Linda Spilker

Whitepaper #038 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research. Additional topics: outer heliosphere; space-based missions/projects; emerging opportunities.

Our solar system has evolved through accretion of dust and gas as the Sun and its protective magnetic bubble – “the heliosphere” - have plowed through interstellar space on its journey through the galaxy. Over the course of its evolution, the solar system has encountered dramatically different interstellar properties resulting in a severely compressed heliosphere with periods of full exposures of interstellar gas, plasma, dust and galactic cosmic rays (GCRs) that all have helped shaped the system we live in today. Our current knowledge lacks the direct measurements necessary to understand how our star upholds its vast heliosphere and its potentially game-changing role in the evolution of our galactic home.Voyager 1 and 2 are now in the Very Local Interstellar Medium (VLISM), where they are expected to operate until the mid-2030’s having uncovered many unexpected discoveries and mysteries. After its paradigm-shifting discoveries at Pluto and Arrokoth, New Horizons is currently the only spacecraft in the outer heliosphere and is following the same heliospheric longitude as Voyager 2, but in the ecliptic plane – a trajectory that intersects the IBEX ribbon. It is projected to operate across the heliospheric termination shock and possible the heliopause with new measurements that will shed light on many of the mysteries of our heliosphere. Now passing 55 au, New Horizons is uniquely positioned to investigate the evolution of the solar wind, energetic particles, GCRs, and, in particular interstellar Pick-Up Ions (PUIs) that Voyager was not equipped to measure, to help constrain the structure and dynamics of the heliosphere. Observations of GCRs offers an opportunity to understand how these scatter strongly in the wavy structure of the “ballerina skirt” of the solar magnetic field leading to the strong modulation as part of the overall heliospheric shielding.As New Horizons continues to travel outward, dust measurements may reveal an interstellar component that will provide the strongest constraint to date on how interstellar dust grains interact with the heliosphere. Now beyond the infrared and UV haze of the circumsolar dust and hydrogen gas, the Alice UV camera holds promise to search for signatures of the hydrogen wall and perhaps even signatures of our neighboring interstellar clouds.New Horizons continues to break new ground in understanding the formation of our solar system by revealing the properties of multiple distant Kuiper Belt Objects and provide critical constraints on the structure of the Sun’s enormous dust disk. Because of its distant position, New Horizons is also providing the unprecedented estimates of the cosmic background.In this presentation we provide an overview of New Horizons’ heliophysics observations in the context of the exploration by Voyager, IBEX, and IMAP. We conclude by providing a status of the future Interstellar Probe mission concept that is now under consideration in the Solar and Space Physics Decadal Survey.

Recently, the Juno and Cassini spacecraft shed light on the interior of both Jupiter and Saturn, the two gas giants of the Solar System. Juno is currently orbiting Jupiter in a highly elliptical 53.5-day orbit, with a perijove altitude of about 4000 km. After the 33rd passage in April 2021 (labeled PJ33), the mission ended its nominal mission and entered its extended mission. On the contrary, the Cassini spacecraft ended its mission on September 15th, 2017 with a deliberate plunge into Saturn’s atmosphere. In its final phase, the Grand Finale, Cassini provided insights on Saturn’s rings, atmosphere, and interior. Out of the 22 proximal orbits, six pericenter passes have been devoted to the determination of the gravity field of the planet. The gravity science experiments on board Juno and Cassini precisely measured, respectively, Jupiter and Saturn zonal gravitational fields. The measured gravity harmonics have been used to constrain the interior structure and atmospheric zonal flow on both planets. The Cassini data analysis have shown the need to include unknown accelerations to properly fit the data to the expected noise (Iess, 2019). Similar unexplained accelerations have been observed also on Juno gravity data (Durante, 2020). Since Jupiter and Saturn are gas giants, unconventional phenomena can be at play, including normal modes, non-zonal atmospheric dynamics, or flows in the dynamo region. The analysis of the accelerations acting on Cassini provided evidence for p-modes on the planet (Markham, 2020), while the analysis of Juno gravity data revealed p-modes on Jupiter and provided upper bounds on lower frequency f-modes. Here, we show the results for normal modes on both Jupiter and Saturn obtained with the analysis of gravity data of both Juno and Cassini.

