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Credit: BBC (https://www.bbc.com/future/article/20140822-the-mission-to-an-un-loved-planet)

New Frontiers-class Orbiter: Exploring the feasibility of achieving multidisciplinary science with a mid-scale mission

Ian J. Cohen1 (240-584-7261, [email protected]), 1The Applied Physics Laboratory (JHU/APL)

Co-authors: Chloe Beddingfield2, Robert Chancia3, Gina DiBraccio4, Matthew Hedman3, Shannon MacKenzie1, Barry Mauk1, Kunio Sayanagi5, Krista Soderlund6, Elizabeth Turtle1, Elena Adams1, Caitlin Ahrens7, Shawn Brooks8, Emma Bunce9, Sebastien Charnoz10, George Clark1, Athena Coustenis11, Robert Dillman12, Soumyo Dutta12, Leigh Fletcher9, Rebecca Harbison13, Ravit Helled14, Richard Holme15, Lauren Jozwiak1, Yasumasa Kasaba16, Peter Kollmann1, Statia Luszcz-Cook17, Kathleen Mandt1, Olivier Mousis18, Alessandro Mura19, Go Murakami20, Marzia Parisi8, Abigail Rymer1, Sabine Stanley21, Katrin Stephan22, Ronald Vervack1, Michael Wong23, and Peter Wurz24

Co-signers: Tibor Balint8, Shawn Brueshaber25, Xin Cao26, Richard Cartwright2, Corey Cochrane8, Alice Cocoros1, Kate Craft1, Ingrid Daubar27, Imke de Pater23, Chuanfei Dong28, Robert Ebert29, Catherine Elder8, Carolyn Ernst1, Gianrico Filacchione19, Jonathan Fortney30, Daniel Gershman4, Jesper Gjerloev1, Matina Gkioulidou1, Athul P. Girija31, George Hospodarsky26, Caitriona Jackman32, Devanshu Jha33, Erin Leonard8, Michael Lucas34, Alice Lucchetti35, Heather Meyer1, Adam Masters36, Kimberly Moore37, Sarah Moran21, Romina Nikoukar1, Maurizio Pajola35, Chris Paranicas1, Wes Patterson1, Carol Paty38, Noemi Pinilla-Alonso39, Gangkai Poh4, Alena Probst8, Lynnae Quick4, Leonardo Regoli1, James Roberts1, Sebastien Rodriguez10, Kirby Runyon1, Todd Smith1, Linda Spilker8, Alan Stern29, Paolo Tortora40, Drew Turner1, Sasha Ukhorskiy1, Sarah Vines1, Roland Young41, and Yongliang Zhang1

2SETI Institute; 3Univ. of Idaho; 4NASA/GSFC; 5Hampton Univ.; 6Univ. of Texas at Austin; 7Univ. of Arkansas; 8JPL/Caltech; 9Univ. of Leicester; 10Paris Diderot Univ.; 11Paris Observatory, CNRS, PSL, France; 12NASA/LaRC; 13Univ. of Nebraska-Lincoln; 14Univ. of Zurich; 15Univ. of Liverpool; 16Tohoku Univ.; 17American Museum of Natural History; 18Aix Marseille Univ.;19National Institute of Astrophysics, Italy; 20JAXA; 21Johns Hopkins Univ.; 22DLR; 23Univ. of California, Berkeley; 24Univ. of Bern; 25Western Michigan Univ.; 26Univ. of Iowa;,27Brown Univ., 28Princeton Univ.; 29SwRI; 30Univ. of California, Santa Cruz; 31Purdue Univ.; 32Dublin Institute for Advanced Studies, 33MVJ College of Engineering, India, 34Univ. of Tennessee; 35INAF - Astronomical Observatory of Padova, Italy; 36Imperial College London; 37Harvard Univ.; 38Univ. of Oregon; 39Florida Space Inst., Univ. of Central Florida; 40Univ. of Bologna; 41United Arab Emirates Univ.

