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The Uranus system hosts a diverse set of large moons, from the enigmatic Miranda with its striking coronae, to the outermost and second-largest moon Oberon with its dark and ancient surface. These moons mostly orbit within Uranus’ magnetosphere, which while relatively depleted in low energy-plasma, was found by Voyager 2 to possess surprisingly intense electron radiation belts. Prominent energetic electron absorptions associated with the Uranian moons were observed by Voyager 2, indicating that these particles interact significantly with the moons. Thus, the surfaces of the moons are continuously bombarded by high energy electrons, which are capable of breaking chemical bonds in surface material, leading to radiolytic chemistry that can alter surface composition. In addition, charged particle bombardment can alter the microstructure of surface ice as well as affect grain sizes by sputtering. Ground-based remote sensing observations have revealed planetocentric and hemispherical trends in several key spectral parameters, including the abundance of CO2 ice, whose concentration on the trailing hemispheres of the moon hint at a possible radiolytic origin. Furthermore, possible signatures of NH3 have been detected on the surfaces of several of the moons, including Ariel, where the sub-observer longitudinal distribution of this species appears to support spatial association with geologic features and terrains. It is known from laboratory experiments that NH3 is readily decomposed by charged particle radiation. If NH3 or NH3-related compounds are present on the moons, it therefore could be an indication of recent emplacement or exposure. Here, we will present possible signatures of charged particle surface modification on the Uranian moons and discuss implications for observations by future missions to the Uranus system.

Since September 2014, NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft has been measuring the in-situ ionospheric constituents of Mars. Recently, MAVEN detected the presence of large-scale plasma depletions (at least ten-fold) within the Martian ionosphere, also known as Plasma Depletion Events (PDEs). Geometrically, the Martian PDEs appear to be bubble-like plasma structures. Although the origin and formation of PDEs are not entirely understood, they are known to occur primarily on the nightside and in regions with stronger crustal magnetic fields.In this study, we analyze the variation of magnetic field magnitude and direction and electric field power (2–100 Hz) associated with PDEs. We show that, in most cases, the magnetic fields do not considerably change within the plasma-depleted region. Conversely, the low-frequency electric field wave power is enhanced by up to two orders of magnitude at the peak depletion. Both ions and electrons within PDEs are highly magnetized. We present possible formation mechanisms of PDEs supported by recent findings.

We investigate the unique magnetosphere of Uranus and its interaction with the solar wind. Following previous seminal work, we developed and validated a simple yet valuable and illustrative model of Uranus’ offset, tilted, and rapidly-spinning magnetic field and magnetopause (nominal and fit to the Voyager-2 inbound crossing point) in three-dimensional space. With this model, we investigated details of the seasonal and interplanetary magnetic field (IMF) orientation dependencies of dayside and flank reconnection along the Uranian magnetopause. We found that anti-parallel (magnetic field shear angle greater than 170-degrees) reconnection occurs nearly continuously along the Uranian dayside and/or flank magnetopause under all seasons of the 84 (Earth) year Uranian orbit and the most likely IMF orientations. Such active and continuous driving of the Uranian magnetosphere should result in constant loading and unloading of the Uranian magnetotail, which may be further complicated and destabilized by sudden changes in the IMF orientation and solar wind conditions plus the reconfigurations from the rotation of Uranus itself. We demonstrate that unlike the other magnetospheric systems that are Dungey-cycle driven (i.e., Mercury and Earth) or rotationally driven (Jupiter and Saturn), global magnetospheric convection of plasma, magnetic flux, and energy flow may occur via three distinct cycles, two of which are unique to Uranus (and possibly also Neptune). Our simple model is also used to map signatures of dayside and flank reconnection down to the Uranian ionosphere, as a function of planetary latitude and longitude. Such mapping demonstrates that “spot”-like auroral features should be very common on the Uranian dayside, consistent with observations from Hubble Space Telescope. We further detail how the combination of Uranus’ rapid rotation and unique and very active global magnetospheric convection should be consistent with fueling of the surprisingly intense trapped radiation environment observed by Voyager-2 during is single flyby. Summarizing, Uranus is a very special magnetosphere that offers new insights on the nature, complexity, and diversity of planetary magnetospheric systems and the acceleration of particles in space plasmas. We still have much to learn about Uranus’ unique and intriguing magnetosphere, which might have important analogs to exoplanetary magnetospheric systems. Our hypotheses can be tested with further work involving more advanced models, new auroral observations, and unprecedented missions to explore the in situ environment from orbit around Uranus. Our results highlight why any future mission to orbit Uranus should include a complement of magnetospheric instruments in the payload.

