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Building Operability into the Jupiter Europa Orbiter Design to Endure a High Radiation Environment

Robert Lock* (Robert.E.Lock@jpl.nasa.gov, 818.393.2525), Kenneth Hibbard† (Kenneth.Hibbard@jhuapl.edu), Robert Rasmussen* (Robert.D.Rasmussen@jpl.nasa.gov), Karla Clark* (Karla.B.Clark@jpl.nasa.gov), Thomas Magner†

(Thomas.Magner@jhuapl.edu), Robert Pappalardo* (Robert.Pappalardo@jpl.nasa.gov), Melissa A. Jones*

(Melissa.A.Jones@jpl.nasa.gov)

*Jet Propulsion Laboratory †Applied Physics Laboratory California Institute of Technology Johns Hopkins University 4800 Oak Grove Drive 11100 Johns Hopkins Rd. Pasadena, CA 91109 Laurel, MD 20723 Abstract—The Europa Jupiter System Mission (EJSM) has been prioritized as the next Outer Planets Flagship Mission that would be devoted to exploring the emergence of habitable worlds around gas giants. This joint NASA and ESA endeavor would focus on the Galilean moons Europa and Ganymede but would also investigate Io, Callisto, and the Jupiter system as a whole. The NASA-contributed Jupiter Europa Orbiter (JEO) and the ESA-contributed Jupiter Ganymede Orbiter (JGO) would be launched on separate launch vehicles in 2020. Here we focus on JEO. 12

After 2-3 years of performing science in the Jovian system, JEO would orbit Europa and would operate in a high radiation environment. The life-limiting radiation environment complicates hardware and software performance as well as operations strategy. These challenges would require the Europa science goals to be met in an efficient duration of 9 months. To maximize the science while in orbit at Europa, JEO needs to develop strategies to make the system easily operable and robust to radiation degradation. In addition to radiation tolerant hardware and software designs along with robust margins, the project has already been considering operability features in its baseline design for JEO.

Operability is the combination of aspects of a system that make it simple and inexpensive to operate, robust to changing system behavior, responsive to modified goals, and adaptive to deviations in expected environments or operating conditions. For JEO, being robust and flexible to meet science goals in the face of a harsh environment is paramount. The mission must be designed to have multiple means of meeting critical science goals; accommodate operation in the face of radiation-based noise, degradation and failure; and be flexible to changing science goals based upon discoveries. As a result, JEO has kept science goals and operability in mind for all system design processes and trade studies. The most stringent and driving operational requirements and constraints for the JEO concept are encountered during Europa Science orbit phase. Some of

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the key operability issues incorporated from the earliest concept studies include:

Make the flight and ground systems operable and survivable for a high intensity, rapid turn-around operations environment in Europa orbit in the presence of radiation based anomalies.

Use modern system engineering methods to model the system behavior as early as possible to balance mission scope with system capability, complexity, risk, and cost. Systems design based on behavior models allows for accommodation of changing behaviors in operating the systems.

Use lessons learned from previous applicable missions to guide design philosophy and trade studies specifically for operability issues.

In this paper, we discuss the key flight system design trades, operations scenarios and lessons learned developed in recent JEO mission studies.

TABLE OF CONTENTS

1. INTRODUCTION ................................................................ 1 2. MISSION AND SYSTEM OVERVIEW.................................. 3 3. OPERABILITY ISSUES....................................................... 7 4. NEXT STEPS ................................................................... 12 5. CONCLUSION.................................................................. 13 6. ACKNOWLEDGEMENTS.................................................. 13 REFERENCES...................................................................... 13 BIOGRAPHY ....................................................................... 13

1. INTRODUCTION Missions to explore Europa have been imagined ever since the Voyager mission first suggested that Europa was geologically very young. Starting in late 1995, the Galileo mission delivered orbit after orbit of new insights into the Jupiter system and the worlds of Io, Europa, Ganymede and Callisto. Extensive architectural studies building on and expanding on Europa, Ganymede, and Jupiter System science have been performed over the past decade. The Galilean satellites are quite diverse with respect to their

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geology, internal structure, evolution and degree of past and present activity. In order to place Europa and its potential habitability in the right context, as well as to fully understand the Galilean satellites as a system, explorations of the two internally active ocean-bearing bodies—Europa and Ganymede—are of significant interest.

In 2008, the NASA Europa Explorer Study and the ESA Laplace Study teams began working together to merge their respective concepts and align the goals through an integrated Joint Jupiter Science Definition Team (JJSDT). The resulting Europa Jupiter System Mission (EJSM) concept complements the Juno mission and allows combined organizational strengths, budgets, and timelines, in order to carry out a systematic and in-depth study of the Jupiter system that aims at a common and overarching theme:

The emergence of habitable worlds around gas giants.

The baseline architecture for EJSM consists of two primary elements operating in the Jovian system at or near the same time: the NASA-led Jupiter Europa Orbiter (JEO), and the ESA-led Jupiter Ganymede Orbiter (JGO).

The JEO mission concept uses a single orbiter flight system that would travel to Jupiter by means of a multiple-gravity-assist trajectory reaching Jupiter and perform a multi-year study of Europa and the Jupiter system, including 30 months of Jupiter system science and a nine month comprehensive Europa orbit phase.

The JEO mission science objectives, as defined by the international EJSM Science Definition Team, include investigations of:

A. Europa’s Ocean: Characterize the extent of the ocean and its relation to the deeper interior

B. Europa’s Ice Shell: Characterize the ice shell and any subsurface water, including their heterogeneity, and the nature of surface-ice-ocean exchange

C. Europa’s Chemistry: Determine global surface compositions and chemistry, especially as related to habitability

D. Europa’s Geology: Understand the formation of surface features, including sites of recent or current activity, and identify and characterize candidate sites for future in situ exploration

E. Jupiter System: Understand Europa in the context of the Jupiter system

In concert with achieving these science objectives, NASA provided constraints for the 2008 Study, including: Launch no earlier than 2020, with preferred flight times to Jupiter of

< 7 years; Use the DSN 34m network for primary science downlink; and carry robust margins in all areas (technical and financial).