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1 Introduction the New Frontiers (NF) class; this of course Uranus presents a compelling case for requires that the NF target list be expanded exploring an system, offering to include Uranus. Both large-scale (Odyssey unique opportunities to answer important Pre-Decadal Mission Concept Study (PMCS)6, scientific questions. The Ice Giants, Uranus & 20107 & 20178 studies) and Discovery () , represent a distinct class of planets missions have been explored for the Ice Giants, within our solar system that has yet to be fully but, no comprehensive study has focused on explored. Characteristics of the Uranian system, determining the scope and feasibility of a NF- include: 1) a dynamically full & haphazard ring- class Uranus orbiter mission. This paper moon system; 2) five major satellites - potential summarizes one of three concepts submitted to ocean worlds with evidence of a rich geologic the 2019 PMCS opportunity9. This paper history1; 3) a complex with demonstrates the potential for a NF-class highly-tilted rotational & magnetic axes2; 4) a orbital mission to address essential scientific bulk planetary composition likely dominated by questions spanning multiple disciplines of Ice heavier “ices” (e.g., H2O, CH4, and NH3); and Giant science (plus cross-cutting heliophysics 5) a unique atmospheric circulation potentially and astrophysics opportunities); additional associated with Uranus’ weak internal heat flux. science could be achieved via of a Centaur Detailed study of Uranus by an orbiter is crucial en route to Uranus. To maximize the prospects to obtain valuable insights into the formation of of meeting launch opportunities by 2034, we our solar system and for bounding the omit scientific objectives that are only possibilities of exoplanetary enviroments3-4. achievable by an atmospheric probe, and focus For these reasons and more, the imperative instead on the excellence of the achievable and timely exploration of Uranus will not science in the broader Uranian system. only enhance our understanding of the Ice V2’s brief encounter with Uranus10 Giants but also extends to planetary origins provided a glimpse at the planet’s complexity and dynamics throughout the universe. and uniqueness, but motivated more The 2013 Planetary Science Decadal Survey5 questions than answers. Our current limited states: “The ice giants are thus one of the great understanding of Uranus is similar to that of remaining unknowns in the solar system, the other planets prior to orbital missions (e.g., after only class of planet that has never been Pioneer & Voyager to & ). explored in detail”. It also recommended Orbital missions have flown to every planet “Uranus Orbiter and Probe” as the third-highest in our solar system except for the Ice Giants. At priority “large-class” mission. Unfortunately, each planet these first orbiters revealed no Ice Giant mission will be implemented surprises that were missed by the initial flybys: before 2022, which makes it challenging for a e.g., the Enceladus geyser11-12, a major mission to make the 2030-2034 launch window discovery of Cassini, which was unnoticed by needed to take advantage of a Jupiter gravity previous flybys. As our understanding of the assist to reach Uranus before it reaches equinox Gas Giants was transformed beyond in 2050; after 2050, the northern hemispheres expectations by dedicated orbiter missions of the satellites not imaged by (V2) (e.g., , , Cassini) and will further will recede into darkness. As recognized improve with future missions (e.g., Europa internationally, the timeliness of a Uranus Clipper, JUICE), so too will our knowledge of orbiter mission is a primary motivation for Uranus expand from the necessary multi- evaluating what science can be done with a year measurements and orbital lower-cost, faster-turnaround mission within investigations.

2 2 Science Objectives Earth-based observations have revealed that Ex. 1 presents a “proto-Science Traceability Uranus hosts a system of dense, narrow rings Matrix” with a broad array of potential science that lack meaningful spacing, diverse broad and objectives and outstanding mysteries, covering finely-structured dusty rings, and the most all areas of the Uranus system (rings, satellites, tightly-packed system of small moons in the magnetosphere, , & interior). An solar system. We still do not understand how the NF-class Uranus orbiter mission is expected to Uranian rings maintain their structure. achieve many of these objectives. The Uranian ring system also boasts ten 2.1 Ring Science narrow and oddly-shaped main rings. Four of Determine the processes that sculpt & these are associated with resonances of small maintain Uranus’ ring-moon system. V2 and moons that likely play a role in shepherding

