Download PDF Fall Issue of The Bridge on Space Exploration September 1, 2021 Volume 51 Issue 3 Close collaboration between engineering and science has enabled marvels of space exploration over decades. Eight exemplary missions are described in this issue, conveying the excitement, challenges, and breakthroughs involved in efforts to better understand the wonders and mysteries of this solar system. The JUICE Mission: Challenges and Expectations Tuesday, September 14, 2021 Author: Athena Coustenis, Olivier Witasse, and Christian Erd Engineering supports the science objectives of JUICE: to investigate Jupiter, its magnetosphere, the icy moons, and their interrelations in all their complexity. The first large mission in the European Space Agency’s Cosmic Vision 2015–2025 program is the Jupiter Icy Moons Explorer (JUICE), which will study the largest planet in this solar system, with emphasis on three of the Galilean[1] icy moons—Ganymede, Callisto, and Europa—to determine whether they might be habitable environments. Building on previous investigations by space missions and ground-based observatories, JUICE is expected to revolutionize understanding of the Jovian system. Historical Context Much was revealed about the Jupiter system by NASA’s Galileo mission from 1995 to 2003. Among other things, Galileo found strong evidence for the existence of subsurface oceans beneath the icy crusts of Europa and Ganymede, as well as perhaps in the partly differentiated interior of Callisto, leading to a new paradigm that considers the icy satellites as potentially habitable. It also discovered an internal magnetic field at Ganymede. It soon became clear that the icy moons around the giant planet harbored many mysteries that begged for further exploration. In 2008 NASA and the European Space Agency (ESA) began jointly exploring the possibility of a large mission to study the satellite systems of the giant planets. Two mission concepts were considered, from which one would be selected: the Europa Jupiter System Mission (EJSM) and the Titan Saturn System Mission (TSSM; ESA 2009). They resulted, respectively, from the ESA “Laplace” mission to Jupiter proposal (Blanc et al. 2009) and a joining of the ESA Titan and Enceladus mission (TandEM; Coustenis et al. 2009) and from NASA’s Titan Explorer concepts. The EJSM mission was selected. The reference mission architecture consisted of two independent flight elements: the Jupiter Ganymede Orbiter (JGO), to be developed, launched, and operated by ESA; and the Jupiter Europa Orbiter (JEO), to be developed, launched, and operated by NASA. The two spacecraft had complementary trajectories and instruments to perform synergistic observations. Eventually the JEO was abandoned by NASA because it was evaluated well beyond its expected cost envelope. ESA reformulated its element of EJSM to become JUICE (figure 1) and conducted further studies in preparation for its implementation in 2012 as the agency’s first large mission in the Cosmic Vision program, along with a science payload selection the following year. In 2015 NASA announced the selection of the Europa Clipper mission concept (Pappalardo et al. 2013) and entered a formulation phase to be implemented by JPL with a launch in late 2024 and arrival expected in spring 2030,[2] at about the same time as JUICE. Based on studies in previous phases that demonstrated that the mission was feasible (Grasset et al. 2013a), in 2014 JUICE was adopted by the ESA Space Program Committee to proceed in development. This was followed with an invitation to tender, and an industrial prime contractor (Airbus) was selected with a kickoff of industrial implementation in 2015. Over the next 5 years, Ariane 5 was chosen as the launcher, scientific instruments were developed and delivered, and spacecraft verification tests started. JUICE is planned to launch from the ESA spaceport Kourou in French Guiana, South America, in the August–September 2022 period for arrival at Jupiter in mid-2031 after five planetary gravity assist flybys. Mission Architecture and Operations JUICE is a 3-axis stabilized spacecraft carrying ten instruments (described below). It will use Venus and Earth gravity assists in its nearly 9-year cruise to Jupiter (table 1). After the orbit insertion in July 2031 the spacecraft will perform a 3½-year tour in the Jovian system, making continuous observations of Jupiter’s atmosphere and magnetosphere. Gravity assists from Callisto and Ganymede will shape the trajectory, while providing opportunities for close science observations of the planet’s moons. Two planned Europa flybys will target regions of interest where the ice may be thin and allow not only examination of the