Exploring planetary atmospheres uncovers important information as to how our solar system formed and evolved. While remote sensing is extensively used, some crucial observations require in situ measurements by an atmospheric probe. Given their scientific importance, probe missions to Saturn, Uranus, and Neptune are under consideration for the coming decades. In anticipation of future probe missions, the software tool Visualization of the Impact of PRobe Entry conditions on the science, mission and spacecraft design (VIPRE) was developed as proof-of-concept to facilitate selection of probe entry locations. Currently, there is no analytical way to identify which interplanetary trajectory from thousands of feasible launch opportunities is optimal for a considered mission concept. The search and decision process for that solution is complex and relies on the intuition of mission designers, who focus on a subset of trajectories to make the trade space manageable. The idea of VIPRE is (1) to generate a multidimensional data cube showing relevant engineering and science parameters simultaneously for thousands of trajectories, and (2) to visualize the data for all entry sites over the body’s envelope. VIPRE lays a foundation to make available the data for browsing in a 3D visualization to identify the best family of solutions for a given mission. This paper introduces the validated and verified core algorithms of VIPRE, published on GitHub Probst. VIPRE serves as a basic framework to be used and extended for different purposes. The paper further presents the motivation for the development and algorithms; it explains the computation and data visualization strategy; and gives a list of suggested functionalities to extend and further develop VIPRE to fully leverage its potential.

The Neptune Odyssey mission concept is a Flagship-class orbiter and atmospheric probe to the Neptune–Triton system. This bold mission of exploration would orbit an ice-giant planet to study the planet, its rings, small satellites, space environment, and the planet-sized moon Triton. Triton is a captured dwarf planet from the Kuiper Belt, twin of Pluto, and likely ocean world. Odyssey addresses Neptune system-level science, with equal priorities placed on Neptune, its rings, moons, space environment, and Triton. Between Uranus and Neptune, the latter is unique in providing simultaneous access to both an ice giant and a Kuiper Belt dwarf planet. The spacecraft—in a class equivalent to the NASA/ESA/ASI Cassini spacecraft—would launch by 2031 on a Space Launch System or equivalent launch vehicle and utilize a Jupiter gravity assist for a 12 yr cruise to Neptune and a 4 yr prime orbital mission; alternatively a launch after 2031 would have a 16 yr direct-to-Neptune cruise phase. Our solution provides annual launch opportunities and allows for an easy upgrade to the shorter (12 yr) cruise. Odyssey would orbit Neptune retrograde (prograde with respect to Triton), using the moon's gravity to shape the orbital tour and allow coverage of Triton, Neptune, and the space environment. The atmospheric entry probe would descend in ∼37 minutes to the 10 bar pressure level in Neptune's atmosphere just before Odyssey's orbit-insertion engine burn. Odyssey's mission would end by conducting a Cassini-like “Grand Finale,” passing inside the rings and ultimately taking a final great plunge into Neptune's atmosphere.

The composition of outer planet atmospheres holds fundamental clues to understanding the formation and evolution of the solar system. Measurements of noble gas abundances and key isotope ratios help constrain formation models, and along with measurements of atmospheric structure and dynamics they reveal formation and evolutionary processes [1], [2]. These enable conclusions about giant planet formation and possible migration during the epoch of solar system formation. With the Galileo Probe laying the foundation of in situ atmospheric measurements of the outer planets by exploring Jupiter, entry probe missions to Saturn, Uranus and Neptune are essential to complete the picture of how our solar system evolved to its present state.   During the development of entry probe missions, the interplanetary and probe approach trajectory, as well as the selection of the entry interface zone, are critical elements to mission success. Both elements are driven by considerations such as spacecraft safety (e.g. avoiding rings), while balancing science and engineering requirements at the same time (e.g. highly interesting science and entry zone vs. optimal communication geometry between probe and relay spacecraft). Due to the complexity of the problem, there is no analytical solution for finding the ‘best’ trajectory. Instead, one relies on the experience and intuition of mission designers to select a few possible interplanetary trajectories, which are then explored in detail to see how they meet science and engineering requirements. This approach leaves a huge trade space unexplored and may find a local, rather than a global optimum trajectory for the mission.   We are addressing this gap by developing a software tool called VAPRE (Visualization of Atmospheric PRobe Entry Conditions for different bodies and trajectories) [3], [4] that allows us to explore those previously unexplored trade spaces. VAPRE can process thousands of trajectories, significantly more than in the currently common mission design processes. Due to its flexible architecture, VAPRE can be adapted and extended to accommodate new science and engineering constraints for different or similar mission scenarios. In our talk, we will present an example of how the tool can be used to design a flyby mission to a giant planet that delivers an atmospheric probe considering opportunities between 2028 and 2042.

