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

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


Introduction
Uranus presents a compelling case for exploring an Ice Giant system, offering unique opportunities to answer important scientific questions. The Ice Giants, Uranus & Neptune, represent a distinct class of planets within our solar system that has yet to be fully explored. Characteristics of the Uranian system, include: 1) a dynamically full & haphazard ringmoon system; 2) five major satellites -potential ocean worlds with evidence of a rich geologic history 1 ; 3) a complex magnetosphere with highly-tilted rotational & magnetic axes 2 ; 4) a bulk planetary composition likely dominated by heavier "ices" (e.g., H2O, CH4, and NH3); and 5) a unique atmospheric circulation potentially associated with Uranus' weak internal heat flux. Detailed study of Uranus by an orbiter is crucial to obtain valuable insights into the formation of our solar system and for bounding the possibilities of exoplanetary enviroments [3][4] . For these reasons and more, the imperative and timely exploration of Uranus will not only enhance our understanding of the Ice Giants but also extends to planetary origins and dynamics throughout the universe.
The 2013 Planetary Science Decadal Survey 5 states: "The ice giants are thus one of the great remaining unknowns in the solar system, the only class of planet that has never been explored in detail". It also recommended "Uranus Orbiter and Probe" as the third-highest priority "large-class" mission. Unfortunately, no Ice Giant mission will be implemented before 2022, which makes it challenging for a mission to make the 2030-2034 launch window needed to take advantage of a Jupiter gravity assist to reach Uranus before it reaches equinox in 2050; after 2050, the northern hemispheres of the satellites not imaged by Voyager 2 (V2) will recede into darkness. As recognized internationally, the timeliness of a Uranus orbiter mission is a primary motivation for evaluating what science can be done with a lower-cost, faster-turnaround mission within the New Frontiers (NF) class; this of course requires that the NF target list be expanded to include Uranus. Both large-scale (Odyssey Pre-Decadal Mission Concept Study (PMCS) 6 , 2010 7 & 2017 8 studies) and Discovery (Trident) missions have been explored for the Ice Giants, but, no comprehensive study has focused on determining the scope and feasibility of a NFclass Uranus orbiter mission. This paper summarizes one of three concepts submitted to the 2019 PMCS opportunity 9 . This paper demonstrates the potential for a NF-class orbital mission to address essential scientific questions spanning multiple disciplines of Ice Giant science (plus cross-cutting heliophysics and astrophysics opportunities); additional science could be achieved via flyby of a Centaur en route to Uranus. To maximize the prospects of meeting launch opportunities by 2034, we omit scientific objectives that are only achievable by an atmospheric probe, and focus instead on the excellence of the achievable science in the broader Uranian system. V2's brief encounter with Uranus 10 provided a glimpse at the planet's complexity and uniqueness, but motivated more questions than answers. Our current limited understanding of Uranus is similar to that of other planets prior to orbital missions (e.g., after Pioneer & Voyager to Jupiter & Saturn).
Orbital missions have flown to every planet in our solar system except for the Ice Giants. At each planet these first orbiters revealed surprises that were missed by the initial flybys: e.g., the Enceladus geyser 11-12 , a major discovery of Cassini, which was unnoticed by previous flybys. As our understanding of the Gas Giants was transformed beyond expectations by dedicated orbiter missions (e.g., Galileo, Juno, Cassini) and will further improve with future missions (e.g., Europa Clipper, JUICE), so too will our knowledge of Uranus expand from the necessary multiyear measurements and orbital investigations.

Science Objectives
Ex. 1 presents a "proto-Science Traceability Matrix" with a broad array of potential science objectives and outstanding mysteries, covering all areas of the Uranus system (rings, satellites, magnetosphere, atmosphere, & interior). An NF-class Uranus orbiter mission is expected to achieve many of these objectives.

Ring Science
Determine the processes that sculpt & maintain Uranus' ring-moon system. V2 and Earth-based observations have revealed that Uranus hosts a system of dense, narrow rings that lack meaningful spacing, diverse broad and finely-structured dusty rings, and the most tightly-packed system of small moons in the solar system. We still do not understand how the Uranian rings maintain their structure. The Uranian ring system also boasts ten narrow and oddly-shaped main rings. Four of these are associated with resonances of small moons that likely play a role in shepherding

