/pixel,1–5 lm,200 km2

FOV

200 m/pixel,1–5 lm,100 km2

FOV

High-res. surfacespectral maps oftectonic features

Identify Io’ssurfacechemistry

Identify volatiles andsilicates present on Io’ssurface

Composition ofsurfaceparticulates

Spectra of dustparticulates


MdM ¼ 10 at1 amu

MdM ¼ 10–50 < 190 amu

Surface particlespectral dataset

Instrument velocity2–40 km/s

Particlesize res.

1 nm 10 nm

Grain sizerange

50 nm–5 lm 20 nm–20 lm

Identify Io’s magmacomposition

Composition ofvolcanic ashparticulates

Spectra of volcanicash particulates


MdM ¼ 10 at1 amu

MdM ¼ 10–50 < 190 amu

Volcanic ashparticle spectraldatasetParticle

size res.1 nm 10 nm

Grain sizerange

50 nm–5 lm 20 nm–20 lm

Identify Io’s crust andmantle composition

Composition ofexposedmagma flows

Spectra of exposedMagma flows


200 m/pixel,1–5 lm,200 km2

FOV

200 m/pixel,1–5 lm,100 km2

FOV

High-res. surfacespectral maps ofexposed magma


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and distribution, particularly in the polar regions of Io,could give further insights into its interior structure. Thesemeasurements could also be used to test tidal dissipationmodels. FIRE flybys # 1, 2, 3 and 10 would target polarregions to obtain the relevant measurements.

Following these unanswered questions, the Decadal Sur-vey identified three objectives that an Io mission shouldaccomplish regarding the interior of Io and tidal dissipa-tion (National Research Council, 2011): (1) Determinethe melt fraction of Io’s mantle; (2) Constrain Io’s tidalheating mechanism; (3) Resolve the state of Io’s corewith improved constraints on intrinsic magnetic fieldgeneration. Data from FIRE’s magnetometer and plasmapackage (Section 4.2) are sensitive to a global inductivemagma ocean and to the possible presence of an intrinsicmagnetic field produced by dynamo action in the core ofIo (Khurana et al., 2011). FIRE’s measurements of thetidal deformation (Section 4.1) are most sensitive to thepresence of global liquid layers, such as a liquid core anda global magma ocean (Moore, 2003). These two data setswould address the three Decadal Survey objectives, withone caveat. The detection of an intrinsic magnetic fieldwould represent the tell- tale sign of a liquid core. How-ever, the core might not undergo dynamo action – and thusnot produce a magnetic field – and still be liquid, leavingonly the tidal measurement sensitive to the presence ofthe liquid core. However, its signal might be masked bythe presence of a global magma ocean, and in this scenarioa conclusive inference about the state of the core might notbe possible.

2.2. Tectonics and volcanic activity

Io’s volcanic activity is predominantly expressed in low-altitude volcanic vents - paterae and shield volcanoes(Radebaugh et al., 2001). Lava flows cover about 28.5%of Io’s surface while Paterae account for 2.5%. Yet 64%of all Ionian hot spots occur within paterae; 45% of allhot spots are associated with dark patera floors (Williamset al., 2011). Mountains are present mostly as isolatedpeaks and are interpreted as tectonic edifices indicative ofa state of compression in the lithosphere (Schenk andBulmer, 1998). A compressive lithosphere should counter-act the flow of melt to the surface, an inference that is atodds with Io being the most volcanically active Solar Sys-tem body (McGovern et al., 2016). Hypotheses have beenadvanced to explain such high volcanic activity on acompression-dominated body but are difficult to test(Carr et al., 1998; Kirchoff et al., 2011; Hamilton et al.,2013; Bland and McKinnon, 2016). For example, deepfaults due to compressive forces could propagate to formscarps on the surface and provide a conduit for magmaascent (Bland and McKinnon, 2016). Repeated high reso-lution imaging by FIRE would be used to determine thetopographical relief associated with these tectonic features

and used to investigate the underlying fault structure (e.g.,Klimczak, 2014) and mechanism of magma ascent.

Optical (e.g., albedo and color) and spectral observa-tions indicate that sulfur and sulfur dioxide are prevalenton Io (e.g., Soderblom et al., 1980), an indication of activesulfur volcanism. High temperatures, strongly indicative ofsilicate volcanism, have been reported for lava flows andhot spots (e.g., Spencer et al., 1997). Dark materials onthe surface, seen in paterae floors, are interpreted to bemagnesium silicate lavas (Geissler et al., 1999), though sil-icates or mafic materials have not been spectroscopicallyidentified on Io. Io’s bulk density and topography doessuggest that the underlying materials are silicates (e.g.,O’Leary and Flandern, 1972; Clow and Carr, 1980) andthere has been speculation that the very high temperaturelavas (up to 1700 K) are due to ultramafic magma compo-sitions (e.g., McEwen et al., 1998; Keszthelyi et al., 2007),similar to komatiitic volcanism on the ancient Earth(Williams et al., 2000). Imaging and monitoring of eruptiontemperatures by FIRE would address questions regardinglava compositions.

The science objectives identified by the Decadal Surveythat are directly relevant to tectonic and volcanic processesare: (1) Study tectonic processes; (2) Study Io’s active vol-canic processes; (3) Investigate endogenic and exogenicprocesses controlling surface composition. FIRE’s visibleand near IR imaging camera (Section 4.3) and dust ana-lyzer (Section 4.4) are directly suitable for achieving theseobjectives.

2.3. Plumes, atmosphere and magnetosphere

The processes that generate plumes on Io, arguably oneof its most spectacular features, are not fully understood.For instance, the observation of the ‘‘wandering plume”,Prometheus (Kieffer et al., 2000), cannot be explained byvolcanic venting alone; another proposed explanation isthat lava flows over sulfur dioxide and/or sulfur ‘‘snow-fields” cause material to vaporize or erupt as plumes(Kieffer et al., 2000). The composition of plume materialscould provide information necessary to determine themechanisms behind these features. In addition, plumematerial compositions could also shed light on the long-term dynamical evolution of the satellite as prolonged vol-canic activity should deplete Io of its volatiles.

Despite the large volume of material released by volcan-ism, Io’s atmosphere is quite thin. A large flux of dust par-ticles from Io continuously escape to circum-Jovian space(e.g., Liu et al., 2016). By identifying and characterizingthe properties of dust particles (e.g., velocity distributionand composition) that are ejected from Io, the mechanismsthat transport material from Io to the plasma torus can beassessed (Smyth and Marconi, 2005). The properties of Io’satmosphere and its outgassing rates could also be deduced.Plasma from Io dominates the Jovian magnetosphere, e.g.bursts of energetic particles observed in the Jovian magne-


totail have been shown to have Io-genic origin (McNuttet al., 2007; Haggerty et al., 2009). In situ sampling of par-ticles throughout the spacecraft’s orbit could provide fur-ther insights on the role of Io-genic plasma in thedynamics and structure of Jupiter’s magnetosphere.

