Near-Earth asteroid (275677) 2000 RS11 was observed over 5 d 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 seven optical light curves collected in 2014 and one light curve from 2023, to create a comprehensive shape and spin-state model for RS11. We find a rotation period of P = (4 . 445 +/- 0 . 001) h around a pole of lambda = (225 +/- 80) degrees and beta = (-80 +/- 9) degrees 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 a geometric albedo of pv = (0 . 16 +/- 0 . 06) and a radar albedo 0 . 08 < r7OC < 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 present JWST Near Infrared Spectrograph (NIRSpec) measurements of the three largest low-albedo main-belt asteroids: (1) Ceres, (2) Pallas, and (10) Hygiea. Their reflectance spectra all have very similar absorptions centered near 2.72 μ m attributed to Mg–OH in minerals. Within this band, Pallas also shows evidence of a sharper, deeper band, also centered near 2.72 μ m. These band positions are similar to those seen in the most aqueously altered carbonaceous chondrites and samples from Ryugu and Bennu. Absorptions in the 2.7–2.9 μ m region due to other cation–OH combinations are weak, if present. The NIRSpec spectrum of Ceres is consistent with the global average spectrum of Dawn, and the similarity between Ceres and Hygiea seen in other wavelength regions continues into the 2.5–2.8 μ m region. This similarity in spectral properties, and thus in interpretations of surface composition, implies that the two bodies may have had similar processes occur and similar histories. This suggests that Hygiea, similar to Ceres, may be associated with the “ocean worlds” despite its relatively small mass. Quantitative estimates of the hydrogen concentrations on the surfaces suggest hydrogen concentrations of roughly 0.5–1 wt%, consistent with CM chondrites. Additional absorptions attributed to ammoniated minerals are seen in Ceres’s and Hygiea’s spectra, as has been reported by others, but are not seen in Pallas’s spectrum. Absorptions are also seen in the 2.5–2.7 μ m region in all three asteroids, likely due to OH combination bands, and from roughly 3.9 to 4.3 μ m in Hygiea, which could be due to carbonates plus an unidentified constituent.

Understanding the properties of near-Earth asteroids (NEAs) is key for many aspects of planetary science, particularly planetary defense. Our current knowledge of NEA sizes and regolith properties is heavily dependent on simple thermal models. These models are often used to analyze data from missions such as NEOWISE because they are well suited to deal with large volumes of data. However, simple model results based on NEOWISE data may be inconsistent with results based on other types of observation in some cases. In this work, we seek to better understand these potential inconsistencies, as well as the situations for which they are most prevalent. We do this by comparing simple model results based on Infrared Telescope Facility SpeX data to similar results based on NEOWISE data. This is carried out for six NEAs that represent a range of spectral types, shapes, and rotation states. We find that models based on SpeX and NEOWISE data for these six objects are inconsistent in some cases, even though the SpeX results are consistent with other methods and observations. We find that objects observed at fainter magnitudes and objects with more primitive compositions are more likely to produce inconsistent fits. These results highlight the importance of better understanding the limitations of simple models as applied to large survey data sets like NEOWISE. This is particularly true as we move into an era where our understanding of the NEA population will be dominated by future large surveys such as NEO Surveyor.

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 greater than or similar to 1 is diagnostic of the high-CPR X complex, which includes all Xe types, while a CPR less than or similar to 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.