The primary challenge of a Europa orbital mission is to operate within Jupiter’s radiation environment, radiation damage being the life limiting parameter for the flight system. Designing for reliability and long life requires key knowledge of the environment, understanding of available hardware, prudent software and hardware design approaches, and a management structure that elevates the coordinated attention to radiation issues to the project office level. Instilling a system-level radiation-hardened-by-design approach very early in the mission concept is necessary to mitigate the pervasive mission and system level effects (e.g., on trajectory, configuration, fault protection, operational scenarios, and circuit design) that can otherwise result in run-away cost and mass growth.

The characteristics of being relatively flexible and robust but affordable to operate needs to be understood from the perspective of several aspects of the mission, all strongly affected by the radiation environment. Accumulated radiation dose limits the lifetime of instruments and electronic components. To achieve full mission success, the challenge will be to meet objectives reliably in just a few months. Science campaigns and observations would be planned in priority order to minimize the risk to their accomplishment. Knowledge and experience from early stages of operations, particularly at Europa, are expected to influence science plans and system operation in later stages. This too must happen quickly, whether it’s the improvement of gravity field knowledge to support later orbit designs, the assessment of whether early goals are accomplished in order to start new investigations, or in direct response to exciting discoveries. The operational and system design flexibility in support of this need is very important to a successful mission.

In response to the radiation environment factors and the science objectives and issues, system operability concerns should:

Accommodate changing system behavior as parts and circuits change characteristics with increasing dose

Respond to low level anomalies and fault responses without stopping science data collection for long periods

Rapidly assess accomplishments, respond to discoveries, and recover missed opportunities

Recover quickly from serious failures and faults

While concept development for a Europa mission was in its early stages, operability issues were approached as a mix of three concerns: operations scenarios considering science

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needs and desired system performance, system design descriptions indicating desired behavior, and guiding principles for system design concepts. Scenarios were analyzed via system performance models to estimate and scope flight system mass, power, data volume and rates, among other things. Key operating characteristics of the system were determined and captured as guidance for future studies. A detailed study of operations lessons learned from recent analogous missions led to a set of recommendations that were applied, where appropriate, to the mission concept. Other recommendations were captured for future use in more detailed concept studies. These approaches stood as proxies in lieu of formal operability studies for architectural consideration.

As the mission study becomes a project and moves into its formulation phases, the focus will shift from mission concept studies using operations concept proxies toward a formal architecture, requirements and design process. Operability issues will continue to be important and will be addressed more formally in these phases.

The following discussion addresses the mission concept overview in the context of operability concerns, the operations concepts and scenarios treated in the concept studies, and the specific recommendations from studies of previous missions used in the current mission concept. A brief discussion of the steps to take in future development phases concludes the paper.

2. MISSION AND SYSTEM OVERVIEW Mission Phases

The JEO mission would be composed of three mission phases. The Interplanetary phase, almost 6 years long, the period in which the orbiter would be launched, performs gravity assist flybys of Venus and Earth, and prepares for

Jupiter Orbit Insertion (JOI) and science operations. The Jovian Tour phase would focus on science activities in the 30 months after JOI and before arrival at Europa. Finally, the Europa Orbit phase would last for 9 months after Europa orbit insertion and return the highest priority science for the mission. A timeline of the notional JEO mission is shown in Figure 1. A summary of the proposed JEO trajectory, tour, and Europa orbit parameters is in Table 1.

After the interplanetary cruise phase, JEO would fly by Io roughly two hours prior to performing JOI. This flyby is designed primarily to give JEO a gravity assist, and would put JEO into an orbit with a period of about 200 days. Near apojove of the first orbit, a maneuver would target JEO to the second Io encounter of the mission, which would be the first Io science encounter of the tour.

In the notional Jovian Tour phase, the flight system would make routine and frequent observations of Jupiter, its satellites, and its environment using a 30-month gravity-assist tour to lower its orbital energy with respect to Europa. The tour begins with an Io Science Campaign involving three Io flybys, and continues with a System Science Campaign that would involve flybys of each of the other Galilean satellites as well as significant Jupiter and system science observations. These would include six encounters with Europa, six with Ganymede, and nine with Callisto. In addition, science observations of the Jovian magnetosphere and atmosphere, and monitoring of Io, would be planned between encounters during the Jovian Tour phase.

The notional Jovian Tour would end with Europa orbit insertion (EOI). The notional science orbit at Europa would be low altitude (100–200 km), near circular, high inclination, with solar incidence angle near 45°. To meet the lighting requirement over the duration of the first three Europa Science Campaigns, a retrograde orbit would be chosen, and the intersection of all the other science

Figure 1 – Notional JEO Mission Phase Timeline

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Table 1. Baseline Mission Design Characteristics

Parameter Value Launch Vehicle Atlas V 551 Earth to Jupiter Trajectory VEEGA

Earth Launch Period 2/29/2020 to 3/20/2020

C3 (km2/s2) Up to 12.8 Interplanetary Deep Space ∆V (m/s) Up to 93 Jupiter Arrival Date 12/21/2025 Declination of Launch Asymptote (deg) <2 Jupiter Arrival V∞ (km/s) 5.5 JOI Earth Range (AU) 4.3 JOI Periapsis Altitude (Rj) 5.2 Jupiter Capture Orbit Period (days) ~200

Tour 12/21/2025 to 7/3/2028

EOI 7/3/2028

Primary Europa Science 7/3/2028 to 3/30/2029

Europa Initial Orbit Inclination (deg) 95 Orbit Altitude, Average (km) 200 100 Orbit Period (min) 138 126 Ground Speed (km/s) 1.2 1.3 Orbits/day 10.4 11.4 Max. Earth Occultation Duration (min) 47 45

constraints puts the required inclination between 95 and 100°. If left uncontrolled, arbitrary orbits with these characteristics would become more eccentric, due primarily to Jupiter’s gravitational perturbations, and generally impact Europa within about a month. These orbits would need to be maintained on a regular basis with perhaps 1-2 orbit trim maneuvers (OTM) per week. This becomes an operability driver, not only because OTM planning can be complex, but also because science planning and execution complexity can be increased by orbit timing and geometry changes resulting from every OTM. Moreover, orbit trims would need to be sustained as radiation effects develop, and even after disrupting faults. Consequently, long episodes of “safing” are inappropriate. After EOI and a busy 5-day engineering assessment and

orbit adjustment period, the Europa science campaigns would be executed as a series of observation campaigns designed to obtain Europa science objectives in priority order. The first three Europa science campaigns would finish after 99 days. The fourth campaign would focus on following up on earlier discoveries and unique observations and would last for up to about 6 months. The rotation rate (and orbit period) of Europa is 3.551 days, referred to as a eurosol, and is a handy planning unit. The notional Europa science campaigns are:

