Context. The third Gaia data release (DR3) contains high-precision, sparse-in-time brightness measurements of over 150000 asteroids. Aims. We employed a light-scattering inversion technique to estimate the rotation periods, spin pole orientations, shapes, and photometric phase function parameters (slope and absolute magnitude) of over 8000 asteroids based solely on DR3 photometry. Methods. Using triaxial ellipsoid and convex shapes, we sought the best-fit shape, spin, and linear slope, along with their uncertainties, via a Markov chain Monte Carlo sampling technique. We also fit H,G12 and H,G1,G2 phase functions from predicted brightnesses derived from the fit shapes. Using previously reported diameters, the Gaia G-band geometric albedos were calculated. Variations in the spin and shape properties were assessed among various families and background populations of the Main Belt. Results. We found that for the vast majority of our objects the best-fit ellipsoid spin poles are comparable to that of a convex shape. We rejected 15% of convex shapes and used the acceptable solutions to investigate differences in the shape distribution of families in the Main Belt. Revisiting the amplitude phase relationship, we found a strong dependence on the shape b/a elongation. The Bond albedo was calculated from the phase integral and shown to correlate well with the G12 slope parameter and known taxonomic classifications. The G-band absolute magnitudes and geometric albedos are systematically fainter than V-band values. Conclusions. The assumption of ellipsoid shape for sparse datasets is sufficient for estimating the spin pole longitude and latitude. We find no correlation between the shapes and spins of main-belt asteroid families and their ages. Absolute magnitudes and phase functions derived from Gaia photometry should be favored over the V band when estimating the solar energy budget, such as in thermal modeling applications. Asteroid taxonomies can be assessed to some degree from the photometric slopes and albedos.

Context. We study the taxonomic classification of asteroids observed by Gaia as a continuation of the lightcurve inversion work presented in Paper I. Aims. We examine the taxonomic classification of asteroids by using both Gaia Data Release 3 (DR3) photometric and spectroscopic data. Particular focus is placed on Ch-class asteroids, as their potentially hydrated nature makes them promising candidates for sample-return missions and the asteroid mining industry. Methods. We utilized the photometric slopes and geometric albedos (via absolute magnitudes) derived from lightcurve inversion, and the Gaia DR3 spectra (from 418 nm to 770 nm) as classification parameters. We also considered how different parameter sets affect classification accuracies for separate asteroid classes. We classified the asteroids with a combination of linear discriminant analysis and a nearest neighbor classifier. Results. We achieve a classification accuracy of 92% for known S-class asteroids and an accuracy of 85% for Ch-class asteroids with a known set of 328 asteroids. Given the three classification parameters, tentative class designations for 1668 previously unclassified asteroids are provided in the Mahlke taxonomy. We also show that the photometric slope values vary significantly within asteroid classes, with a standard deviation three to four times the mean slope uncertainties. Conclusions. We show that the combination of photometry and spectroscopy can be useful in the taxonomic classification of asteroids observed by Gaia. Further studies of the surface roughness at different scales could help clarify the potential of the photometric slope in classification efforts.

We present new post-perihelion polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1; hereafter 3I), obtained using FORS2 at the Very Large Telescope, ALFOSC at the Nordic Optical Telescope, and FoReRo2 at the 2 m Ritchey-Chrétien-Coudé telescope. The observations span phase angles 1-30^∘, providing the most extensive polarimetric phase angle coverage obtained for an interstellar object to date. The post-perihelion measurements reveal that the unusual polarimetric properties of 3I persist across perihelion and match recent independent post-perihelion observations, indicating no significant evolution in the polarimetric properties. The increased phase angle coverage allowed us to further constrain the minimum polarisation to -2.9% at phase angle 5.5^∘. Multi-band BVRI observations reveal a wavelength dependence of polarisation, with the negative polarisation branch becoming deeper and shifted toward smaller phase angles at longer wavelengths. The polarimetric colour is predominantly red and increases with phase angle, consistent with behaviour observed in other comets. Imaging and polarimetric maps show a substantially more extended coma after perihelion, while the polarisation distribution itself remains smooth and spatially homogeneous. Numerical modelling suggests that the unusually deep polarisation phase curve originates from moderately porous dust aggregates consisting of weakly absorbing sub-micron monomers, resulting in much brighter dust than that in typical solar system comets.