The relevant science objectives identified in the DecadalSurvey are: (1) Study Io’s active volcanic processes; (2)Investigate interrelated volcanic, atmospheric, plasma-torus, and magnetospheric mass-and energy-exchange pro-cesses; (3) Investigate endogenic and exogenic processescontrolling surface composition. A combination of mea-surements from FIRE’s magnetometer and plasma pack-age (Section 4.2) and dust detector (Section 4.4) wouldaddress some of these objectives.

3. Design exercise

During the exercise, the science objectives were decom-posed into measurement objectives and mission functionalrequirements summarized in the STM (Table 2). JPL’sTeam X facility allowed the team to work in networkedworkstations where the project model was updated in realtime in a collaborative fashion among the different subsys-tems (i.e., propulsion, communication, structure, naviga-tion, etc.). This approach allowed for efficientdevelopment and rapid evaluation of mission modifications(McGuire et al., 2012). Parameters of flight system compo-nents were tracked in a system spreadsheet, part of which issummarized in Table 3. Refinements and optimization ofthe mission design were made as the team checked compo-nents against constraints on resources. Most of the space-craft subsystems were based on flight proven heritagesystems whose specification were available in the NASA

Table 3Summary of masses and contingencies for spacecraft components (overallmasses in bold).

Spacecraft components Mass (kg) Contingency (%)

Attitude control 41.3 30Command and data 21.3 6

Power 134.3 12Propulsion 101.3 5

Structures and mechanisms 351.8 30Cabling 54.2 30Telecom 47.1 17Thermal 54.5 25Bus total 817.9 22

Science payload total 23.1 15Spacecraft total (dry) 841.0 22

Subsystem heritage contingency 184.3 22System contingency 177.3 21

Propellant & pressurant 915.3 0Spacecraft total (wet) 2118

Launch vehicle-side adapter 36.5Launch mass 2154

Launch vehicle (Atlas V 401) capacity 2265 5(Launch vehicle margin 110.5)

(JPL design principles

margin 36%)

Space Science Data Coordinated Archive (NSSDCA) Mas-ter Catalog (http://nssdc.gsfc.nasa.gov/nmc/). Marginsand contingencies were incorporated to allow flexibilityas the design progresses in accordance with JPL designprinciples (JPL-D-17868, 2004–2008). During the exercise,trades were also made at the system and subsystem leveland the science payload underwent several iterations inorder to find the best design to achieve the missionobjectives.

Some constraints present in the 2009 AO were relaxedbased on later calls and guidance which allowed the teamto explore non-conventional design options. For instance,Advanced Stirling Radioisotope Generators (ASRGs) wereutilized as the power source for the mission. ASRGs werenot a power option in the 2009 New Frontiers AO but wereadded in the 2010 Discovery Call and provided a cost sav-ing. The most recent New Frontiers AO allows the use ofradioisotope power sources (National Aeronautics andSpace Administration (NASA), 2016).

3.1. System design

Due to FIRE’s encounter with Io in the equatorial planeof Jupiter (at 5.9 Jovian radii (RJ) from Jupiter), whichcoincides with the highest radiation zone, the spacecraftwould accumulate a total ionizing dose (TID) of �60 kradper flyby, assuming 100 mil aluminium shielding. Long-term exposure of spacecraft electronics and materials tosuch harsh radiation environment may degrade the hard-ware (JPL-D-17868, 2004–2008). Several instrumentsonboard the Galileo spacecraft suffered from malfunctionas a result of radiation exposure (Fieseler et al., 2002). Dur-ing the first of Galileo’s three close encounters with Io, thespacecraft went into safe-mode due to a radiation event.Galileo’s overall science return from Io was reduced dueto radiation induced instrument malfunction and data loss.Other spacecrafts that have observed Io (Pioneer 10 and11; Voyager 1 and 2) did not approach closer than Euro-pa’s orbit. Juno’s polar orbit is designed so that the space-craft avoids the harshest regions of the radiation belt,though the spacecraft is expected to accumulate a TIDwhich is ten times greater than that of Cassini and fourtimes that of Galileo (Kayali et al., 2012).

To mitigate radiation damage of the FIRE spacecraft,we adopted the strategies of previous missions and missionconcepts that addressed survival in high radiation environ-ments, e.g. Juno (Dodge et al., 2007; Kayali et al., 2012), IoVolcano Observer (IVO) (McEwen et al., 2014; Adamset al., 2012), Europa Jupiter System Mission (EJSM)(Pappalardo et al., 2008), and the Van Allen probes(Spence et al., 2013). The main strategies adopted includea radiation vault and radiation shielding of sensitive com-ponents (see also discussion in Section 6.1). In addition,most of FIRE’s subsystems derived flight heritage fromrobust predecessors such as subsystems on Juno and MEr-cury Surface, Space ENvironment, GEochemistry, andRanging (MESSENGER).

http://nssdc.gsfc.nasa.gov/nmc/


Redundancy of components was incorporated into thedesign to increase the chances of operational success. Forexample, two star trackers are included because these com-ponents are particularly sensitive to radiation damage andare critical for navigation. Two avionic computers are alsoincluded in the design. There is a 36% design margin on theoverall system and contingencies at the subsystem level (seeTable 3) allowing for additional shielding mass to be addedas the design progresses. These issues are discussed in moredetails in Sections 6.1 and 7. Lessons learned from the Junomission will contribute to the success of a future dedicatedIo mission. Technological development in radiation shield-ing and radiation tolerant electronics could boost thechances of success of a future Io mission.

Gimbaled solar arrays similar to those on Juno wereconsidered to provide mission power needs. But a tradewas made for ASRGs as a power source early in the exer-cise since they offered a reduction in mass, lowered missionrisk and an increase in the mission science return. Othermission concepts for the Jovian system have also consid-ered ASRGs, for example, the IVO (McEwen et al.,2014) and Ganymede Interior Structure, and Magneto-sphere Observatory (GISMO) (Jones et al., 2011). AsASRGs have not been flown, their flight performance isnot fully characterized. To address this issue, four ASRGSwere included in the design: two to power spacecraft oper-ations and two spares in case the primary units failed.Later, it was determined that downsizing to three ASRGswould not significantly affect the mission risks and wouldallow the payload to fit onto a smaller launch vehicle andremain under the New Frontiers cost cap. Another poweroption that could be available for a future Io mission isthe new ultra-light, flexible solar arrays called Roll OutSolar Arrays (ROSA) (Bailey and Landis, 2014). However,these panels would serve best for an Io flyby mission with ahighly inclined orbit that would minimize ROSA exposureto radiation.