The results of two radiation test campaigns on a low-cost commercial off-the-shelf magnetometer are presented. The test setup and the total ionization dose (TID) levels studied were designed to meet the requirements of a mission to land on Europa. Based on the Europa Lander Science Definition Team report, instruments inside an aluminum vault at the surface of Europa would need to withstand TID of up to 300 krad(SI). In order to evaluate the performance of the PNI RM3100 magnetometer, nine separate sensors were irradiated at two different facilities during two separate campaigns and under different configurations, including passive and active tests. Of the nine sensors, seven survived the TID of 300 krad(SI) while the other two sensors started presenting failures after reaching 150 krad(SI). Post-irradiation tests showed that eight of the nine sensors continued to work without appreciable degradation after stopping exposure, while one sensor stopped working altogether.

International audience; Saturn is permanently surrounded by 6 discrete proton radiation belts that are rigidly separated by the orbits of its inner moons and dense rings. These radiation belts are ideal environments to study the details of radial diffusion and the CRAND source process, yet progress has been hindered by the fact that the energy spectra are not known with certainty: Reanalysis of the response functions of the LEMMS instrument on-board the Cassini orbiter has shown that measurements of ≲ 10 MeV protons may be easily contaminated by ≳ 10 MeV protons and that many available measurements characterize a very broad energy range, so that the calculation of an energy-resolved spectrum is not as straightforward as previously assumed. Here we use forward modeling of the measurements based on the instrument response and combine this technique where useful with numerical modeling of the proton belt physics in order to determine Saturn's spectra with higher certainty. We find significant proton intensities up to ≈ 1 GeV. While earlier studies reported on proton spectra roughly following a power law with exponent ≈ - 2 , our more advanced analysis shows harder spectra with exponent ≈ - 1 . The observed spectra provide independent confirmation that Saturn's proton belts are sourced by CRAND and are consistent with the provided protons being subsequently cooled in the tenuous gas originating from Saturn or Enceladus. The intensities at Saturn are found to be lower than at Jupiter and Earth, which is also consistent with the source of Saturn being exclusively CRAND, while the other planets can draw from additional processes. Our new spectra can be used in the future to further our understanding of Saturn's proton belts and the respective physical processes that occur at other magnetized planets in general. Also, the spectra have applications for several topics of planetary science, such as space weathering of Saturn's moons and rings, and can be useful to constrain properties of the main rings through their production of secondary particles.

The Galileo mission to Jupiter discovered magnetic signatures associated with hidden sub-surface oceans at the moons Europa and Callisto using the phenomenon of magnetic induction. These induced magnetic fields originate from electrically conductive layers within the moons and are driven by Jupiter’s strong time-varying magnetic field. The ice giants and their moons are also ideal laboratories for magnetic induction studies. Both Uranus and Neptune have a strongly tilted magnetic axis with respect to their spin axis, creating a dynamic and strongly variable magnetic field environment at the orbits of their major moons. Although Voyager-2 visited the ice giants in the 1980s, it did not pass close enough to any of the moons to detect magnetic induction signatures. However, Voyager-2 revealed that some of these moons exhibit surface features that hint at recent geologically activity, possibly associated with sub-surface oceans. Future missions to the ice giants may therefore be capable of discovering sub-surface oceans, thereby adding to the family of known “ocean worlds” in our solar system. Here, we assess magnetic induction as a technique for investigating sub-surface oceans within the major moons of Uranus. Furthermore, we establish the ability to distinguish induction responses created by different interior characteristics that tie into the induction response: ocean thickness, conductivity, and depth, and ionospheric conductance. The results reported here demonstrate the possibility of single-pass ocean detection and constrained characterization within the moons of Miranda, Ariel, and Umbriel, and provide guidance for magnetometer selection and trajectory design for future missions to Uranus.