OVIRS [1, 2] acquired visible to near-infrared spectra of asteroid Bennu’s surface showing an asymmetric absorption band centered at 2.74 ± 0.01 μm [3], attributed to the presence of hydrated phyllosilicates. This feature is widespread across Bennu’s surface. Such an absorption band has been detected in some carbonaceous chondrite meteorites [4, 5].In this study, we report the results from two distinct methods to estimate the hydration of Bennu’s surface. We calculated the normalized optical path length (NOPL) as well as the effective single particle absorption thickness (ESPAT) [6, 7, 8] on Bennu’s hydration band and on the selected meteorite spectra. For both methods, we compare meteorite results with their H2O/OH– H content, to estimate a H2O/OH– H content of Bennu’s average surface. Carbonaceous chondrite meteorite H2O/OH– H contents are derived from laboratory studies [9, 10]. Bennu spectra. Analysed spectra were acquired by OVIRS during Equatorial Station 3 (EQ3) of the Detailed Survey mission phase, on May 9, 2019, at 12:30 pm local solar time [11]. The reflectance spectra have been calibrated and photometrically corrected to an incidence angle of 0°, emission angle of 30°, and phase angle of 30°, using a McEwen photometrical model [12].Meteorite spectra. We used three sets of meteorite absolute reflectance spectra, from [4,5], [8], and [13]. For each set, powdered meteorite sample spectra were measured under vacuum (asteroid-like conditions).We selected over 40 meteorites for which bulk H values have been independently measured [9, 10]. In the case of Orgueil, Bells, and Tagish Lake, several samples were analysed and several H contents were ultimately derived [9, 14], all of which were used. Normalized Optical Path Length (NOPL). The NOPL parameter was calculated as described in [6, 7, 8] on each meteorite spectrum, each individual Bennu reflectance spectrum, and the global average spectrum of Bennu. A linear continuum was fitted from 2.67 to 3.3 μm. The wavelength, at which the NOPL parameter is calculated, is the mean band minimum position for the EQ3 data set at 2.73 μm. Methods used to locate the band minimum are described in [3].Effective Single Particle Absorption Thickness (ESPAT). The ESPAT parameter was calculated following the method of [6, 7, 8]. Absolute reflectance spectra of meteorites and Bennu’s surface were first converted into single-scattering albedo spectra [6, 7, 8]. A linear continuum was then fitted from 2.67 to 3.3 μm and the ESPAT parameter is calculated at 2.73 μm as well. Our analyses do not include the organic absorption bands, present longwards of ~3.3 μm [11, 15]. Thus, we compare NOPL and ESPAT results with the hydrogen content of H2O/OH– groups in hydrated phyllosilicates only, measured for the selected meteorites [9, 10]. Figure 1 shows the NOPL parameter variations across Bennu’s surface using EQ3 spectra.Figure 1: Map of NOPL values computed at 2.73 μm for each EQ3 spectrum of Bennu.We find a linear correlation (Figure 2) between the NOPL parameter calculated at 2.73 μm on meteorite spectra and the meteorite H2O/OH– H content.Using this linear correlation, for the NOPL calculated on Bennu’s EQ3 average spectrum, we estimate a H2O/OH– H content for Bennu’s average surface of 0.54 ± 0.11 wt.%. Figure 2: Linear correlation between NOPL calculated at 2.73 μm and H2O/OH– H content of the seven selected meteorites (in colored points), and for average Bennu (blue circle).As with the NOPL parameter, we also find a linear correlation between the ESPAT parameter calculated at 2.73 μm on meteorite spectra and the meteorite H2O/OH– H content. We therefore estimate a H2O/OH– H content for Bennu’s average surface of 0.49 ± 0.13 wt.%, using Bennu’s EQ3 mean ESPAT value and the latter correlation. Discussion and ConclusionThe H2O/OH– H content for Bennu’s average surface obtained using NOPL parameters is consistent with the range obtained with the ESPAT parameter. Both methods are based on estimating global H content (in H2O/OH– groups of hydrated phyllosilicates) by analogy with meteorite data. The values of H2O/OH– H content of Bennu’s average surface we obtained are 0.54 ± 0.11 and 0.49 ± 0.13 wt.% using the NOPL parameter and the ESPAT parameter, respectively. From our results (Figure 2), Bennu’s average surface is most similar to heated CMs and Tagish Lake. Both estimated H2O/OH– H content ranges of Bennu’s average surface are more consistent with those of CM meteorites (0.46–1.36 wt%), Tagish Lake (0.50–0.69 wt.%), CR meteorites (0.30–1.20 wt.%), and CO meteorites (0.49–0.52 wt.%) [3, 9]. The gaussian modeling of the hydration band will complete those results. AcknowledgementsThis material is based on work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. AP, MAB, FM, SF, PH and JDPD acknowledge funding support by CNES. INAF participation was supported by Italian Space Agency grant agreement n. 2017-37-H.0. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible. References[1] Lauretta D. S. et al. (2017) Space Sci. Rev. 212, 925-984. [2] Reuter D. C. et al. (2018) Space Sci. Rev. 214, 54. [3] Hamilton V. E. et al. (2019) Nat. Astron. 3, 332. [4] Takir D. et al. (2013) Meteorit. Planet. Sci. 48, 1618–1637. [5] Takir D. et al. (2019) Icarus 333, 243–251. [6] Milliken R. E. et Mustard J. F. (2005) JGR, 110, E12001. [7] Milliken R. E. et al. (2007) JGR, 112, E08S07. [8] Garenne A. et al. (2016) Icarus, 264, 172-183. [9] Alexander C.M.O’D. et al. (2012) Science, 337, 721-723. [10] Alexander C.M.O’D. et al. (2013) Geochim. Cosmochim. Acta, 123, 244-260. [11] Simon et al. (in revision) Science. [12] Zou X.-D. et al., this meeting. [13] Potin S. et al. (2020) Icarus, 348, 113826. [14] Gilmour C. M. et al. (2019) Meteorit. Planet. Sci. 54, 1951–1972. [15] Kaplan et al. (2020) LPSC LI, 1050.