How is Uranus' interior structured below the clouds and how does it behave?
Understand the configuration & evolution of Uranus' magnetic field. (1 & 3) • Map of the intrinsic magnetic field, including spherical harmonic coefficients • Temporal evolution of the intrinsic magnetic field • Low-degree (<10) odd and high degree (>10) even gravitational harmonics • Internal heat flux as a function of latitude them 13 . The mechanisms confining the other ring edges and the nature of their present locations remain a mystery. Uranus also features a complex system of faint dusty rings 14 about which little is known. They likely originate from micrometeoroid bombardment ejecta of the small inner moons and dense rings themselves 15 . Determining the rates and sources of the dusty ring production and distribution within the system will help to determine the lifecycle of ring and moon material. Thirteen small moons orbit between the main rings and larger classical moons of Uranus 16 , within a tight radial range of <18,000 km. This arrangement is unstable on relatively short timescales and depends on the moons' unknown masses 17 . Many of these moons orbit inside Uranus' co-rotation radius. Thus, these moons' tidal interactions with Uranus cause inward migration towards the Roche limit, where they may fragment into new rings or interact with existing rings. They may also be driven outward through strong resonant torques if a more massive ring develops. Determining how the ring-moon system may undergo recycling throughout its lifetime is critical to understand how planetary ring-moon systems operate under myriad configurations.
Determine the composition & origin of Uranus' rings & small satellites. The rings and small moons of Uranus are dark, and their compositions are unknown. Observations 18 have revealed H2O and CO2 ice spectral features on Uranus' larger moons, whereas the rings' spectra are flat 19 . Limited observations of the small moons have not revealed if they are more akin to the larger moons or the rings.

Satellite Science
Determine whether the classical Uranian satellites have signatures indicative of oceans & determine their surface compositions. Uranus has five classical satellites (Miranda, Ariel, Umbriel, Titania, and Oberon). These moons may have common compositional surface ices like those of the Pluto-Charon system (i.e., widespread H2O ice, CH4 and other volatiles, hints of NH3-hydrates and possible tholins 20-21 ). However, further investigation of these moons may provide insight to a different icy evolution than Kuiper Belt Objects (KBOs), mainly due to the limited knowledge of CO2 as a volatile ice at Uranus [22][23] , as opposed to CO on KBOs 21 . The widespread evidence for resurfaced terrains on the classical Uranian satellites, hypothesized global heating events, and the possible presence of NH3-hydrates on their surfaces indicate that these moons are possible ocean worlds 24 . Heat flux estimates for Miranda [25][26] and Ariel 27 indicate that these moons experienced heating events in the past, possibly sustaining subsurface liquid H2O. Additionally, groundbased spectroscopic observations of the Uranian satellites hint at the presence of NH3-bearing species on their surfaces 20 . NH3-rich material in the lithosphere would dramatically lower the interior freezing temperature (vs. pure H2O ice), assisting in the sustainability of subsurface oceans. If oceans are present in these satellites' interiors, either globally or locally, they may have interacted, or currently interact with the surface in the form of plumes, cryovolcanic flows, and/or tectonic features indicative of nonsynchronous rotation.
Understand what processes formed & modify the surfaces of the classical Uranian satellites. The geologic processes of the Uranian satellites are complex, as indicated by large tectonic and possibly cryovolcanic features imaged by V2. These features extend well past the terminator in the V2 images, as revealed by enhanced nightside "Uranus-shine" processing techniques 28 . Miranda exhibits three unique "coronae", large polygonal shaped regions of surface deformation containing subparallel ridges and troughs that are highlighted by high and low albedos. These are made up of complex sets of tectonic features 29-30 and may contain cryovolcanic flows 31 . Ariel exhibits complex canyon systems thought to be a result of internal processes driving tectonism [32][33] , while the formation of the large canyons on Oberon and Titania remain poorly understood 34-35 .

Magnetosphere Science
Understand the fundamental structure & dynamics of Uranus' magnetosphere and the importance of internal vs. external drivers. Uranus' magnetosphere offers a unique configuration that provides an opportunity to understand the drivers of magnetospheric dynamics throughout the solar system. The axis of the approximately dipolar portion of its internal magnet is offset and tilted ~59° from the planetary rotation axis, which itself is inclined by 98° relative to the ecliptic (cf. these two axes are nearly aligned at Saturn). Such an orientation presents an asymmetrical obstacle to the impinging solar wind 36 . Plasma transport within a planetary magnetosphere may be driven by external and/or internal dynamics. The former suggests convection-driven plasma acceleration and circulation 37 , whereas the latter would be subjected to centrifugal forces as the plasma is accelerated and energized. The magnetospheres of terrestrial planets with intrinsic magnetic fields (i.e., Earth & Mercury) are primarily driven by solar wind forcing, while the magnetospheres of Gas Giants (Jupiter & Saturn) are driven predominantly by internal plasma sources & fast planetary rotation and a mixture of internal/external processes, respectively. V2 observations suggest that Uranus may be solar wind-driven [38][39] , but it is a mystery why, unlike all other planets, no solar wind alpha particles were seen by observed higher energies 40 . Uranus' fast (~17 hr) rotation forces the magnetic configuration to open and close to the solar wind diurnally; this suggests that internal drivers must play a role, even though solar wind-driven transport can be decoupled near the solstices [41][42] . A mission arriving within a decade of 2049 would observe a different configuration than V2, as the alignment of the planet's rotation axis changes seasonally.
Understand what processes generate Uranus' intense electron radiation belt. Planetary radiation belts provide an in-situ laboratory to study the universal process of particle acceleration, providing conditions that are hard to reproduce on Earth and remain inaccessible in astrophysical phenomena. Radiation belts are defined by trapped energetic charged particle populations and their energy density and composition are a fingerprint of a planet's diverse interactions with internal and external drivers. Uranus' radiation belts are especially interesting as V2 observations did not confirm our expectations. For the particles to accumulate to high intensities, the radiation belts need to draw from a large reservoir of lower energy plasma and/or lose the accelerated particles only very slowly. Neither appeared to be the case at Uranus, which possesses a uniquely low-density plasma magnetosphere and where waves result in efficient particle losses. The mystery remains as to why Uranus' electron belts appear so intense 43 (cf. Saturn & Neptune at energies ≲2 MeV) while its ion belts show low intensities, despite sharing physical processes 44 .
Uranus' unexpected behavior may stem from its unique magnetospheric configuration resulting in the dominance of surprising processes. For example, whistler mode chorus waves are thought play a role for electron acceleration and loss, but the intensities of these waves measured at Uranus were surprisingly higher than those observed at any other planet 45 .