We present the first polarimetric observations of the third discovered interstellar object (ISO), 3I/ATLAS (C/2025 N1, or 3I), obtained preperihelion with FORS2 at the Very Large Telescope, ALFOSC at the Nordic Optical Telescope, and FoReRo2 at the 2 m Ritchey-Chr & eacute;tien-Coud & eacute; telescope, over a phase angle range of 7 .degrees 7-22 .degrees 4. This marks the second-ever polarimetric study of an ISO, the first distinguishing 2I/Borisov from most solar system comets by its higher positive polarization. Our polarimetric measurements as a function of phase angle reveal that 3I is characterized by a deep and narrow negative polarization branch, reaching a minimum value of -2.7% at phase angle 7 degrees, and an inversion angle of 17 degrees-a combination unprecedented among asteroids and comets, including 2I/Borisov. At very small phase angles, the extrapolated slope of the polarization phase curve is consistent with that of certain small trans-Neptunian objects and Centaur Pholus, consistent with independent spectroscopic evidence for a red, possibly water-ice-bearing object. Imaging confirms a diffuse coma present from our earliest observations, though no strong polarimetric features are spatially resolved. These findings may demonstrate that 3I represents a distinct type of comet, expanding the diversity of known interstellar bodies.

The destructive power of an impacting asteroid is primarily estimated by knowledge of its size. Asteroid 2024 YR4 reached a peak 2032 impact probability with Earth of 3%, motivating a desire to determine its size. Owing to its infrared capabilities, JWST is uniquely suited for such assessment, especially for decameter-scale objects. We used JWST to observe 2024 YR4 and find a diameter of 60 ± 7 m. This size range corresponds to an albedo for 2024 YR4 of 8%–18%, consistent with observation that it is an S-type asteroid (International Asteroid Warning Network, https://iawn.net/obscamp/2024YR4/index.shtml ), if at the low-albedo end of that asteroid taxon. Future observations through 2025 May will help further refine the asteroid’s thermophysical model.

Estimation of asteroid sizes from the observed thermal emission are performed by comparing the flux modeled from simple thermal models and thermophysical models (TPMs). The accuracy of the diameter estimate is dependent on the accuracy of the modeled surface temperatures, which are calculated differently among these models. Simple thermal models, like the often-chosen NEATM, do not account for subsurface heat conduction whereas thermophysical models explicitly account for it. This neglect of heat transport can, generally speaking, result in the overestimation of diameters when simple thermal models are used (Spencer, et al. 1989) because nighttime thermal emission is neglected by simpe thermal models (the NESTM model being the exception). Mommert et al. (2019) showed that NEATM diameter fits to NEAs are reasonably accurate (i.e. little systematic bias) when the solar phase angle of observation is 65°. On the other hand, TPMs are thought to provide highly accurate diameters regardless of the observing circumstances or physical properties of the surface because they include heat conduction.Nearly all TPMs compute surface temperatures over a diurnal timescale, over which the solar energy input is held constant. However, as the case with high-eccentricity NEAs, a potentially overlooked effect on diameter determination is the influence of rapidly-change in solar heating. Because these objects experience large, rapid changes in the amount of absorbed sunlight, heat conducted into the subsurface at small heliocentric distances will affect surface tempertures at larger heliocentric distances. Figure 1 below demonstrates that this effect is more pronounced for larger orbital eccentricities.Figure 1. Discrepancy (orbTPM-diTPM) in the maximum daily temperature at different heliocentric distances for a hypothetical asteroid with varying orbital eccentricity. Dots indicate time steps separated by ~2 days. Open circles show the median values of temperature discrepancy and heliocentric distance. A positive value for the discrepancy indicates a larger orbTPM temperature.In this work we compare two TPMs: one with a fixed solar energy input over a diurnal timescale, diTPM, and another that models surface temperatures over an entire orbit, orbTPM. We compare the disk-integrated fluxes of each model in order to investigate and characterise any discrepancy. Emitted thermal flux is directly proportional to size, thus the flux discrepancy between the two models is used to quantify any bias in diameter determination when the diTPM is used.The potential effects of thermal inertia, rotation period, spin axis, and phase angle on this discrepancy are considered. Finally, we employ the orbTPM on a few asteroids observed in the thermal infrared and postulate on the implications on thermal modelling of NEAs as a population.

On 2020 April 29, the near-Earth object (52768) 1998 OR2 experienced a close approach to Earth at a distance of 16.4 lunar distances (LD). 1998 OR2 is a potentially hazardous asteroid of absolute magnitude H = 16.04 that can currently come as close to Earth as 3.4 LD. We report here observations of this object in polarimetry, photometry, and radar. Our observations show that the physical characteristics of 1998 OR2 are similar to those of both M- and S-type asteroids. Arecibo's radar observations provide a high radar albedo of sigma OC= 0.29 +/- 0.08, suggesting that metals are present in 1998 OR2 near-surface. We find a circular polarization ratio of mu c = 0.291 +/- 0.012, and the delay-Doppler images show that the surface of 1998 OR2 is a top-shape asteroid with large-scale structures such as large craters and concavities. The polarimetric observations display a consistent variation of the polarimetric response as a function of the rotational phase, suggesting that the surface of 1998 OR2 is heterogeneous. Color observations suggest an X-complex taxonomy in the Bus-DeMeo classification. Combining optical polarization, radar, and two epochs from the NEOWISE satellite observations, we derived an equivalent diameter of D = 1.80 +/- 0.1 km and a visual albedo p v = 0.21 +/- 0.02. Photometric and radar data provide a sidereal rotation period of P = 4.10872 +/- 0.00001 hr, a pole orientation of (332.degrees 3 +/- 5 degrees, 20.degrees 7 +/- 5 degrees), and a shape model with dimensions of (2.08-0.10+0.10,1.93-0.10+0.10,1.60-0.05+0.05) km.