Science Objective Potential Measurement Types Outstanding Mystery Potential Observables (Relevant V&V Science Goal) A B C D E F G H I J K L • Ring particle size distribution, planet/moon tidal parameters • Ring internal structures (e.g., density/satellite wakes) Determine the processes that sculpt & • Rings' non-circular shapes & pattern speeds X X X X X X X X X maintain Uranus’ ring-moon system. (1) • Discovery of new moons & moon shapes, light-curves, & orbital elements RINGS Why is the architecture of the • Dusty ring spatial density & periodic structures Uranian ring-moon system so • Magnetic field orientation, components, & periodicities dynamically full & haphazard? • Spectral absorption in moon & ring spectra Determine the composition & origin of Uranus’ • Crater density on small moons X X X X X X X rings & small satellites. (1) • Micrometeoroid impact flux & composition • Radiation belt location & flux • Tectonic & geomorphologic structures, tidal flexing, plume activity, physical libration, Determine whether the classical Uranian thermal anomalies Do any of Uranus’ classical satellites satellites have signatures indicative of • Topography X X X X X X sustain a subsurface ocean? subsurface oceans. (6) • Spectroscopic indications of outsourcing from interior SATELLITES • Induced magnetic field & satellite tidal number/degree of compensation • Compositional mapping & associations (or lack) w/ geologic features/topographic lows Determine the surface compositions of the • Regional distributions (leading vs. trailing hemisphere) of dark material X X X classical Uranian satellites. (4) Which processes formed the • Compositional trends w/ distance from Uranus extremely dark & resurfaced terrains • Units & surface features/structures of the five classical Uranian Understand what processes formed & modify • Topography & stratigraphy satellites? the surfaces of the classical Uranian satellites. X X X X X X • Relative age of units & features (estimated from cross-cutting relations & crater density) (4 & 5) • Incident plasma & energetic particle spectra (moon-magnetosphere interactions) MAGNETOSPHERE Understand the fundamental structure and • Temporal & spatial variabilities in plasma & magnetic fields How does plasma transport work in dynamics of Uranus’ magnetosphere and the • Plasma & energetic ion composition Uranus’ unique magnetospheric X X X importance of internal vs. external drivers. • Particle energization & acceleration configuration? (1 & 3) • Times, durations & depths of satellite/ring microsignatures How does Uranus generate such an Understand what processes generate Uranus’ • Plasma & low-frequency & wave power distributions X X X X intense electron radiation belt? intense electron radiation belt. (1 & 3) • Plasma and energetic electron & ion pitch-angle distributions & energy spectra • Map of the intrinsic magnetic field, including spherical harmonic coefficients Understand the configuration & evolution of • Temporal evolution of the intrinsic magnetic field X X X Uranus' magnetic field. (1 & 3) • Low-degree (<10) odd and high degree (>10) even gravitational harmonics • Internal heat flux as a function of latitude

INTERIOR • Noble gas abundances (incl. He) – requires entry probe How is Uranus’ interior structured Determine the bulk composition & the • Bulk enrichments of C, N, and S (requires entry probe) & remote sensing above clouds below the clouds and how does it X X X X X X X distribution of materials within Uranus. (1 & 2) • Low-degree (<10) even gravitational harmonics behave? • Map & temporal evolution of the intrinsic magnetic field • Reflectivity at multiple phase angles & latitudes Understand Uranus’ global energy balance & • Thermal emission at multiple latitudes X X X X internal heat flow. (1) • Temperature/density profiles • Distribution of absorbers & temperature lapse rate in upper troposphere/stratosphere • Mapping of entire planetary “surface” Understand Uranus’ atmospheric heat • Upper atmospheric density & wave inventory X X X X X X transport mechanisms. (1 & 3) • Tracking of storms, clouds, and eddies in reflected sunlight ATMOSPHERE • Thermal profile, upward & downward radiative flux – requires entry probe