Fast-rotating asteroids (FRAs) are considered to be small bodies having a rotation period (P) faster than the spin barrier of about 2.3 h, starting at diameters of less than 300 m. We selected the 20 fastest Arecibo radar-observed targets, with P<0.13 h (∼8 min). Some key measurements and calculations obtained from radar observations include: the Doppler bandwidth, the circular polarization ratio, radar astrometry, and (with enough signal-to-noise ratio) delay-Doppler images of the object. Rotation period data available from the Light Curve Database for the selected objects combined with the radar observations allow us to constrain the possible diameters and confirm the periods. Of the objects in this sample, the median absolute magnitude (H) is 24.9, and the median calculated diameter is 32 meters.The apparent rotation period indicated by the bandwidth can appear slower than the object’s true rotation, but not faster, providing an upper limit to the true rotation period. Asteroid cohesion required to prevent rotational disruption depends on rotation rate, density, and diameter; we performed calculations for the minimum values for cohesion via the Drucker–Prager (D-P) cohesion criterion. Most of these objects need a few to a few hundred pascals of cohesion; however four cases stand out: 2014 TV, 2015 RF36, 2015 GS2 and 2017 EK, needing a minimum cohesion on the order of a few kilopascals. These are comparable to very weak Earth rocks, and are larger than previously reported values for NEAs.

We report physical and mutual orbit characteristics of near-Earth binary asteroid (163693) Atira. Using S-band (2380 MHz, 12.6 cm) radar observations from the Arecibo Observatory and several epochs of lightcurve observations from 2003 to 2019 with SHAPE modeling software, we determine the shape, size, rotational period, and mutual orbit of the primary and secondary components and the density of the primary component. We confirm the primary's sidereal rotation period to be 3.398521 +/- 0.000003 hr, and we find a likely spin axis orientation of ecliptic longitude and latitude (187 degrees, -53 degrees) +/- 12 degrees. We find the primary's volume-equivalent diameter to be 4.92 +/- 0.95 km and the secondary's volume-equivalent diameter to be 0.80 +/- 0.30 km. We find the primary component's density to be 1.43 +/- 0.87 g cm-3. We also find that the secondary has a semimajor axis of 7.8 +/- 0.5 km and a sidereal orbital period of 15.577 +/- 0.003 hr based on orbital calculations using delay and Doppler offsets between the primary and secondary and the timing of mutual events observed in lightcurve data. This work represents the first detailed analysis of the shape of an Atira-class asteroid.

Near-Earth asteroids (NEAs) are a key test bed for investigations into planet formation, asteroid dynamics, and planetary defense initiatives. These studies rely on understanding NEA sizes, albedo distributions, and regolith properties. Simple thermal models are a commonly used method for determining these properties; however, they have inherent limitations owing to the simplifying assumptions they make about asteroid shapes and properties. With the recent collapse of the Arecibo Telescope and a decrease of direct size measurements, as well as future facilities such as LSST and NEO Surveyor coming online soon, these models will play an increasingly important role in our knowledge of the NEA population. Therefore, it is key to understand the limits of these models. In this work we constrain the limitations of simple thermal models by comparing model results to more complex thermophysical models, radar data, and other existing analyses. Furthermore, we present a method for placing tighter constraints on inferred NEA properties using simple thermal models. These comparisons and constraints are explored using the NEA (285263) 1998 QE2 as a case study. We analyze QE2 with a simple thermal model and data from both the NASA IRTF SpeX instrument and NEOWISE mission. We determine an albedo between 0.05 and 0.10 and thermal inertia between 0 and 425J m ^−2 s ^−1/2 K ^−1 . We find that overall the simple thermal model is able to well constrain the properties of QE2; however, we find that model uncertainties can be influenced by topography, viewing geometry, and the wavelength range of data used.