surface composition but also subsurface sounding of the moon. Additionally, gravity assists at Callisto will be used to raise the orbit inclination up to 33° above Jupiter’s equatorial plane, where all the Galilean moons are, and will allow for observations of the planet’s polar regions. The main events of the trajectory are listed in table 1. The mission will culminate in a dedicated 9-month orbital tour around Ganymede (December 2034–September 2035) during which JUICE will perform detailed investigations of this moon and its environment. The orbits will include an elliptical phase, then a circular orbit at 5000 km altitude, followed by a second elliptical phase. A final maneuver will put the spacecraft into a circular orbit at 500 km altitude. Mission extension at the final or lower orbit (target 200 km) may be feasible, depending on available propellant, the state of the solar array, and the spacecraft’s overall performance. Once the mission ends, JUICE will eventually impact the surface of Ganymede in an uncontrolled way. Scientific Goals of the Mission The science objectives for JUICE (table 2) span a variety of disciplines—from geology to astrobiology and magnetospheric/plasma and atmospheric science—and call for a large number of measurements. The high-level goals are to investigate the gas giant, its magnetosphere, the icy moons, and their interrelations in all their complexity. The mission will seek to characterize the conditions that may have led to the emergence of habitable environments around gas giants and in particular on three Galilean satellites—Ganymede, Europa, and Callisto—that are expected to harbor liquid water oceans beneath their surfaces. Ganymede and Europa are believed to be internally active, due to a strong tidal interaction with Jupiter and to other energy sources present. A particular emphasis is on Ganymede, the largest natural satellite in the solar system, as a planetary body and potential habitat. It provides a natural laboratory for the investigation of a possible habitable world, exhibits unique magnetic and plasma interactions with the surrounding environment, and plays a role in the Laplace resonance with Io and Europa, permitting tidal heating (Showman and Malhotra 1997).[3] The instruments on board JUICE will enable investigation of the formation, evolution, and chemical composition of the planet’s and icy moons’ surfaces and subsurface oceans, and of the processes that have affected the satellites and their environments through time. Study of the subsurface oceans will enhance understanding of their chemical composition and the possible sources and cycling of chemical and thermal energy. The mission will also characterize the diversity of processes in the Jovian system that may provide a stable environment on the icy moons at geologic time scales, including gravitational coupling between the Galilean moons and their long-term tidal influence on the system as a whole. This study will be supplemented by information about Io and the smaller moons, acquired through remote sensing. The tidal response of the satellites’ icy shells strongly depends on the properties of the shell and the ocean thickness. The Galilean moons Io, Ganymede, and Europa are locked in a stable 1:1 spin-orbit resonance, but slight periodic variations in the rotation rate (physical librations) and the amplitudes associated with these librations may provide further evidence for subsurface oceans and their characteristics. JUICE will measure precisely the rotation rate, pole position, obliquity, and libration amplitude of Ganymede. This will define the dynamical history of the satellite (e.g., effects such as despinning, resonance capture, nonsynchronous rotation of the icy shell) besides yielding information about the subsurface ocean and deeper interior. The Science Instruments The JUICE spacecraft will carry 10 state-of-the-art remote sensing, geophysical, and other instruments engineered to the challenge of performing measurements in Jupiter’s intense radiation environment (table 3, figure 2). It will also include an experiment that uses the spacecraft telecommunication system with ground-based instruments. The remote sensing suite includes imaging (JANUS) and spectral-imaging capabilities from ultraviolet to submillimeter wavelengths (MAJIS, UVS, SWI). A geophysical package consists of a laser altimeter (GALA) and radar sounder (RIME) for exploring the surface and subsurface of the moons, and a radio science experiment (3GM) to probe the atmospheres of Jupiter and its satellites and to measure the gravity fields. The PRIDE experiment will use ground-based very-long-baseline interferometry to precisely determine the spacecraft