The Solar Orbiter spacecraft crossed Comet Leonard’s ion tail on 2021 December 17 near 1 au. In and near the ion tail, significant amounts of singly charged oxygen (O ^+ ) ions were detected by the Heavy Ion Sensor on board the spacecraft. These ions are likely the result of outgassed water molecules from the comet that became ionized and disassociated into protons and O ^+ ions and that were then picked up by the interplanetary magnetic field and advected outward with the solar wind. At this time, the spacecraft was also located amidst the denser parts of the interstellar helium (He) neutrals that are focused here by their gravitational interaction with the Sun. Pickup He ^+ ions in greater numbers are also found in this region and form when neutrals interact with solar photons. Newly ionized ions can generate waves that propagate mainly along the background magnetic field before the waves scatter the ions toward isotropy. Spectral magnetic field analyses are employed to find mainly elliptically polarized waves associated with O ^+ and He ^+ from ring-beam instabilities. Waves associated with He ^+ are identified, but O ^+ waves are not seen. Visibility is concluded to be affected by the relative amplitude of the waves to turbulence, and the visibility increases when the sampling direction is more aligned with the background magnetic field.

We present a 3D reconstruction of the ejecta plume generated by the DART impact on Dimorphos based on LUKE images acquired by LICIACube. Using adaptive histogram equalization and geometric coregistration from multiple vantage points, we identified and tracked extended ejecta features and reconstructed their 3D spatial distribution with a voxel-based method. The reconstructed ejecta field shows strong anisotropy. An azimuth-matched comparison with the published elliptical cone highlights coherent, direction-dependent departures from a simple conic surface, despite broad agreement on the global cone orientation. This quantifies nonaxisymmetric structures that cone models cannot capture. We derived lower limits to ejecta velocities ranging from 20 to 50 m s ^−1 , consistent with numerical simulations of hypervelocity impacts into weak, porous targets. Compared to laboratory experiments, these values correspond to few-meter launch depths, suggesting a significant role of near-surface boulders in the ejecta distribution. Complementing this ejecta filament analysis, we studied 105 isolated comoving diffuse ejecta features, tracked up to 7 km from the impact site. These features followed highly tangential (85 ^∘ –92 ^∘ from the impact direction) trajectories relative to Dimorphos’s surface. Their velocity distribution peaks around 50 m s ^−1 , indicating that crater excavation persisted throughout the imaging sequence. Our results challenge and refine previous cone-based ejecta distribution models, offering new insights into the complex nature of impact-induced ejecta and improving constraints relevant for planetary defense strategies. Given these new insights, we urge the designers of future planetary defense missions to take into consideration anisotropic ejecta models for more realistic estimates of the imparted momentum via a kinetic impactor technique.

We report ejecta mass estimates produced by the Double Asteroid Redirection Test impact with Dimorphos, the secondary of the Didymos asteroid binary system. This first demonstration of an asteroid deflection technique was witnessed by the Light Italian Cubesat for Imaging of Asteroids (LICIACube). The LICIACube Unit Key Explorer (LUKE) RGB camera images are used to estimate the ejecta mass, critical to understanding the impact conditions and physical properties of the asteroid. This requires realistic scattering properties for the ejecta particles, which were obtained from model and laboratory scattering analog particles (sizes <1 mu m to similar to 1 cm). The particle size distribution (PSD) of ejecta is constrained from a series of 18 images acquired at widely separated phase angles (50 .degrees 6 to 110 .degrees 2) using the size dependence of the analog scattering phase functions. Total plume radiance in the LUKE RGB channels is determined by spatially integrating over the field of view in each image. Plume radiances are then used to retrieve ejecta mass assuming an optically thin plume, giving lower limit estimates of 0.85 to 1.19 x 10(7) kg. Results indicate that the PSD of ejecta in the nascent plume follows a single power-law coefficient (k approximate to -2.5), rather than the broken power law inferred from later observations. Using a single, high signal-to-noise image, the observed areal mass-brightness relation is extrapolated inward, yielding an increase of approximate to 77% for ejecta mass in the optically thick inner region. Our ejecta mass estimates are consistent with Dimorphos having weak cohesive strength (<5000 Pa but more likely similar to 50 Pa).