Europa Campaign 1, Global Framework at 200 km orbit for 8 eurosols (28 days)

Europa Campaign 2, Regional Processes at 100 km orbit for 12 eurosols (43 days)

Europa Campaign 3, Targeted Processes at 100 km for 8 eurosols (28 days)

Europa Campaign 4, Focused Science at 100 km for 46 eurosols (165 days)

When the orbit maintenance fuel is depleted or the flight system ceases to function, the orbiter would eventually impact the surface of Europa (appropriate planetary protections measures having been taken).

Flight System Overview

The JEO flight system concept is based on a wealth of work performed in the last several years: the 2007 Europa Explorer study, which in turn was based on the 2006 Europa Explorer Design study, as well as from Europa Geophysical Explorer (2005), Europa Orbiter (2001), and numerous trade studies conducted over the past decade. The technology to fly such a mission has advanced in that time, especially in areas of launch vehicles, avionics, power sources, and detectors. While showing incremental improvements, the overall design has become remarkably stable, suggesting that the requirements are well understood.

Key design drivers on the flight system are Jupiter’s radiation environment, planetary protection, high propulsive needs to get into Europa orbit, the large distance from the Sun and Earth, and the accommodation of the instrument payload. Some important high-level constraints and assumptions on the JEO flight system design are as follows:

The flight system design employs technology that either exists already or is under development and is planned for qualification early in the JEO project lifecycle.

The mission reference radiation design dose (referenced to 100 mil aluminum shell) is less than 3 Mrad.

The required total ∆V is no more than 2260 m/s.

Approximately 7.3 Gbits of science data is returned per Earth-day during the Europa science phase and ~3.6 Gbits per Earth-day during the Jupiter tour phase.

34 m DSN antennas are used during normal operations, with limited 70 m antenna (or equivalent) use for critical or emergency events.

Heliocentric operating range is 0.7 AU to 5.5 AU, with a maximum Earth range of 6.5 AU.

These driving constraints are, generally, operability drivers as well, either directly or indirectly. The existing technology constraint in tandem with the very high radiation

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dose, for example, limits the mass memory designs to current low density, rad-hard parts. This would significantly reduce the choices and flexibility of science data acquisition for the Europa science phase. Less tolerant memory, available in much higher densities, can be used to improve options for the earlier tour phase, but the dual use brings other operability issues to the system. In another example, the multiple constraints of radiation dose, science data needs, 34 m antenna use, and long range to Earth combine to challenge the system performance, and therefore drive operability significantly. Radiation dose strongly limits flight system lifetime, causing the need to achieve science goals very quickly and the need to reduce system down-time to very low levels. The radiation dose also changes system behavior and increases the frequency of recoverable but anomalous behaviors. High Earth range and 34 m DSN antenna constraints limit downlink of the data volumes acquired in such a short mission. Finding fast, easy ways to accommodate all of these issues at the same time will be the operability challenge for JEO.

Radiation is the key defining challenge and life limiting consideration for the flight system. Due to the high radiation environment at Jupiter, the flight system must be designed from the outset to address radiation tolerance. The JEO conceptual radiation approach has to go well beyond conventional approaches to address a mission in such a harsh environment. Radiation protection for the JEO flight system requires several measures: a mission design that considers radiation dose while meeting JEO science objectives, a significant program to judiciously select radiation hardened parts and materials, detailed shielding mass composition design, deliberate component placement within assemblies, and systematic refinement of reliability assessment modeling of the electronics and subassemblies from the ground up. All inherited electronics would need to be redesigned to incorporate rad-hard parts. Analyses and packaging would need to be re-done. Therefore, no off-the-shelf electronics are assumed. Failure in the end would likely be due to cumulative radiation effects, so the ability to predict and optimize lifetime becomes a significant systems engineering tool. System lifetime analyses have been performed and provide the basis for projected mission duration of the JEO mission concept.

Even with extensive shielding and radiation hardened parts, total dose, transient effects such as SEUs, and noise effects would cause degraded performance and anomalous behaviors while the system survives. The operability of the system would need to be considered to mitigate these issues. The appropriate use of automated on-board and on-ground functions to isolate and manage faults, monitor and characterize changing system behavior, and keep the system running during science operations is a key operability tool to balance mission safety and success, and operations team size and cost.

The radiation shielding approach is to communally shield assemblies of similar rad-hardness. This allows grouping of similarly rad-hard assemblies together in separate enclosures as opposed to using a single vault for all assemblies, regardless of their need. Shield mass would be reduced by avoiding a heavier shield mass penalty from having to shield everything down to the “lowest common denominator” part tolerance level. This would also allow placement of electronics in strategic locations, such as the traveling wave tube amplifiers (TWTAs) on the back of the high gain antenna (HGA).

The flight system is comprised of an orbiter and a science payload. The orbiter would be a mostly redundant, 3-axis stabilized spacecraft powered by Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs). The model payload has 11 instruments, including the radio system for gravity science investigations.

The high propulsive requirements to get into Jupiter orbit and subsequently into Europa orbit drive the large propellant load required and the dry mass of the propulsion subsystem to hold the propellant. The dual-mode, bi-propellant propulsion system would use hydrazine (N2H4) fuel and nitrogen tetroxide (N2O4) oxidizer. The main engine would be 2-axis gimbaled. Small thrusters would be used to reduce post-launch separation rates, provide attitude control during cruise, execute small ∆V maneuvers, and manage reaction wheel momentum.

Attitude sensors include stellar reference units (SRU), inertial measurement units (IMU) or gyros, and multiple sun sensors, all of which would be selected based on their radiation tolerance.

