Asteroid Thermal Model Research Articles

Size, albedo, and rotational period of the Hayabusa2# target (98943) 2001 CC21

This study aims to determine the size, albedo, and rotational period of (98943) 2001 CC21, a target of the Hayabusa2 extended mission, using thermal data from the Spitzer Space telescope and ground-based observations. The Spitzer data were acquired with the Infrared Spectrograph in the 6-38 mu m range, reduced using the Spitzer pipeline, and modeled with the near-Earth asteroid thermal model to determine the asteroid size and albedo. The absolute magnitude and rotational period were determined thanks to new observations carried out at the 3.5m New Technology Telescope, the 1.2m Observatoire de Haute Provence, and the 0.7m Abastumani telescope. Three complete light curves were obtained in 2023 and 2024 at the last-mentioned telescope. We determine an absolute magnitude of H=18.94pm 0.05 and a rotational period of 5.02124pm 0.00001 hours, with a large light curve amplitude of sim 0.8 mag. at a phase angle of 22$^o$, indicating a very elongated shape with an estimated a/b semiaxis ratio geq 1.7, or a close-contact binary body. The emissivity of 2001 CC21 is consistent with that of silicates, and its albedo is 21.6pm 1.6 <!PCT!>. Finally, the spherical-equivalent diameter of 2001 CC21 is 465pm 15 m. The albedo value and emissivity determined here, coupled with results from polarimetry and spectroscopy from the literature, confirm that 2001 CC21 is an S-complex asteroid, and not an L-type one as was previously suggested. The size of 2001 CC21 is less than 500 m, which is smaller than its first size estimation (sim 700 m). These results are relevant in preparation of the observing strategy for 2001 CC21 of the Hayabusa2 extended mission.

The Atacama Cosmology Telescope: Millimeter Observations of a Population of Asteroids or: ACTeroids

We present fluxes and light curves for a population of asteroids at millimeter wavelengths, detected by the Atacama Cosmology Telescope (ACT) over 18,000 deg2 of the sky using data from 2017 to 2021. We utilize high cadence maps, which can be used in searching for moving objects such as asteroids and trans-Neptunian Objects, as well as for studying transients. We detect 170 asteroids with a signal-to-noise of at least 5 in at least one of the ACT observing bands, which are centered near 90, 150, and 220 GHz. For each asteroid, we compare the ACT measured flux to predicted fluxes from the near-Earth asteroid thermal model fit to WISE data. We confirm previous results that detected a deficit of flux at millimeter wavelengths. Moreover, we report a spectral characteristic to this deficit, such that the flux is relatively lower at 150 and 220 GHz than at 90 GHz. Additionally, we find that the deficit in flux is greater for S-type asteroids than for C-type.

Thermal Models of Asteroids with Two-band Combinations of Wide-field Infrared Survey Explorer Cryogenic Data

We used the reparameterized Near-Earth Asteroid Thermal Model to model observations of a curated set of over 4000 asteroids from the Wide-field Infrared Survey Explorer in two wavelength bands (W2–3 or W3–4) and compared the results to previous results from all four wavelength bands (W1–4). This comparison was done with the goal of elucidating unique aspects of modeling two-band observations so that any potential biases or shortcomings for planned two-band surveys (e.g., the NASA Near-Earth Object Surveyor Mission) can be anticipated and quantified. The W2–3 two-band fits usually yielded slightly smaller diameters than the four-band fits, with a median diameter difference of −10%, with the 5% and 95% quantiles of the distribution at −32% and −1.5%, respectively. We conducted similar comparisons for W3–4, in part because the longest wavelength bands are expected to provide the best two-band results. We found that the W3–4 two-band diameters are slightly larger than the four-band results, with a median diameter difference of 11% and the 5% and 95% quantiles of the distribution at −2.1% and 26%, respectively. The diameter uncertainty, obtained with bootstrap analysis, is larger by 30% and 35% (median values) for the W2–3 and W3–4 fits, respectively, than for the corresponding four-band fits. Using 23 high-quality stellar occultation diameters as a benchmark, we found that the median errors of W2–3 and W3–4 diameter estimates are −15% and +12%, respectively, whereas the median error of the four-band fits is 9.3%. Although the W2–3 and W3–4 diameters appear to have greater systematic errors and uncertainties than their four-band counterparts, two-band estimates remain useful because they improve upon diameter estimates obtained from visible photometry alone.