The choice of orbit (near equatorial in the Jovian plane,see Section 5) was determined by the delta-v availabilityand the ability to perform gravity science measurementswith this orbital geometry. The IVO concept (McEwenet al., 2014), assessed the use of a polar orbit in order toavoid the Io plasma torus and the most intense regionsof the radiation belt, while being able to obtain high lati-tudes measurements. However, this orbital geometry hasa higher delta-v requirement than could be afforded byour design constraints. The eccentricities of the selectedorbits for FIRE (Table 11) also allows the spacecraft toavoid the highest radiation zone for most of the space-craft’s orbit.

3.2. Science payload design

After formulating the science objectives, it was evidentthat an instrument suite focused on gravity field, magneticfield and particles was necessary. With the STM as a guide,the team selected a set of instruments to accomplish the

science goals. During the exercise, the science payloads ofprevious mission concepts for Io, as well as missions orconcepts designed to operate in extreme radiation environ-ments, were considered. The team explored a differentinstrument suite from the one which was proposed forIVO, which included a narrow and wide angle camera, flux-gate magnetometer, thermal mapper and an Ion and Neu-tral Plasma Spectrometer (INMS).

We utilized instruments that would take advantage ofthe in situ environment at Io to accomplish the mission’sscience objectives. We opted for an advanced radio sciencepackage, which would include a gimbaled high gainantenna, and a measurement plan that could provide accu-rate gravity field data to resolve Io’s interior structure.Though not an available option during our study, a fanbeam antenna such as the one on MESSENGER and pro-posed for Europa Clipper would have saved weight andpossibly allowed the inclusion of another instrument suchas a thermal mapper (see discussion below). We alsoselected a magnetometer package with the measurementcapability to resolve the interacting fields and particlesaround Io. A dust analyzer was added to measure the char-acteristics of the ubiquitous dust particles throughout theFIRE orbit. A visible/near IR camera was selected forthe imager. Table 4 summarizes the main specifications ofthe science payload while Table 5 lists the projected datareturn for each experiment planned for FIRE.

Additional instruments were assessed for the sciencepayload but were not included in the final design. A ther-mal mapper such as the one proposed for IVO and EuropaClipper was proposed to provide temperature, compositionand heat flow measurements. Due to mass, power and costconstraints, a visible-IR camera/spectrometer was choseninstead. This instrument could accomplish some of thescience objectives to study tectonics via imaging in the vis-ible and high temperatures volcanism with near IR imaging(see Section 4.3). The dust analyzer would also aid in deter-mining the composition of material ejected in plumes orsputtered from the surface. Though a scanning platformcould have improved the camera performance it was notincluded due to budget constraints but could be consideredfor a later iteration. A laser altimeter was also consideredfor providing quantitative topographic information. Thisinstrument was withdrawn from the design due to highmass, power and cost requirements. Plans were imple-mented to coordinate the mission with telescopic observa-tions of Io to increase the overall science return of FIRE.

The imaging camera chosen for FIRE is based on theNew Horizons Ralph instrument (Reuter et al., 2008),which has both visible and spectral imaging capabilities.Ralph is compact, has no moving parts and could be fur-ther customized for the radiation environment with addi-tional shielding. The instrument has redundant electroniccomponents and the thermal and optical stability necessaryfor a deep space environment. These features made thisinstrument suitable for a long duration flyby reconnais-sance mission to Io. Though not an available option during

Table 4Summary of the science payload on FIRE.

Instrument Mass (kg) Peak power (W) Peak data rate (Mbp/s)

FLARE 44 124 50VOLCANO 10.5 7.1 12CALDERA 8.5 9 0.5MAGMAS:

Fluxgate magnetometer 1.05 4.2 56Energetic particle detector 1.47 2.49 91

Table 5Summary of projected data sufficiency for each science experiment on FIRE.

Data type Science goals Data qualityrequirement

Data quality Data quantityrequirement

Data quantity

Gravity field map(Doppler shift anomaly map)

Tidal heating;Interior structure

0.1 mm/s 0.1 mm/s 6.5 Mb 13 Mb

Magnetic induction signal Interior structure;Magnetic field

0.5 nT 0.1 nT 180 kb 360 kb

Global surfaceIR map

Tidal heating;Volcanism

Global coverage at1 km/pix6 bit depth

Global coverage at720 m/pix12 bit depth

13 images6 bit depth=78 Mb


Global surfacevisible

image map

Volcanism;Tectonics




100 images12 bit depth=1.2 Gb

High resolutionvisible targeted

images

Volcanism;Tectonics

Local coverage at250 m/pix

50 km features6 bit depth

100 m/pix50 km features12 bit depth


500 images12 bit depth

=6 Gb

High resolutionIR

targeted images

Volcanism;Tectonics;

Surface chemistry

Local coverage at500 m/pix

50 km features6 bit depth

200 m/pix50 km features12 bit depth


500 images12 bit depth

=6 Gb

Particle characteristicdata set

(impact speed,charge, direction,composition,impact time)

Volcanism;Surface chemistry;Atmospheric and

ionosphericdynamics

20 degree resolutionover 12 � 12 degree

FOV

20 degree resolutionover 12 � 12 degree

FOV

53 Mb 106 Mb

Table 6Performance specifications for the radio science experiment on Juno(Mukai et al. 2012), which is the basis for FLARE.

Parameter Value

Design Dual band reflectorX and Ka

Diameter 2.5 mSensitivity 1 mm/s


our study, a camera customized for Io such as ‘‘Rcam”,(e.g., McEwen et al., 2012; Adams et al., 2012) wouldimprove the scientific return of a future Io mission. Thiscamera is designed to be radiation tolerant and uses a com-plementary metal–oxide–semiconductor (CMOS) detectorwith extremely fast readout time to minimize noise.