We examine the dynamics of energetic magnetospheric ions in the highly perturbed and asymmetric electromagnetic environment of the Jovian moon Callisto. The Alfvenic interaction of the (nearly) corotating magnetospheric plasma with Callisto's ionosphere and induced dipole generates electromagnetic field perturbations near the moon, the structure of which vary as a function of Callisto's orbital position. For this study, these perturbations are obtained from the AIKEF hybrid model (kinetic ions, fluid electrons) which has already been successfully applied to Callisto's local plasma environment (Liuzzo et al., 2015, 2016, 2017). To isolate the influence of Callisto's ionosphere and induced dipole field on energetic ion dynamics, we analyze the trajectories of energetic hydrogen, oxygen, and sulfur ions exposed to various configurations of the locally perturbed electromagnetic fields. We present spatially resolved surface maps that display accessibility of Callisto to these ion populations for select energies from 1 keV to 5000 keV. The Alfvenic plasma interaction with (i) Callisto's induced magnetic field, (ii) Callisto's ionosphere, and (iii) the combination of both, all leave distinct imprints in these accessibility patterns. Draping of the magnetospheric field around Callisto's ionosphere partially shields the moon's trailing (ramside) hemisphere from energetic ion impacts, and the induced field tends to focus energetic ion impacts near Callisto's Jupiter-facing and Jupiter-averted apices. Depending on the nature of Callisto's Alfvenic plasma interaction, the accessibility of its surface to energetic protons may evolve non-monotonically with increasing energy. We also present maps of energetic ion accessibility and the resulting energy deposition onto Callisto at the time of the Galileo C3, C9, and C10 flybys. Our findings show that the shielding of Callisto's surface from energetic ion impacts is most effective during flybys that took place while the moon was located at an intermediate distance to the center of Jupiter's magnetospheric current sheet. In this case, the ionosphere and induced dipole field both make substantial contributions to the electromagnetic field perturbations near Callisto.

We present the first investigation and quantification of the photoionization loss process to Mercury's sodium exosphere from spacecraft and ground‐based observations. We analyze plasma and neutral sodium measurements from NASA's MESSENGER spacecraft and the THEMIS telescope. We find that the sodium ion (Na+) content and therefore the significance of photoionization varies with Mercury's orbit around the Sun (i.e., true anomaly angle: TAA). Na+ production is affected by the neutral sodium solar‐radiation acceleration loss process. More Na+ was measured on the inbound leg of Mercury's orbit at 180°–360° TAA because less neutral sodium is lost downtail from radiation acceleration. Calculations using results from observations show that the photoionization loss process removes ∼1024 atoms/s from the sodium exosphere (maxima of 4 × 1024 atoms/s), showing that modeling efforts underestimate this loss process. This is an important result as it shows that photoionization is a significant loss process and larger than loss from radiation acceleration., Key Points Photoionization can be a significant loss process to the sodium exosphere with peak loss estimates of 4 × 1024 atoms/sThe photoionization loss process of Mercury's sodium exosphere varies throughout the planet's orbit around the SunMore sodium is lost due to photoionization on the inbound leg (true anomaly angle of 180°–360°) of Mercury's orbit than the outbound leg

Thanks to the Cassini-Huygens mission, Titan, the pale orange dot of Pioneer and Voyager encounters has been revealed to be a dynamic, hydrologically-shaped, organic-rich ocean world offering unparalleled opportunities to explore prebiotic chemistry. And while Cassini-Huygens revolutionized our understanding of each of the three layers of Titan--the atmosphere, the surface, and the interior--we are only beginning to hypothesize how these realms interact. In this paper, we summarize the current state of Titan knowledge and discuss how future exploration of Titan would address some of the next decade's most compelling planetary science questions. We also demonstrate why exploring Titan, both with and beyond the Dragonfly New Frontiers mission, is a necessary and complementary component of an Ocean Worlds Program that seeks to understand whether habitable environments exist elsewhere in our solar system., Submitted to the PSJ Focus Issue on Ocean World Exploration

Cassini plasma wave and charged particle observations are combined with magnetometer measurements to investigate Titan's induced magnetosphere. Electric field emissions close to Titan are identifie...