Understanding the zenith angle dependence of the Martian surface radiation environment is crucial for planning future human exploration missions to Mars. In our previous research (Wimmer et al. 2015; Guo et al. 2021; Khaksarighiri et al. 2023) we extensively studied the zenith-angle dependence of the Martian surface radiation dose rate. Leveraging the same validated radiation model, calibrated with data from the Radiation Assessment Detector (RAD) on Mars, we calculated the flux of secondary downward particles reaching to the surface of Mars from various zenith angles resulting from the interaction of primary particles with the Martian atmosphere. These flux of secondary particles, coming from different zenith angles, can be integrated into a comprehensive topographic map of Mars, providing a detailed depiction of the global radiation landscape.The construction of this radiation map requires careful consideration of various factors, including atmospheric column density, local and large-scale topography offering potential shielding effects, and the input spectrum is affected by heliospheric modulation. Additionally, accounting for seasonal pressure cycles and daily atmospheric surface pressure due to thermal tides is essential. Our model specifically focused on the influence of zenith angle on atmospheric column depth and simulations tailored to the Gale Crater region, a region explored by the Curiosity rover. Applying this methodology allows us to create lookup tables of all secondary particles reaching the Martian surface from various zenith angles and evaluate the atmospheric impact. Employing these matrices alongside the incident spectrum enables the calculation of secondary particle flux from all zenith angles on the Martian surface.This method provides valuable insights into the fluctuations in radiation flux on Mars, facilitating thorough assessments of potential radiation hazards. Mission planners can leverage these data, obtaining vital information to identify secure landing areas and sheltered regions for astronauts on the Martian surface.

Kinetic deflection is a planetary defense technique delivering spacecraft momentum to a small body to deviate its course from Earth. The deflection efficiency depends on the impactor and target. Among them, the contribution of global curvature was poorly understood. The ejecta plume created by NASA's Double Asteroid Redirection Test impact on its target asteroid, Dimorphos, exhibited an elliptical shape almost aligned along its north-south direction. Here, we identify that this elliptical ejecta plume resulted from the target's curvature, reducing the momentum transfer to 44 ± 10% along the orbit track compared to an equivalent impact on a flat target. We also find lower kinetic deflection of impacts on smaller near-Earth objects due to higher curvature. A solution to mitigate low deflection efficiency is to apply multiple low-energy impactors rather than a single high-energy impactor. Rapid reconnaissance to acquire a target's properties before deflection enables determining the proper locations and timing of impacts.

On 26 September 2022 the Double Asteroid Redirection Test (DART) spacecraft collided with Dimorphos, the moon of the near-Earth asteroid 65803 Didymos, in a full-scale demonstration of a kinetic impactor concept. The companion LICIACube spacecraft documented the aftermath, capturing images of the expansion and evolution of the ejecta from 29 to 243 s after the impact. We present results from our analyses of these observations, including an improved reduction of the data and new absolute calibration, an updated LICIACube trajectory, and a detailed description of the events and phenomena that were recorded throughout the flyby. One notable aspect of the ejecta was the existence of clusters of boulders, up to 3.6 m in radius, that were ejected at speeds up to 52 m/s. Our analysis of the spatial distribution of 104 of these boulders suggests that they are likely the remnants of larger boulders shattered by the DART spacecraft in the first stages of the impact. The amount of momentum contained in these boulders is more than 3 times that of the DART spacecraft, and it is directed primarily to the south, almost perpendicular to the DART trajectory. Recoil of Dimorphos from the ejection of these boulders has the potential to change its orbital plane by up to a degree and to impart a non-principal axis component to its rotation state. Damping timescales for these phenomena are such that the Hera spacecraft, arriving at the system in 2026, should be able to measure these effects.