The plume of Enceladus is unique in the solar system in providing direct access to fresh material from an extraterrestrial subsurface ocean. The Cassini Mission, though not specifically designed for it, was able to take advantage of the plume to conduct the best characterization to date of an extraterrestrial ocean. Evidence gathered from multiple instruments points to a global, subsurface liquid water ocean rich in salts and organic compounds, with water-rock interactions occurring presumably in hydrothermal systems at or below the moon’s sea floor. Meeting the criteria of “extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy source(s) to sustain metabolism,” the ocean of Enceladus can therefore be considered habitable. It is also the only confirmed place beyond the Earth where we can easily sample fresh material from a demonstrably habitable environment without the complications of digging or drilling. The next step is to investigate whether Enceladus’ ocean is actually inhabited. Here, we summarize the evidence for Enceladus’ ocean and its habitability, identify constraints and outstanding questions on the detectability of life within its ocean, and recommend a return to Enceladus with a dedicated search-for-life mission (or missions).

Enceladus’s long-lived plume of ice grains and water vapor makes accessing oceanic material readily achievable from orbit (around Saturn or Enceladus) and from the moon’s surface. In preparation for the National Academies of Sciences, Engineering and Medicine 2023–2032 Planetary Science and Astrobiology Decadal Survey, we investigated four architectures capable of collecting and analyzing plume material from orbit and/or on the surface to address the most pressing questions at Enceladus: Is the subsurface ocean inhabited? Why, or why not? Trades specific to these four architectures were studied to allow an evaluation of the science return with respect to investment. The team found that Orbilander, a mission concept that would first orbit and then land on Enceladus, represented the best balance. Orbilander was thus studied at a higher fidelity, including a more detailed science operations plan during both orbital and landed phases, landing site characterization and selection analyses, and landing procedures. The Orbilander mission concept demonstrates that scientifically compelling but resource-conscious Flagship-class missions can be executed in the next decade to search for life at Enceladus.

In the pursuit of deciphering the formation of our solar system, the exploration of the compositional and dynamical structure of planetary atmospheres with entry probes plays a crucial role. A probe's measurements provide insight into an atmosphere's deeper composition and dynamical processes not accessible via remote sensing, providing key information on the origin and possible migration of planets during early formation phase. A planetary entry probe mission has been in discussion in several Planetary Science Decadal Surveys, one to Saturn has been identified as a mission of highest priority in the current one 2013-2022, and a mission to Uranus and/or Neptune carrying a probe is being considered as a Flagship mission in the next one spanning 2023-2032.In the development of such missions, the probe approach and delivery trajectory is a critical element to mission success, including ring avoidance, and targeting of highly desirable regions in the atmosphere, while balancing other requirements such as providing an optimal communication geometry between the probe and the relay spacecraft while meeting the mission's science objectives. Due to the complexity of the problem, mission concept studies are usually limited to the investigation of a limited number of specific trajectories and probe delivery opportunities to a very small, pre-defined range of latitudes while leaving a huge trade space unexplored.The tool VAPRE (Visualization of Atmospheric PRobe Entry conditions) has been developed to enable a fast and wide-range evaluation of entry conditions for planetary probes, spanning the complete range of latitudes for each of the three planets. VAPRE allows a rapid assessment of feasible entry sites by evaluating a large number of arrival trajectories based on their hyperbolic arrival velocities with respect to parameters such as the flight path angle and the relative entry velocity of the probe at the entry interface point. VAPRE facilitates the mission design process by combining the evaluation of technical feasibility and science value for the investigated scenarios to assess potential entry sites. VAPRE is developed in the framework of IPED (Impact of the Probe Entry zone on the trajectory and probe Design), which is a two- to three-year research study to investigate both the impact of interplanetary and approach trajectories on the feasible range of entry sites as well as on probe design, considering Saturn, Uranus, and Neptune as target bodies.In this paper we fully demonstrate the functionalities of the VAPRE tool on a case scenario for a mission to the Ice Giants.The presented research was supported by an appointment to the NASA Postdoctoral Program (NPP) at the Jet Propulsion Laboratory (JPL), California Institute of Technology, administered by Universities Space Research Association (USRA) under contract with National Aeronautics and Space Association (NASA). © 2020 All rights reserved.