We report on the local structure of the Martian subsolar Magnetic Pileup Boundary (MPB) from minimum variance analysis of the magnetic field measured by the MAVEN spacecraft for six orbits. In particular, we detect a well defined current layer within the MPB’s fine structure andprovide a local estimate of its current density and compare these results with the current density obtained by multi-fluid simulations.This current is of the order of hundreds of nAm-2 which results in a sunward Lorentz force of the order of 10-14 Nm-3. We compare these results with multifluid numerical simulations.This force is associated with the gradient of the magnetic pressure, it accounts for the deflection of the solar wind ions near the MPB and for the acceleration of solar wind electrons which carry the interplanetary magnetic field through the MPB into the MPR. We also find that thethickness of the MPB current layer is of the order of both the upstream (magnetosheath) solar wind proton inertial length and convective gyroradius. The former is consistent with the demagnetization of the ions due to the Hall electric field, an effect observed recently at the Earth magnetopause, while the latter would imply kinetic processes are important at the MPB.This study supports recent results that report the presence of a steady current system around Mars in a similar way to the Earth.

Mercury has a global dayside exosphere, with measured densities of 10−2 cm−3 at ~1500 km. Here we report on the inferred enhancement of neutral densities (, Mercury has a global dayside exosphere that is very tenuous and does not extend far from the planet. Here, the authors show enhancement of neutral densities at high altitudes inferred from pickup ions that is most likely caused by the impact of a meteroid.

Pickup ion detection at Titan is challenging because ion cyclotron waves are rarely detected in the vicinity of the moon. In this work, signatures left by freshly produced pick up heavy ions (m/q ∼ 16 to m/q ∼ 28) as detected in the plasma data by the CAPS/IMS instrument on-board Cassini are analyzed. In order to discern whether these correspond to ions of exospheric origin, one of the flybys during which the reported signatures were observed is investigated in detail. For this purpose, ion composition data from time of flight measurements and test particle simulations to constrain the ions' origin are used. After being validated, the detection method is applied to all the flybys for which the CAPS/IMS instrument gathered valid data, constraining the region around the moon where the signatures are observed. The results reveal an escape region located in the anti-Saturn direction as expected from the nominal corotation electric field direction. These findings provide new constraints for the area of freshly produced pickup ion escape, giving an approximate escape rate of 3.3−2+3×1023ions·s−1.

International audience; Cassini discovered Saturn's innermost radiation belt during the end of its mission. The belt is populated with relativistic protons, probably up to the trapping limit of ≈20 GeV. It extends from Saturn's dense atmosphere into and throughout the D-ring. The A-C rings separate this belt entirely from the previously known radiation belts, suggesting that the innermost radiation belt is populated entirely via cosmic ray albedo neutron decay. We find that the proton pitch angle distributions are consistent with being shaped by losses to the D-ring and the upper atmosphere rather than, for example, wave-particle interactions. This supports that the main loss process of this new radiation belt is energy loss in neutral material, different from Saturn's other radiation belts. This property constrains the overall scale height of Saturn's exosphere to

Cassini's final phase of exploration The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere. Science , this issue p. eaat5434 , p. eaat1962 , p. eaat2027 , p. eaat3185 , p. eaat2236 , p. eaat2382

A new sensor for measuring low-amplitude magnetic fields that is ideal for small spacecraft is presented. The novel measurement principle enables the fabrication of a low-cost sensor with low power consumption and with measuring capabilities that are comparable to recent developments for CubeSat applications. The current magnetometer, a software-modified version of a commercial sensor, is capable of detecting fields with amplitudes as low as 8.7 nT at 40 Hz and 2.7 nT at 1 Hz, with a noise floor of 4 pT∕Hz at 1 Hz. The sensor has a linear response to less than 3 % over a range of ±100 000 nT. All of these features make the magneto-inductive principle a promising technology for the development of magnetic sensors for both space-borne and ground-based applications to study geomagnetic activity.