The topographic influence of the radiation environment on the Martian surface radiation is crucial for future human exploration. Topographic maps help assess radiation flux variations, aiding in hazard evaluation. Creating a global radiation map requires accounting for seasonally varying atmospheric density, heliospheric modulation, and topography. Here, we use a radiation model to derive the flux of secondary downward particles generated by the interaction of primary protons with the Martian atmosphere. Our model examines two key factors: (a) the dependence of atmospheric column depth on the zenith angle, affecting radiation directionality as horizon-arriving particles traverse more atmosphere than vertical ones and (b) atmospheric conditions at surface heights in Gale Crater, crucial for developing radiation dose maps that incorporate topographic effects. Our model is validated against Radiation Assessment Detector measurements and benchmarked with existing models. We construct response matrices representing the ratio of secondary particles at the Martian surface to primary inputs across zenith angles, assessing atmospheric effects. We combine these matrices with the incident spectrum to compute secondary particle fluxes from all zenith angles for Galactic Cosmic Rays and Solar Energetic Particles. These fluxes will be integrated into a topographic map of Mars in a follow-up study, providing a detailed representation of surface radiation levels across different terrains. This approach aids mission planners in identifying safe landing sites for astronauts.

Over decades of solar wind observations, heavy ions have been observed to have a higher temperature and flow faster than protons in the solar corona and heliosphere. Remote observations have largely been limited to the low corona (<4 R _⊙ ), while in situ observations for heavy ions ( Z > 2) have only been sampled at 1 au and beyond. As a result, theories that address heavy ion heating and acceleration remain largely unconstrained. With the launch of Solar Orbiter, heavy ion kinetics can be probed closer to the Sun, as close as the orbit of Mercury (65 R _⊙ ), to examine their radial behavior. Through a statistical analysis of O ^6+ , this work provides a comprehensive analysis of the velocity and temperature of O ^6+ from 0.3 to 1 au. The study finds that the O ^6+ relative drift, normalized to the local Alfvén speed, and its temperature compared to protons both decrease with distance from the Sun and show some speed dependence. The O ^6+ temperature is well fit by a single temperature adiabatic profile across all wind speeds, suggesting that there is no significant heating at these heliocentric distances, which is in contrast to what is observed for protons and He ^2+ . Alfvénic fluctuations, some with full 180 ^∘ field rotation, create momentary negative differential streaming where the speed of O ^6+ trails the protons. The amount of negative differential streaming gradually increases at larger distances. These results provide critical constraints on the proposed mechanisms seeking to describe ion heating and acceleration in the solar wind.

Kinetic deflection is a planetary defense technique that delivers spacecraft momentum to a small body to deviate its course from Earth. The deflection efficiency depends strongly on the impactor and target. Among them, the contribution of global curvature was poorly understood. The ejecta plume created by NASA's DART impact on its target asteroid, Dimorphos, exhibited an elliptical shape almost aligned along its north-south direction. Here, we identify that this elliptical ejecta plume resulted from the target’s curvature, reducing the momentum transfer to 44±10% along the orbit track compared to an equivalent impact on a flat target. We also find lower kinetic deflection of impacts on smaller Near-Earth objects (NEOs) due to higher curvature. A solution to mitigate low deflection efficiency is to apply multiple low-energy impactors rather than a single high-energy impactor. Rapid reconnaissance to acquire a target's properties before deflection enables determining the proper locations and timing of impacts.

NASA's Double Asteroid Redirection Test (DART) mission was the first to demonstrate asteroid deflection, and the mission's Level 1 requirements guided its planetary defense investigations. Here, we summarize DART's achievement of those requirements. On 2022 September 26, the DART spacecraft impacted Dimorphos, the secondary member of the Didymos near-Earth asteroid binary system, demonstrating an autonomously navigated kinetic impact into an asteroid with limited prior knowledge for planetary defense. Months of subsequent Earth-based observations showed that the binary orbital period was changed by –33.24 minutes, with two independent analysis methods each reporting a 1 σ uncertainty of 1.4 s. Dynamical models determined that the momentum enhancement factor, β , resulting from DART's kinetic impact test is between 2.4 and 4.9, depending on the mass of Dimorphos, which remains the largest source of uncertainty. Over five dozen telescopes across the globe and in space, along with the Light Italian CubeSat for Imaging of Asteroids, have contributed to DART's investigations. These combined investigations have addressed topics related to the ejecta, dynamics, impact event, and properties of both asteroids in the binary system. A year following DART's successful impact into Dimorphos, the mission has achieved its planetary defense requirements, although work to further understand DART's kinetic impact test and the Didymos system will continue. In particular, ESA's Hera mission is planned to perform extensive measurements in 2027 during its rendezvous with the Didymos–Dimorphos system, building on DART to advance our knowledge and continue the ongoing international collaboration for planetary defense.