Five MMRTGs would power the flight system, providing about 540 W of electrical power at end of mission (EOM) with an unregulated, nominal 28 Vdc main power bus (22–36 VDC). Redundant 12 Ah lithium-ion batteries would provide for energy storage to handle transient demands for power throughout the mission, such as during Europa Science phase when simultaneously operating science instruments and communicating with Earth. Grounding would be established for a balanced bus, with both high side and return floating from spacecraft chassis for additional fault tolerance.

Integrated power and thermal management is expected to be a significant complicating factor for operability. Waste heat from the MMRTGs would be used for thermal control to the maximum extent practical, in order to reduce electrical power that would otherwise be allocated for heaters. Radioisotope Heater Units (RHUs) and Variable RHUs would also be used for the same reason. In addition, the thermal design would use multilayer insulation (MLI), thermal surfaces, thermal conduction control, thermal louvers (both external and internal), electric heaters and

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thermo-stats/engineering sensors to thermally control the spacecraft.

The 4.2 to 6.5 AU variation in distance from Earth during the Jupiter orbital mission would require a very capable telecommunications system to return the significant data required to meet the science objectives. The flight system would use Ka-band for the highest rate science data return and X-band for high and low rate communications system during cruise, safing, critical events, and uplink commanding. One design feature for operability is the use of a 2-axis gimbal for the main downlink antenna. This mainly decouples the data acquisition activities from the data return strategies, reducing the need for higher power links, large mass memory and complex operations to manage simultaneous data acquisition and return activities

The data processing and handling architecture includes a high-speed computer, such as a RAD750 running at 200 Mhz, that would be capable of performing all science and engineering functions including limited science data compression. It would include high data rate connections (using SpaceWire for example), lower data rate interfaces (eg, 1553) and considerable redundancy and cross-strapping. Data storage would be implemented using a hybrid Solid State Recorder (SSR). A notional SSR, for instance, might contain 3 Gb of non-volatile chalcogenide random access memory (CRAM) and 16 Gb of volatile synchronous dynamic RAM (SDRAM). 1 Gb of the CRAM would be allocated for science data storage during the Europa science phase when, presumably, the SDRAM would have failed at some point due to radiation dose. All 16 Gb of SDRAM would be dedicated to science use during the Jovian Tour phase.

Flight software together with compatibly designed ground system functions is a key provider of operability for the mission. Some functions needed to improve operability that are over and above those needed for less demanding missions include: more refined on-board functions to isolate faults and anomalous behaviors and manage system response, redundancy and re-configuration; decoupling of fault management functions, to the extent practical, from on-going science and engineering activities on the flight system; reduction of complexity and workload for ground operators of the management of detailed configuration and capability of the flight system; and the parallel design and execution of individual spacecraft and instrument observations and engineering activities.

The conceptual configuration of the baseline flight system is shown in Figure 2. Major configuration drivers were:

Nadir pointing fields-of-view for remote sensing instruments at Europa

Simultaneous pointing of instruments while pointing the HGA at Earth

Large boom and radar antenna accommodation

Atlas V fairing envelope and access door size and number, accommodating 5 MMRTGs and the HGA

MMRTGs view of each other and to space with maximum distance to instruments

Eight RCS thruster clusters with placement driven by a coupling requirement and plume impingement avoidance of structure and instruments

Figure 2 – Configuration of JEO Flight System

Model Payload

The JEO model payload (Table 2) is a speculative set of instruments consistent with science objectives that has been used to quantify engineering aspects of the mission and spacecraft concept, and to analyze candidate operational scenarios to obtain the data necessary to meet the science objectives. The instruments, while notional, were defined to demonstrate a viable approach to meeting the measurement objectives, to perform in the radiation environment at Europa, and to meet planetary protection requirements. The actual JEO instrument suite would ultimately be the result of a solicitation through a NASA Announcement of Opportunity.

Table 2. Science Model Payload Instruments

Model Payload

Laser Altimeter (LA)

Radio Science (RS)

Ice Penetrating Radar (IPR)

Visible-IR Spectrometer (VIRIS)

Ultraviolet Spectrometer (UVS)

Ion and Neutral Mass Spectrometer (INMS)

Thermal Instrument (TI)

Narrow Angle Camera (NAC)

Wide and Medium Angle Camera (WAC + MAC)

Magnetometer (MAG)

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Plasma and Particles (PPI)

The model payload consists of a notional set of remote sensing instruments and in situ instruments, as well as X-band and Ka-band telecommunications functions that would provide Doppler and range data for accurate orbit reconstruction and radio science. All remote sensing instruments would be co-aligned and pointed for compatible observations and simplification of operations. Instrument articulation required for target motion compensation, limb viewing or other purposes was assumed to be implemented within the instruments. All instruments would be mounted on the nadir-facing deck of the spacecraft with the exception of the Magnetometer that would be located on a 10-m boom. The high-gain antenna would be deployed well clear of instrument fields of view and would be articulated in 2 axes to decouple instrument pointing from the radio link to Earth.

3. OPERABILITY ISSUES Several approaches were taken to develop a system concept that would be operable and provide the capability to achieve the science objectives of the mission in a short observation span. First, basic operations scenarios for the demanding Europa science phase were constructed and these included the known constraints from radiation issues, distance, spacecraft resources, orbital geometry and the needs of multiple instruments. Second, known approaches for improving system operability were applied to the system design.

The science operations scenarios were used to determine the operational methods to acquire and return the science data, size key flight system components and resources, help the science team measure the accomplishment of science goals over time, and capture key operational constraints on the mission. Simulations based on performance and behavior models of the system were used to determine how well scenarios under discussion performed.

In order to sort among potential operability improvements, a retrospective study (ref 4) was performed to assess the desirable and undesirable operability aspects of mission design, management and project organization, and flight system characteristics. Four recent analogous missions were assessed and recommendations made for JEO to consider for use in its mission concept.

Science Operations Scenarios

The development of the operations scenarios was a central part of the JEO Mission Concept Study from the start. The development was an interactive collaboration among the members of the Joint Jupiter Science Definition Team (JJSDT) and engineers from the JEO study team.

During JJSDT meetings, science objectives and instrument characteristics for the planning payload were developed. Simulations were run to determine how well scenarios under discussion performed. For example, it was discovered that instrument data rates were too high for single orbit repetitive observing strategies, but by alternating orbits for certain instruments, global imaging coverage and profile distribution would meet science goals.