The Gaia Data Release 3 (DR3) contains high-precision sparse-in-time photometric data of more than 150 000 asteroids and spectroscopy of more than 60 000 asteroids (Tanga et al., A&A, 674, A12, 2023). We have fitted simple triaxial and general convex shapes to estimate the rotational periods, spin axes, and linear photometric slopes of the asteroids with the lightcurve inversion algorithm developed by Muinonen et al. (A&A, 642, A138, 2020). Sample selection of the asteroids follows Cellino et al. (A&A, in press, 2024), and we use the ellipsoid results from their genetic evolution algorithm to initiate our ellipsoid and convex shape inversion solutions. In total, we have modelled linear photometric slopes for 8660 asteroids, which have at least 25 Gaia observations (MacLennan et al., in preparation). The slopes are used to compute H,G1,G2 phase functions (Muinonen et al., Icarus, 209, 542, 2010) to derive absolute magnitudes for the asteroids. As the absolute magnitudes have been calculated using Gaia’s G-band, they account for a larger portion of the incident solar spectrum compared to e.g. V-band. This is particularly useful for estimating the amount of absorbed sunlight for thermal infrared studies of asteroids via calculation of the Bond albedo. Selecting the asteroids for which there are spectroscopic data as well, gives us 2381 asteroids to analyse further. We have kept the data from the Gaia red and blue photometers (RP and BP) separate due to problems merging them. The wavelength range of both ends have also been cut to eliminate unreliable behaviour in the spectra. We are left with the following wavelengths: 418, 462, 506, 550, 594 nm (in the BP), and 638, 682, 726, 770 nm (in the RP). We have taken asteroid diameters from NASA’s NEOWISE survey (Mainzer et al., NEOWISE Diameters and Albedos V2.0, 2019) and the absolute magnitudes derived from the lightcurve inversion to calculate geometric albedos for the asteroids. Bond albedos are calculated from the geometric albedo and using the phase integral, q(G1,G2), from Muinonen et al. (Icarus, 209, 542, 2010). Traditionally, geometric albedos have been used to examine the inclusion of asteroids in asteroid families (Masiero et al., ApJ, 770, 7, 2013). Yet, taken together with the mean photometric slope of the families there seems to be a steep inverse linear trend with slopes smaller than 2 mag/rad at α = 20◦. We now investigate the parameters further by creating bins of the photometric slope and geometric albedo pair using Principal Component Analysis (PCA) on the data of individual asteroids. The asteroid spectra within the bins are examined in order to evaluate how well the parameter pair aids in the classification of asteroids.