The Didymos binary asteroid was the target of the Double Asteroid Redirection Test (DART) mission, which intentionally impacted Dimorphos, the smaller member of the binary system. We used the Near-Infrared Spectrograph and Mid-Infrared Instrument instruments on JWST to measure the 0.6–5 and 5–20 μ m spectra of Didymos approximately two months after the DART impact. These observations confirm that Didymos belongs to the S asteroid class and is most consistent with LL chondrite composition, as was previously determined from its 0.6–2.5 μ m reflectance spectrum. Measurements at wavelengths >2.5 μ m show Didymos to have thermal properties typical for an S-complex asteroid of its size and to be lacking absorptions deeper than ∼2% due to OH or H _2 O. Didymos’ mid-infrared emissivity spectrum is within the range of what has been measured on S-complex asteroids observed with the Spitzer Space Telescope and is most consistent with emission from small (<25 μ m) surface particles. We conclude that the observed reflectance and physical properties make the Didymos system a good proxy for the type of ordinary chondrite asteroids that cross near-Earth space, and a good representative of likely future impactors.

We successfully observed 191 near-Earth asteroids using the Arecibo Observatory’s S-band planetary radar system from 2017 December through 2019 December. We present radar cross sections for 167 asteroids; circular-polarization ratios for 112 asteroids based on Doppler-echo-power spectra measurements; and radar albedos, constraints on size and spin periods, and surface-feature and shape evaluation for 37 selected asteroids using delay-Doppler radar images with a range resolution of 75 m or finer. Out of 33 asteroids with an estimated effective diameter of at least 200 m and sufficient image quality to give clues of the shape, at least 4 (∼12%) are binary asteroids, including 1 equal-mass binary asteroid, 2017 YE5, and at least 10 (∼30%) are contact-binary asteroids. For 5 out of 112 asteroids with reliable measurements in both circular polarizations, we measured circular-polarization ratios greater than 1.0, which could indicate that they are E-type asteroids, while the mean and the 1 σ standard deviation were 0.37 ± 0.23. Further, we find a mean opposite-sense circular-polarization radar albedo of 0.21 ± 0.11 for 41 asteroids (0.19 ± 0.06 for 11 S-complex asteroids). We identified two asteroids, 2011 WN15 and (505657) 2014 SR339, as possible metal-rich objects based on their unusually high radar albedos, and discuss possible evidence of water ice in 2017 YE5.

We conducted radar observations of near-Earth asteroid 2019 OK on 2019 July 25 using the Arecibo Observatory S-band (2380 MHz, 12.6 cm) planetary radar system. Based on Arecibo and optical observations the apparent diameter is between 70 and 130 m. Combined with an absolute magnitude of H = 23.3 ± 0.3, the optical albedo of 2019 OK is likely between 0.05 and 0.17. Our measured radar circular polarization ratio of μ C = 0.33 ± 0.03 indicates 2019 OK is likely not a V- or E-type asteroid and is most likely a C- or S-type. The measured radar echo bandwidth of 39 ± 2 Hz restricts the apparent rotation period to be approximately between 3 minutes (0.049 hr, D = 70 m) and 5 minutes (0.091 h, D = 130 m). Together, the apparent diameter and rotation period suggest that 2019 OK is likely not a rubble-pile body bound only by gravity. 2019 OK is one of a growing number of fast-rotating near-Earth asteroids that require some internal strength to keep them from breaking apart.

We present evidence, via a large survey of 191 new spectra along with previously published spectra, of a divide in the 3 μ m spectral properties of the low-albedo asteroid population. One group (“sharp types,” or STs, with band centers <3 μ m) has a spectral shape consistent with carbonaceous chondrite meteorites, while the other group (“not sharp types,” or NSTs, with bands centered >3 μ m) is not represented in the meteorite literature but is as abundant as the STs among large objects. Both groups are present in most low-albedo asteroid taxonomic classes, and, except in limited cases, taxonomic classifications based on 0.5–2.5 μ m data alone cannot predict whether an asteroid is an ST or NST. Statistical tests show that the STs and NSTs differ in average band depth, semimajor axis, and perihelion at confidence levels ≥98% while not showing significant differences in albedo. We also show that many NSTs have a 3 μ m absorption band shape like comet 67P and likely represent an important small-body composition throughout the solar system. A simple explanation for the origin of these groups is formation on opposite sides of the ammonia snow line, with the NST group accreting H 2 O and NH 3 and the ST group only accreting H 2 O, with subsequent thermal and chemical evolution resulting in the minerals seen today. Such an explanation is consistent with recent dynamical modeling of planetesimal formation and delivery and suggests that much more outer solar system material was delivered to the main asteroid belt than would be thought based on the number of D-class asteroids found today.