position and velocity data for complementary gravity science. Particle and field investigations will be performed by the particle environment package (PEP), a magnetometer (J-MAG), and a radio and plasma wave instrument (RPWI), including electric field and magnetic field sensors and a Langmuir probe. This suite of instruments is provided by multinational teams led by institutes in France, Germany, Italy, the Netherlands, Sweden, the United Kingdom, and the United States, illustrating the large international collaboration around the project. Engineering Challenges JUICE will build on scientific and technological knowledge from previous ESA and NASA missions engineered for harsh planetary environments. Aside from the programmatic challenges to maintain cost and schedule, the most challenging engineering problems are the solar array performance in a cold and intense radiation environment, protection of electronics against Jupiter’s harsh radiation environment, and electromagnetic cleanliness on the spacecraft so as to not disturb the sensitive measurements to be obtained. The 85 m2 solar arrays consist of two wings each with five solar panels, protected from the high-radiation environment by a 150 µm cover glass. Following extensive tests and careful selection, power production relies on a specific solar cell modified from telecommunication technology (triple junction GaAs cells) tuned for use in the low-intensity, low-temperature application. JUICE is designed for Jupiter’s radiation environment, which is dominated by high-energy electrons in the magnetosphere; about 50 percent of the radiation dose will result from the Ganymede orbital phase and 20 percent from the two Europa flybys. Sensitive electronics are installed in two protective vaults in the spacecraft, and significant shielding (>200 kg) will limit the radiation effects, with spot shielding for less tolerant components. The strong scientific interest in icy moons in the outer solar system, based on recent findings by the Cassini mission on Saturn’s satellites and on Europa from the Hubble Space Telescope for instance, led to a series of studies (e.g., Raulin et al. 2019) that resulted in recommendations inviting a revision of the planetary protection requirements for missions to Europa and Enceladus, given their strong habitability potential. These were validated by and implemented in an updated policy of the Committee on Space Research (COSPAR) Panel on Planetary Protection.[4] Ganymede is a Category II object in the COSPAR Planetary Protection Policy: a target body for which there is significant interest in the chemical evolution and origin of life, for all types of missions (gravity assist, orbiter, lander), and only a remote chance that contamination carried by a spacecraft could compromise future investigations (Grasset et al. 2013b). Europa (and Enceladus) are now classified in Category III for orbiters and Category IV for landers: they are protected target bodies of interest for chemical evolution and/or the origin of life, but scientific studies indicate a significant chance of contamination that could compromise future investigations. It is therefore strongly recommended to avoid any interference with these bodies, which is the reason for ending the Europa Clipper mission on Ganymede rather than risking an encounter with Europa (similar to what was done with Cassini, when it was sent into Saturn instead of remaining in the vicinity of the Kronian icy moons). On the operation side, the mission controllers will need to perform a complex satellite navigation in both the inner solar system and the Jupiter system. The cruise phase includes four flybys of Earth and one of Venus (table 1). The first Earth flyby will see the spacecraft skimming over our Moon at only 300 km distance. The Jupiter system tour (a first for an ESA mission) comprises two orbit insertions and a moon flyby nearly every month, with the Europa flybys spaced by about 2 weeks. The design of the trajectory required a lot of effort and iterations to optimize propellant use, reduce radiation dose, limit solar eclipse durations, and maximize the science return by optimizing the operation of the ten instruments. These were all governed by limitations in power and data volume, possibilities for accurate pointing, scientific priorities depending on the mission phase, and navigation constraints. Exciting Expected Outcomes from Multiple Missions The complex system around Jupiter is the target of several space missions and ground-based observations, including NASA’s Juno mission (begun in 2016), JUICE, and Europa Clipper. Juno is expected to continue its investigation of the solar system’s largest planet through