4. Science instruments

4.1. FLARE - FieLd Analysis through Radio Exploration

FieLd Analysis through Radio Exploration (FLARE) isthe gravity science experiment on FIRE that derives her-itage from the radio subsystem on Juno (Mukai et al.,2012). The instrument consists of a 2.4 m gimbaled highgain antenna (HGA) with uplink/downlink capabilities inKa (32 GHz) and X (8.4 GHz) band. This gravity scienceexperiment relies on the deep space network (DSN) groundcomponent, particularly the antenna at Goldstone, to pre-cisely quantify the Doppler effect of the radio signal trans-

mitted to Earth during closest approach of Io. Changes inspacecraft velocity of less than 1 mm/s can be resolved.Simultaneous X and Ka band tracking data will be twoorders of magnitude more accurate relative to Galileo mea-surements (Asmar et al., 2009) and will also be less suscep-tible to dispersive noise (Mukai et al., 2012). The HGA onFIRE would be gimbaled to afford the possibility of per-forming the radio-science experiment while collecting datawith the other instruments. This configuration is differentfrom the design of the Cassini communications, which

Table 7Performance specifications for the fluxgate magnetometer on MESSEN-GER (Anderson et al., 2007), which is the model for the fluxgate

Fig. 2. Plot of Io mean anomaly at the time of closest approach for eachof the ten FIRE flybys.


needed dedicated radio-science flybys in order to performradio science (Iess et al., 2014; Kliore et al., 2004) as radiotracking from Earth was not allowed during remote sensingobservations. Performance specifications of FLARE aresummarized in Table 6.

Doppler shifted transmitted signals can provide a mea-surement of the tides of the satellite. The tidal responsedepends on the density and state of the core and mantle,and on the rheology of the interior (e.g., Padovan et al.,2014). The mission flight profile has been chosen in orderthat the flybys would occur at different phases of Io’s orbit(see Fig. 2), which is the best orbital configuration toobtain a measurement of the solid tide of Io. Given theflyby latitudes, FLARE could potentially provide uncorre-lated estimates for the degree-2 coefficients of the gravita-tional field of Io. These measurements would alsoimprove the description of Io’s gravity field compared toGalileo measurements, which were analyzed under theassumption of hydrostatic equilibrium, since the gravita-tional coefficients J2 and C22 were highly correlated andthus their determination was not independent (Andersonet al., 1996).

Other measurements permitted by FLARE includeDoppler limb profiles and spacecraft radio occultationmeasurements, that could determine structure of the atmo-sphere and ionosphere. Similar measurements have beencarried out by Pioneer and Galileo to probe the atmo-spheric structure of the Jovian moons (Kliore et al., 1975;Kliore et al., 1997). There is also the possibility ofspacecraft- to-spacecraft radio occultations with otherspacecraft that might be present in the Jovian system inthe coming decade (Asmar et al., 2009), e.g., JUICE (JUpi-ter ICy Moon Explorer) (Grasset et al., 2013) and theEuropa Clipper mission (Pappalardo et al., 2015).

magnetometer in the MAGMAS package.

Parameter Value

Design Triaxial 3.5 m boomResolution of

amplitude variation�0.1 nT

Absolute accuracy �0.047 nTFrequency coverage few HzFine dynamic range �51,300 nT

Coarse dynamic range �1530 nT

4.2. MAGMAS (Multi-Axis Geophysical MAgnetometer

and plasma Suite)

The main goal of this package is to measure Io’s mag-netic induction signal. Io’s magnetic signature is influencedby the surrounding plasma currents. The ion flow velocityand mass density of the plasma are therefore required to

accurately determine the induction signal (e.g., Khuranaet al., 2011). Our approach to this problem was to adaptstrategies from missions that successfully carried or consid-ered these measurements in the Jovian system, e.g. Voy-ager, Galileo, Juno and the upcoming Europa Clipper(Pappalardo et al., 2015). MAGMAS is composed of threesensors: a tri-axial fluxgate magnetometer to characterizemagnetic fields and two instruments to characterize plasmacurrents and energetic particles.

The triaxial fluxgate magnetometer with heritage fromMESSENGER (Anderson et al., 2007) measures magneticfield strength and direction. The instrument has bothcoarse (�51,300 nT) and fine (�1530 nT, resolution0.047 nT) measurement range modes to cover differentmagnitudes of fields that are present close to Io. By com-parison, Galileo’s magnetometer had a fine measurementmode with 512 nT range and 0.25 nT resolution. Theexpected magnitudes of Io’s intrinsic and induced fields(Khurana et al., 2011) are within the range of the instru-ment’s capability. Specifications of this instrument aresummarized in Table 7.

A Langmuir probe similar to the one on Mars Atmo-sphere and Volatile Evolution Mission (MAVEN)(Andersson et al., 2015) was initially considered for its abil-ity to measure the basic properties of plasma for a broadrange of magnetic field conditions. However, this instru-ment would not be able to characterize plasma flow veloc-ity due to its cylindrical symmetric design. A more suitableinstrument would include two or more Faraday cups,which could measure the ion flow velocity, mass density,and temperature of plasma. Such an instrument, Plasmainstrument for Magnetic Sounding (PiMS), will be partof the science payload for the Europa Clipper mission(Pappalardo et al., 2015). PiMS will be able to measureand correct for the strong plasma induced fields surround-ing Europa in support of magnetic sounding (Westlakeet al., 2014).

An energetic particle detector (EPD), based on NewHorizons’ Pluto Energetic Particle Spectrometer ScienceInvestigation (PEPSSI) (McNutt et al., 2008), would char-acterize the energetic particle population (i.e., energy >10 keV). The instrument can measure ions, with composi-tional information, electrons (from tens of keV to1 MeV), and record particle events at a rate of at least


103 per second. With the combination of time-of-flightmeasurements and energy measurements it is possible todistinguish particle events with varying masses, for exam-ple protons, electrons, and heavier particles such as Na+.EPD measurements could also characterize the neutralatmosphere and the ionosphere. The instrument would becapable of observing ion mass loading from Io plasmasources into circum-Jovian space. Specifications are inTable 8. A similar instrument, Jupiter Energetic ParticleDetector Instrument (JEDI), is currently onboard Juno(Mauk et al., 2013).

4.3. VOLCANO (Visible OpticaL Camera And Near-IRObserver)

VOLCANO is designed to carry out observations withsub-kilometer resolution to provide insights into the rela-tionships between geology, composition and silicate vol-canism on Io. VOLCANO is a compact visible-IR imagerand IR spectrometer modeled after the Ralph camera onNew Horizons (Reuter et al., 2008). The instrument is aframing camera with several band passes in the visibleand infrared. Color images (blue (400–550 nm), red (540–700 nm), near IR (780–975 nm)) provide compositionalinformation about Io’s surface. The IR spectral imagerprovide measurements in the 1.25–2.5 lm range, corre-sponding to temperatures relevant to Io’s high temperaturevolcanic eruptions (up to 1800 K). Additional filters andband passes could be included in the camera as the designis refined to observe at wavelengths corresponding to lower

Table 9Performance specifications for the Ralph camera on New Horizon (Reuteret al., 2008), after which VOLCANO is modeled.

