Nancy L. Chabot

On 2022 September 26, the NASA/DART spacecraft performed the first-ever test of the kinetic impactor technique by hitting Dimorphos, the moon of the asteroid (65803) Didymos. The Italian Space Agency (ASI)/Light Italian CubeSat for Imaging of Asteroids (LICIACube) witnessed the event in situ. In this work, we used images from both Didymos Reconnaissance and Asteroid Camera for Opnav (DRACO) and LICIACube Explorer Imaging for Asteroid (LEIA) instruments on board the two spacecraft to search for small objects that may be present around the primary before the impact, to provide hints on the complete dynamical characterization of the system. This will be compared to the future environment the European Space Agency (ESA)/Hera mission will encounter in late 2026. We found no unknown objects larger than ∼50 m in the entire Hill sphere. When considering closer regions with respect to the primary object, the limiting size gets smaller, reaching a minimum diameter equal to ∼2 m within a radial distance of ∼2 km from Didymos. Despite the dynamical environment in the system being extremely perturbed and unstable, there is still a small chance that large objects could find a stable niche in the phase space of the Didymos–Dimorphos system, but the null result of our work is a strong indication that this niche is either nonexistent or empty.

Mercury’s radar-bright polar deposits provide some of the strongest evidence for volatile accumulation on the innermost planet. Ground-based radar observations first identified highly reflective features associated with permanently shadowed regions, and MESSENGER later confirmed that many of these cold traps host water ice and other volatile-bearing materials. However, while recent work has shown that Mercury’s north polar deposits are heterogeneous at local scales, a comparable analysis of the south polar deposits has remained limited by the quality of available topographic, illumination, thermal, and radar data.Here we present ongoing work focused on a selected set of Mercury’s south polar craters, combining newly recovered Arecibo S-band radar datasets with refined and enhanced local digital elevation models derived from MESSENGER MDIS NAC images. A key component of this effort is the recovery of individual same-circular and opposite-circular polarization images, enabling an analogous analysis to recent north-polar work in which the two polarizations were used independently to characterize radar-bright deposits, scattering behavior, and local-scale heterogeneity. Additionally, high-resolution DEMs can now be used to measure the local radar incidence angle, allowing for radar scattering modeling that can be used to further constrain the properties of the deposits. A major challenge is the lack of MESSENGER altimetry at southern latitudes, leaving many scientifically important areas, including permanently shadowed regions, poorly sampled or unsampled. Rather than relying on simple interpolation, which can produce unrealistic crater morphologies, we apply inpainting techniques to recover plausible local topography while preserving consistency with the surrounding terrain. These enhanced DEMs support modeling of illumination, radar viewing geometry, and thermal environments, allowing us to examine how radar-bright deposits relate to local topography, incidence geometry, shadowing, and predicted temperatures, and to assess whether south polar deposits show spatial patterns comparable to those identified in the north.This work also provides a timely framework for the BepiColombo era. The refined DEMs, recovered Arecibo radar products, and combined radar–illumination–thermal analysis developed here can provide local context for future BepiColombo observations of Mercury’s south polar cold traps. New BepiColombo measurements will also offer an opportunity to test, refine, and extend these methods, enabling more detailed characterization of Mercury’s polar volatile inventory and its relationship to surface geology, thermal environment, and volatile delivery or preservation processes.

Origins, Worlds, and Life: Planetary Science and Astrobiology in the Next Decade identified a Uranus Orbiter and Probe as the highest-priority strategic mission for the decade 2023–2032, as it enables broad cross-disciplinary science in the largely unexplored Uranian system. The mission architecture evaluated by the Decadal Survey was a singular proof of concept demonstrating that a moderately instrumented mission could deliver Decadal-priority science with a reduced cost and risk posture by leveraging existing technologies to the maximum extent possible. With revised assumptions since the Decadal, we have explored a large trade space including launch vehicles, propulsion options, cruise trajectories, available power sources, viable concept of operations, and science data return for later launch dates without a Jupiter gravity assist. The most repeatable trajectory solutions employ either a commercially derived solar electric propulsion (SEP) transfer stage or the availability of a more capable launch vehicle under development, such as the SpaceX Starship. Orbit insertion has been moved farther from Uranus to acknowledge the remaining uncertainty in Uranian ring structure. A streamlined, SEP-adaptable, orbiter design was developed using two Next Gen Radioisotope Thermoelectric Generators, and the probe design was matured, reducing the entry gravitational acceleration, and assuming the largest Decadal-recommended payload to provide margin for future instrument selections. With this updated design, we also constructed a detailed concept of operations for three representative science cases, returning 13–15 Gbit of science data and spacecraft telemetry per ∼34 day orbit.