September 2025 (or until the spacecraft’s end of life), exploring the full Jovian system—Jupiter and its rings and moons—with multiple rendezvous performed or planned for Ganymede, Europa, and Io. Juno’s observations—mainly from far away, but also from a limited number of flybys, such as the one of Ganymede in early June 2021, which returned some high-resolution close-up images—will be very helpful for the detailed planning of observations by JUICE and Europa Clipper in a number of areas related to the Jovian moons: identification of possible surface changes since the Galileo flybys, properties of the surface material and composition, and latest information on the radiation and space environment. In particular, Juno will provide insights into Ganymede’s composition, ionosphere, magnetosphere, and ice shell. The detailed JUICE characterization of the wider Jovian system will build on Juno’s focused study of Jupiter’s interior and inner magnetosphere. ESA is looking forward to contributing to this multimission exploration using proven engineering, science, and mission operation approaches demonstrated by Cassini and Galileo at Jupiter in 2000, Cluster in the Earth’s magnetosphere, Mars Express, and the Trace Gas Orbiter around Mars that combined observations of the same object by two or more space missions. This extended multimission approach is extremely useful for magnetospheric studies because it makes it possible to disentangle spatial from temporal variations, a necessity in plasma physics. JUICE and Europa Clipper will provide ground-breaking information, through their monitoring of the Jovian system elements, about the planet’s atmosphere, magnetosphere, and plasma environment and about the exospheres of the icy moons. Observations from the two planned Europa flybys by JUICE, combined with the more extensive Europa Clipper investigations, will enhance confidence in the findings and yield breakthrough observations about habitability conditions of the satellite as well as moon-Jupiter interactions via Jupiter’s magnetic field lines. Having two spacecraft at the same time around Jupiter will also improve the data on the ephemerides, which describe the position of the Galilean moons, increasing knowledge of the dynamics, resonances, and tidal heating in the Jovian system. Finally, unique data—for instance, from Europa Clipper’s SUrface Dust Analyzer (SUDA) instruments, on dust particles, and based on differences in spectral and spatial resolution provided by the two missions—will significantly enhance scientific understanding of the Jovian system. These advances will be possible thanks to the operations of three missions in the same decade and at times simultaneously. Conclusion JUICE’s extensive new studies of Jupiter’s atmosphere and magnetosphere and their interaction with the satellites will enhance understanding of the evolution and dynamics of the Jovian system. Following in the footsteps of the Cassini-Huygens mission, JUICE is an especially exciting mission because it invites international collaboration of a large number of people interested in a variety of important science themes. JUICE is the necessary step for future exploration of the outer solar system, and it is on track for launch in 2022. JUICE is the necessary step for future exploration of the outer solar system, and it is on track for launch in 2022. In the longer term, ESA’s future planetary program will build on the strengths and capabilities that have allowed it to achieve breakthroughs in understanding of all bodies in the solar system—from the Sun (Solar Orbiter) to Mercury (BepiColombo), Venus (Venus Express and EnVision), Mars (Mars Express, ExoMars Trace Gas Orbiter, and Rosalind Franklin rover), the Saturnian system (Cassini-Huygens), and comets (Rosetta/Philae). ESA exploration is on the verge of a new era with the Voyage 2050 program,[5] which will include a thorough investigation of the habitability potential of one of the icy moons in the outer solar system, building on the findings of the Cassini-Huygens mission to Saturn and JUICE (ESA 2021). Although the specific targets are not yet identified, the mission will involve significant international collaboration and use advanced instrumentation to characterize the association between ocean-bearing moon interiors with their near-surface environments and to search for possible biosignatures. The mission profile might include an in situ element, such as a lander or a drone, as in the case of NASA’s Dragonfly mission to Titan (Lorenz et al. 2021). Other possible investigations of the outer solar system might aim for longer monitoring of plasma processes (with possibly two or more probes at the same