What mechanisms • Temperature & ortho/para-H2 mapping drive Uranus' large- & small-scale Understand Uranus’ zonal & meridional • Tracking of clouds X X X X X X X atmospheric dynamics? circulation patterns. (1 & 3) • 3D maps of key volatiles and tracers (e.g., CH4, H2S, NH3, H2O, CO, para-H2) • 2-cm brightness temperature • Aerosol structure mapping Determine the thermodynamics & chemistry • 3D maps of key volatiles and tracers (e.g., CH , H S, NH , H O, CO, para-H ) X X X X X X of Uranus’ clouds and hazes. (1 & 3) 4 2 3 2 2 • Abundances of hydrocarbons in upper atmosphere ExhibitPotential Measurements1. A New FrontiersKey: A: Near-class-IR imaging/spectroscopy orbiter mission; Bcould: Thermal addressIR imaging/spectroscopy many of the; outstandingC: Visible imaging/spectroscopy cross-disciplinary; D: UV imaging/spectroscopymysteries at Uranus.; E: Gravity science; F: Radio occultations; G: Magnetic field; H: Plasma; I: Energetic particles; J: Plasma/low-frequency waves; K: Dust; L: In-situ atmospheric probe

3 13 20-21 them . The mechanisms confining the other hints of NH3-hydrates and possible tholins ). ring edges and the nature of their present However, further investigation of these moons locations remain a mystery. Uranus also may provide insight to a different icy evolution features a complex system of faint dusty rings14 than Objects (KBOs), mainly due to about which little is known. They likely the limited knowledge of CO2 as a volatile ice originate from micrometeoroid bombardment at Uranus22-23, as opposed to CO on KBOs21. ejecta of the small inner moons and dense rings The widespread evidence for resurfaced terrains themselves15. Determining the rates and sources on the classical Uranian satellites, hypothesized of the dusty ring production and distribution global heating events, and the possible presence within the system will help to determine the of NH3-hydrates on their surfaces indicate that lifecycle of ring and moon material. these moons are possible ocean worlds24. Heat Thirteen small moons orbit between the main flux estimates for Miranda25-26 and Ariel27 rings and larger classical moons of Uranus16, indicate that these moons experienced heating within a tight radial range of <18,000 km. This events in the past, possibly sustaining arrangement is unstable on relatively short subsurface liquid H2O. Additionally, ground- timescales and depends on the moons’ unknown based spectroscopic observations of the Uranian masses17. Many of these moons orbit inside satellites hint at the presence of NH3-bearing 20 Uranus’ co-rotation radius. Thus, these moons’ species on their surfaces . NH3-rich material in tidal interactions with Uranus cause inward the lithosphere would dramatically lower the migration towards the Roche limit, where they interior freezing temperature (vs. pure H2O ice), may fragment into new rings or interact with assisting in the sustainability of subsurface existing rings. They may also be driven outward oceans. If oceans are present in these satellites’ through strong resonant torques if a more interiors, either globally or locally, they may massive ring develops. Determining how the have interacted, or currently interact with the ring-moon system may undergo recycling surface in the form of plumes, cryovolcanic throughout its lifetime is critical to understand flows, and/or tectonic features indicative of how planetary ring-moon systems operate under nonsynchronous rotation. myriad configurations. Understand what processes formed & modify Determine the composition & origin of the surfaces of the classical Uranian satellites. Uranus’ rings & small satellites. The rings and The geologic processes of the Uranian satellites small are dark, and their are complex, as indicated by large tectonic and compositions are unknown. Observations18 possibly cryovolcanic features imaged by V2. have revealed H2O and CO2 ice spectral These features extend well past the terminator features on Uranus’ larger moons, whereas the in the V2 images, as revealed by enhanced rings’ spectra are flat19. Limited observations of nightside “Uranus-shine” processing the small moons have not revealed if they are techniques28. exhibits three unique more akin to the larger moons or the rings. “coronae”, large polygonal shaped regions of surface deformation containing subparallel 2.2 Satellite Science ridges and troughs that are highlighted by high Determine whether the classical Uranian and low albedos. These are made up of complex satellites have signatures indicative of oceans & sets of tectonic features29-30 and may contain determine their surface compositions. Uranus cryovolcanic flows31. exhibits complex has five classical satellites (Miranda, Ariel, canyon systems thought to be a result of internal Umbriel, Titania, and Oberon). These moons processes driving tectonism32-33, while the may have common compositional surface ices formation of the large canyons on Oberon and like those of the -Charon system (i.e., Titania remain poorly understood34-35. widespread H2O ice, CH4 and other volatiles,

