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Abstract:

NASA’s Lucy mission [1] successfully completed a flyby encounter with the main-belt asteroid (52246) Donaldjohanson on April 20, 2025, collecting data as part of a full-scale operational test for Lucy’s future Trojan encounters.  Donaldjohanson was known to be a C-type asteroid and based on our ground-based observations, to have a Fe-bearing phyllosilicate 0.7 µm absorption. Similar absorptions in spectra of CI, CM, and CR carbonaceous chondrites are indicative of aqueously altered mafic silicates [2-4]. Donaldjohanson is also a member of the 155 Mya Erigone family [5], which is dominated by objects that have also been inferred to be aqueously altered based on their visible 0.7 µm absorptions [6].The Multi-spectral Visible Imaging Camera (MVIC), part of Lucy’s L’Ralph instrument [7-8], was specifically designed to include a filter covering the 0.7 µm absorption to detect evidence of aqueous alteration on the mission’s primary Trojan targets. The Donaldjohanson encounter is thus an excellent opportunity to compare the performance and calibration of MVIC to ground-based data. Here, we will report on both these validation efforts and our exploration of the spatial variability of the 0.7 µm phyllosilicate absorption across the imaged surface of Donaldjohanson to understand potential variability with surface features and photometry, and in relation to other Erigone family objects.References: [1] Levison et al. (2021) PSJ. [2] Cloutis et al. (2011a) Icarus. [3] Cloutis et al. (2011b) Icarus. [4] Cloutis et al. (2012) Icarus. [5] Marchi et al., (2025) PSJ. [6] Morate, D., et al. (2016) A&A. [5] Reuter et al. (2023), SSR. [6] Simon, A.A., et al. 2025 PSJ.Acknowledgments: The Lucy mission is funded through the NASA Discovery Program (Contract No. NNM16AA08C).

Abstract:

Introduction: On the 20th of April 2025, NASA’s Lucy mission [1] flew by the C-type main-belt asteroid (52246) Donaldjohanson (hereafter DJ). The encounter’s goal was to test the spacecraft and instruments during an observation sequence commensurate with those to be used on Lucy’s main targets – Jupiter’s Trojan asteroids. Data returned from the panchromatic Lucy LOng Range Reconnaissance Imager (L’LORRI, 450-850 nm, [2]) during this testing sequence reveal the asteroid to be bi-lobed and elongated shape (Fig. 1).DJ is a member of the Erigone collisional family, named after the parent body asteroid (163) Erigone (see references in [3]). Ground-based color observations (Fig. 2) show it to decrease in color towards shorter wavelengths, possibly due to the presence of hydrated materials [4].In this work, we present an analysis of color images taken by Lucy’s Multispectral Visible Imaging Camera (MVIC). MVIC consists of six time delay integration (TDI) charge-coupled devices (CCDs). TDI works by synchronizing the transfer rate of the image between CCD rows and the relative motion of the instrument allowing a high signal to noise image to be built up even for fast scans. It covers wavelengths between 375 nm and 950 nm using five color filters and a panchromatic one (see Table 1).Color Analysis: We focus our analysis on images acquired with the four wide band filters: violet, green, orange and near-IR. Our results will provide resolved color variations and contextualise DJ’s color with respect to ground-based observations of DJ, Erigone (Fig. 2), other members of the Erigone family, and the broader asteroid and small body populations.Filter Wavelength Violet 375-480 Green 480-520 Orange 520-625 Phyllosilicate 625-750 Near-IR 750-950 Panchromatic 350-950 Table 1 – MVIC filters [5]Figure 1 – (52246) Donaldjohanson as seen by the panchromatic Lucy L’LORRI instrument, taken on April 20, 2025 at 17:51 UTC.  Figure 2 – Ground-based normalized (at 0.55 µm) visible spectrum of DJ (blue) acquired with the Gran Telescopio Canarias compared to the Bus-DeMeo’s Cg-type (black) and the mean spectrum of the C-type members within the Erigone family (grey). Taken from [6]. Acknowledgments: The Lucy mission is funded through the NASA Discovery program on contract No. NNM16AA08C.References: [1] Levison et al. (2021) PSJ 2, 171. [2] Weaver et al. (2023), SSR 219, 82. [3] Marchi et al., (2025) PSJ 6, 59. [4] Vilas (1995) Icarus 115, 217-218. [5] Reuter et al. (2023), SSR 219, 69. [6] Souza-Feliciano et al. (2020), Icarus 338, 113463.