Parameter Value

FOV 10 mrad @ 10 lrad/pixelFilters R, G, B, NIR, Panchromatic � 2

IR spectral range 1.5–5 lmAperture 75 mmCCD 1024 � 1024 pixels

Resolution 12 bitRead-out- time 1.8 s

Table 8Performance specifications for the energetic particle detector (EPD) onNew Horizons (McNutt et al., 2008), the basis for the one included inMAGMAS.

Species Energy range

Electron 25–500 keVProton 24 keV–1 MeV

(TOF measurements1 keV–1 MeV)

Atomic ions,e.g., CNO group,Mg+, Si+,Ne+

60 keV–1 meV(TOF measurements

15 keV–1 MeV/nucleon)Molecular ions

e.g., Nþ2 , O

þ2

100 keV–1 MeVTOF measurements

30 keV–1 MeV/nucleon

temperatures. Stereo pairs would be obtained by repeatedimaging of a target on multiple passes to provide topo-graphic information via stereo-photogrammetry (e.g.,Kirk et al., 2015). Topographic information could also bederived from the images via photoclinometry (e.g., Kirket al., 2003; Giese, 2010). The camera specifications aresummarized in Table 9.

Fig. 3 shows VOLCANO’s closest approach imagingplan. Global mosaic images, M1 (900 m/pix) and M2(360 m/pix) are acquired before closest approach. Globaltopography, compositional, and thermal variations arederived from these mosaics. Targeted imagery, T1 (1 km/pix), T2 (900–350 m/pix) and T3 (300–100 m/pix) occurin the intervals between the mosaic modes. Topographicfeatures and volcanically active regions including faults,scarps, lava channels, and plume deposits are resolved intargeted images. Nightside imagery permits observationsof hotspots and lava without sunlight contamination. Sur-face heat flux is derived from photometry of the IR spectralimages. Temperatures are derived by fitting the observedspectra to single temperature blackbody spectra as doneby Tsang et al. (2014). Eclipse and limb observations ofplumes could also be carried out to investigate Io’s atmo-spheric properties. Possible VOLCANO targets includeactive volcanic centers such as Pele, Wayland Patera andPrometheus (see ground tracks in Fig. 6). Observations ofheat flux from some of these areas are discussed inVeeder et al. (2012). These volcanic centers would beobserved throughout the mission duration to quantify tem-poral changes.

Though information on silicate volcanism on Io can beobtained in the near IR (Keszthelyi et al., 2009), a thermalIR camera is a better choice for studying passive back-ground temperatures and heat flow. One of our main mea-surement goals was to acquire observations of hightemperature eruptions (e.g. 1400–1800 K), which couldconstrain silicate lava composition and the state of Io’smantle (e.g., Keszthelyi et al., 2007). Observations madeat VOLCANO bandpasses could provide information onhigh temperature eruptions and different styles of pyroclas-tic emplacement (e.g., Keszthelyi et al., 2009; Davies et al.,2010). Similar studies were carried out with observationsmade by Ralph during the 2007 New Horizons approachof Io. Peak temperatures of lava and hotspots and tempo-ral variability of thermal emission from some features werederived from these observations (Tsang et al., 2014;Rathbun et al., 2014). Heatflows derived from these studieshave also been used in combination with other datasets totest tidal dissipation models for Io (e.g., Rathbun et al.,2017). However, for a dedicated Io mission many changeswould be needed to improve Ralph’s radiation-toleranceand signal to noise. In future mission design iterations, athermal IR camera such as the one proposed for EuropaClipper, E-Themis would be better suited for studyingbackground heatflow in the mid to far IR (Rathbunet al., 2016). Additionally, a radiation-hardened camerawith a CMOS detector (Adams et al., 2012; McEwen

Table 10Performance specifications for the dust analyzer on the Cassini missin(Srama et al., 2004), the model for CALDERA.

Parameter Value

Particle massthreshold

5� 10�19 kg

Size 20 nmSpeed 670 km/s

Dynamic range 106

Sensitive area (m2) 0.10

Fig. 3. Illustration of the close approach imaging plan for VOLCANO (Io not to scale).


et al., 2012) such as that proposed for IVO (McEwen et al.2014) would permit high-resolution imagery for detailedsurface maps and topography studies.

4.4. CALDERA (Comprehensive AnaLysis of Dust from

ERuptions and Atmosphere)

CALDERA is an impact-ionization dust analyzer withtime-of-flight mass spectra capability, similar to the CosmicDust Analyzer (CDA) onboard Cassini (Srama et al.,2004). The instrument can measure the mass, composition,electric charge, speed and direction of dust particles.Impacting materials are transformed into a mixture of par-ticle fragments and plasma that are then separated by anelectric field and focused onto a multiplier, which convertsthe signal to a mass spectrum. CALDERA directly samplessub-micron sized particles reliably at speeds of 670 km/s,though measuring particles at higher speeds is possible withincreased uncertainties in the measurements. Volcanicplumes on Io are a continuous source of dust in the Joviansystem (Liu et al., 2016) and understanding dust composi-tion and kinematics is an important aspect of the mission’sscience objectives. The instrument samples dust duringclose approach and throughout the spacecraft orbit. Theinstrument performance specifications are summarized inTable 10.

CALDERA’s measurements provide a database of dustparticles characteristics (e.g., charge and velocity distribu-tions) that can be used to study the nature of the materialemitted from Io. The mass resolving power of the time-of-flight measurements is between 10 and 50, which should besufficient to discriminate between species of geologicimportance that are expected to be present in Io-genic dustparticles; these species may include Na+, Fe2+, and varioussilicate or silica species. The composition of species identi-fied in the dust reflects Io’s volcanic processes. For exam-

ple, the identification of silica in plume ejecta wouldprovide additional evidence for the presence of basalticor ultramafic volcanism (McEwen et al., 1998; Keszthelyiand McEwen, 1997). Mineralogy deduced from the compo-sition of dust particles would add constraints to the degreeof melting, differentiation, and volatile loss that Io’s inte-rior has undergone (Keszthelyi et al., 2007). Continuousmonitoring of the characteristics of dust emission fromplumes would quantify Io’s mass flux at different timescalesto assess the factors affecting its variability (e.g., Krugeret al., 2003). Dust particle characteristics are relevant tounderstanding the mechanisms that transfer material fromIo’s atmosphere to circum-Jovian space. Surface composi-tion would also be studied through sampling material sput-tered from the surface.

Having a dust analyzer onboard an Io mission wouldenhance the mission’s hazard mitigation and would allowin situ sampling of plumes. However, due to the risks posedby a plume fly-through, this maneuver would not be per-formed until the final orbits of the mission (Section 7).