Interior Science
Understand the configuration & evolution of Uranus' magnetic field. V2 showed that the intrinsic magnetic field of Uranus is multipolar and non-axisymmetric. However, only the largest scales were resolved, so its detailed configuration remains unknown and its temporal evolution unconstrained. Dynamo region thickness and convective vigor are two hypotheses to explain Uranus' unique magnetic field configuration [46][47][48] , testable by magnetic field measurements resolving both large-and small-scale components and its secular variation. In addition to determining the planet's compositional and thermal structure, gravitational field measurements would further test generation mechanisms by establishing how deep zonal flows extend into the interior 49 .
Determine the bulk composition & the distribution of materials within Uranus. Standard three-layer structure models of Uranus infer that the planet consists of ~2 M⊕ of H-He; although this estimate puts important limits on the planetary metallicity, it is not known which elements dominate the deep interior 50 . Alternative structure models suggest that Uranus could have a density profile without discontinuities 51 and that a large fraction of water is not needed fit the observed properties. It is of particular importance to determine the global ice-to-rock ratio, which can also be used to address Uranus' formation -a long-standing problem for planet formation theory 52 . Beyond the ice-to-rock ratio, the abundances of key species such as He tell us about the environment in which Uranus formed, and bulk enrichments of C, N, and S provide additional information on the planet formation process. However, ground-based attempts to constrain the bulk composition from measurements of atmospheric disequilibrium species (e.g., CO) have been inconclusive 53 ; and Uranus' ice-torock ratio remains loosely constrained [50][51] .
Understand Uranus' global energy balance & internal heat flow. Uranus is the only Giant Planet that is in approximate equilibrium with solar insolation 54 . However, a more precise energy balance determination is necessary because large uncertainties exist in the V2 bond albedo and thermal emission measurements. This balance presently suggests that Uranus' interior may not be fully convective and/or contains compositional gradients that hinder convection 55 , although atmospheric phenomena may also be responsible 56 . If convective inhibition is at play, then Uranus' internal heat flux may vary seasonally, and given that recent ground-based observations reveal many episodic convective events, an orbiter mission arriving during an active period may measure a higher heat flux, especially since episodic convective events have been revealed by recent ground-based observations 57 .