Abstract:

On November 1, 2023, NASA’s Lucy spacecraft successfully imaged the Main-Belt asteroid (152830) Dinkinesh and its moon, Selam. Dinkinesh is an S- or Sq-type asteroid with multiple geologic features (i.e., craters, central ridge, and trough) [1].  The Dinkinesh system is complex, with satellite that itself is a contact binary [1]. Broadband visible (0.35-0.95 µm) and near-IR (0.97-3.95 µm) hyperspectral images collected by the L’Ralph instrument showed absorption features near 1-, 2-, and 3-µm [2, 3].  The vibrational absorption between 2.6 and 3.3 µm in asteroid spectra has generally been interpreted as OH and H2O (i.e., hydration). This ~3.0 µm band, has been a crucial tool of characterization to understand the degree of hydration on the surface of asteroids [4]. Detection of hydration or volatile-rich materials on S-type objects is surprising due to the expected high temperature at which these bodies formed in the main-belt and presence of anhydrous silicates. Ground-based facilities have provided crucial detections and insights about the 3.0 µm band on S-type asteroids [5,6], yet much remains unknown about its origin. Dinkinesh’s close approach by Lucy offers a fortuitous opportunity to better understand the hydration of these bodies and assess any spatial variation on the surface that might be related to geologic features. The Lucy L’Ralph Dinkinesh observations can help differentiate the source of hydration. For example, exogenous material (e.g., carbonaceous or cometary material) is expected to appear in discrete areas associated with specific surface features such as craters [7]. Alternatively, solar wind implantation on asteroids occurs when high H+ fluxes doses from the Sun interact with surface minerals, embedding hydrogen atoms and potentially leading to the formation of OH or H2O in the regolith [8]. We will report on the spectral analysis of Dinkinesh, with a focus on the shape model registration of hyperspectral images from the L’Ralph Multi-spectral Visible Imaging Camera (MVIC) and Linear Etalon Imaging Spectral Array (LEISA). We will present colors, spectral slopes, and band depth to look for possible spectral heterogeneities associated with geologic morphologies. Results: We registered the digital shape model of Dinkinesh to the L’Ralph instrument detectors. Figure 1 shows a preliminary example of the MVIC panchromatic filter frame during the close approach registered to the respective incidence angle backplane obtained using SpiceyPy [9]. Figure 2 shows an example of a LEISA-calibrated frame (e.g., I/F) registered to Dinkinesh’s shape model.  After registration, the 3 µm absorption feature is analyzed for each facet by computing the absorption strength (e.g., band depth) and looking for correlations with surface morphologies provided by stereophotogrammetry of L’LORRI images. Similarly, we obtained MVIC color maps and overlayed them on the shape model. Our preliminary analysis suggests a 3 µm detection across the entire imaged surface, showing variabilities in band depth. We will further explore such variability to find its possible relationship with surface morphologies, local color variations, and illumination geometry.Figure 1. MVIC panchromatic frame of Dinkinesh overlayed with the SpiceyPy incidence angle backplane.Figure 2. Left: Dinkinesh shape model with overlayed LEISA cross-track I/F frame 700 during close approach.  [1] Levison, H.F. et al. 2024. A contact binary satellite of the asteroid (152830)Dinkinesh. Nature 629, 1015–1020.[2] Simon, A. et al. 2025. Lucy L'Ralph In-flight Calibration and Results at (152830) Dinkinesh. Planet. Sci. J.  6, 7.[3] Kaplan, H., et al. 2024.  "Multi-spectral imaging observations of asteroid (152830) Dinkinesh by the Lucy Mission." Proceedings of the Lunar and Planetary Science Conference 2024,abstract #1474. Houston, TX: Lunar and Planetary Institute.[4] Rivkin, A. S. et al. 2018. Evidence for OH or H2O on thesurface of 433 Eros and 1036 Ganymed. Icarus 304, 74–82.[5] McGraw, L. E. et al. 2022. 3 μm Spectroscopic Survey of Near-Earth Asteroids. Planet. Sci. J. 3, 243.[6] McAdam, M. et al. 2024. Detection of Hydration on Nominally Anhydrous S-complex Main Belt Asteroids. Planet. Sci. J. 5, 254.[7] De Sanctis, M. C. et al. 2015. Mineralogy of Marcia, the youngest large crater of Vesta: Character and distribution of pyroxenes and hydrated material. Icarus 248, 392–406.[8] Hibbits, C. A., et al., 2011. Thermal stability of water and hydroxyl on the surface of the Moon from temperature-programmed desorption measurements of lunar analog materials. Icarus, 213, 64-72.[9] Annex, A. M., et al., 2020. SpiceyPy: a Pythonic Wrapper for the SPICE Toolkit. Journal of Open Source Software, 46, 2050.