A major operability challenge recognized for the Europa radiation environment is the limited amount of on-board data storage available. Mass memories of 1–2 Gb can be reasonably accommodated in the flight system design. For the purposes of the mission study, 1 Gb of mass memory was allocated to science for data collection and return. Because of this, operations constraints were developed to manage the acquisition and return of very large daily data volumes. For the Europa Science phase these constraints would include:

Downlink of all data on the orbit collected

Collection of data mainly during downlink sessions

Preclusion of mass memory allocations for data retransmission

Scheduling of continuous DSN 34 m tracking (or equivalent)

Use of Ka-band for highest link rates

These operations constraints would remove consideration for data retransmission, non-continuous DSN coverage, and prioritization and queuing of data products. On-the-fly data reduction, compression, processing, packetization, and management could still be used if included in the hardware design.

Four types of analysis and simulation were used to characterize Europa science scenarios. First, a minute-by-minute simulation for one or two orbits was constructed to model the data flows of the instruments, mass memory and telecom downlink. This was used to assess alternating orbit scenarios. Second, using SOAP (Satellite Orbit Analysis Program), the geometric performance of the imaging fields of view and ground track spacing for profiling instruments was simulated for the same scenarios used in the data flow models. Third, a simple model was used to estimate, the number of coordinated targets that could be acquired within the memory resources of the orbiter. The model was based on the strategy of using residual downlink data volume after repetitive mapping operations were scheduled. This was an average value and care was taken to evaluate how often targets could be collected. Fourth, an accounting model was used to evaluate the overall campaign-based strategy for the entire science phase. Based on campaign durations, data rate changes due to changing Europa-Earth range over time, and campaign priorities, summary data was estimated

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for the overall science performance with respect to stated goals.

Table 3 shows the potential instrument characteristics of raw data rate, data reduction factor, observation duty cycle and generated data volumes per orbit for the model payload. The example shown is for Campaign 1 at 200 km orbit altitude. Campaigns 2, 3, and 4 are similar but at 100 km orbit altitude. Some instrument rates are twice as fast at the lower altitudes because the pixel rates are faster due to

Table 3. Example Instrument Characteristics for a two orbit scenario in Campaign 1. Data rates, duty cycles, and data reduction factors are used as inputs to the data flow model. Inputs WAC MAC NAC IPR VIRIS UVS TI LA INMS PPI MAG S/C TLMRaw data rate (Mb/s) 0.27 1.40 13.5 30 0.1 0.010 0.009 0.002 0.002 0.002 0.004 0.002Mapping orbit duty cycle 40% 0.0% 0.00% 35% 35% 14% 100% 100% 50% 100% 100% 100%Data reduction rate 4 4 24 107 2.5 2 3 1 1 1 1 1Uncompressed Dvol (Mb) 907 0 0 86940 290 12 75 17 8 15.0 30.0 15.0Compressed Rate (Mb/s) 0.068 0.35 0.563 0.280 0.040 0.0050 0.0030 0.0020 0.0020 0.0020 0.0040 0.0020Total Dvol/Orbit #1 (Mb) (0.17) 226.8 0.0 0.0 0.0 117.6 6.0 24.8 16.6 8.3 16.6 33.1 16.6Total Dvol/Orbit #2 (Mb) (0.17) 0.0 0.0 0.0 824.3 0.0 0.0 24.8 16.6 8.3 16.6 33.1 16.6Total Dvol/2Orbit (Mb) 226.8 0.0 0.0 824.3 117.6 6.0 49.7 33.1 16.6 33.1 66.2 33.1

range and ground speed. This table is used as the instrument input to the data flow model for instrument characteristics.

The data flow simulation results for Campaign 1 are shown as an example in Figure 3. Similar analyses were performed for all campaigns. The red plot line shows the available accumulated downlink data volume (occultations are shown and include DSN lockup times). The green line shows the data collected as an accumulation to compare with the downlink capability. The dark blue line shows the state of the SSR at each minute. Each instrument's data collection scenario is represented in the plot and the simultaneous and accumulated impacts are characterized. The example shows accumulation in the SSR during occultations when only a few low rate instruments are operating. These scenarios show a small 10-15% depth of use on the SSR and only during occultations, leaving ample room for coordinated target data collection with either the ~400 Mb imaging type

or the 900Mb radar type on most orbits. As the Earth-Europa range decreases over the mission, data rates increase and subsequent campaigns gain data volume benefits.

An example of global coverage for the WAC in the baseline Campaign 1 is shown in Figure 4. Global color coverage could be complete in as little as 3 eurosols or about 10 days. Global stereo coverage could be achieved in another 10 days, leaving 8 days in Campaign 1 for margin. A several day delay in the start of mapping could be tolerated and still achieve the Campaign 1 science goals. Operability goals of the science planning system would allow the addition of observations to achieve other mission goals ahead of schedule.

The performance of the baseline mission is represented by measures of daily data volume for global mapping and profiling goals, and for coordinated targets and the totals for each campaign.

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Figure 3 – Example Data Flow Simulation Results. 2-orbit simulation with one sample per minute. Plot shows downlink data volume accumulation, data collection accumulation for each instrument and for all data collected (after data reduction), and SSR volume state for each minute. Orbit period, occultations, and downlink telemetry lockup durations are included.

Figure 4 – Example Global Coverage for the Wide Angle Camera (WAC). It is simulated using an orbit analysis program that uses timing from the baseline scenario.

The science scenarios for Europa orbit, validated through the above process were able to achieve the desired goal of obtaining the highest-priority observations early in the Europa Science phase. The earliest and highest priority goals, to be accomplished in the first 4 weeks, include 2 global maps, 1–2 degree global grids from the 5 profiling instruments, and more than a hundred coordinated targets of high interest sites. In a similar way, each campaign was simulated and validated to be able to obtain the desired data.

In the scenarios for the Europa science campaigns, science data collection would be continuous and repetitive with

continuous fields and particles, altimetry, thermal imaging, and infrared spectroscopy profile data collection, along with alternating orbit global imaging and radar sounding. This repetitive data collection would represent about two thirds of the daily average downlink data volume. On orbits when additional data volume is available, targeted data acquisitions would be collected. Except for the low rate instruments, all observations would be taken with Earth (and the DSN) in view, enabling rapid downlink of high rate science data while bypassing the limited SSR.