time), characterization of the minor moons, and more thorough investigation of Jupiter’s inner magnetic field in the equatorial region. Such goals would require long-lived orbiters in addition to landers on Europa or Ganymede, for instance (current studies are examining such in situ elements[6]; see also NASA 2017), as well as the development of new technologies to enable landing on potentially rough terrain, radiation tolerance, drilling capabilities deep into the ice crusts, delivery of probes into subsurface oceans, power generation, and communications improvements. All of these missions and international collaborations have built technological expertise and talent that enables the European Space Agency to undertake challenging future projects in the solar system and beyond. Weblinks of Interest https://sci.esa.int/web/juice https://sci.esa.int/web/juice/-/59342-juice-mission-poster https://cosmos.esa.int/web/juice https://www.youtube.com/channel/UClK7xrwF0-XVl5IsG9 SFKEA https://www.flickr.com/photos/europeanspaceagency/albums/ 72157714008474857 https://twitter.com/esa_juice www.esa.int/Science_Exploration/Space_Science/ESA_s_Cosmic_ Vision https://cosparhq.cnes.fr/scientific-structure/panels/panel- on-planetary-protection-ppp/ References Blanc M, Pappalardo R, Fujimoto M, Sasaki S, Zelenyi L, Alibert Y, André N, Atreya S, Beebe R, Benz W, and 32 others. 2009. Laplace: A mission to Europa and the Jupiter system for ESA’s Cosmic Vision programme. Experimental Astronomy 23:849–92. Coustenis A, Atreya S, Balint T, Brown RH, Dougherty M, Ferri F, Fulchignoni M, Gautier D, Gowen R, Griffith C, and 145 others. 2009. TandEM: Titan and Enceladus mission. Experimental Astronomy 23:893–946. ESA [European Space Agency]. 2009. TSSM In Situ Elements: ESA Contribution to the Titan Saturn System Mission. ESA/SRE(2008)4. Paris. ESA. 2014. JUICE: JUpiter ICy moons Explorer—Exploring the Emergence of Habitable Worlds around Gas Giants. ESA/SRE(2014)1. Paris. ESA. 2021. Voyage 2050 sets sail: ESA chooses future science mission themes. Jun 11. Paris. Grasset O, Dougherty MK, Coustenis A, Bunce EJ, Erd C, Titov D, Blanc M, Coates A, Drossart P, Fletcher L, and 8 others. 2013a. JUpiter ICy moons Explorer (JUICE): An ESA mission to orbit Ganymede and to characterise the Jupiter system. Planetary and Space Science 78:1–21. Grasset O, Bunce E, Coustenis A, Dougherty M, Erd C, Hussmann H, Jaumann R, Prieto-Ballesteros O. 2013b. Planetary protection requirements at Ganymede. Astrobiology 13(10):991–1004. Lorenz RD, MacKenzie SM, Neish CD, Le Gall A, Turtle EP, Barnes JW, Trainer MG, Werynski A, Hedgepeth J, Karkoschka E. 2021. Selection and characteristics of the Dragonfly landing site near Selk Crater, Titan. Planetary Science Journal 2(1):24. NASA [National Aeronautics and Space Administration]. 2017. Report of the Europa Lander Science Definition Team (JPL D-97667). Washington. Pappalardo RT, Vance S, Bagenal F, Bills BG, Blaney DL, Blankenship DD, Brinckerhoff WB, Connerney JEP, Hand KP, Hoehler TM, and 12 others. 2013. Science potential from a Europa lander. Astrobiology 13(8):740–73. Raulin F, Coustenis A, Kminek G, Hedman N, eds. 2019. Planetary protection: New aspects of policy and requirements. Life Sciences in Space Research 23. Showman AP, Malhotra R. 1997. Tidal evolution into the Laplace resonance and the resurfacing of Ganymede. Icarus 127(1):93–111. [1] In 1610 Galileo was the first to observe the four largest of Jupiter’s moons, Callisto, Europa, Ganymede, and Io. [2] https://www.jpl.nasa.gov/missions/europa-clipper [3] https://en.wikipedia.org/wiki/Orbital_resonance#Laplace_ resonance [4] https://cosparhq.cnes.fr/assets/uploads/2021/07/PPPolicy_ 2021_ 3-June.pdf [5] https://www.esa.int/Science_Exploration/Space_Science/ Voyage_2050_sets_sail_ESA_chooses_future_science_ mission_themes [6] The National Academies of Sciences, Engineering, and Medicine are conducting a Planetary Science and Astrobiology Decadal Survey 2023–2032 (https://www.nationalacademies.org/our-work/planetary- science-and-astrobiology-decadal-survey-2023-2032) . The study committee members include this article’s lead author as well as this issue’s guest editor, Steven Battel, and fellow contributor John Grunsfeld. About the Author:Athena Coustenis is director of research of the French Centre National de la Recherche Scientifique (CNRS) at the Laboratoire d’études spatiales et d’instrumentation en astrophysique (LESIA) of the Paris Observatory. Olivier Witasse is the JUICE project scientist and Christian Erd is the JUICE system engineering manager, both at the European Space Agency’s Science Directorate at the European Space Research and Technology Centre (ESTEC) in the Netherlands.