Studying materials released from Jupiter-family comets (JFCs)—as seen in their inner comae, the envelope of gas and dust that forms as the comet approaches the Sun—improves the understanding of their origin and evolutionary history. As part of a coordinated, multiwavelength observing campaign, we observed comet 45P/Honda–Mrkos–Pajdušáková during its close approach to Earth in 2017 February. Narrowband observations were taken using the Bok 90″ telescope at Kitt Peak National Observatory on February 16 and 17 UT, revealing gas and dust structures. We observed different jet directions for different volatile species, implying source region heterogeneity, consistent with other ground-based and in situ observations of other comet nuclei. A repeating feature visible in CN and C2 images on February 16 was also observed on February 17 with an interval of 7.6 ± 0.1 hr, consistent with the rotation period of the comet derived from Arecibo Observatory radar observations. The repeating feature’s projected gas velocity away from the nucleus is 0.8 km s−1, with an outflow velocity of 0.5 km s−1. A bright compact spot adjacent to the nucleus provides a lower limit of the amount of material released in one cycle of ∼9.2 kg, depending on composition—a quantity small enough to be produced by repeated exposure of nucleus ices to sunlight. This repeating CN jet, forming within 400 km of the nucleus, may be typical of inner-coma behavior in JFCs; however, similar features could be obscured by other processes and daughter product species when viewed from distances further than the scale length of CN molecules.

Ground-based planetary radar observations have enabled and continue to facilitate the exploration of our solar system through characterization of planets and their moons.This includes spacecraft landing-site characterization, e.g., for Viking (Simpson et al., 1978) and most recently InSight (Putzig et al., 2017) at Mars, and improved target astrometry, such as for Europa (Brozović et al., 2020).The power of radar for planetary geology is its ability to sense subsurface features buried beneath regolith, such as cryptomare on the Moon (Campbell & Hawke, 2005), and surface features obscured by a dense atmosphere, e.g., Venus (

Ground-based planetary radar systems are a crucial tool for post-discovery high-precision astrometry and physical characterization of near-Earth asteroids and comets.Radar ranging and imaging capabilities exceed those of any other ground-based instruments.As described in more detail in the Decadal white paper (henceforth WP) Mainzer et al. and WP Taylor et al., detection and characterization of potentially hazardous asteroids was mandated by the US Congress through George E. Brown, Jr. Near-Earth Object Survey Act in 2005.While optical telescopes survey for new asteroid detections, post-discovery characterization using planetary radar can secure the heliocentric orbit information, extending the interval of reliable orbit estimation by decades to centuries for most objects and preventing recently-discovered objects from being lost.Radar observations also provide constraints on the object's size, rotation state, morphology, satellites, and near-surface geology as well as regolith properties, making groundbased radar systems a powerful, low-cost complement to spacecraft destined for specific targets.Furthermore, they can be used for characterizing the nuclei of comets, which are typically obscured by the dust coma at optical wavelengths, and probe the decimeter-scale coma particle abundance, which is relevant especially for the study of disintegrating comets.These characteristics are invaluable information for understanding the formation and evolution of asteroids and comets, which are the building blocks of our Solar System, for evaluating impact risk and developing impact mitigation technologies, and for ensuring safe spacecraft encounters.In the next decade, in addition to continuing as a state-of-the-art astrometric tool for planetary defense, we expect ground-based radar observations to expand our knowledge of the physical characteristics of the near-Earth asteroid population in terms of their shape, binarity, and near-surface structure.We recommend that facilities with ground-based radar systems, planetary radar programs, and research and analysis of radar observations receive full support from NASA and NSF to ensure their availability to the planetary science community and as a resource for planetary defense initiatives through the next decade.