Some active asteroids have been proposed to be formed as a result of impact events(1). Because active asteroids are generally discovered by chance only after their tails have fully formed, the process of how impact ejecta evolve into a tail has, to our knowledge, not been directly observed. The Double Asteroid Redirection Test (DART) mission of NASA(2), in addition to having successfully changed the orbital period of Dimorphos(3), demonstrated the activation process of an asteroid resulting from an impact under precisely known conditions. Here we report the observations of the DART impact ejecta with the Hubble Space Telescope from impact time T + 15 min to T + 18.5 days at spatial resolutions of around 2.1 km per pixel. Our observations reveal the complex evolution of the ejecta, which are first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and subsequently by solar radiation pressure. The lowest-speed ejecta dispersed through a sustained tail that had a consistent morphology with previously observed asteroid tails thought to be produced by an impact(4,5). The evolution of the ejecta after the controlled impact experiment of DART thus provides a framework for understanding the fundamental mechanisms that act on asteroids disrupted by a natural impact(1,6).

The Double Asteroid Redirection Test (DART) mission will impact the moon Dimorphos of the (65803) Didymos binary and demonstrate the kinetic impactor technique for planetary defense. Observations from the ride-along CubeSat companion, LICIACube, of the spatial structure and temporal evolution of the resulting ejecta plume will help determine the vector momentum transfer and constrain physical properties such as strength and porosity. The optical scattering properties of the ejecta particles used in plume simulations will dictate the critical relation between observed brightness and inferred ejecta mass (and momentum). Here we examine the scattering behavior of plausible analogs for Dimorphos ejecta particles, including laboratory-measured “millimeter grains” selected based on a comparison with Bennu particle photometry. Over the range of phase angles observable by LICIACube (≲120°), brightness simulations for optical depths ranging from 0.01 to 10 show that scattering from these analogs is considerably dimmer than from grains scattering isotropically. Therefore, adopting realistic scattering properties for ejecta particles is critical for accurately interpreting and understanding observations of the DART impact.

Around 17 December 2021, the Solar Orbiter spacecraft was predicted to have had its closest approach to comet C/2021 A1 (Leonard) with a minimum streamline distance < 0.01 AU. This encounter provided an unprecedented opportunity to investigate in situ comet Leonard's interaction with the solar wind and the composition of pick-up ions produced by ionization and dissociation of outgassed neutrals from its coma. It was a long-period comet originating from the Oort Cloud with a nucleus about 1 km in diameter, with ground-based telescope observations after its perihelion pass (at ~0.62 AU on 3 January 2022) indicating that it had subsequently disintegrated. Prior to perihelion, outbursts had been reported as well as variations in brightness, which had resulted in speculation about an impending disintegration. However, the dimming in November 2021, before the Solar Orbiter encounter, was argued to be due to a transition from outgassing dominated by carbon dioxide to water. Comet Leonard was the brightest comet of the year and noted for its spectacular ion tail with complex structures, including knots and streamers. Preliminary analysis of in situ Solar Orbiter observations have revealed tell-tale signatures of a cometary encounter around the time of predicted closest approach, such as evidence for magnetic field line draping. However, the clearest evidence has come from Solar Wind Analyzer-Heavy Ion Sensor (SWA-HIS) observations of singly-charged oxygen ions, which are typically not of solar origin and are usually produced when the solar wind interacts with a comet or other Solar System body. In this presentation we use SWA-HIS and EDP-STEP data to investigate aspects of the solar wind interaction and composition of cometary pick-up ions from this active, long-period comet shortly before its disintegration.