The Double Asteroid Redirection Test (DART) was NASA's first planetary defense test mission, designed to validate the technologies and methods associated with using a kinetic impactor to deflect asteroids that pose a threat to Earth. On September 26, 2022, DART intentionally collided with Dimorphos, the moonlet of the near-Earth asteroid Didymos. Observations over the following weeks and months confirmed that DART's impact changed Dimorphos's orbit around Didymos, reducing its speed by 2.6 mm$/$s. In addition, the DART impact provided the opportunity to observe the creation and evolution of the resulting ejecta, to investigate the dynamics within a perturbed binary system, and to study a natural-scale asteroid impact experiment. Additionally, the Didymos-Dimorphos system was the first binary near-Earth asteroid system investigated by spacecraft, and Dimorphos is the smallest asteroid visited by spacecraft to date. We detail the science and planetary defense findings of DART and look to future advancements. ▪ NASA's Double Asteroid Redirection Test (DART) mission was the first to demonstrate asteroid deflection, autonomously navigating to impact the asteroid Dimorphos on September 26, 2022. ▪ DART's successful planetary defense test validated that the kinetic impactor (KI) technique is a viable means to alter an asteroid's future path, which could potentially be used to prevent a future Earth impact. ▪ Analysis of the DART impact event and the resulting aftermath showed that the efficiency of the deflection achieved by the KI technique depended on the asteroid's properties. ▪ Prior information about the asteroid and its properties, such as by a reconnaissance mission, has the potential to allow optimization of the KI technique for asteroid deflection. ▪ Warning time is key for utilizing the KI technique to prevent a potential Earth impact, stressing the need for a planetary defense strategy that includes searching for asteroids and characterizing them in addition to developing mitigation approaches. ▪ In addition to advancing planetary defense objectives, the DART mission provided the first close-up visit to a binary asteroid system and measurements on the smallest asteroid visited by a spacecraft to date. ▪ The success of the DART mission relied on an interdisciplinary and international team, demonstrating the value of wide-ranging cooperation for planetary defense efforts.

Permanently shadowed regions near Mercury's poles are thought to harbor significant deposits of water ice, the origin of which remains to be conclusively determined. One leading hypothesis is that most of the water observed today may have been delivered by a relatively recent, volatile‐rich impact comparable in scale and age to that which formed Hokusai, a prominent, rayed, northern‐hemisphere crater 97 km in diameter. In this work, we model the transport, loss and deposition of water on Mercury in two different scenarios. We first perform a baseline simulation to update older estimates of the efficiency of exospheric transport of water on Mercury, and find that in the optically thin limit, ∼3% of gravitationally bound water released at perihelion from a point source in Hokusai crater (57.7°N, 16.8°E) migrates to polar cold traps, primarily at the nearest pole. In contrast, we find that a volatile‐rich, Hokusai‐scale impact gives rise to a transient, optically thick atmosphere in which lower photolysis rates due to atmospheric self‐shielding allow for ∼31% of gravitationally bound water to be cold‐trapped and more evenly distributed between north and south polar regions. Cold‐trap deposition is largely completed within one solar day, and the total mass of water deposited is within the range estimated to be present at Mercury's poles today. However, simulation results also suggest that in order to emplace deposits that are sufficiently thick to be radar bright, a larger, slower impact than the 17 km diameter, 30 km/s scenario modeled here may be necessary.

In September 2022, NASA’s Double Asteroid Redirection Test (DART) spacecraft crashed into Dimorphos and demonstrated the kinetic impact method of protecting Earth from asteroids. A fraction of the impulse delivered to Dimorphos was also imparted onto the Didymos system’s barycenter, changing its heliocentric orbit. Here, we present the first-ever measurement of human-caused change in the heliocentric orbit of a celestial body. Thanks to stellar occultation and radar measurements, we estimate that the Didymos system experienced an along-track velocity change of −11.7 ± 1.3 micrometers per second. We constrain the heliocentric momentum enhancement factor for DART at 2.0 ± 0.3 and the bulk densities of Didymos and Dimorphos at 2600 ± 140 and 1540 ± 220 kilograms per cubic meter, respectively. Our results demonstrate that targeting the secondary asteroid in binary systems constitutes a possible strategy for kinetic impact deflection, adding to humanity’s planetary defense capabilities.