We examine the dynamics of energetic magnetospheric electrons exposed to the highly perturbed and asymmetric plasma environment of Jupiter’s moon Callisto. The interaction of the (nearly) corotating magnetospheric plasma with Callisto’s ionosphere and induced dipole locally generates intense electromagnetic pileup and draping signatures which vary as a function of the moon’s distance to the center of Jupiter’s magnetospheric current sheet. In our study, these field perturbations are represented using output from the AIKEF hybrid (kinetic ions, fluid electrons) model of Callisto’s interaction with the corotating plasma. In order to constrain the influence of Callisto’s variable electromagnetic environment on the dynamics of energetic electrons, we trace the trajectories of more than 6.7 million test particles as they travel through a distinct configuration of the locally perturbed fields. We present spatially resolved maps that display the accessibility of Callisto to electrons at energies E between 10 1 keV ≤ E ≤ 10 5 keV for multiple sets of these perturbed fields, corresponding to select distances of the moon to the center of the Jovian current sheet. The electromagnetic field perturbations near Callisto play a crucial role in generating inhomogeneous precipitation of energetic electrons onto the top of the moon’s atmosphere. In particular, Callisto’s Jupiter-facing and Jupiter-averted hemispheres are partially protected from energetic electron precipitation: when located far above or below the center of Jupiter’s current sheet, Callisto’s induced dipole shields the apices of these hemispheres, whereas near the center of the sheet, strong field line draping protects these regions. In contrast to this, the apex of Callisto’s trailing hemisphere is exposed to intense energetic electron precipitation at any distance to the center of the Jovian current sheet. We also present maps of energetic electron accessibility during the Galileo C3, C9, and C10 flybys, and calculate the intensity of energetic electron flux onto multiple locations at the top of Callisto’s atmosphere. These non-uniform electron fluxes likely cause inhomogeneous ionization of Callisto’s atmosphere and may even contribute to irregular erosion of the surface. This paper is a companion to Liuzzo et al. (2019), who studied the dynamics of energetic ions near Callisto.

A significant difference in Titan's ionospheric electron density is observed between the T118 and T119 Cassini nightside flybys. These flybys had similar geometry, occurred at the same Saturn local time and while Titan was exposed to similar EUV and ambient magnetic field conditions. Despite these similarities, the RPWS/LP measured density differed a factor of 5 between the passes. This difference was present, and similar, both inbound and outbound. Two distinct electron peaks were present during T118, at 1150 km and 1200 km, suggesting very localised plasma production. During T118, from 1200-1350 km and below 1100 km, the lowest electron density ever observed in Titan's ionosphere are reported. We suggest that an exceptionally low rate of particle impact ionisation in combination with increased dynamics in the ionosphere could be the cause. This is, however, not verified by measurements and the measured ambient high energy particle pressure is in fact higher during T118 than during T119., Comment: 11 pages, 3 figures

Over several decades spacecraft that have reached end of life have been adding space debris. There are spacecraft with precious functional payloads on-board but have lost the ability to actively control their attitude and orbit. Proposals have been made to either mitigate them or when possible extend their life by docking them with a service spacecraft. On-orbit servicing involves the challenge of bringing an active satellite in contact with a non-cooperative one. This requires the servicer to make observations of the target to be able to characterize target's motion and to prepare itself for docking or grasping maneuvers. The criticality of proximity operations require high availability of relative navigation information in real-time. This paper focuses on applying Photonic Mixer Device, a Time of Flight based 3D imaging sensor, for relative motion estimation of the target. While much research has been done on evaluation and characterization of PMD and other ToF based cameras, they are mostly restricted to terrestrial applications. The objective of the research activities done here are to understand the dynamics of the sensor and to adapt the camera to optimize its performance for scenarios typically encountered in RvD.