Aims. Solar Orbiter launched in February 2020 with the goal of revealing the connections between the Sun’s interior, atmosphere, and the heliosphere. The Solar Orbiter Heavy Ion Sensor (HIS) is a time-of-flight ion mass spectrometer dedicated to measuring heavy ions in the solar wind. Methods. We present an overview of the first measurements of heavy ion composition from HIS, reviewing the methods used to transform the spectra obtained on board into scientific data products and examining two solar wind case studies as well as the statistical properties of the heavy ion composition observed by HIS. We also carried out a comparison with prior measurements of heavy ions at L1. Results. The HIS data set provides the first mass- and charge-resolved heavy ion measurements in the inner heliosphere. Conclusions. These high temporal resolution data have the potential to transform our understanding of the connections between the solar wind and its origin at the Sun, as well as the interaction between the solar wind and the environment around planets, comets, and in the interstellar medium.

Lessons from the Apollo program showed that dust perturbed by human activities on the lunar surface can significantly interfere with the operation of mechanical, thermal and optical systems. During the Artemis program, monitoring the local dust environment created by surface activities will be critical to understanding and mitigating problems associated with lunar dust. This could be accomplished at many locations using in situ dust detectors; however, a complementary, and arguably more comprehensive, approach would be to measure the intensity of scattered sunlight from dust. Such measurements could be obtained using modest cameras and yield the abundance of dust along an observer line-of-sight. Observations along several look-directions would reveal the dust spatial distribution and constrain the minimum grain size from the angular width of the forward scattering lobe. Perhaps most importantly, these measurements would constrain spatial and temporal variations in dust ejection and deposition rates. Using scattering properties for realistically shaped lunar dust grains, this study simulates spectral intensities for a plausible steady-state distribution of low-speed (similar to m s(-1)) dust around an exploration site. This is used to assess the feasibility of using commonly available wide-angle optics and commercial off-the-shelf (COTS) image sensors to create a notional dust monitoring camera. The dust detection sensitivity of cameras constructed using components with flight heritage is modeled; e.g., the PL1 image sensor aboard the Light Italian CubeSat for Imaging of Asteroids (LICIACube) part of NASA's Double Asteroid Redirect Test (DART) mission. Results indicate that a simple but well-baffled camera can successfully detect a steady-state dust cloud created by human (artificial) activity with a peak concentration of < 1 mu g m(-3) for a polar site on the Moon-well below what might be expected during Artemis operations.

The existence of a “dense” lunar ionosphere has been controversial for decades. Positive ions produced from the lunar surface and exosphere are inferred to have densities that are ≲106 – 107 m−3 near the surface and smaller at higher altitudes, yet electron densities derived from radio occultation measurements occasionally exceed these values by orders of magnitude. For example, about 4% of the single-spacecraft radio occultation measurements from Kaguya/SELENE were consistent with peak electron densities of ~3×108 m−3. Space plasmas should be neutral on macroscopic scales, so this represents a substantial discrepancy. Aditional observations of electron densities in the lunar ionosphere are critical to resolving this longstanding paradox. Here we theoretically assess whether radio occultation observations using two-way coherent S-band radio signals from the Lunar Reconnaissance Orbiter (LRO) spacecraft could provide useful measurements of electron densities in the lunar ionosphere. We predict the uncertainty in a single LRO radio occultation measurement of electron density to be ~3×108 m−3, comparable to occasional observations by Kaguya/SELENE of a dense lunar ionosphere. Thus an individual profile from LRO is unlikely to reliably detect the lunar ionosphere; however, averages of multiple (~10) LRO profiles acquired under similar geophysical and viewing conditions should be able to make reliable detections. An observing rate of six ingress occultations per day (~2000 per year) could be achieved with minimal impact on current LRO operations. This rate compares favorably with the 378 observations reported from the single-spacecraft experiment on Kaguya/SELENE between November 2007 and June 2009. The large number of observations possible for LRO would be sufficient to permit wide-ranging investigations of spatial and temporal variations in the poorly understood lunar ionosphere. These findings strengthen efforts to conduct such observations with LRO.

All LIDAR instruments are not the same, and advancement of LIDAR technology requires an ongoing interest and demand from the community to foster further development of the required components. The purpose of this paper is to make the community aware of the need for further technical development, and the potential payoff of investing experimental time, money and thought into the next generation of LIDARs.