The primary objective of the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer ( OSIRIS ‐ RE x) mission is to return to Earth a pristine sample of carbonaceous material from the primitive asteroid (101955) Bennu. To support compositional mapping of Bennu as part of sample site selection and characterization, we tested 95 spectral indices on visible to near infrared laboratory reflectance data from minerals and carbonaceous meteorites. Our aim was to determine which indices reliably identify spectral features of interest. Most spectral indices had high positive detection rates when applied to spectra of pure, single‐component materials. The meteorite spectra have fewer and weaker absorption features and, as a result, fewer detections with the spectral indices. Indices targeting absorptions at 0.7 and 2.7–3 μm, which are attributable to hydrated minerals, were most successful for the meteorites. Based on these results, we identified a set of 17 indices that are most likely to be useful at Bennu. These indices detect olivines, pyroxenes, carbonates, water/ OH ‐bearing minerals, serpentines, ferric minerals, and organics. Particle size and albedo are known to affect band depth but had a negligible impact on interpretive success with spectral indices. Preliminary analysis of the disk‐integrated Bennu spectrum with these indices is consistent with expectations given the observed absorption near 3 μm. Our study prioritizes spectral indices to be used for OSIRIS ‐ RE x spectral analysis and mapping and informs the reliability of all index‐derived data products, including a science value map for sample site selection.

Proximity observations by OSIRIS-REx and Hayabusa2 provided clues on the shape evolution processes of the target asteroids, (101955) Bennu and (162173) Ryugu. Their oblate shapes with equatorial ridges, or the so-called top shapes, may have evolved due to their rotational conditions at present and in the past. Different shape evolution scenarios were previously proposed; Bennu's top shape may have been driven by surface processing, while Ryugu's may have been developed due to large deformation. These two scenarios seem to be inconsistent. Here, we revisit the structural analyses in earlier works and fill a gap to connect these explanations. We also apply a semi-analytical technique for computing the cohesive strength distribution in a uniformly rotating triaxial ellipsoid to characterize the global failure of top-shaped bodies. Assuming that the structure is uniform, our semi-analytical approach describes the spatial variations in failed regions at different spin periods; surface regions are the most sensitive at longer spin periods, while interiors fail structurally at shorter spin periods. This finding suggests that the shape evolution of a top shape may vary due to rotation and internal structure, which can explain the different evolution scenarios of Bennu's and Ryugu's top shapes. We interpret our results as the indications of top shapes' various evolution processes.

Although solar-analog stars have been studied extensively over the past few decades, most of these studies have focused on visible wavelengths, especially those identifying solar-analog stars to be used as calibration tools for observations. As a result, there is a dearth of well-characterized solar analogs for observations in the near-infrared, a wavelength range important for studying solar system objects. We present 184 stars selected based on solar-like spectral type and V-J and V-K colors whose spectra we have observed in the 0.8-4.2 micron range for calibrating our asteroid observations. Each star has been classified into one of three ranks based on spectral resemblance to vetted solar analogs. Of our set of 184 stars, we report 145 as reliable solar-analog stars, 21 as solar analogs usable after spectral corrections with low-order polynomial fitting, and 18 as unsuitable for use as calibration standards owing to spectral shape, variability, or features at low to medium resolution. We conclude that all but 5 of our candidates are reliable solar analogs in the longer wavelength range from 2.5 to 4.2 microns. The average colors of the stars classified as reliable or usable solar analogs are V-J=1.148, V-H=1.418, and V-K=1.491, with the entire set being distributed fairly uniformly in R.A. across the sky between -27 and +67 degrees in decl.

Asteroid (3200) Phaethon is an active near-Earth asteroid and the parent body of the Geminid Meteor Shower. Because of its small perihelion distance, Phaethon's surface reaches temperatures sufficient to destabilize hydrated materials. We conducted rotationally resolved spectroscopic observations of this asteroid, mostly covering the northern hemisphere and the equatorial region, beyond 2.5-µm to search for evidence of hydration on its surface. Here we show that the observed part of Phaethon does not exhibit the 3-µm hydrated mineral absorption (within 2σ). These observations suggest that Phaethon's modern activity is not due to volatile sublimation or devolatilization of phyllosilicates on its surface. It is possible that the observed part of Phaethon was originally hydrated and has since lost volatiles from its surface via dehydration, supporting its connection to the Pallas family, or it was formed from anhydrous material.