Characterization of Thermal-infrared Dust Emission and Refinements to the Nucleus Properties of Centaur 29P/Schwassmann–Wachmann 1

Abstract We present analyses of Spitzer observations of 29P/Schwassmann–Wachmann 1 using 16 μm IRS “blue” peak-up (PU) and 24 and 70 μm MIPS images obtained on UT 2003 November 23 and 24 that characterize the Centaur’s large-grain (10–100 μm) dust coma during a time of non-outbursting “quiescent” activity. Estimates of ϵ f ρ for each band (16 μm (2600 ± 43 cm), 24 μm (5800 ± 63 cm), and 70 μm (1800 ± 900 cm)) follow the trend between nucleus size versus ϵ f ρ that was observed for the WISE/NEOWISE comet ensemble. A coma model was used to derive a dust production rate in the range of 50–100 kg s−1. For the first time, a color temperature map of SW1's coma was constructed using the 16 and 24 μm imaging data. With peaks at ∼140 K, this map implies that coma water-ice grains should be slowly sublimating and producing water gas in the coma. We analyzed the persistent 24 μm “wing” (a curved southwestern coma) feature at 352,000 km (90″) from the nucleus attributed by Stansberry et al. to nucleus rotation and instead propose that it is largely created by solar radiation pressure and gravity acting on micron-sized grains. We performed coma removal to the 16 μm PU image in order to refine the nucleus’ emitted thermal flux. A new application of the Near Earth Asteroid Thermal Model at five wavelengths (5.730, 7.873, 15.80, 23.68, and 71.42 μm) was then used to refine SW1's effective radius measurement to R = 32.3 ± 3.1 km and infrared beaming parameter to η = 1.1 ± 0.2, respectively.

The Moon at thermal infrared wavelengths: a benchmark for asteroid thermal models

Thermal-infrared measurements of asteroids, satellites, and distant minor bodies are crucial for deriving the objects’ sizes, albedos, and in some cases, also the thermophysical properties of the surface material. Depending on the available measurements and auxiliary data, such as visual light curves, spin and shape information, or direct size measurements from occultations or high-resolution imaging techniques, a range of simple to complex thermal models are applied to achieve specific science goals. However, testing these models is often a difficult process and the uncertainties of the derived parameters are not easy to estimate. Here, we make an attempt to verify a widely accepted thermophysical model (TPM) against unique thermal infrared (IR), full-disk, and well-calibrated measurements of the Moon. The data were obtained by the High-resolution InfraRed Sounder (HIRS) instruments on board a fleet of Earth weather satellites that serendipitously scan the surface of the Moon. We found 22 Moon intrusions, taken in 19 channels between 3.75 μm and 15.0 μm, and over a wide phase angle range from −73.1° (waxing Moon) to +73.8° (waning Moon). These measurements include the entire Moon in a single pixel, seen almost simultaneously in all bands. The HIRS filters are narrow and outside the wavelength regime of the Christiansen feature. The similarity between these Moon data and typical asteroid spectral-IR energy distributions allows us to benchmark the TPM concepts and to point out problematic aspects. The TPM predictions match the HIRS measurements within 5% (10% at the shortest wavelengths below 5 μm) when using the Moon’s known properties (size, shape, spin, albedo, thermal inertia, roughness) in combination with a newly established wavelength-dependent hemispherical emissivity. In the 5–7.5 μm and in the 9.5–11 μm ranges, the global emissivity model deviates considerably from the known lunar sample spectra. Our findings will influence radiometric studies of near-Earth and main-belt asteroids in cases where only short-wavelength data (from e.g., NEOWISE, the warm Spitzer mission, or ground-basedM-band measurements) are available. The new, full-disk IR Moon model will also be used for the calibration of IR instrumentation on interplanetary missions (e.g., for Hayabusa-2) and weather satellites.

Near-earth asteroid: (285263) 1998 QE2

We present the results of an integrated effort to constrain the thermal and surface properties of near-Earth asteroid (285263) 1998 QE2 using the NASA Infrared Telescope Facility (IRTF) during a historically approach to the Earth in May 2013. The near-infrared spectrum of (285263) 1998 QE2 is red, featureless, and exhibits a strong thermal excess beyond 2.0 μm. We model this thermal excess using the Near-Earth Asteroid Thermal Model and retrieved a model visual albedo of 0.04 ± 0.01, consistent with previous observations. Comparison with newly obtained laboratory spectra of the CM2 chondrites Murchison and the recently fallen Aguas Zarcas as well as archival spectra of Nogoya reveal that QE2 has a more Murchison/Aguas Zargas-like spectrum throughout the NIR. However, our spectrum of (285263) 1998 QE2 does not show the 0.7-μm feature observed by others and also visible in the laboratory spectra of CM2 carbonaceous chondrites. A better understanding of QE2's physical properties could help constrain models of the formation of other low-albedo NEA binaries.