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Plans for repetitive mapping activities would be uplinked weekly. Lists of targets to be acquired via on-board targeting software would be developed and uplinked to the flight system every few days. Quick look data processing, mapping assessment, and target selection processes would all be rapid, needing about one day each. Data return would be via continuous 34 m tracking and data rates would be determined every orbit based on the conditions for DSN elevation angle and Jupiter radio (hot body) noise for that orbit. These variable data rates increase the average data volume returned by nearly 100% over traditional methods.

The scenarios are used to size the flight system resources and result in operations design constraints for the flight and ground segments of the system concept. These constraints will become, in future studies, some of the main drivers for operability concerns for the system architecture, leading to specific allocations of required performance and behavior in various systems.

Sizing the Mission and Systems Using Scenarios

The scenario analysis models were intended to allow the assessment of flight system performance parameters. The power generation by the radioisotope power system was sized based on the power use profile as shown in Figure 6. Time varying power consumption was modeled over two orbits and required system margins of 43% were applied. For times when payload power consumption rose above the power generation levels, battery use and depth of discharge was modeled. This modeling determined whether system power was adequate or insufficient for the mission scenarios. If power availability was insufficient, decisions on whether to change the power subsystem design or to constrain the payload complement, power modes, or observing scenarios could be made based on this analysis. Excess power availability was tracked for potential use in scenario trades or for potential reductions in the power system design.

Power use varies primarily by instrument modes, observing profiles, and telecom downlink profiles for TWTA power and HGA gimbal motion. With the addition of power modes for targeted data collection, the same simulation that was used for data flow analysis (see Figure 3) was used to evaluate the power profile.

Flight system mass is generally insensitive to operations scenarios (except for delta-V for orbit changes, which is analyzed separately). There is a secondary effect of data flow analysis and power modeling in that changes to desired data rate can lead to changes in HGA diameter (mass impact) or TWTA power level (potential mass impact from power system).

Recommendations for Operability Improvements

In an effort to improve operability and reduce operations costs associated with the next Outer Planets Flagship

Mission (OPFM), Jet Propulsion Laboratory (JPL) tasked the Johns Hopkins University Applied Physics Laboratory (JHU/APL) to lead a study of the Cassini mission operation cost drivers and those of other planetary space missions, including two missions currently operated at APL, MESSENGER and New Horizons, and JPL’s Mars Reconnaissance Orbiter.

The study team derived a comprehensive list of space mission operations costs drivers and through the evaluation of each mission found the following to be the top cost drivers:

a. Mission architecture: Includes mission trajectory, type, duration, number of flybys or gravity assist maneuvers.

b. Management and project organization: Considers organization structure, geographical boundaries, and organization conduct.

c. Flight system interfaces:

Systems: Includes number of flight vehicles, system redundancy, complexity of fault protection systems, number of engineering calibrations.

Guidance and control system design: sensors, actuators, control modes, pointing constraints and accuracy, momentum management scheme, number of tunable parameters, articulating mechanisms.

Command and data handling: Number of fight software modules or sub-elements, stored command management or scripting capabilities, type of data recorder, data storage margin, memory margin for commands, number of tunable parameters, data identification and tracking.

Payload: Number and type of instruments, degree of shared instrument interfaces and processing, number of instrument mechanisms.

Science operations: Includes science mission duration, science team structure, number of interfaces between instrument teams, number and density of science observations, type of observations, level of post launch science operations development, instrument data volume, data latency requirements, number of instrument and calibration and maintenance operations, data quality requirements.

The study’s most valuable end products are the numerous, tangible recommendations for reducing the cost and complexity for future space operations, including the next Outer Planet Flagship Mission. Many of these recommendations are based on successful approaches utilized on the missions under study. Application of many of those recommendations to the development and

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operations phases will permit future JEO operations to be conducted in a significantly more efficient manner.

The JEO design would make use of these recommendations to the flight system where appropriate. A few examples of flight system features intended to answer operability issues are included below.

Simplified and independent attitude pointing constraints among subsystems significantly improve spacecraft operability. The JEO Guidance and Control (G&C) system concept includes reaction wheels and coupled thrusters that greatly reduce trajectory perturbations (resulting in reduced coupling of observation pointing design and attitude control activities). On-board ephemeris-based pointing enables rapid observation planning and updates without ground based generation of detailed pointing commands. Not only are observations more rapidly planned, but modifications needed for ephemeris knowledge updates over time are inherently handled in the on-board pointing capability.

The Flight System would utilize co-aligned remote sensing instruments and independent pointing of the communications system to simplify attitude orientation planning, and to enable continuous pointing and operation of science instruments while communicating with the Earth (via 2 axis HGA gimbal). In the Tour Science phase this is important for radio science (gravity Doppler and occultations) data collection while also collecting remote sensing data. In Europa orbit, this would allow the payload to be nadir pointed continuously and monitor the local environment from a consistent attitude. The independent communications pointing also permits Earth communication during propulsive events such as major burns, trajectory control maneuvers, and reaction wheel desaturation events.

Operability must not only factor into the design choices, but into the architecting process as well, ensuring that the design is well-defined and modular to support alternate solutions throughout the development phases when difficult choices are often required to remain within cost and schedule. Potential alternatives to the 2-axis HGA solution above, for example, would not only impact the telecommunication subsystem design for power level and antenna placement but would potentially affect design assumptions for mass storage volumes, science observation and data return strategies, and ground system activities.

The solid state recorder (SSR) concept facilitates all science phases. The SSR is a hybrid design using a 1 Gb radiation hardened CRAM based partition for use in the Europa Science phase and a somewhat less radiation tolerant 16 Gb SDRAM partition for the earlier Tour Science phase when higher data volume storage is needed to meet science goals. The expanded capability of the hybrid SSR allows more data collection and extended playback in the Tour phase when it is needed, but also provides additional flexibility for all activities until it degrades in the increased radiation

environment near Europa. Additionally, standard file-based data storage and transfer protocols would be used for data management and delivery interactions with the ground system. These utilities enable automated file uplink and downlink capabilities, streamlining and simplifying operations.