4 2.3 Magnetosphere Science particle acceleration, providing conditions that Understand the fundamental structure & are hard to reproduce on Earth and remain dynamics of Uranus’ magnetosphere and the inaccessible in astrophysical phenomena. importance of internal vs. external drivers. Radiation belts are defined by trapped energetic Uranus’ magnetosphere offers a unique charged particle populations and their energy configuration that provides an opportunity to density and composition are a fingerprint of a understand the drivers of magnetospheric planet’s diverse interactions with internal and dynamics throughout the solar system. The axis external drivers. Uranus’ radiation belts are of the approximately dipolar portion of its especially interesting as V2 observations did not internal magnet is offset and tilted ~59° from confirm our expectations. For the particles to the planetary rotation axis, which itself is accumulate to high intensities, the radiation inclined by 98° relative to the ecliptic (cf. these belts need to draw from a large reservoir of two axes are nearly aligned at Saturn). Such an lower energy plasma and/or lose the accelerated orientation presents an asymmetrical obstacle to particles only very slowly. Neither appeared to the impinging solar wind36. Plasma transport be the case at Uranus, which possesses a within a planetary magnetosphere may be uniquely low-density plasma magnetosphere driven by external and/or internal dynamics. and where waves result in efficient particle The former suggests convection-driven plasma losses. The mystery remains as to why Uranus’ acceleration and circulation37, whereas the latter electron belts appear so intense43 (cf. Saturn & would be subjected to centrifugal forces as the Neptune at energies ≲2 MeV) while its ion belts plasma is accelerated and energized. The show low intensities, despite sharing physical of terrestrial planets with processes44. intrinsic magnetic fields (i.e., Earth & Mercury) Uranus’ unexpected behavior may stem from are primarily driven by solar wind forcing, its unique magnetospheric configuration while the magnetospheres of Gas Giants resulting in the dominance of surprising (Jupiter & Saturn) are driven predominantly by processes. For example, whistler mode chorus internal plasma sources & fast planetary waves are thought play a role for electron rotation and a mixture of internal/external acceleration and loss, but the intensities of these processes, respectively. V2 observations waves measured at Uranus were surprisingly suggest that Uranus may be solar wind-driven38- higher than those observed at any other planet45. 39 , but it is a mystery why, unlike all other 2.4 Interior Science planets, no solar wind alpha particles were seen Understand the configuration & evolution of 40 by observed higher energies . Uranus’ fast Uranus’ magnetic field. V2 showed that the (~17 hr) rotation forces the magnetic intrinsic magnetic field of Uranus is multipolar configuration to open and close to the solar and non-axisymmetric. However, only the wind diurnally; this suggests that internal largest scales were resolved, so its detailed drivers must play a role, even though solar configuration remains unknown and its wind-driven transport can be decoupled near the temporal evolution unconstrained. Dynamo 41-42 solstices . A mission arriving within a region thickness and convective vigor are two decade of 2049 would observe a different hypotheses to explain Uranus’ unique magnetic configuration than V2, as the alignment of the field configuration46-48, testable by magnetic planet’s rotation axis changes seasonally. field measurements resolving both large- and Understand what processes generate small-scale components and its secular Uranus’ intense electron radiation belt. variation. In addition to determining the Planetary radiation belts provide an in-situ planet’s compositional and thermal structure, laboratory to study the universal process of gravitational field measurements would further