Abstract:

Introduction: Understanding surface temperatures on airless planetary bodies is crucial for interpreting thermal observations and constraining surface properties. We present TEMPEST (Thermal Evolution Model for Planetary Environment Surface Temperatures), a modular, open-source Python model that simulates diurnal and non-periodic thermal evolution on irregular bodies. Unlike traditional 1D periodic solvers, TEMPEST handles transient heating events such as eclipses, non-synchronous rotations such as tumbling asteroids, and seasonal variations. Key capabilities include surface roughness modelling via hemispherical craters, multiple thermal conduction schemes, and modular scattering using lookup tables (LUTs). TEMPEST has been used to analyse data from the Lucy mission and has been validated against the well-established Spencer 1D thermal model, thermprojrs [1].Figure 1: TEMPEST allows the user to select a facet to view any of its time varying properties including insolation, temperature and radiance. The diurnal temperature curves (right) are those of the corresponding outlined facets selected by the user in the interactive pane (left).Methods: TEMPEST calculates surface temperatures by solving a surface energy balance that includes solar flux, thermal emission, vertical heat conduction, and (optionally) radiative self-heating. Figure 1 shows the user interface once the model has completed a run. Key components include:Thermal solvers: Includes a standard 1D periodic conduction scheme influenced by the widely used thermprojrs [1] and a non-equilibrium solver, designed for better performance and stability in non-periodic cases. Scattering treatments: Utilises precomputed LUTs for various scattering laws (e.g., Lambertian, Lommel-Seeliger). This structure allows users to incorporate empirical bi-directional reflectance function (BRDF) data (e.g., from goniometer measurements of lunar regolith) or test the impact of different scattering assumptions, which can be particularly important for investigating the temperature of shadowed regions, as shown in Figure 2. The modularity also facilitates user modification for specific research needs. Surface roughness: Implemented via hemispherical sub-facet craters with adjustable rim angle to match roughness with a specified RMS slope angle. Non-periodic and time-dependent conditions: Supports time-dependent boundary conditions, including periodic scenarios such as eclipses and seasonal variations due to orbital eccentricity, as well as non-periodic cases including tumbling rotation, endogenic heating, and, or other user-defined transient heating scenarios. Designed for efficient parallel execution, the model runs effectively on multi-core personal computers and can efficiently simulate shape models with tens of thousands of facets. It has also been deployed on high-performance computing clusters for larger-scale models on the order of 1 million facets. Input configuration files are simple and flexible, allowing integration into larger analysis pipelines.Figure 2: An example insolation curve from a 1666 facet model of the bilobate comet 67P. The effects of scattered light can be seen either side of the main peak, this is particularly important for permanently shadowed regions. The selected facet is shown with a blue outline; sunlight direction is shown with a yellow arrow.Results: We validated TEMPEST by comparing temperature time series with Spencer’s 1D model thermprojrs [1] under idealised conditions, showing consistent results – see Figure 3. Applied to high-resolution shape models of 67P/Churyumov-Gerasimenko and 101955 Bennu, the model produces detailed temperature maps that reflect the significant influence of self-shadowing and local geometry, quantifying, for example, the temperature reduction in shadowed craters. Non-periodic simulations have been run to explore rotational transitions and eclipse effects, enabling new modes of comparison with observational datasets. The modular scattering and roughness components offer a powerful way to assess how sub-resolution scale parameters impact apparent thermal inertia and surface radiative behaviour. TEMPEST is already being used to interpret thermal data from recent missions, including Lucy, and can be adapted for upcoming datasets from targets like those of Comet Interceptor and Europa Clipper.Figure 3: TEMPEST shows good agreement with ‘industry standard’ thermophysical models in 1 dimension.TEMPEST is open-source and available at:github.com/duncanLyster/TEMPEST/Acknowledgement: This work was made possible by 91̽�� from the UK Science and Technology Facilities Council. References:[1] Spencer, J.R., Lebofsky, L.A., and Sykes, M.V., 1989. Systematic biases in radiometric diameter determinations. Icarus, 78(2), pp.337-354.[2] Lyster, D., Howett, C., & Penn, J. (2024). Predicting surface temperatures on airless bodies: An open-source Python tool. EPSC Abstracts, 18, EPSC2024-1121.[3] Lyster, D.G., Howett, C.J.A., Spencer, J.R., Emery, J.P., Byron, B., et al. (2025). Thermal Modelling of the Flyby of Binary Main Belt Asteroid (152830) Dinkinesh by NASA’s Lucy Mission. Submitted to EPSC Abstracts, 2025.