The frequency of unique onboard maintenance events, such as orbit trim maneuvers, reaction wheel desaturation, and HGA antenna rewinds, are common operations cost and complexity drivers. To meet orbit maintenance needs for example, the orbiter would be expected to perform orbit trim maneuvers one to two times per week while in Europa orbit. Reaction wheel desaturations would be limited to no more than every other day during all mission phases. HGA rewinds would be needed every orbit. Selecting flexible methods for managing these activities (e.g., automation) and limiting their frequency, when possible, decreases complexity of on-board activities by making them repeatable and predictable and decreases development effort and error checking on the part of the operations teams.

Operability and consequent cost savings are increased any time resources can be allocated to or isolated between elements, reducing the need to collaborate and coordinate between these elements (thus limiting the number of iterative development cycles). This allows each element to plan independently and manage their resources. JEO would plan to use independent commanding for individual instruments and spacecraft activities, segregating data acquisition and return, and isolating science operations from the standard housekeeping activities. Operability is greatly enhanced when science and spacecraft activities can be developed in parallel and merged as late as possible prior to testing and uplink to the flight system.

Onboard autonomy would be employed for fault protection and science operations. As is typical for deep space mission operations, the use of autonomy ensures the spacecraft can handle a range of faults, ideally a broad enough set that no single fault could result in the loss of the mission. In JEO’s case, autonomy would improve operability by responding to a variety of faults and anomalous behaviors while continuing to collect science data and preserve key engineering functions, including OTMs and momentum management, to the greatest extent possible. Faults with individual instruments or specific flight systems should not disrupt science collection unless the health and safety of the entire mission is jeopardized. Autonomy would facilitate achievement of the mission objectives without an over-reliance on the ground to engage in every problem that may arise.

The MOS design and implementation is informed by the OPFM Lessons Learned and by other science operations concept studies, with recommendations incorporated where appropriate. Perhaps the most significant recommendation is to include operations personnel, and operability

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considerations from the very start of the project. Operability will be one of the principle concerns factored into the JEO architecture, and used during project and system evaluations throughout the development phases.

All system trades (spacecraft, operations, science, etc.) would be treated as mission trades to work toward best cost/risk for the overall mission (rather than optimizing an element and adding significant cost/risk to another without making the conscious trade). This begins by viewing JEO operations as those necessary to operate the spacecraft as a system and not as a collection of subsystems; it is essential to understand and manage the interactions and effects between the various flight systems rather than treating each as an isolated entity. JEO plans to utilize a model based engineering approach and state analysis tools to support its architecting, implementation, and testing from concept development through operations.

Careful consideration would also be given to the operations environment, tools, and processes. To start off, JEO anticipates a rich online collaboration system to facilitate remote planning and operations support, required due to the globally distributed science and engineering teams and to the long mission duration. Communal spacecraft analysis tools would be used by mission planners and system analysts, and these would be made available to science teams early to ensure all players are using the same tools (and versions) when planning. Post-launch efforts to incorporate new technologies, tools, and capabilities would be entertained.

JEO would incorporate specific techniques and approaches into the project’s operations concept to foster operability throughout Phase E execution. The planning process would be developed to be efficient for orbital operations, the “stress” case for the system, and modify this process as necessary for cruise and tour operations. In addition to extensive testbed activities, JEO plans to use early cruise gravity assisted flybys to test and demonstrate science and instrument interfaces and operations.

Science operations can be one of the most expensive elements of a deep space flight mission, and often becomes complex and inefficient as the “catch-all” for achieving the mission objectives, once the flight and ground systems are beyond the point they can be less easily modified (or improved). To combat this potential problem, maintain operability, and keep costs low JEO would implement a commanding architecture that allows modular and parallel activities for instrument operations, and allows non-interactive, independent development and uplink of selected activities, where possible (with automated coordination, otherwise). All instrument providers would be furnished a standard instrument ground data system interface. From the project’s start the intent is to constrain planning time, model flight constraints, allocate contentious resources (such as pointing and SSR space), and develop science observation

constructs for coordinated multi-instrument activities within the program’s available resources. A streamlined arbitration process would reduce communication delays and iterations. Science and mission planning tools enabling short (1 week, or less) planning cycles would be available early for operations process testing and would be updated throughput the mission. They could even become an effective systems engineering tool during development.

4. NEXT STEPS The mission studies from 2006 through 2008 were concerned for the most part in developing and documenting feasible mission concepts that would overcome the development challenges of a mission to Jupiter and Europa. In other words, each study created a conceptual design for the mission. Now that a mission concept has been selected to go forward, the project will be formulating an architectural framework from which the requirements and designs for an actual mission will emerge. In this architectural framework, the key concepts and properties of concern will be described for all aspects of the mission. Many of these properties will be extracted from the results of the previous mission studies. Others will be developed as the concept matures and to support the need for well formed requirements and high level designs.

The concerns of the studies for increasing the operability of the system by identifying key cost and complexity drivers, responding to science needs, and mitigating radiation issues, will be increasingly expressed as desired operability characteristics. Other related properties of concern including Survivability, Extensibility and Expandability, Scalability, Interoperability, and Adaptability, among others will be defined and expressed based in part on the concepts developed in the mission studies. These properties will emerge from the needs of various JEO stakeholders, captured in a structured architecting process that will derive not only the system requirements, but the relationships and interactions that will permit the project to make well informed trades while understanding the broad-reaching impacts of decisions made.

Operators, both for the spacecraft and the science payload, will be key stakeholders in the JEO architecture. As such, their concerns will factor into the needs and subsequent requirements that the design must satisfy. Rather than being given a system that they must learn to operate efficiently and effectively, experienced operators will be part of the design team throughout the development phases to ensure that operability concerns are given due consideration, and are factored into the tough cost and schedule choices that will inevitably be encountered.

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5. CONCLUSION The various Europa mission concept studies for the last several years, including the 2008 JEO component of the Europa Jupiter System Mission, have incorporated operability concerns to ensure that the mission can be executed with a low level of development and cost risk. The operability concerns were addressed by emphasizing the development of detailed operations concepts and science operations scenarios intended to drive out key issues and requirements, by incorporating the operations concepts and scenarios into system behavior models and validating the system design concept, and by reducing operations cost and complexity through the use of recommendations based on the hard won lessons learned from recent similar science missions.