5. Flight profile

FIRE launches with an Atlas V lift vehicle and uses aVenus-Earth-Earth Gravity Assist (VEEGA) transfer tra-

Table 12Proposed flight schedule for FIRE.

Maneuver Date

Launch 2/29/2020VGA 6/28/2020EGA-1 4/24/2021EGA-2 6/23/2023

Jupiter Arrival 12/21/2025End of mission 4/17/2026

Fig. 4. Illustration of the interplanetary trajectory of FIRE (orange) with the main maneuvers and relative schedules indicated. There is one VGA: Venusgravity assist and two EGAs: Earth gravity assist. Orbits of Earth, Venus and Jupiter are shown. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)


jectory (Fig. 4). The cruise phase is a long duration flightwith characteristic energy C3, a measure of the excessspecific energy of the spacecraft as it departs the earth, of12.8 km/s2. There are minimum power requirements forscience instruments that would be operating in safe modeduring this mission phase. Telecommunications are period-ically turned on for tests. The attitude control subsystem(ACS) is used minimally for small course corrections dur-ing cruise. There are few, if any, maneuvers until arrivalat Jupiter six years after launch. Table 12 summarizes theproposed flight schedule.

FIRE arrives at Jupiter with a v1 of 11.15 km/s. Itencounters Io three hours before Jupiter orbital insertion(JOI), at an altitude of 500 km and velocity of 300 m/s.The spacecraft fires the main engine for the JOI burn atabout 5.16 RJ. The delta-v for this maneuver is 1.3 km/s.It enters into a near equatorial orbit with an initial period

Table 11Parameters for the 10 science orbits of FIRE. Angular quantities are in degre

Orbit # Duration(days)

Closest approachflyby altitude

(km)

Lattitu

1 32 300 76 S2 32 100 79 S3 37 100 80 S4 21 100 8 S5 14 100 24 N6 21 100 28 S7 15 100 49 S8 15 100 89 S9 21 100 50 S10 21 100 86 N

of 32 days. The orbital inclinations would range between0.62� and 4.50�. The selected orbit is optimized for delta-v savings and traverses the plasma torus for only shortamounts of time, during which the imager is turned off asa safety precaution. The parameters for the ten scienceorbits are summarized in Table 11 and shown in Fig. 5; fly-

es.

de Longitude Inclination Eccentricity

216 W 3.18 0.87185 W 1.67 0.87261 W 0.62 0.89119 W 0.65 0.84113 W 0.8 0.79293 W 0.7 0.84118 W 1.68 0.8136 W 3.28 0.81300 W 4.50 0.84175 W 2.89 0.84

Fig. 5. FIRE orbit trajectories in Jupiter reference frame (x and y axis aredistances in terms of Jovian radii (RJ)) with the 10 primary science orbitslabeled. Closest approach trajectory is indicated in green; Io’s orbit is blueand plasma torus outlined by red circles. Direction of the sun is indicatedby the yellow line.


bys are separated by intervals of 12 to 37 days. The closestapproach paths allow FIRE to cover a wide latitudinalrange of Io, permitting widespread imaging of the surface,including polar features (see groundtracks in Fig. 6).FIRE’s encounter with Io at different phases of its orbitprovides the optimal geometries for sampling of Io’s grav-ity fields (Fig. 2). The flyby geometries also permit mag-netic field measurement.

6. Flight system

6.1. Structure

The FIRE orbiter is in a standard spacecraft bus madeof an aluminum honeycomb structure (Fig. 7). The totalspacecraft wet mass is 2154 kg and fits in a 4 m faring, com-

Fig. 6. Altitude ground track plot showing the ten flybys over a USGS basemaltitude. The red areas with upside down triangles indicate where intense volcolour in this figure legend, the reader is referred to the web version of this a

patible with multiple launch vehicles. Several componentsare mounted on the outside of the spacecraft, for example:the ASRGs; the high gain antenna; thrusters; fuel and oxi-dizer tanks. Structural reinforcements are included for theASRGs and a shelf is included to support the reactionwheels. Support structure and mechanisms are includedfor power, telecommunications and the main engine. Thereis integration hardware including fasteners, adapters, and aharness. The magnetometer is placed on an extendableboom.

Radiation was a major design driver for the spacecraftstructure as FIRE accumulates 60 krad per orbit. Integralto the design is a radiation hardened titanium vault thatwould protect the electronics and other sensitive equipment(e.g., Kayali et al., 2012). The walls of the vault andplacement geometries of the instruments are optimized toattenuate radiation. Spot shielding is implemented withthe placement of radiation resistant material over sensitivecomponents such as power supplies and micro-controllers.Some radiation hardened parts are placed directly on thebus. This design plan allows the radiation dosage insidethe vault to be reduced to 150 krad for the mission dura-tion (or no more than about 15 krad per orbit). In additionto these measures, several components and electronics areredundant, for example, star trackers, main computersand ASRGs.

6.2. Power and thermal

The spacecraft is powered by three ASRGs, each havingredundant electronics (Shaltens and Wong, 2007). Thesedevices use the heat from decaying Plutonium-238 to gen-erate electricity. They have been developed as a low mass

ap of Io. The curves show the flyby ground tracks while color representscanic activity has been observed. (For interpretation of the references torticle.)

Table 13Summary of the mission power modes and resource requirements.

Power modes Time (Hours) Power (W)

Launch 3 438Safe mode 0.5 487Cruise 2.4 279

Cruise with telecom 6 414Orbit insertion 1.5 520

Science flyby w/o radio science 4 450Science flyby with radio science 4 637

Science orbit, torus 0 210Orbit with telecomm 25 423Orbit w/o telecom 255 216

Fig. 7. A diagram of the FIRE spacecraft with the main components labeled. Most of the sensitive electronics are located in the radiation hardened vault.


alternative to power small missions. Given the orbital con-figuration and science objectives of FIRE, the ASRGs offera significant advantage over deployable solar panels suchas the those on Juno, which are vulnerable to radiationdamage.

Based on the selected instrument suite and spacecraftoperational requirements, the average power required perorbit is 179 W; with 43% design margin added, this totals256 W. The ASRGs provide a total power of 390 W witha margin of 52%. They have battery support from two 38A-Hr Li-ION batteries. ASRG power production isexpected to degrade at 0.8%/yr (Shaltens and Wong,2007). A baseline mission (Table 1) would still be possiblewith two ASRGs if one unit were to fail. Power is cycledthroughout the mission to ensure that batteries are chargedand that power is available for science observations, com-munication and other spacecraft operations. Several powermodes were considered during the design exercise and theseare summarized in Table 13.