Abstract:

Introduction: The Lucy mission's first asteroid flyby provided a unique and unexpected opportunity to study a binary asteroid system up close. Originally expected to encounter a single target, Dinkinesh, the discovery of its small, tidally locked moon, Selam, introduced additional opportunity and complexity to the interpreting flyby observations [1]. We present thermal modelling of the binary system, quantifying how the presence of Selam influenced radiance measurements and indicating its possible impact on thermal inertia estimates. Thermal inertia (TI) offers insight into surface properties such as grain size and regolith structure. Determining the TI of Dinkinesh adds to our understanding of small S-type asteroids and enables comparison within a binary, potentially revealing differences driven by tidal effects or surface evolution.Methods: We modelled the flyby geometry and instrument measurements using the new TESBY (Thermal Emissions Spectrometer flyBY) module of TEMPEST (the Thermophysical Equilibrium Model for Planetary Environment Surface Temperatures) [2] to simulate the thermal radiance of both bodies and assess their combined effect on interpretation of data from the Lucy Thermal Emission Spectrometer (L’TES) instrument [3].The Thermal Model: Dinkinesh and its satellite, Selam, were modelled in TEMPEST. A stereo-photogrammetric shape model is available for the primary target – Dinkinesh [4], with ~2 m lateral and ~0.5 m vertical resolution, covering ~60% of the surface. This shape model was downsampled to a dimensionally accurate model with 1266 facets with a resolution of ~35 m. A sphere of representative diameter (230 m [1]) was used for the satellite Selam.Figure 1: TESBY visualization of flyby. Global view of the flyby trajectory (left), and the FOV of the instrument (centre), with corresponding L’LORRI image for comparison [1] taken 0.54 minutes before closest approach (right). Input is the TEMPEST [5] result for the shape model of Dinkinesh, and representative diameter sphere for Selam. Parameters used: solar distance = 2.19 AU, rotation periods = 3.74 hours (Dinkinesh) and 52.7 hours (Selam) [1] thermal inertia (provisional) = 40 J m-2 s-1/2 K-1, geometric albedo = 0.27Flyby geometry: Building on the TEMPEST framework, the TESBY module is given the geometry information for the flyby and the thermal data from TEMPEST. Based on the 7.3 mrad Field-of-View (FOV) of the L’TES instrument [3] TESBY produces simulated radiance measurements by computing a weighted sum of blackbody curves from each visible facet, based on its temperature, projected area, and emission angle. Matching these modelled radiances to the instrument data allows us to fit for the thermal inertia of the asteroid. A complicating factor in this study is that the sensitivity of L’TES is not uniform across its FOV, including this effect in the model is the subject of ongoing work.Figure 2: Preliminary modelled radiance results (blue line) compared to L’TES observation (red) using the same model settings as Fig. 