Michael C. Nolan

Unexpectedly following launch, the Lunar Trailblazer mission experienced software anomalies that led it to orient solar panels away from the sun and lose communication with Earth. This paper describes efforts to determine the spacecraft state and attempt recovery of the mission's science at the Moon. First, ground observatories at optical and radar wavelengths were engaged to maintain custody of the spacecraft and knowledge of its trajectory. Second, viability of recovery of the mission science objectives was established via testbed work to understand system behavior in fault conditions and determination of trajectories and maneuvers that would enable lunar orbit insertion. Third, optical photometry and radar doppler broadening were employed to determine Lunar Trailblazer's spin and orientation, using approaches similar to those in asteroid studies, to establish when solar panels might again receive sufficient power to boot the spacecraft and initialize its radio. Fourth, X‐band‐capable groundstations in addition to the NASA Deep Space Network were engaged to monitor for the spacecraft's radio carrier signal nearly continually, including crowd‐sourced monitoring and tip‐and‐cue style commanding. Lunar Trailblazer left the Earth‐Moon system and is in a 14‐year Earth return, heliocentric orbit. As it moved further away from Earth prospects for recovery became formidable; ultimately, the ability of the telecom system to return telemetry to Earth would have been insufficient to enable actions to recover the spacecraft, and the recovery attempt ended 6 July 2025. Lunar Trailblazer's mission recovery efforts illuminate capabilities in characterizing a 1–3.5 m ³ size non‐cooperative object in cis‐lunar space.

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.

Polarimetric radar measurements are sensitive to viewing geometry, wavelength-scale roughness, and composition. As a result, radar imaging enables characterization of planetary surfaces, but the multiple contributors to backscatter complicate the interpretation of metrics commonly treated as proxies for roughness, such as the circular polarization ratio (CPR). Motivated by renewed interest in lunar surface exploration, here we evaluate how meter-scale rock-related surface roughness influences polarimetric radar backscatter by comparing global monostatic S -band observations from LRO’s Mini-RF with Diviner-derived rock abundance (RA). We identified three scattering regimes across the lunar highlands and maria. Over rock-poor terrains, backscatter is insensitive to increasing RA and is controlled by incidence angle and, in the maria, TiO ₂ content. For 0.35 ≲ RA ≲ 1.5%, CPR increases rapidly with RA, loses sensitivity to incidence angle, and is driven by enhanced depolarized backscatter, consistent with an increasing population of wavelength-scale scatterers. Over rock-rich terrains, though, CPR plateaus and weakly decreases. Indeed, in some block-rich, fresh crater ejecta, CPR is negatively correlated with RA. In these terrains, the polarimetric response exhibits suppression of double-bounce and depolarized scattering, resulting in decreasing CPR. In contrast, some rock-rich regions sustain sufficient depolarized scattering to offset double-bounce suppression, resulting in a CPR plateau. In the maria, enhanced attenuation further modulates the balance between scattering mechanisms, leading to lower CPR. As such, we find that CPR is not a monotonic indicator of rock-related surface roughness, but an emergent property of competing scattering mechanisms reflective of target properties. Nevertheless, multiple polarimetric properties alongside CPR can resolve interpretive degeneracies.

Near-Earth asteroid (275677) 2000 RS₁₁ was observed over 5 days in March 2014 with both the Arecibo (2380 MHz, 12.6 cm) and Goldstone (8560 MHz, 3.5 cm) planetary radar systems. The continuous-wave spectra and delay-Doppler images collected revealed a sub-km-sized object with a strongly bifurcated shape. We used these radar observations, in combination with 7 optical lightcurves collected in 2014 and one lightcurve from 2023, to create a comprehensive shape and spin-state model for RS₁₁. We find a rotation period of P = (4.445 ± 0.001) hours around a pole of λ = (225 ± 80)^○ and β = ( − 80 ± 9)^○ relative to the plane of the ecliptic. The shape of RS₁₁ is unusual in that it does not resemble many of the other near-Earth asteroids modelled with ground-based radar. Whilst RS₁₁ consists of a largely spherical, smaller lobe attached to an elongated, larger lobe via a narrow neck, the smaller lobe is not aligned with the long axis of the larger lobe, but is closer to the larger lobe’s shortest principal axis. In combination with a large concavity observed on the outer face of the larger lobe, this may point to an unusual formation or event in the object’s past. We estimate that RS₁₁ has an geometric albedo of p_v = (0.16 ± 0.06) and a radar albedo 0.08 < η_OC < 0.16. Analysis of its gravitational environment reveals that for standard S-type asteroid densities, we would not expect rotational instability and it is possible for RS₁₁ to be a low tensile strength rubble-pile asteroid.