5 test generation mechanisms by establishing how convective events have been revealed by recent deep zonal flows extend into the interior49. ground-based observations57. Determine the bulk composition & the 2.5 Atmosphere Science distribution of materials within Uranus. Understand Uranus’ atmospheric heat Standard three-layer structure models of Uranus transport mechanisms. Many atmospheric infer that the planet consists of ~2 M⊕ of H-He; processes cause downward (e.g., solar although this estimate puts important limits on insolation) and upward (e.g., thermal radiation the planetary metallicity, it is not known which and cumulus convection) flux of energy. These 50 elements dominate the deep interior . processes provide local perturbations that shape Alternative structure models suggest that atmospheric features such as cloud bands and Uranus could have a density profile without vortices. The connection between local 51 discontinuities and that a large fraction of atmospheric events and the global energy water is not needed fit the observed properties. balance remains an outstanding mystery. It is of particular importance to determine the Because the molecular weight of condensable global ice-to-rock ratio, which can also be used species is heavier than the background H-He to address Uranus’ formation – a long-standing atmospheric mixture, moist convection is problem for planet formation theory52. Beyond generally inhibited and tends to happen in the ice-to-rock ratio, the abundances of key episodic bursts58. Given this time-variability, a species such as He tell us about the environment new mission may find that local episodic in which Uranus formed, and bulk enrichments convection leads to a higher global heat flux. of C, N, and S provide additional information In the middle and upper atmosphere, our on the planet formation process. However, ignorance of heat transport processes is ground-based attempts to constrain the bulk symptomized as the “energy crisis”: V2 stellar composition from measurements of occultations revealed that Uranus’ atmospheric disequilibrium species (e.g., CO) thermosphere is hot59. Although this “crisis” 53 have been inconclusive ; and Uranus’ ice-to- exists at all four giant planets, it is particularly rock ratio remains loosely constrained50-51. surprising for Uranus because of its large axial Understand Uranus’ global energy balance tilt; given that the thermosphere is hot in both & internal heat flow. Uranus is the only Giant summer and winter hemispheres, solar heating Planet that is in approximate equilibrium with cannot be the cause60. The vertical temperature 54 solar insolation . However, a more precise gradient may point to the nature of the unknown energy balance determination is necessary heating61, but V2 data cannot distinguish because large uncertainties exist in the V2 bond between candidate heating mechanisms. albedo and thermal emission measurements. Understand Uranus’ zonal & meridional This balance presently suggests that Uranus’ circulation patterns. These circulations are interior may not be fully convective and/or critical for understanding vertical heat transport contains compositional gradients that hinder and energy balance, as well as producing a 55 convection , although atmospheric phenomena coherent model of atmospheric dynamics. 56 may also be responsible . If convective Uranus’ zonal wind profile – with retrograde inhibition is at play, then Uranus’ internal heat winds at the equator and a single prograde peak flux may vary seasonally, and given that recent in each hemisphere – starkly contrasts with ground-based observations reveal many Jupiter and Saturn. Uranus’ winds also exhibit a episodic convective events, an orbiter mission surprising hemispheric asymmetry near the arriving during an active period may measure a poles57, which may be seasonally driven. higher heat flux, especially since episodic Whereas the cloud bands of Jupiter and Saturn are loosely associated with the zonal jets due to