On 26th September 2022, NASA’s Double Asteroid Redirection Test (DART) mission actively impacted the small moon Dimorphos of the Didymos-Dimorphos binary asteroid system. The impact caused a variation of about 33 minutes in its orbital period around Didymos and demonstrated the feasibility of moving an asteroid in space. As part of an international observing campaign devoted to its physical characterization, in this work we investigate the visible spectral properties of the Didymos-Dimorphos system soon after the impact. Rotationally-resolved spectra have been obtained 25 days after the impact and two months later. Geometric investigations have been performed to look for the source region corresponding to each spectrum. Here we show an overall agreement with pre-impact spectra and S-type classification despite subtle variations emerging, possibly correlated with mutual events. Furthermore a small long term color evolution from October to December spectra tentatively interpreted as related to dust cloud grain size effects or alternatively attributed to an uneven deposition of freshly exposed material from the impact on Didymos.

The Double Asteroid Redirection Test (DART) mission was a successful planetary defense demonstration of a kinetic impactor on Dimorphos, the satellite of binary near-Earth asteroid 63803 Didymos (Daly et al. 2023). The DART impact changed not only the orbit of the satellite Dimorphos about Didymos (Thomas et al. 2023), but also the orbit of the Didymos system about the Sun (Makadia et al. 2024). We report quantitative results of this heliocentric deflection, leading to a revised estimate of the momentum enhancement factor β as well as an estimate of the bulk density ρ of the target Dimorphos.In the months following the DART impact, a series of stellar occultation campaigns led to a total of 18 observed occultations of the Didymos system from 2022-Oct-15 to 2023-Jan-22. These observations represent an exquisite astrometric data set, with reported errors of no more than a few milliarcseconds. Three of these observations were reported with

There are a number of possible mitigation strategies that have been identified in the event a hazardous asteroid is discovered. NASA’s DART mission recently demonstrated the kinetic impactor technique [1]. The gravity tractor (GT) is attractive as the next technology for demonstration since other techniques may be prohibited by cost and legality [2]. A GT demonstration mission would align with NASA’s goal to "develop preliminary mission designs for future NEO deflection mission campaigns” [3]. Here, we present the design of a mission that would demonstrate a GT by changing the orbit of the secondary in an asteroid binary system.The GT concept for deflecting asteroids involves bringing a spacecraft near an asteroid and controlling the spacecraft so that the asteroid’s orbit is altered by the spacecraft’s gravity [4]. This slow-pull mitigation strategy can achieve greater precision in an asteroid’s post-deflection orbit than impulsive mitigation techniques. GT also has the benefits of being agnostic to the material properties of the asteroid and not requiring contact between the spacecraft and asteroid. A GT may be used as the “primary” mitigation technique for hazardous asteroids that are found sufficiently far in advance of their Earth impact dates, or as a “secondary” mitigation technique applied after a “primary” impulsive technique to ensure the avoidance of gravitational keyholes. Inspired by the success of the DART mission, we are studying whether GTs will be more easily tested in a binary asteroid system in the same way that kinetic impactors are: a small velocity change on the order of what would be necessary in a real emergency is more easily detected and measured on an asteroid satellite's orbit than it is on a single asteroid’s heliocentric orbit [5]. We report on the design of a GT demonstration mission to a binary asteroid system. We identify three main goals that a GT mission to a binary asteroid should achieve: (1) guide and navigate the spacecraft to the vicinity of the secondary and precisely control its relative position (within a few body radii), (2) measure the change in the secondary’s orbit due to the GT, (3) demonstrate long-duration tractoring operations in close proximity to the asteroid. We present mission requirements needed to achieve these mission goals. These requirements are used to define a concept of operations for a binary asteroid system “characterization phase” and “tractor phase,” which would lead to a measurable deflection of the secondary within a 12-month timeframe for asteroid proximity operations. We present the mission design and baseline payload that would meet these investigation requirements. In sum, this report outlines a demonstration mission of a GT at reasonable cost that will accomplish NASA’s goal of demonstrating a slow-pull asteroid deflection technique. References[1] Chabot, N., et al. (2024). Planet. Sci. J. 5 49[2] Abell, P. and Frazier (2021). Planetary Defense Missions: Rapid Mission Architecture Study. Planetary Science Decadal Survey: Mission Concept Study Report.[3] NASA Planetary Defense Strategy and Action Plan (2023). NASA. https://go.nasa.gov/3UO2mmt[4] Lu, E. T. and Love, S. G. (2005). Nature, 438, 177–178.[5] Merrill, C., et al., Planetary Defense Conference 2025.