As the mission study moves forward and becomes a project in the formulation phase, operability will continue to be a focus for its architecture development

6. ACKNOWLEDGEMENTS This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, in partnership with the Applied Physics Laboratory, Johns Hopkins University under a contract with the National Aeronautics and Space Administration. The authors would like to acknowledge the hard work completed by all of the members of the JJSDT, the NASA JEO study team, the ESA JGO study team, and the Outer Planets Flagship Mission (OPFM) Mission Operations Lessons Learned Study team.

REFERENCES [1] Clark, K. et al., “Return to Europa: Overview of the

Jupiter Europa Orbiter Mission,” 2009 IEEE Aerospace Conference Proceedings, March 14-18, 2009.

[2] “Jupiter Europa Orbiter Mission Study 2008: Final Report,” NASA Report, Task Order #NMO710851, January 2009. (http://opfm.jpl.nasa.gov/library)

[3] “Europa Jupiter System Mission – Joint Summary Report,” NASA-ESA Report, Joint Science Definition Team, January 2009. (http://opfm.jpl.nasa.gov/library)

[4] M. Holdridge, R. Lock, G. Tan-Wang, K. Hibbard, B. Jai, G. Welz, N. Pinkine, M. Shafto, D. Artis, “Outer Planets Operations Lessons Learned,” AIAA SPACE 2009 Conference, AIAA-2009-6417.

[5] R. E. Lock, R. T. Pappalardo, and K. B. Clark. “Europa Explorer operational scenarios development,” SpaceOps 2008 Conference, AIAA 2008-3307, CD-ROM, 2008.

BIOGRAPHY Rob Lock received his B.S. degree in Mechanical Engineering from Cal Poly, San Luis Obispo in 1985. After graduation, he worked as a systems engineer in the Engineering Economic Analysis Group at Martin Marietta Aerospace Corporation. He provided cost estimates to NASA’s Space Station and DOD Star Wars projects and systems engineering analysis for the Attitude and Articulation Subsystem on the Magellan project. He came to the Jet Propulsion Laboratory in 1988 where he works as a systems engineer. He was a mission planner and later the mission planning team chief for the Magellan mission and was the lead mission planner for the Mars Reconnaissance Orbiter mission during development and early operations. He has supported many advanced mission concept studies and has been a systems engineer for the JEO mission studies since 2006 and where he currently leads the systems engineering team.

Kenneth Hibbard received his B.S. degree in Aerospace Engineering from the Pennsylvania State University in 1996. Upon graduating, he worked on the Advanced Composition Explorer (ACE), SOlar and Heliospheric Observatory (SOHO), and Swift spacecraft as a spacecraft systems and operations engineer at the NASA Goddard Space Flight Center (GSFC) from 1996 until moving over to APL in 2004. At APL, Mr. Hibbard served as the MESSENGER Deputy Mission Operations Manager, and as an assistant section supervisor in the Integration and Operations Group of the APL Space Department. In 2008 he moved over to the Space Systems Applications Group where he currently serves as a systems engineer supporting multiple programs, proposals, and mission studies

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including JEO since 2008. In December 2009 Mr. Hibbard earned his M.S degree in Systems Engineering from the Johns Hopkins University in Baltimore, Maryland.

Robert Rasmussen is a Fellow of the Jet Propulsion Laboratory. He joined JPL in 1975 after receiving his Ph.D. in Electrical Engineering from Iowa State University. Since then, he has contributed to several JPL missions, including Voyager, Galileo, and Cassini, with significant roles in spacecraft system design, guidance and control, avionics and computer systems, test and flight operations, and automation and autonomy. He supervised the Subsystem Design and Integration Group in the Guidance and Control section, was lead systems engineer for Galileo attitude control, was Cognizant Engineer for the Cassini Attitude & Articulation Control Subsystem, and initiated and advised the Remote Agent autonomy experiment on Deep Space 1. In addition to his flight system experience, he has led research in fault-tolerant multicomputers, was Chief Technologist for the Information Technologies and Software Systems Division, and was Chief Architect of the Mission Data System project. He is presently Chief Engineer of JPL’s Systems and Software Division.

Karla Clark received her B.S. degree in chemical engineering from Rice University in Houston, Texas. After graduation, Ms. Clark worked at Hughes Aircraft Company developing flight batteries for communications satellites. Ms. Clark continued her education and received M.S. degrees in both mechanical engineering and engineering management from the University of Southern California. She joined JPL in 1987 where she has held numerous technical and managerial roles. Primary technical roles include system engineering efforts for the Cassini Power System & Outer Planets/Solar Probe Project. Management roles have ranged from Task Manager for Flight Battery Research; Power System Technical Manager for Cassini; Power Electronics Engineering Group Supervisor; Flight System Manager for Europa Orbiter; Spacecraft Manager and Contract Technical Manager for the Prometheus Project and her current position as study lead for JEO.

Robert Pappalardo is a Senior Research Scientist in the Planetary Ices Group, at the Jet Propulsion Laboratory. His research focuses on processes that have shaped the icy satellites of the outer solar system, especially Europa and the role of its probable subsurface ocean. In 1986 he received his B.A. in Geological Sciences from Cornell University, and in 1994 he obtained his Ph.D. in Geology from Arizona State University. As an affiliate member of the Galileo Imaging Team while a researcher at Brown University, he worked to plan many of the Galileo observations of Jupiter's icy Galilean satellites. From 2001-2006, he was an Assistant Professor of Planetary Sciences in the Astrophysical and Planetary Sciences Department of the University of Colorado at Boulder, and he continues to mentor graduate student researchers.

Melissa Jones is a senior engineer in the Planetary and Lunar Mission Concepts Group at the Jet Propulsion Laboratory. Current work includes systems engineering on the Jupiter Europa Orbiter (JEO) Outer Planets Flagship Mission, development of small Lunar lander concepts, and staffing various concept studies as a systems engineer on Team X, JPL’s mission design team. Melissa graduated from Loras College with a B.S. in Chemistry and a Ph.D. in Space and Planetary Science from the University of Arkansas.