The near-Earth asteroid (3200) Phaethon is classified as an active asteroid and is one of the largest objects with a perihelion within the orbit of Mercury. A dust tail has been observed during each of the last 3 closest approaches with the Sun. The activity, which is likely not driven by volatile sublimation, strongly suggests that Phaethon is the most likely parent body of the annual Geminid meteor shower. Phaethon is the main target for the JAXA DESTINY+ mission that will measure the dust environment, among other goals.Regolith properties in the near-surface of asteroid can be inferred from the thermal inertia, Γ = √kρc. The effective thermal conductivity, k, is the most influential controlling factor in thermal inertia, as opposed to heat capacity (c) and bulk density (ρ). The effective thermal conductivity can expressed as a sum of the radiative conductivity and solid conductivity, which are controlled by the grain size and porosity of the regolith: specifically, the conduction through contacts through the regolith grains and the radiative transfer within the pores between them. Gundlach and Blum (2013) presented a thermal conductivity model that accurately models both these effects for planetary regoliths.Because the radiative component of conductivity is temperature dependent (T3) it implies that thermal inertia is also temperature dependent. By extension, the strong inverse dependence that temperature has on heliocentric distance means that thermal inertia will increase with decreasing heliocentric distance. Rozitis et al., (2018) calculated this dependence for 3 NEAs: (1036) Ganymed, (1580) Betulia, and (276049) 2002 CE26. They found a wide variation in the variability, with Ganymed showing a strong dependence and Betulia showing a weak dependence.The thermal inertia of Phaethon was previously estimated to be 600 ± 200 J m-2 K-1 s-1/2 by Hanus et al. (2018) and 880+580-330 J m-2 K-1 s-1/2 by Masiero et al. (2019). While Hanus et al. (2018) used a convex shape model derived from lightcurve inversion techniques, Masiero et al. (2019) used assumed a spherical shape. Furthermore these two different thermal inertia estimates were derived using two distinct datatsets: thermal emission spectrum from Spitzer and photometric data from IRAS and UKIRT Green et al (1995) was used by Hanus et al. (2018) and 5 epochs of WISE/NEOWISE photometry was used by Masiero et al. (2019). Both of these works alos reported diameter estimates of Phaethon (~5.1 and ~4.6 km) that are noticeably smaller than the reported effective diameter from delay-Doppler radar observations (5.5 - 6 km; Taylor et al. 2017).Using a radar-derived shape model and thermal infrared observations from 10 observing epochs, we estimate Phaethon's thermal inertia for each epoch. We independently derive an effective diameter of ~5.5-5.6 km that is consistent with the radar observations and find that Phaethon's thermal inertia increases with decreasing heliocentric distance. Using the regolith thermal conductivity model presented by Gundlach and Blum (2013), we model the thermal inertia as a function of heliocentric distance for various grain sizes and porosities (Figure 1). We find that larger grain sizes are consistent with smaller heliocentric distances and smaller grain sizes are consistent with larger heliocentric distances.Figure 1. Phaethon's thermal inertia as a function of heliocentric distance (black points) and modeled thermal inertia for different regoolith grain sizes and porosities.We consider two other possibilities for Phaethon's regolith: 1) a depth-dependent layered model and 2) a two component latitude-dependnt model. The layered model consists of a fine-grained regolith covering solid bedrock and the two component model consists of two distinct grain sizes for each of Phaethon's northern and southern hemispheres. The effective thermal inertia of the layered regolith model exhibits a stronger dependence on heliocentric distance, as expected, but does not fit the observed thermal inertia estimates well. On the other hand, the model that consisters distinct grain sizes is very consistent with Phaethon's observed thermal inertia. We conclude that Phaethon's northern hemisphere consists of larger regolith grains compared to its southern hemisphere.

The thermal inertia and roughness are two parameters that are included in thermophysical models (TPMs) in order to interpret the infrared emission of asteroid surfaces. These parameters depend on the size of particles and degree of roughness for size scales larger than the thermal skin depth, which is typically on the order of a few centimeters (Delbo et al., 2015). For example, large decimeter-sized boulders have elevated thermal inertia values and increase the overall degree of roughness. On the other hand, particles that are smaller than the thermal skin depth act to lower the estimated thermal inertia, but are less influential to the overall surface roughness. Several studies have focused on the interpretation of only the thermal inertia from both disk-resolved and disk-integrated thermal infrared observations. In principle, accurate estimates of both these parameters can be used to develop a heuristic tool for estimating the abundance of boulders on an asteroid surface (or any other airless body).Our primary goal is to develop a consistent framework for which to compare roughness scales as constrained by different wavelength regimes: micron scale from light scattering (e.g. Zubko et al., 2007), centimeter scale from thermal infrared, and decimeter scale from active radar sensing (Virkki et al. 2022). The degree of roughness for visible and radar observations is determined by scattering behavior at the scale of the wavelength, whereas the roughness constrained by thermal infrared emission is affected by deviations in surface temperature from a smooth surface. These temperature deviations are caused by topographical features larger than the skin depth. In practice, TPMs calculate the temperature distribution for terrains which incorporate the effects of scattered light, shadowing of roughness elements, and self-heating (thermal emission from one roughness element that is re-absorbed by another). Model terrains for representing roughness include, for example, spherical section craters and self-affine (fractal) surfaces.The symmetry of a spherical crater geometry lends to computational efficiency in TPMs, yet may not be an ideal physical or geological representation of asteroid surface roughness. Yet, hemispherical craters have been successfully employed to fit the disk-resolved surface emission of Bennu (Rozitis et al. 2020), whereas a random Gaussian surface has been used to model lunar emission (Bandfield et al. 2013). These terrains are often characterized by the root mean squared (RMS) of their surface slopes, even though their surface slope distributions may differ in some ways. In these studies of Bennu and the Moon, the best-fit thermal roughness was reported as RMS of 40o and 20 — 35o, respectively. However, it is not trivial to compare the results of these two distinct roughness implementations, or when the thermal skin depths are somewhat different (1 – 5 cm for Bennu and 6 cm for the Moon). A study of the roughness of Eros showed a higher degree of roughness at the scale of the thermal skin depth compared to the extrapolated roughness at larger spatial scales (Rozitis, 2016). Finally, radar observations show that surface roughness differs among spectral classes (Benner et al. 2008; Virkki et al., 2022).In our study, we compare the thermal emission profiles of spherical craters and randomly-generated fractal surfaces (Virkki, 2024) in order to investigate the effects of increasing thermal infrared roughness for craters and fractal surfaces. A one-dimensional heat diffusion model is used to calculate temperatures for craters and fractal surfaces with differing surface slopes. The craters are described by their opening angle (90o being the largest for hemispherical craters), and the fractal terrains are characterized by Hurst exponent and RMS height of the surface (Figure 1; Figure 2). Multiple scattering and self-heating effects are implemented using the so-called view factors; defined as the fraction of energy leaving one roughness element that reaches another. The edges of the fractal surface are determined such that a periodic boundary condition can be implemented to simulate an infinite surface. In this way, energy can be exchanged between elements on opposite edges of the mesh.One intriguing difference between these two roughness terrains is the degree of shadowing and self-heating in response to changes in the surface slope distribution. The view factor for every crater element is identical, whereas the view factors for a rough fractal surface are unique for each elements and thus follow some distribution. Furthermore, the effects of shadowing of roughness elements at large incident angles can have different effects between the two terrains, even if the surface slope distributions (or RMS slopes) are similar. The variations in the thermal emission profgiles of these two models have important implications for the interpretation of both disk-resolved and disk-integrated thermal infrared datasets. Given that the thermal skin depth is dependent on the rotation period, it can differ from one asteroid to the next and create challenges when comparing disk-integrated roughness values among the asteroid population.Figure 1. A fractal surface with H = 0.5 and rms height = 0.4.Figure 2. Surface slope distribution for the surface in Figure 1.References:Bandfield, J., et al., 2015. Icarus 248, 357–372.Benner, L., et al., 2008. Icarus 198, 294–304.Delbo, M., et al., 2015. In Asteroids IV.Rozitis, B., et al. 2016. Monthly Notices of the Royal Astronomical Society 464, 1.Rozitis, B., et al. 2020. Science Advances 6, eabc3699.Virkki, A., et al. 2022. The Planetary Science Journal, 3:222, 36 pp.Virkki, A. 2024. Remote Sensing 2024, 16, 890.Zubko, E., et al. 2007. Journal of Quantitative Spectroscopy & Radiative Transfer 106, 604–615.