Radar-bright features near Mercury’s poles were first identified through ground-based radar observations, which suggested the presence of water ice at or near the surface in these regions [1,2]. Subsequent data from MESSENGER revealed a strong correlation between these radar-bright deposits and deep impact craters that remain in persistent shadow – areas where sunlight never reaches the crater floors and where surface temperatures are low enough for water ice to be thermally stable [3,4]. This stability is especially likely if the ice is insulated by a layer of organic-rich volatile compounds, as suggested by topographic, thermal, and spectral measurements from MESSENGER [5-8].In these permanently shaded regions (Figure 1), the main source of illumination is solar Lyman-α radiation at 121.6 nm, produced by the scattering of solar photons by interplanetary hydrogen. This process generates a diffuse ultraviolet glow that uniformly bathes Mercury’s surface. If water ice is present, it is expected to induce detectable variations in surface albedo at this wavelength, as the Lyman-α reflectance from icy areas should be lower than that of the surrounding regolith.Figure 1: North Polar Stereographic view of Mercury. The pink circles represent the Permanent Shadowed Regions (PSRs). The EUV (Extreme Ultraviolet) detector of the PHEBUS (Probing the Hermean Exosphere By Ultraviolet Spectroscopy) instrument on board BepiColombo [9], operating in the 55–155 nm wavelength range, is well-suited to investigate this phenomenon. PHEBUS/EUV will enable the detection of variations of surface reflectance variations at Lyman-α, offering a unique opportunity to confirm the presence of surface or near-surface water ice in Mercury’s polar regions. In this study, we identify potential observation opportunities for these PSRs with PHEBUS/EUV during the nominal science phase of the BepiColombo mission, starting in April 2027. Observations of Mercury’s polar regions on the night side, near the terminator, with PHEBUS are scheduled during the so-called “terminator seasons” (Figure 2). However, these observations come with a risk that needs to be carefully considered: because the observations involve pointing close to the illuminated surface of Mercury, even a small misalignment could allow direct sunlight to enter the baffle and irreversibly damage the detector. Scheduling the observations toward the end of the mission would ensure that most exospheric data will have already been acquired, minimizing the risk of detector degradation.Figure 2: The diagram shows the orbit of Mercury as seen from the Sun's north pole and the polar orbits of BepiColombo (green). It also indicates the boundaries of the four seasonal phases during Mercury's 88-day orbital period: in blue, the perihelion season, in orange the aphelion season and in magenta, the terminator seasons. References:[1] Harmon, J. K., & Slade, M. A. (1992). Radar mapping of Mercury: Full-disk images and polar anomalies. Science, 258(5082), 640-643.[2] Slade, M. A., Butler, B. J., & Muhleman, D. O. (1992). Mercury radar imaging: Evidence for polar ice. Science, 258(5082), 635-640.[3] Chabot, N. L., Ernst, C. M., Denevi, B. W., et al. (2012). Areas of permanent shadow in Mercury's south polar region ascertained by MESSENGER orbital imaging. Geophysical Research Letters, 39(9).[4] Chabot, N. L., Ernst, C. M., Harmon, J. K., et al. (2013). Craters hosting radar‐bright deposits in Mercury's north polar region: Areas of persistent shadow determined from MESSENGER images. Journal of Geophysical Research: Planets, 118(1), 26-36.[5] Neumann, G. A., Cavanaugh, J. F., Sun, X., et al. (2013). Bright and dark polar deposits on Mercury: Evidence for surface volatiles. Science, 339(6117), 296-300.[6] Paige, D. A., Siegler, M. A., Harmon, J. K., et al. (2013). Thermal stability of volatiles in the north polar region of Mercury. Science, 339(6117), 300-303.[7] Chabot, N. L., Ernst, C. M., Denevi, B. W., et al. (2014). Images of surface volatiles in Mercury’s polar craters acquired by the MESSENGER spacecraft. Geology, 42(12), 1051-1054.[8] Chabot, N. L., Ernst, C. M., Paige, D. A., et al. (2016). Imaging Mercury's polar deposits during MESSENGER's low‐altitude campaign. Geophysical Research Letters, 43(18), 9461-9468.[9] Quémerais, E., Chaufray, J. Y., Koutroumpa, D., et al. (2020). PHEBUS on Bepi-Colombo: post-launch update and instrument performance. Space Science Reviews, 216, 1-23.