Several machine vision based techniques have been developed in the past to estimate relative motions and pose among spacecrafts and to assist them in Rendezvous and Docking maneuvers. These techniques have typically relied on stereo vision for range information and image processing techniques primarily for object and pattern recognition and their tracking among the others. Development of state-of-the-art 3D imaging sensors such as PMD sensor based cameras opened up possibilities for new and innovative techniques in the field of machine vision. This paper presents an innovative approach for estimating the relative position and pose of a spacecraft by combining the strengths of a proven edge detection algorithm with the advantages of using a 3D imager in real-time machine vision applications.

Several new terthiophene-based, highly anisotropic chromophores were synthesized and dispersed in ultrahigh molecular weight polyethylene (UHMWPE). The chromophores were axially oriented by drawing the polymer matrix and showed an extremely pronounced dichroic behavior. In a pseudo-affine deformation scheme the dyes did well approximate the ultimate properties in terms of molecular orientation; on the other hand, the observed thermochromism and other experimental evidence indicated the persistence of a phase segregation from the polymer matrix.

The dispersibility of terthiophene alkyl derivatives in polyethylene films was comparatively investigated by calorimetry, differential scanning calorimetry (DSC) and microscopy. Polyethylene affinity of the terthiophenes increases with increasing linear alkyl chain length, but the increased tendency to separate crystallization gives rise to distinct phases and dishomogeneity. Drawing of the film at higher temperature than chromophore and poly(ethylene)(PE) melting points (141 °C) improves dispersibility with parallel orientation of chromophore molecules and PE macromolecules. These phenomena are important in determining the high dichroism of new polymeric host–guest polarizers based on blends of ultra-high-molecular-weight poly(ethylene)(UHMWPE) with linear anisotropic molecules based on terthiophene nuclei functionalized with push-pull and alkyl groups. The PE affinity of the chromophores favored by their alkyl chains is one of the key factors in allowing their dispersion in the oriented poly(ethylene) films which show good performances as linear optical polarizers. Copyright © 2001 John Wiley & Sons, Ltd.

This paper presents a collision detection system especially designed for Rendezvous and Docking (RvD) maneuvers to passive objects in space. It is based on 3D images provided by a Photonic Mixer Device (PMD) and makes use of the output of a pose estimation algorithm developed particularly for this 3D information. The algorithm on-line predicts the trajectory of the observed object and detects possible collisions along this trajectory. The intersection tests performed for that purpose consist of examining multiple oriented bounding boxes that surround target and observer spacecraft. Different scenarios including approaching, departing, rotating and by-passing were conducted in a laboratory setup with two robot manipulators holding a client mockup model and a PMD camera. By the help of these scenarios the functionality of the developed algorithm in common RvD maneuvers was evaluated and confirmed.

In this paper a new approach for estimating the pose of an unknown object using Photonic Mixer Devices (PMD) based cameras is presented. The algorithm works on-line by making use of the amplitude and depth information provided at high frame rates by the camera. The aim of the work is to provide a robust pose estimation as an input for control systems in Rendezvous and Docking (RvD) maneuvers in space. The performance of the developed algorithm is evaluated in six different simple scenarios corresponding to the six degrees-of-freedom (DoF) of the object. Final results with errors less than 1% are obtained for the distance estimation and errors ranging from less than 1% up to less than 9% for the velocity estimation.

On-Earth testing of space scenarios is one of the key steps during the development of new technologies, being the reproduction of space-like conditions one of the main challenges. In this paper, a testbed designed and developed for testing of proximity operations scenarios such as rendezvous and docking (RvD) or formation flying is presented. The testbed is based on two industrial manipulators and has been developed and used for the validation of pose estimation algorithms for RvD with non-cooperative targets at the University of Wurzburg.