The exposure to irradiation from high-energy particles alters the reflectance properties of asteroid surfaces and is referred to as space weathering. This process leads to an increase in spectral slope in visible and near-infrared wavelengths. However, changes in the regolith particle size, which can vary dramatically among the asteroid population, are known to influence the spectral properties of meteorites and asteroids. In this context, we investigate the changes in spectral slope and absorption band depths of fresh and irradiated ordinary chondrite meteorites to quantitatively compare the effects of space weathering and grain size variations. To do so, we develop and employ the Spectral Analysis for Asteroid Reflectance Investigation routine that calculates the band parameters of reflectance spectra. We then formulate a parameter called the Space Weathering Index (SWI) that is designed to encapsulate spectral changes due to space weathering. We find that the SWI is strongly dependent on the spectral slope which complicates the interpretation of asteroid spectra in the context of grain size variations and space weathering. We also show that a second parameter, the Band Depth Index, is indicative of petrologic type. Finally, we use a linear discriminant analysis to classify asteroid reflectance spectra into H, L, LL, and unequilibrated ordinary chondrites.

Near-Earth asteroid (3200) Phaethon exhibits activity during its perihelion passage at 0.14 au from the Sun and is the likely parent body of the annual Geminid meteor shower. Its low albedo and featureless B-type reflectance spectrum indicate a primitive composition, but a definitive meteorite analogue is currently indeterminate. Here we analyse a mid-infrared emissivity spectrum of Phaethon and find that it most closely matches the Yamato group (CY) of carbonaceous chondrites. The CY chondrites experienced aqueous alteration and recent thermal metamorphism in which extreme temperatures caused mineral decomposition, resulting in the production of gas species. Temperatures within Phaethon during its close approach to the Sun are conducive to the thermal decomposition of carbonates, iron sulfides and phyllosilicates that release CO2, S2 and H2O gas, respectively. Spectral detection of these minerals strongly implies that gas release from mineral decomposition is capable of triggering dust ejection. The planned flyby of Phaethon by the DESTINY+ spacecraft in 2028 will allow us to verify this hypothesis. The altered and thermally metamorphosed CY chondrites are shown to be the meteoritic analogue of asteroid Phaethon. This suggests that Phaethon's activity is driven by gas released from the decomposition of near-surficial material heated at perihelion, whereas the interior is kept relatively unaltered and hydrated.