6 eastward jet peaks acting as transport barriers, 3 NF-class Design Considerations Uranian cloud bands are seemingly not tied to Maintaining balance across the research the smooth wind structure and associated disciplines described above should be a high temperature gradients, which may hint at priority for a NF Uranus orbiter. Based on unresolved peaks in the zonal wind structure61. work from previous studies, a NF-class orbiter Uranus’ overturning meridional circulation mission should be feasible. For example, the structure remains unknown. Methane gas costs in the 2010 study7 suggest ~$1.1B depletion around the poles suggests that Uranus (FY15$) for Phases A-D for an orbiter mission has a single deep circulation cell in each with a flagship-class payload without an hemisphere, in which air rises from the deep atmospheric probe (assuming 30% reserves) atmosphere at low latitudes, clouds condense without the launch vehicle costs. Appropriately out, and dry air is transported to high latitudes scoping the payload to accommodate NF-class where it descends63. However, such a pattern is science would reduce both the payload and inconsistent with observed cloud distributions, spacecraft costs. From a mission design implying that the meridional circulation must be standpoint, the potential use of a solar electric more complex. High-resolution measurements propulsion (SEP) stage with a cruise of ~14 of the wind field may reveal coupling between years could reduce the spacecraft’s chemical zonal/meridional circulation via eddies. propulsion burden, while still leaving enough Determine the thermodynamics & chemistry Radioisotope Power System (RPS) lifetime for of Uranus’ clouds and hazes. During the V2 the baseline mission, to be feasible within the flyby, Uranus appeared almost featureless. The NF cost. A NF Uranus orbiter could be subsequent presence of unexpected bright undertaken with current technologies, given storms63 has revealed that Uranus has an active, appropriate trades in design and scope. temporally dynamic, and poorly understood Power is perhaps the most limiting constraint weather layer. Clouds and hazes occur on a Uranus orbiter mission and addressing preferentially at specific latitudes, and the power within cost is the primary obstacle to the banding pattern of tropospheric hazes is feasibility of a NF Uranus orbiter mission. apparently not tied to the zonal wind structure. Previous Ice Giant mission studies7-8 have Vertically, clouds and tropospheric hazes are resulted in architectures requiring >350 W-e not found at the altitudes predicted by end-of-life power, which required three or more thermochemical equilibrium models64; in fact, now-cancelled Enhanced Multi-Mission the compositions of Uranus’ upper cloud layers Radioisotope Thermoelectric Generator remain unclear61. The thermodynamics and (eMMRTGs). Owing to the relative inefficiency chemistry of the clouds have far-reaching and significant cost of current RPSs, any design implications for connecting the atmosphere to should attempt to reduce the needed end-of-life the planet’s bulk composition, and for power; this will have significant impact on both understanding the global energy balance. the spacecraft and orbit design as well as the V2 and Spitzer measurements65 revealed communication subsystem and payload. Hence, hydrocarbon densities much lower in Uranus’ accelerating the development and expanding stratosphere than at any other giant planet due the efficiency and lifetime (and potentially to the stark differences in atmospheric mixing. reducing the cost) of next-generation RPSs Hydrocarbons are the main source of would significantly enhance the mission. For photochemical haze, act as EUV continuum example, the recent Neptune Odyssey mission absorbers, and trace vertical transport. concept uses three next-generation RPSs6, suggesting that a NF-class Uranus mission