1. Scaled radiances (dotted line) are also provided (see main text for more information).Results: An example of the currently predicted model radiance is given by Figure 2. As it shows, there is a notable offset between the predicted and observed radiances. Accounting for the position of the targets in the L’TES FOV is expected to resolve the observed discrepancy in absolute radiance levels. However, as the scaled model shows, the predicted radiances are able to capture the shape of the L’TES radiance.We find that due to the slower rotation rate of Selam, the maximum surface temperatures on the satellite can be as much as 25 K higher than those on Dinkinesh (Fig. 1), meaning that despite the small size (lobe diameter of only 230 m, compared with 790 m for Dinkinesh [1]), the contribution to measured radiance is significant. This effect is highlighted by investigation of the integrated radiances of the targets throughout the flyby (Fig. 3), where the entry and exit of Selam within the FOV is visible, as well as the dip in integrated radiance while Selam is partially eclipsed by Dinkinesh. Our results demonstrate the importance of considering the full system in flyby analysis, informing techniques for similar encounters in the future. This work highlights how the thermal signature of even a small secondary body can significantly impact observations, shaping our understanding of asteroid surface properties and thermal environments.Continued analysis will focus on the use of TEMPEST/TESBY to constrain the thermal inertia of this binary asteroid from L’TES flyby observations.  Figure 3: Variation in integrated wavelength for Dinkinesh (target, blue), Selam (satellite, red) and combined effect (green). Radiances were integrated over the 200–1500 cm⁻¹ spectral range. The results show that despite its small size, Selam makes a significant difference to the spectral radiance, particularly at shorter wavelengths. The dip in combined spectral radiance at observations 3315-3320 is due to Selam being eclipsed by Dinkinesh.The thermal model code is open source and available at: github.com/duncanLyster/TEMPEST/Acknowledgement: This work was made possible by 91̽�� from the UK Science and Technology Facilities Council.  References: [1] Levison, H.F., Marchi, S., Noll, K.S. et al. A contact binary satellite of the asteroid (152830) Dinkinesh. Nature 629, 1015–1020 (2024).[2] Lyster, D., Howett, C., & Penn, J. (2024). Predicting surface temperatures on airless bodies: An open-source Python tool. EPSC Abstracts, 18, EPSC2024-1121.[3] Christensen, P. R., et al. The Lucy Thermal Emission Spectrometer (L’TES) Instrument, Space Sci. Rev. (2023)[4] Preusker, F. et al. (2024). Shape Model of Asteroid (152830) Dinkinesh from Photogrammetric Analysis of Lucy’s Frame Camera L’LORRI. 55th Lunar and Planetary Science Conference, Abstract #1903.[5] Lyster, D., Howett, C., & Penn, J. (2025). TEMPEST: A Modular Thermophysical Model for Airless Bodies with Support for Surface Roughness and Non-Periodic Heating. Submitted to EPSC Abstracts, 2025