Atmosphere Science
Understand Uranus' atmospheric heat transport mechanisms. Many atmospheric processes cause downward (e.g., solar insolation) and upward (e.g., thermal radiation and cumulus convection) flux of energy. These processes provide local perturbations that shape atmospheric features such as cloud bands and vortices. The connection between local atmospheric events and the global energy balance remains an outstanding mystery. Because the molecular weight of condensable species is heavier than the background H-He atmospheric mixture, moist convection is generally inhibited and tends to happen in episodic bursts 58 . Given this time-variability, a new mission may find that local episodic convection leads to a higher global heat flux.
In the middle and upper atmosphere, our ignorance of heat transport processes is symptomized as the "energy crisis": V2 stellar occultations revealed that Uranus' thermosphere is hot 59 . Although this "crisis" exists at all four giant planets, it is particularly surprising for Uranus because of its large axial tilt; given that the thermosphere is hot in both summer and winter hemispheres, solar heating cannot be the cause 60 . The vertical temperature gradient may point to the nature of the unknown heating 61 , but V2 data cannot distinguish between candidate heating mechanisms.
Understand Uranus' zonal & meridional circulation patterns. These circulations are critical for understanding vertical heat transport and energy balance, as well as producing a coherent model of atmospheric dynamics. Uranus' zonal wind profile -with retrograde winds at the equator and a single prograde peak in each hemisphere -starkly contrasts with Jupiter and Saturn. Uranus' winds also exhibit a surprising hemispheric asymmetry near the poles 57 , which may be seasonally driven. Whereas the cloud bands of Jupiter and Saturn are loosely associated with the zonal jets due to eastward jet peaks acting as transport barriers, Uranian cloud bands are seemingly not tied to the smooth wind structure and associated temperature gradients, which may hint at unresolved peaks in the zonal wind structure 61 .
Uranus' overturning meridional circulation structure remains unknown. Methane gas depletion around the poles suggests that Uranus has a single deep circulation cell in each hemisphere, in which air rises from the deep atmosphere at low latitudes, clouds condense out, and dry air is transported to high latitudes where it descends 63 . However, such a pattern is inconsistent with observed cloud distributions, implying that the meridional circulation must be more complex. High-resolution measurements of the wind field may reveal coupling between zonal/meridional circulation via eddies.
Determine the thermodynamics & chemistry of Uranus' clouds and hazes. During the V2 flyby, Uranus appeared almost featureless. The subsequent presence of unexpected bright storms 63 has revealed that Uranus has an active, temporally dynamic, and poorly understood weather layer. Clouds and hazes occur preferentially at specific latitudes, and the banding pattern of tropospheric hazes is apparently not tied to the zonal wind structure. Vertically, clouds and tropospheric hazes are not found at the altitudes predicted by thermochemical equilibrium models 64 ; in fact, the compositions of Uranus' upper cloud layers remain unclear 61 . The thermodynamics and chemistry of the clouds have far-reaching implications for connecting the atmosphere to the planet's bulk composition, and for understanding the global energy balance.
V2 and Spitzer measurements 65 revealed hydrocarbon densities much lower in Uranus' stratosphere than at any other giant planet due to the stark differences in atmospheric mixing. Hydrocarbons are the main source of photochemical haze, act as EUV continuum absorbers, and trace vertical transport.

NF-class Design Considerations
Maintaining balance across the research disciplines described above should be a high priority for a NF Uranus orbiter. Based on work from previous studies, a NF-class orbiter mission should be feasible. For example, the costs in the 2010 study 7 suggest ~$1.1B (FY15$) for Phases A-D for an orbiter mission with a flagship-class payload without an atmospheric probe (assuming 30% reserves) without the launch vehicle costs. Appropriately scoping the payload to accommodate NF-class science would reduce both the payload and spacecraft costs. From a mission design standpoint, the potential use of a solar electric propulsion (SEP) stage with a cruise of ~14 years could reduce the spacecraft's chemical propulsion burden, while still leaving enough Radioisotope Power System (RPS) lifetime for the baseline mission, to be feasible within the NF cost. A NF Uranus orbiter could be undertaken with current technologies, given appropriate trades in design and scope.
Power is perhaps the most limiting constraint on a Uranus orbiter mission and addressing power within cost is the primary obstacle to the feasibility of a NF Uranus orbiter mission. Previous Ice Giant mission studies 7-8 have resulted in architectures requiring >350 W-e end-of-life power, which required three or more now-cancelled Enhanced Multi-Mission Radioisotope Thermoelectric Generator (eMMRTGs). Owing to the relative inefficiency and significant cost of current RPSs, any design should attempt to reduce the needed end-of-life power; this will have significant impact on both the spacecraft and orbit design as well as the communication subsystem and payload. Hence, accelerating the development and expanding the efficiency and lifetime (and potentially reducing the cost) of next-generation RPSs would significantly enhance the mission. For example, the recent Neptune Odyssey mission concept uses three next-generation RPSs 6 , suggesting that a NF-class Uranus mission could close with fewer. This of course assumes that a sufficient supply of plutonium is available for future space exploration missions. With current technology (i.e., 14-year MMRTG flight design life), a typical baseline would be a <12year cruise (potentially with a Centuar flyby) and a 2-year mission at Uranus with a system tour that enables surface mapping of the large satellites as well as spatial coverage of the planet & rings/small moons; this baseline could be significantly lengthened if the lifetime of future RPSs were improved.
Another significant driver is determining the total mass that can be put into Uranus orbit within the NF cost cap. Because significant propellant mass is needed for Uranus orbit insertion, mass efficiency is critical. A realistic ~60-kg payload (e.g., 5 instruments & radio science) using current technologies, would provide closure to numerous mysteries summarized in Ex. 1. However, cost reduction and increases in capability & availability of launch vehicles (e.g., SLS) could be enabling for a NF-class Uranus orbiter mission launching outside of windows with Jupiter gravity assists. Lastly, the risk-vs.-benefit of using aerocapture for orbit insertion should be analyzed as it can greatly increase the delivered payload 66-67 and shorten flight times 68 .