The near-Earth asteroids (NEAs) (3200) Phaethon and (155140) 2005 UD are thought to share a common origin, with the former exhibiting dust activity at perihelion that is thought to directly supply the Geminid meteor stream. Both of these objects currently have very small perihelion distances (0.140 and 0.163 au for Phaethon and 2005 UD, respectively), which results in them having perihelion temperatures of or exceeding 1000 K. NEA population models compared to observation suggest that low-perihelion objects are destroyed over time by a temperature-dependent mechanism that becomes relevant at heliocentric distances < 0.3 au. Thus, the current activity from Phaethon is relevant to the destruction of NEAs close to the Sun, which most likely has produced meteor streams linked to asteroids in the past.In this work, we model the past thermal characteristics of Phaethon and 2005 UD using a detailed thermophysical model (TPM) and orbital integrations of each object. Our aim is to investigate and inform a temperature-dependent mechanism responsible for Phaethon's dust activity and the destruction of NEAs at small heliocentric distances. We consider volatile sublimation and thermal fracturing as potential candidate processes. First, a TPM is used to calculate temperatures (surface and subsurface) along an entire orbit for a spherical object, given its semimajor axis and eccentricity (a and e). Temperature characteristics such as maximum daily temperature, maximum thermal gradient, and temperature at varying depths are extracted from the model, which is run for a predefined set of a and e. Next, dynamical integrations of orbital clones of Phaethon and 2005 UD are used to estimate the past orbital elements of each object. These dynamical results are then combined with the temperature characteristics to model the past evolution of thermal characteristics.We find that predictions of the orbital history for these objects is reasonably accurate up to ~100,000 yr in the past, and is characterized by cyclic changes in e resulting in perihelia values periodically shifting between present-day values and 0.3 au. The thermal history of the maximum surface temperatures, for example, thus follows a pattern of extreme heating (up to 1000 K) every 20,000 yr. Currently, Phaethon is experiencing relatively large degrees of heating compared to the recent 20,000 yr. We find that even temperatures at-depth are too large over these timescales for water ice to be stable-unless actively supplied somehow and that thermal fracturing may be extremely effective at breaking down surface regolith. Observations of dust activity from the DESTINY+ flyby mission will provide important constraints on the mechanics of dust-loss.Past estimates of Phaethon's dust tail and mass-loss rate assume particle size of ≈1 micron and are insufficient to explain the entire mass of the Geminid stream of its ~1,000 year lifetime. However, observations of Geminid meteors show that it consists of a wide range of particle sizes (from micron-sized up to a few centimeters). Assuming a similar particle size distribution as the Geminids for Phaethon's dust tail we re-evaluate the mass-loss rate. We find that the annual dust activity from Phaethon may be sufficient to actively supply the Geminid stream in steady-state.

Spacecraft missions to asteroids have revealed surfaces that have variations in albedo and spectral properties. Such variations are also detected across the asteroid population with ground-based observations, and are controlled by the physical characteristics of the regolith and by processes such as space weathering. Here, we investigate how space weathering and regolith grain size influence the spectra of ordinary chondrite-like asteroids observed from ground-based spectroscopy. The estimation of diagnostic band parameters from asteroid visible and near-infrared reflectance spectra allow us to estimate the degree of space weathering and their compositions, using results from an accompanying study (MacLennan et al., 2024). We use grain size estimations gleaned from the thermal inertia to show that regolith particle size differences have similar effect as space weathering on asteroid spectra. Finally, we quantify changes in spectral slope and band depth among asteroids using the space weathering index developed by MacLennan et al (2024), and reassess the importance of previously-proposed surface freshening mechanisms.

Near-Earth asteroids can become warm enough to emit radiation at near-infrared wavelengths, close to 2.5 & mu;m. Thermal radiation can interfere with reflectance measurements in these wavelengths, and should be evaluated and corrected for. Current methods for correcting disk-resolved measurements either rely on previous Earth-based observations or perform heavy computations to find the thermally emitted spectral radiance. Using results based on disk-integrated observations may lead to errors for some cases where the target asteroid surface is not ho-mogeneous. Computational efficiency is desirable for those future missions where data processing is to be per-formed on-board the spacecraft due to a limited downlink budget, such as missions employing small spacecraft. We propose to predict the temperature of an asteroid surface element from its observed spectral radiance using a convolutional neural network. The thermal spectral radiance emitted by the asteroid surface can be approximated using the temperature, and subsequently subtracted from the original spectral radiance. The model was tested using OSIRIS-REx measurements of asteroid (101955) Bennu with promising results. The performance of the model should be validated further in the future as asteroid missions produce suitable data. Both accuracy and speed of the method could likely be increased significantly with further development.

Asteroid surfaces are subjected to mechanical weathering processes that result in the development and evolution of regolith. Two proposed mechanisms—impact bombardment and thermal fatigue—have been proposed as viable and dominant weathering processes. Previously, we compiled and estimated thermal inertias of several hundred asteroids (mostly in the main belt) for which we determined dependencies on temperature, diameter, and rotation period. In this work, we estimate grain sizes of asteroid regoliths from this large thermal inertia data set using thermal conductivity models. Following our previous work, we perform multivariate linear model fits to the grain size data set and quantify its dependency on diameter and rotation period. We find that the preferred model indicates that asteroid grain sizes are inversely dependent on object size for <10 km asteroids and exhibit no relationship above this size cutoff. Rotation period and grain size show a positive relationship when the rotation period is greater than ∼5 hr and an inverse relationship below this rotation period. These results indicate that both impact weathering and thermal fatigue are relevant regolith evolution mechanisms. We run post-hoc t -tests between spectral groups to infer the influence of composition on regolith grain sizes. We find that M-type (including suspected metal-rich objects) and E-type asteroids have larger grain sizes relative to our population sample and that P-type asteroids have distinctly smaller grains than other groups.