We investigate the feasibility of a spacecraft mission to conduct a flyby of 3I/ATLAS, the third macroscopic interstellar object discovered on 2025 July 1, as it traverses the solar system. There are both ready-to-launch spacecraft currently in storage on Earth, such as Janus, and spacecraft nearing the end of their missions at Mars. We calculate minimum Δ V single-impulse direct transfer trajectories to 3I/ATLAS both from Earth and from Mars. We consider launch dates spanning 2025 January through 2026 March to explore obtainable and hypothetical mission scenarios. Postdiscovery Earth departures require a challenging Δ V ≳ 24 km s ⁻¹ to fly by 3I/ATLAS. By contrast, Mars departures from 2025 July to 2025 September require Δ V ∼ 5 km s ⁻¹ to achieve an early October flyby—which is more feasible with existing propulsion capabilities. We further calculate the phase angle and flyby velocity for these trajectories, noting that the resulting flyby speeds would impose significant observational and engineering challenges that a mission would need to overcome. We discuss how existing spacecraft could be used to observe 3I/ATLAS and how spacecraft at other locations in the solar system could be repurposed to visit future interstellar objects on short notice.

Introduction: NASA’s Origins, Spectral Interpretation, Resource Identification, and Security– Regolith Explorer (OSIRIS–REx) mission characterized and collected a sample from asteroid (101955) Bennu [1,2]. Twenty minutes after releasing its sample return capsule to Earth’s surface on September 24, 2023, the spacecraft diverted into an orbit around the Sun that allows for subsequent close Earth flybys. On this trajectory, the spacecraft will approach Earth alongside asteroid (99942) Apophis in 2029, enabling a follow-on mission to another asteroid with the same unique capabilities that led to OSIRIS- REx’s groundbreaking scientific results: OSIRIS– Apophis Explorer, or “APEX” [3].Apophis will be well placed for an extensive ground-based telescopic campaign ahead of its Earth encounter on April 13, 2029, but observing conditions degrade just hours after the closest approach as the asteroid passes within 20° of the Sun. Immediate aftereffects of the tidal interaction will be impossible to examine from the ground—but will be distantly witnessed by APEX.Figure 1. View of the APEX spacecraft, Earth, and Apophis trajectories around the Sun during the cruise phase. APEX will use a series of Earth gravity assists (EGAs) to approach Apophis. APEX will follow the right-hand small ellipse 3 times around the Sun over 2 years. EGA3 alters APEX’s orbit to the larger yellow path, which brings the intersection point with Earth’s orbit to the left side of the figure after 1.5 orbits. EGA4 shifts the orbit to the left-hand small ellipse for 3 orbits over 2 Earth years. About one hour after Apophis’ close approach to Earth, EGA5 puts APEX on a nearly identical trajectory as Apophis. The Journey to Apophis: The Earth divert maneuver performed just after release of the OSIRIS- REx sample return capsule, and just before the mission’s second Earth gravity assist (EGA2), put the spacecraft on an eccentric orbit that passes to within 0.5 au of the Sun at perihelion (Fig. 1)—much closer to the Sun than the 0.77 au limit analyzed for the OSIRIS-REx mission. To mitigate this, the spacecraft adopts a “fig- leaf” attitude in which