In conjunction with power is the thermal subsystem thatmaintains flight system components within their operatingtemperature range. The thermal subsystem consists of bothpassive and active elements. Instrument shelves and thevault are heated using waste heat from the ASRGs(500 W per unit) through a series of conductance heatpipes. Heaters on the instrument shelves are thermostat

controlled, as are the dedicated heaters on the propulsiontanks and feed lines. Some of the internal and external sur-faces are covered by multilayered insulated material to fur-ther regulate temperature. There are temperature sensorsand thermostats throughout the spacecraft. Any excessheat is discharged from the spacecraft by being radiatedaway. If one ASRG failed, there would still be ample spareheat to regulate the spacecraft temperature.

6.3. Attitude control system

The attitude control system (ACS) provides the posi-tional and pointing accuracy necessary for conducting tar-

Table 14Proposed science instrument data volumes and science data plan.

Instrument(Data type)

Peak datarate (Mbps)

Max datarate (Mbps)

Raw datavolume perflyby (Gb)

Archive

Camera-visible(visible Images)

10 10 2.4 Imaging node, PDS

Camera-IR(IR images)

10 10 2.4 Imaging node, PDS

Dust analyzer(particle spectra)

0.025 1 0.54 Planetary plasmainteraction node, PDS

Magnetometer(induced magnetic field

signal)

0.000025 1 0.005 Geosciences node, PDS

HGA(doppler shift anomalies)

– – – Radio science node, PDS

Total (Capacity) 5.35 (10 % margin)


geted science measurements and to accomplish correctturning maneuvers. The ACS consist of two star trackers,four micro-sun sensors, four reaction wheels, an inertialmeasurement unit (IMU) and the gimbal drive electronics.Sun sensors are included in case the star trackers are mis-guided by false signals induced by electrons. Two of themicro-sun sensors are placed on the high gain antenna toensure accurate pointing of that instrument. Four reactionwheels are positioned on the spacecraft in a pyramid for-mation for three-axis stabilization and the wheels have a20% margin after one wheel failure. Software that detectsfalse stars due to radiation is implemented as a safety mea-sure. Spot shielding is added to ACS components in orderto minimize radiation damage.

During the cruise stage, stellar inertial attitude determi-nation is employed using the star trackers and reactionwheels. Thrusters are used to turn the spacecraft duringcruise owing to their minimal use of propellant and thuspreserve the reaction wheels for science measurements onceFIRE arrives at Io.

FIRE pointing requirements are dictated by the camera,VOLCANO, and the high gain antenna, FLARE. Bore-sight control is 0.12� (3 r) and the boresight knowledge is0.06� (3 r). The per-axis stability required is 4 arcsec/s(3 r). The spacecraft can slew 180� using the reactionwheels in eight to ten minutes. These specifications ensurethat images are neither smeared more than one pixel norover- saturated (exposure times are <1 s).

6.4. Propulsion

The propulsion system is designed as a dual-mode sys-tem and is comprised of: (1) a bi-propellant main rocketengine with four mono-propellant hydrazine thrusters forlarge correction maneuvers and (2) an array of eighthydrazine-nitrogen tetroxide thrusters for smaller maneu-vers once in flyby range. Smaller thruster components areplaced in a propulsion vault for protection. Oxidizer andfuel pressurants are stored in dedicated tanks, and theirflow is controlled by valves. The subsystem is designed to

be robust and is expected to function reliably in a highradiation environment. As a Class B mission (NationalResearch Council, 2012), the spacecraft carries a sparemain engine for full redundancy. These two engines havea protective covering to prevent damage by micro-meteoroid impacts.

There are six planned propulsion maneuvers: Launchcheckout, cruise, JOI, JOI cleanup (repointing and recon-figuring after successful JOI), ten flybys (maintenance),and end of mission impact into Io. The propellant is sizedfor launch vehicle capability at 2265 kg, and total delta-vrequired for all propulsion maneuvers is calculated to be1495 m/s.

6.5. Communications, command and data, and ground data

systems

The communication subsystem on FIRE is similar tothat on the Juno spacecraft (Mukai et al., 2012;Vacchione et al., 2012). The system consists of: one 2.5 mhigh gain antenna (HGA), a medium gain antenna(MGA), and three low gain antennae (LGA). Multipleantennae provide redundancy in the event of problemsdue to dust or radiation. The HGA has dual X (8 GHz)and Ka (32 GHz) feeds. Two small deep space transpon-ders (SDST) provide redundancy for both communicationand gravity science. The Ka/Ka translator (KaTS) unit isused primarily for gravity science and is inside theradiation-shielded vault. Other components include awaveguide transfer switch, coax transfer switch and a highisolation X-band diplexer. The subsystem has a total massof 44 kg with power requirements of 25 W for data down-link and 30 W for dual band radio science.

FIRE has a reliable, heritage-based command and datahandling subsystem. The system is designed as low com-plexity with a single 4-interface card, 2 dedicated camerainterfaces and an integrated 32 GB solid state recorder(SSR). Table 14 shows the projected data rate requirementsfor the science instruments. All hardware and softwarecomponents including drivers, operating system (OS) and


dual-hot switching are inherited from the Mars ScienceLaboratory (MSL) with minor modification for instru-ments support and data compression.

Standard ground system operating procedures areplanned for the mission, with heritage from Juno missionoperations. Deep Space Network (DSN) passes are bud-geted to ensure a 95% data return rate, with onboardSSR storage capable of holding four orbits worth of datain the event of a missed DSN pass, or other failures. Down-link capacity is allotted such that all data from a singleorbit can be transmitted in a single DSN pass with thedownlink rate on the X-band of 10 to 15 kb/s. The averagedownlink capacity is 7.2 Gb average per orbit with 1.5DSN passes per week. The planned data return rates andvolumes for the mission are well within projected values,including a 10% margin (see Table 14). All mission datawould be archived on the Planetary Data System (PDS).Mission operations would have high level of inheritancefrom Juno mission.

7. Risk management

In flight, FIRE faces several risks, most of which areaddressed by a trade and mitigation strategy, resulting inthe mission being categorized as relatively low risk. Risksare designated by color with green representing the leastrisk, yellow a medium level of risk, and red a high levelof risk. One yellow risk and three green risks were identi-fied based on their impact and likelihood.

Yellow (medium) risk comprises the degradation of atti-tude knowledge due to high radiation in the Jupiter system.The radiation could damage the star trackers by generatingfalse star detections and endanger navigation and sciencedata collection. Tests would have to be made with the startracker during development and software implemented todetect false stars. In the event of a false star being gener-ated, a plan would be in place for ACS to coordinate withthe IMU data to help to maintain accurate navigation.