The aim of this project is to derive reference phase functions and their parameters using data from the ATLAS survey. The reference phase function corrects for the observation geometry by removing the influence of the asteroid shape by normalizing it to a sphere. (Muinonen et al. 2020) The ATLAS survey performed photometric observations in two filters: cyan (420-650 nm) and orange (560 - 820 nm) for over 180 000 asteroids at phase angles even below 1 deg (Heinze et al. 2018). Mahlke et al. (2021) derived over 1270 000 phase curve parameters using the ATLAS photometry, but they were corresponding to different viewing geometries, so they cannot be directly compared with each other. Traditional phase curves are derived based on lightcurve brightness maximum (or mean) values at a given phase angle. When using sparse photometry (e.g., Gaia, ATLAS), the observational geometry can substantially change between observations and objects. As a result, it is challenging to compare phase curves obtained for different asteroids (even if they were observed at the same epoch). If enough photometry is available, one can account for brightness changes due to shape, rotation, and aspect changes by moving to a reference phase function, which can be directly compared with the phase functions of other objects. (Muinonen et al. 2020, Martikainen et al. 2021, Wilawer et al. 2022) We derive the reference phase functions for ~2750 asteroids with models derived by Ďurech et al. (2020) using ATLAS photometric data. As a result, for each object, we will derive two reference phase functions: one for each ATLAS filter. This work has been supported by grant No. 2017/25/B/ST9/00740 from the National Science Centre, Poland. References Ďurech, J., J. Tonry, N. Erasmus, L. Denneau, A. N. Heinze, H. Flewelling, and R. Vanco. ‘Asteroid Models Reconstructed from ATLAS Photometry’. Astronomy & Astrophysics 643 (November 2020): A59. https://doi.org/10.1051/0004-6361/202037729. Heinze, A. N., J. L. Tonry, L. Denneau, H. Flewelling, B. Stalder, A. Rest, K. W. Smith, S. J. Smartt, and H. Weiland. ‘A First Catalog of Variable Stars Measured by the Asteroid Terrestrial-Impact Last Alert System (ATLAS)’. The Astronomical Journal 156, no. 5 (November 2018): 241. https://doi.org/10.3847/1538-3881/aae47f. Mahlke, Max, Benoit Carry, and Larry Denneau. ‘Asteroid Phase Curves from ATLAS Dual-Band Photometry’. Icarus 354 (January 2021): 114094. https://doi.org/10.1016/j.icarus.2020.114094. Martikainen, J., K. Muinonen, A. Penttilä, A. Cellino, and X.-B. Wang. ‘Asteroid Absolute Magnitudes and Phase Curve Parameters from Gaia Photometry’. Astronomy & Astrophysics 649 (May 2021): A98. https://doi.org/10.1051/0004-6361/202039796. Muinonen, K., J. Torppa, X.-B. Wang, A. Cellino, and A. Penttilä. ‘Asteroid Lightcurve Inversion with Bayesian Inference’. Astronomy & Astrophysics 642 (October 2020): A138. https://doi.org/10.1051/0004-6361/202038036. Wilawer, E, D Oszkiewicz, A Kryszczyńska, A Marciniak, V Shevchenko, I Belskaya, T Kwiatkowski, et al. ‘Asteroid Phase Curves Using Sparse Gaia DR2 Data and Differential Dense Light Curves’. Monthly Notices of the Royal Astronomical Society 513, no. 3 (May 2022): 3242–51. https://doi.org/10.1093/mnras/stac1008.

Thermal infrared emission and thermophysical modeling techniques are powerful tools in deciphering the surface properties of asteroids. The near-Earth asteroid (3200) Phaethon is an active asteroid with a very small perihelion distance and is likely the source of the Geminid meteor shower. We estimate and interpret the thermal inertia of this extraordinary asteroid using observations that span ten distinct sightings. The variation in thermal inertia over these sightings is inconsistent with the expected temperature-dependent thermal inertia theorized from radiative heat transfer within the regolith. Thus, we test whether the variation in thermal inertia can be explained by modeling a regolith layer over bedrock and two spatially heterogeneous scenarios. We find that the model in which Phaethon's north and south hemispheres have distinctly different thermophysical properties can sufficiently explain the thermal-inertias determined herein. In particular, we find that a boundary located between latitudes -30 deg and +10 deg separates fine-grained southern latitudes from a northern hemisphere that is dominated by coarse-grained regolith and/or a high coverage of porous boulders. We discuss the implications related to Phaethon's activity and potential association with 2005 UD.