the solar arrays shadow critical spacecraft components when the spacecraft is within 0.65 au of the Sun (Fig. 2). As of January 2025, OSIRIS- APEX has successfully completed two of six close perihelion passages, and all instruments and spacecraft systems continue to operate nominally.Figure 2. The APEX spacecraft in the fig-leaf attitude as viewed from the Sun. The solar panels shade critical spacecraft components. Apophis Arrival and Proximity Operations: APEX will begin observing Apophis as a point source by April 2, 2029, at a distance of 5×106 km. Orbital mechanics prohibit an earlier rendezvous. APEX will observe Apophis from 50,000 km away during its close Earth encounter on April 13 and capture the evolution of its spin state in real time, revealing the consequences of a near-Earth object undergoing tidal disturbance by a planet (see presentation by Adam et al., this meeting) and continue observing as Apophis grows in its field of view. APEX will arrive at Apophis and begin its detailed study on June 5, 2029. Observations will uncover any signs of mass wasting that the tidal encounter triggered, revealing centimeter-scale topography via lidar and millimeter-scale morphology via images (Fig. 3),Figure 3. The APEX spacecraft was designed to obtain global high-spatial-resolution imagery of small asteroids.Chronicling the tidal encounter is only the beginning of APEX’s journey with Apophis. Having already challenged our fundamental understanding of carbonaceous (C-complex) asteroids during its exploration of Bennu, the spacecraft instrument suite will provide first-of-its-kind high-resolution data of a stony (S-complex) asteroid—dramatically advancing our knowledge of this asteroid class and its connection to the meteorite collection. Global spectral mapping at meter scales and across a wide range of wavelengths (0.4–100 μm) will determine the composition of Apophis and identify any volatiles on its surface. Optical and radiometric tracking data will reveal Apophis’ mass and structure. We will also search for signatures of mass shedding, whether due to the tidal encounter or an episodic process like that observed at Bennu. After 15 months of orbital operations, APEX will perform the Spacecraft Thruster Investigation of Regolith (STIR), mobilizing surface material by means of its backaway thrusters, as demonstrated at Bennu. Observations during and after excavation will provide otherwise inaccessible insight into space weathering and the surface strength of stony asteroids.Although scientific discovery is APEX’s prime motivator, Apophis’ bulk structure and surface strength have critical implications for planetary defense. As an S-complex object, Apophis represents the most common class of potentially hazardous asteroids, and knowledge of its properties can inform mitigation strategies. Monitoring Apophis after Earth approach provides the first opportunity to witness any change in an asteroid’s Yarkovsky force—a nongravitational effect that influences an asteroid’s likelihood of striking Earth.Acknowledgement: The OSIRIS-APEX mission is supported by NASA Contract NNM10AA11C.References:[1] Lauretta, D.S., DellaGiustina, D.N. et al. (2019). Nature 568, 55–60, doi:10.1038/s41586-019-1033-6.[2] Lauretta, D. S. et al. (2022). Science 377, 285–291, doi:10.1126/science.abm1018.[3] DellaGiustina, D. N. et al. (2023). PSJ 4, 198, doi:10.3847/PSJ/acf75e.