could close with fewer. This of course assumes

7 that a sufficient supply of plutonium is available 20 Cartwright+(2018), doi:10.1016/j.icarus.2018.06.004 for future missions. With 21 Grundy+(2016), doi:10.1126/science.aad9189 current technology (i.e., 14-year MMRTG flight 22 Cartwright+(2015), doi:10.1016/j.icarus.2015.05.020 design life), a typical baseline would be a <12- 23 Sori+(2017), doi:10.1016/j.icarus.2017.02.029 24 Hendrix+(2019), doi:10.1089/asr.2018.1955 year cruise (potentially with a Centuar flyby) 25 Beddingfield+(2015), doi:10.1016/j.icarus.2014.09.048 and a 2-year mission at Uranus with a system 26 Ruiz, J.(2005), doi:10.1016/j.icarus.2005.03.021 tour that enables surface mapping of the large 27 Peterson+(2015), doi:10.1016/j.icarus.2014.11.007 satellites as well as spatial coverage of the 28 Stryke&Stooke (2008), 2008 LPSC, abstract #1362 planet & rings/small moons; this baseline could 29 Schenk(1991), doi:10.1029/90JB01604 be significantly lengthened if the lifetime of 30 Pappalardo+(1997), doi:10.1029/97JE00802 future RPSs were improved. 31 Jankowski&Squyres(1988), doi:10.1126/science.241.4871.1322 Another significant driver is determining the 32 Peterson+(2015), doi:10.1016/j.icarus.2014.11.007 total mass that can be put into Uranus orbit 33 Croft&Soderblom (1991), in Uranus, pp. 561–628 34 within the NF cost cap. Because significant McKinnon 1988), doi:10.1038/333701a0 35 Janes&Melosh(1988), doi:10.1029/JB093iB04p03127 propellant mass is needed for Uranus orbit 36 Cao&Paty (2017), doi:10.1002/2017JA024063 insertion, mass efficiency is critical. A realistic 37 Dungey(1961), doi:10.1103/PhysRevLett.6.47 ~60-kg payload (e.g., 5 instruments & radio 38 Mauk+(1987), doi:10.1029/JA092iA13p15283 science) using current technologies, would 39 DiBraccio&Gershman(2019), doi:10.1029/2019GL083909 provide closure to numerous mysteries 40 Krimigis+(1986), doi:10.1126/science.233.4759.97 summarized in Ex. 1. However, cost reduction 41 Selesnick&Richardson (1986), doi:10.1029/GL013i007p00624 and increases in capability & availability of 42 Vasyliuñas(1986), doi:10.1029/GL013i007p00621 launch vehicles (e.g., SLS) could be enabling 43 Mauk&Fox(2010), doi:10.1029/2010JA015660 44 Mauk(2014), doi:10.1002/2014JA020392 for a NF-class Uranus orbiter mission 45 Kurth&Gurnett (1991), doi:10.1029/91JA01819 launching outside of windows with Jupiter 46 Holme&Bloxham (1996), doi:10.1029/95JE03437 gravity assists. Lastly, the risk-vs.-benefit of 47 Stanley&Bloxham(2006), doi:10.1016/j.icarus.2006.05.005 using aerocapture for orbit insertion should be 48 Soderlund+(2013), doi:10.1016/j.icarus.2013.02.014 analyzed as it can greatly increase the delivered 49 Kaspi+(2013), doi:10.1038/nature12131 payload66-67 and shorten flight times68. 50 Nettelmann+(2013), doi:10.1016/j.pss.2012.06.019 References 51 Helled+(2011), doi:10.1088/0004-637X/726/1/15 1 Ahrens+(2020), PS Decadal Survey White Paper 52 Helled&Bodenheimer(2014), doi:10.1088/0004-637X/789/1/69 2 Kollmann+(2020), PS Decadal Survey White Paper 53 Cavalie+(2014), doi:10.1051/0004-6361/201322297 3 Batalha+(2011), doi:10.1088/0067-0049/204/2/24 54 Pearl+(1990), doi:10.1016/0019-1035(90)90155-3 4 Rymer+(2018), arXiv:1804.03573 55 Nettelmann+(2016), doi:10.1016/j.icarus.2016.04.008 5 Nat’l Res. Council(2011), doi:10.17226/13117 56 Gierasch&Conrath(1987), doi:10.1029/JA092iA13p15019 6 Rymer+(2020), Neptune- Odyssey PMCS Report 57 Sromovsky+(2015), doi:10.1016/j.icarus.2015.05.029 7 Hubbard(2010), Mission Study Report for 2013 PS Decadal 58 Li+(2018), doi:10.1175/JAS-D-17-0257.1 8 Hofstadter+(2017), Ice Giants Mission Study Report 59 Broadfoot+(1986), doi:10.1126/science.233.4759.74 9 Leonard+(2020), PS Decadal Survey White Paper 60 Stevens+(1993), doi:10.1006/icar.1993.1005. 10 Stone&Miner(1986), doi:10.1126/science.233.4759.39 61 Sicardy+(1985), doi:10.1016/0019-1035(85)90040-5. 11 Dougherty+(2006), doi:10.1126/science.1120985 62 Fletcher+(2020), doi:10.1007/s11214-020-00646-1 12 Porco+(2006), doi:10.1126/science.1123013 63 de Pater+(2015), doi:10.1016/j.icarus.2014.12.037 13 Porco&Goldreich(1987), doi:10.1086/114354 64 de Pater+(1991), doi:10.1016/0019-1035(91)90020-T 14 de Pater+(2006), doi:10.1126/science.1125110 65 Bishop+(1990), doi:10.1016/0019-1035(90)90094-P 15 Esposito&Colwell(1989), doi:10.1038/339605a0 66 Spilker+(2016), NASA PSD Report (JPL D-97058) 16 Showalter&Lissauer(2006), doi:10.1126/science.1122882 67 Hall+(2005), doi:10.2514/1.4118 17 French+(2015), doi:10.1088/0004-6256/149/4/142. 68 Girija+(2020), doi:10.31224/osf.io/bxvgz 18 Grundy+(2006), doi:10.1016/j.icarus.2006.04.016 19 de Kleer+(2013), doi:10.1016/j.icarus.2013.07.016

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