The km-scale near-Earth object (1566) Icarus has an extremely eccentric orbit with a perihelion of q = 0.187 au and is classified as a potentially hazardous asteroid (PHA). It has been suspected to be the larger component of an asteroid pair, with the smaller object 2007 MK6, that is dynamically adjacent to the Taurid-Perseid meteor shower (Ohsutka et al., 2007; Kasuga & Jewitt, 2019). The low radar albedo of ~2% and photometric behavior at high phase angles together suggest a high-porosity surface with a high macroscopic roughness (Greenberg, et al. 2017; Ishiguro, et al., 2017). Delay-Doppler and visible lightcurve observations indicate a retrograde spin with a rapid rotation period of ~2.26 hr (Greenberg, et al. 2017; Warner et al., 2009). Combining visible spectrophotometry from the 24-Color Asteroid Survey (Chapman et al., 2020) and MITHNEOS near-infrared reflectance spectra (Binzel et al., 2019), we classify Icarus (Figure 1) as a slightly space weathered LL chondrite via a band parameter analysis routine (MacLennan, et al. in prep.). Using archived lightcurve observations of Icarus collected in 1968 and 2015 (Lagerkvist et al., 1993; Warner et al., 2009), and informed by spin axis constraints, we implement a Bayesian lightcurve inversion approach (Muinonen, et al. 2020) to construct a convex shape model of Icarus (Figure 2). Figure 1. Combined visible spectrophotometry and near-infrared reflectance spectra of Icarus and reflectance spectrum of the LL4 ordinary chondrite Hamlet from the RELAB database. Figure 2. Convex shape model of Icarus from inversion of lightcurve photometry. We incorporate thermal infrared data from the Spitzer Space Telescope (IRAC photometry and IRS spectra) and the NEOWISE survey in order to characterize Icarus’s thermophysical properties. We estimate the effective diameter and thermal inertia to be 1.4 ± 0.2 km and 60 ± 40 J K-1 m-2 s-1/2, respectively, with moderate surface roughness. The relatively low thermal inertia is consistent with a high porosity surface and/or a fine-grained lunar like surface. The latter interpretation is in contradiction to the polarization-phase relationship that suggests larger regolith grains (Ishiguro et al., 2007). We attempt to reconcile these different measurement results in our presentation. The physical characteristics of this extreme object are important for informing various resurfacing processes that have been proposed to be relevant for rapidly rotating objects, near-Sun asteroids, and spectrally-fresh Q-type asteroids (Graves et al., 2018, 2019). We thus consider our results in the context of these resurfacing processes. References: Binzel, R.P., et al. (2019) “Compositional distributions and evolutionary processes for the near-Earth object population: Results from the MIT-Hawaii Near-Earth Object Spectroscopic Survey (MITHNEOS)” Icarus, 324, 41–76. Chapman, C.R., Gaffey, M., and McFadden, L. (2020) 24-color Asteroid Survey V1.0. urn:nasa:pds:gbo.ast.24-color-survey::1.0. NASA Planetary Data System. Greenberg, A., et al. (2017) “Asteroid 1566 Icarus’s Size, Shape, Orbit, and Yarkovsky Drift from Radar Observations” AJ, 153:108. Graves, et al. (2018) “Resurfacing asteroids from YORP spin-up and failure”, Icarus, 304 (2018) 162–171. Graves, et al. (2019) “Resurfacing asteroids from thermally induced surface degradation”, Icarus, 322 (2019) 1–12. Ishiguro, M., et al. (2017) “Polarimetric Study of Near-Earth Asteroid (1566) Icarus”, AJ, 154:180. Kasuga, T. & Jewitt, D. (2019) “Asteroid-Meteorite Complexes”, In Meteoroids: Sources of Meteors on Earth and Beyond (Ed. G. Ryabova, D. Asher, & M. Campbell-Brown). Lagerkvist, C.-I. and Magnusson, P., Eds., (2011) Asteroid Photometric Catalog V1.1. EAR-A-3-DDR-APC-LIGHTCURVE-V1.1. NASA Planetary Data System. MacLennan, E.M., et al. (in prep) “Empirical characterization of space weathering on ordinary chondrite-like asteroids”. Muinonen , J. Torppa , X.-B. Wang , A. Cellino , and A. Penttilä (2020) “Asteroid lightcurve inversion with Bayesian inference”, A&A, 642, A138. Ohtsuka, K. “Apollo asteroids 1566 Icarus and 2007 MK6: Icarus Family Members?” AJ, 668: L71–L74. Warner, B.D., Harris, A.W., Pravec, P. (2009). “The Asteroid Lightcurve Database”, Icarus 202, 134-146.