One green (low) risk is the impact of a high radiationenvironment on electronic parts and subcomponent relia-bility. The flash memory could suffer from possible dataloss while damaged star trackers and micro-sun sensorscould result in navigational errors. Mitigation of these risksincludes increasing spot shielding on several instruments. Ashielded vault for sensitive electronics further reduces thisrisk from the high radiation environment. Radiation hard-ened avionics and non-volatile memory are used wherepossible. Redundancy is implemented for star trackers,micro-sun sensors, microprocessors, ASRGs and mainengines. There are dual hot strings on the engines and theonboard computer during the close approaches for rapidrecovery from radiation upsets.

The second green (low) risk concerns damage from dustparticles during the low altitude flybys. To mitigate thisrisk, sensitive equipment are inactive during plume or torusencounters early in the mission. Plume sampling is done inthe final flybys to maximize instrument and mission safety.

According to a study by Lorenz (2015), dust hazards isminimal above an altitude of 100 km. The third green riskis scheduling delays from the increased analysis and legalplanetary protection approval required for the ASRGs.Sufficient time has been allotted for this analysis duringPhase A of the high-level mission schedule.

Use of heritage components is a strategy that lowers theoverall mission risk. Most of the science instrumentationand spacecraft subsystems are derived from flight-provenpredecessors and are available off-the-shelf or with minimalcustomization. This strategy greatly lowers developmentcosts as well. FIRE would be considered a Planetary Pro-tection Category II mission (National Research Council,2012), due to Io’s proximity to Europa. In order to complywith planetary protection, the spacecraft ends its life bycontrolled de-orbiting into Io to reduce the risk of contam-inating other satellites.

8. Schedule and cost

The primary schedule driver for the FIRE mission is theoptimal launch window, available every eighteen months.The proposed mission follows a conservative New Fron-tiers class schedule with no major schedule constraints fore-seen. Schedule reserves include one month reserve per year;a two-month reserve for assembly, test and launch opera-tions; and additional months for radiation design marginand ASRG integration. As summarized in Fig. 4, the mis-sion cruise phase involves a Venus gravity assist after fourmonths, followed by two Earth gravity assists 14 monthsand 2.4 years later to arrive in the Jovian system 5.4 yearsafter launch. The science phase begins 28 days after JOIwith the first of ten Io flybys. With an orbital period of12–37 days, adequate margin is available for data downlinkbetween flybys.

An Io observer has been classified as a New Frontiersmission (National Research Council, 2011) with a desig-nated cost cap of $927 M (in fiscal year (FY) 2012 dollars).The cost cap could be increased under specific situations;one of these would be the performance range of the launchvehicle. As the mission concept outlined here makes use ofthe lowest performance range launch vehicle with a 4 mfairing, the cost cap would increase to $991 M (FY2012).

Using a quasi-grass roots cost estimate model based onNew Frontiers and Planetary Science Decadal Surveyrequirements (National Research Council, 2011), applying50% cost reserves on Phases A-D and 25% on Phases E-F,the best cost estimate for the proposed FIRE mission isbelow the cost cap ($990 M (FY2012)). For the develop-ment costs ($842.5 M for Phases A-D), the main cost driveris the flight system ($357.2 M), specifically the power sub-system ($107.4 M), accounting for almost one third of thetotal flight system costs. In terms of the operations costs($126.5 M for Phases E-F), the main cost driver is missionoperation, which is almost half of the total projected oper-ations costs ($54.8 M). Nevertheless, the cost best estimatefor FIRE is below the cost cap and allows significant


reserves. Large reserves in Phase A allow for flexibility inlater stages of the mission design process.

9. Conclusions

FIRE is a low complexity mission within the New Fron-tiers cost cap, designed to answer fundamental science ques-tions regarding Jupiter’s moon Io. The scientific knowledgeof Io would be advanced by the improved measurementcapabilities of several instruments in the science payload.The science payload includes a gimbaled HGA, a magne-tometer, particle and plasma package, a visible-IR camera,and a dust analyzer that doubles as a science instrumentand an additional safeguard against dust hazards. Thespacecraft design uses heritage-based radiation-tolerantequipment to maximize scientific return while reducing radi-ation damage. The mission design utilizes an innovativeorbit with lowdelta-v requirements and rapid passes throughhigh radiation regions, permitting measurements of gravityand magnetic fields. A mission such as FIRE would alsobe among the first missions to utilize ASRG technology.

The one-week PSSS mission design exercise was aninvaluable educational exercise which established the basisfor the mission concept further developed here. While thismission design concept is by no means ideal, the designexercise highlights the necessary tradeoffs between keyscience questions, technical challenges, costs, and risks ofa successful Io flyby mission. The team was able to articu-late the advantages and limitations of a specific set ofinstruments that have not been proposed together for Iobefore. We have also broadened the scope of the discussionto include additional instrument possibilities for futuremission concept iterations.

Several technological advances could add to the feasibil-ity of an Io mission in the future. A large design marginsuch as that of FIRE, 36%, would allow for flexibility dur-ing the development stage of a mission including the possi-bility of other science instruments. Coordination with otherobservations such as Earth-based or space telescope moni-toring could increase science return. With several missionsin or en route to the outer Solar System, e.g., Juno, JUICEand Europa Clipper mission, there will be opportunities tomake synergistic and comparative studies in the Jovian sys-tem. For example, gravity fields, magnetic fields, and in situ

particle sampling could be measured in different orbitalconfigurations.

There is exciting science to be learned about Io’s volca-noes, interior structure, atmosphere, plasma and magneto-spheric environment. Using flight validated architectureand other risk mitigating strategies, a radiation tolerantspacecraft with a robust instrument suite could functionreliably for several months to acquire measurements ofIo. The potential science return of this concept wouldgreatly enhance the current understanding of Io andaddress several of the science goals put forward in the Dec-adal Surveys (National Research Council, 2011).

Acknowledgements

This exercise was carried out for educational purposes atNASA JPL and does not represent an actual mission pro-posal or mission. We wish to thank Trisha Steltzner, KarlMitchell, and the JPL Team X members for their mentor-ship and a stimulating learning experience. We also thankthe JPL review panel: Gregg Vane, Ed Miller, Keith Gro-gan, Jay Goguen, Richard Bennett and Steve Vance, fortheir generous and insightful comments. Thank you toKrishnan Khurana for providing valuable information onthe magnetometer package, William B. Moore for provid-ing insights in the possibility of investigating the interiorwith the radio-science experiment, David J. Stevenson fordiscussion on the magnetic field and gravity measurements,and Rainee Simons for discussion on the telecommunica-tion and power subsystems. We would like to acknowledgeAssociate Editor, David T Blewett, David A. Williams andtwo anonymous Reviewers for comments which helped toimprove the quality of the manuscript.

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