Studies of the regolith of near-Earth asteroids (NEAs) can give us insight into the evolutionary processes that affect these bodies. The regolith’s physical, mechanical, thermal, and scattering properties can tell us about, e.g., the asteroid’s collisional environment, its overall structure, how its surface has responded to space weathering, and the geophysics of the topographic features.Asteroid (433) Eros, with diameter 34-by-11-by-11 km, is in particular a useful target for such studies as it was extensively observed by the NEAR Shoemaker rendezvous mission, which had a visit with Eros in 2000-2001. NEAR firmly established (among other things) Eros’s shape, spin-state, reflectance, and albedo [e.g. 1,2,3,4]. All of these make it possible to put Earth-based observations of Eros, which are point-source hemispherical-averages, into better context.The work we report here is part of a broader study of Eros at wavelengths that were not sampled by NEAR. In earlier work [5], we analyzed an extensive dataset of 25 epochs of near-to-mid-IR spectroscopy. Each spectrum simultaneously sampled the reflected and thermal parts of the asteroid’s spectral energy distribution. Those data cover numerous sub-Earth latitudes and rotational longitudes, which, along with the known physical parameters, allowed us to use thermal modeling [6] to assess Eros’s global thermophysical properties as well as to look for hints of heterogeneity in the regolith.Here we report our analysis of multi-epoch radar observations of Eros, extending our investigation of regolith into the decimeter- and meter-scale [7,8]. Dual-polarization, S-band (at wavelength 12.6 cm) radar observations of Eros were made at the Arecibo Observatory 305-m telescope in early 2019. We have CW data from January 25 to 31 and February 8, 13, and 16, and delay-Doppler imaging from those January dates. We obtained data at each observing session for up to 2.6 h. Eros’s sub-Earth latitude was –30º to –20º during that time span. With so many observation epochs we saw virtually all Erosographic longitudes at least once. (Eros’s rotation period is 5.2703 h.) Eros was 0.21-0.26 au from Earth during this span. Our analysis so far has focused on two products. First, we have used the CW spectra to derive the rotationally-resolved SC/OC polarization ratio. We note that the ratio varies and is correlated with the broad-end vs short-end of the asteroid. Our second product is an assessment of the scattering law that is most applicable to Eros. Normally when interpreting radar data, the dependence of the differential radar cross section σ0 (a.k.a. dσ/dA) on the incidence angle must be assumed since there is no a-priori, independent shape model to use that is detailed enough to take its effect out. A typical assumption is that σ0 is proportional to the cosine of the incidence angle all raised to an assumed exponent [9,10]. Since Eros’s shape is already well-known, we can instead independently constrain this. We will present our analysis of Eros’s scattering behavior using both the CW and delay-Doppler data.Acknowledgments: We acknowledge support from NASA’s YORPD program via award 80NSSC21K0658, NASA’s SSERVI program via award 80NSSC19M0214, and from NSF’s AAG program via award 1856411. We also thank Arecibo Observatory staff and observers for their efforts in acquiring the data presented here. References: [1] Veverka, J. et al. (2000) Science, 289, 2088-2097, [2] Miller J. K. et al. (2001) Icarus, 155, 3-17, [3] Riner M. A. et al. (2008) Icarus, 198, 67-76, [4] Li J. et al. (2004) Icarus, 172, 415-431, [5] Hinkle M. L. et al. (2022) Icarus, 382, 114939, [6] Magri C. et al. (2018) Icarus, 303, 203-219, [7] Magri C. et al. (2001) Meteoritics & Planet. Sci., 36, 1697-1790, [8] Virkki A. et al. (2023) Remote Sensing, 15, 5605, [9] Mitchell D. L. et al. (1996) Icarus, 124, 113-133, [10] Nolan M. C. et al. (2013) Icarus, 226, 629-640.

The radar circular polarization ratio (CPR) is an indicator of near-surface, wavelength-scale complexity. Interestingly, previous surveys also noted an association between near-Earth asteroid (NEA) disk-integrated CPR and spectral taxonomic class, potentially reflecting differences in regolith mechanical properties and/or surface histories. Here, using Arecibo S-band radar measurements, we revisit this relationship, as well as associations with orbital and physical parameters, to test the impact of compositional class and surface processes on CPR. We found that the X complex, and potentially V-type NEAs, follow a bimodal CPR distribution, while the other studied taxonomies are unimodal. Considering S, C, Q, L, V, and X types, a CPR ≳ 1 is diagnostic of the high-CPR X complex, which includes all Xe types, while a CPR ≲ 0.4 is shared by S, C, Q, and L types. V types are not reliably identified by CPR alone. The high CPR tied to some X types may be reflective of significant roughness over multiple scales, but further analysis, including experimental insights, is needed to understand this signature. Furthermore, we found strong evidence that Aten NEAs have both a broader distribution and a higher median CPR than objects outside of Earth’s orbit. This may suggest that these Earth-crossing asteroids have their surficial physical properties altered by planetary encounters, indicating that such processes act over centimeter-to-decimeter scales. The 2029 close approach of NEA (99942) Apophis presents a unique opportunity to integrate ground-based radar observations with spacecraft measurements, such as by the Apophis Explorer (OSIRIS-APEX) mission, to reveal how resurfacing events alter NEA radar properties.