ATM: An open-source tool for asteroid thermal modeling and its application to NEOWISE data

We publicly release ATM, a Python package designed to model asteroid flux measurements to estimate an asteroid's size, surface temperature distribution, and emissivity. A number of the most popular static asteroid thermal models (NEATM, STM, FRM) are implemented with the reflected solar light contribution and Kirchhoff's law accounted for. Priors for fitted parameters can be easily specified and the solution, including the full multi-dimensional posterior probability density function, is found using Markov Chain Monte Carlo (MCMC). We describe the package's architecture and discuss model and fitting validation. Data files with ~10 million WISE flux measurements for ~150,000 unique asteroids and additional Minor Planet Center data are also included with the package, as well as Python Jupyter Notebooks with examples of how to select subsamples of objects, filter and process data, and use ATM to fit for the desired model parameters. The entirety of the analysis presented here, including all the figures, tables, and catalogs, can be easily reproduced with these publicly released Notebooks. We show that ATM can match the best-fit size estimates for well-observed asteroids published in 2016 by the NEOWISE team (Mainzer et al., 2016) with a sub-percent bias and a scatter of only 6%. Our analysis of various sources of random and systematic size uncertainties shows that for the majority of over 100,000 objects with WISE-based size estimates random uncertainties (precision) are about 10%; systematic uncertainties within the adopted model framework, such as NEATM, and with assumed emissivity for WISE W3 and W4 bands, are likely in the range of 10–20%. Hence, the accuracy of WISE-based asteroid size estimates is approximately in the range of 15–20% for most objects, except for unknown errors due to a possibly over-simplified modeling framework (e.g., spherical asteroid approximation). We compare model families to data in WISE color-color diagrams and derive a simple method to estimate size that only uses WISE W3 band flux; we show that it matches estimates based on all four WISE bands to within 10%. We also study optical data collected by the Sloan Digital Sky Survey (SDSS) and show that correlations of optical colors and WISE-based best-fit model parameters indicate robustness of the latter. Our analysis gives support to the claim by Harris & Drube (2014) that candidate metallic asteroids can be selected using the best-fit temperature parameter and infrared albedo. We investigate a correlation between SDSS colors and optical albedo derived using WISE-based size estimates and show that this correlation can be used to estimate asteroid sizes with optical data alone, with a precision of about 17% relative to WISE-based size estimates. After accounting for systematic errors, the difference in accuracy between infrared and optical color-based size estimates becomes less than a factor of two.

Modeling of Surface Temperature for Near-Earth Asteroid 2016HO3

On April 18, 2019, the China National Space Administration (CNSA) published the plan of asteroid exploration, which would select the near-earth asteroid 2016HO3 as one of its targets. On this paper, the parameters of 2016HO3 for thermal environment analysis were discussed. By investigating the current international observation data, the environmental parameters of 2016HO3 are obtained preliminarily. Then the near-Earth asteroid thermal model(NEATM) and asteroid thermal physics model (TPM) were used to model and analyze the surface temperature of 2016HO3, and the upper boulder of the temperature was 412 K. Considering the spinning effect, the day and night temperature difference at different locations was simulated and analyzed, and the temperature analysis method under the polar night conditions was proposed, then the maximum temperature difference at any position was determined to be about 30 K. Last on the basis of TPM, by the idea of the average illumination during the rotation period, the temperature analysis method of the polar night was proposed, and lower limit of temperature of 2016HO3 was achieved.

Physical Characterization of Active Asteroid (6478) Gault

Abstract Main belt asteroid (6478) Gault has been dynamically linked with two overlapping asteroid families: Phocaea, dominated by S-type asteroids, and Tamara, dominated by low-albedo C-types. This object has recently become an interesting case for study after images obtained in late 2018 revealed that it was active and displaying a comet-like tail. Previous authors have proposed that the most likely scenarios to explain the observed activity on Gault were rotational excitation or merger of near-contact binaries. Here we use new photometric and spectroscopic data of Gault to determine its physical and compositional properties. Lightcurves derived from the photometric data showed little variation over three nights of observations, which prevented us from determining the rotation period of the asteroid. Using WISE observations of Gault and the near-Earth Asteroid Thermal Model (NEATM) we determined that this asteroid has a diameter <6 km. Near-infrared spectroscopic data obtained with the Infrared Telescope Facility showed a spectrum similar to that of S-complex asteroids, and a surface composition consistent with H chondrite meteorites. These results favor a compositional affinity between Gault and asteroid (25) Phocaea, and rules out a compositional link with the Tamara family. From the spectroscopic data we found no evidence of fresh material that could have been exposed during the outburst episodes.

Thermal Emission from the Uranian Ring System

Abstract The narrow main rings of Uranus are composed of almost exclusively centimeter- to meter-sized particles, with a very small or nonexistent dust component; however, the filling factor, composition, thickness, mass, and detailed particle size distribution of these rings remain poorly constrained. Using millimeter (1.3–3.1 mm) imaging from the Atacama Large (sub-)Millimeter Array and mid-infrared (18.7 μm) imaging from the Very Large Telescope VISIR instrument, we observed the thermal component of the Uranian ring system for the first time. The ϵ ring is detected strongly and can be seen by eye in the images; the other main rings are visible in a radial (azimuthally averaged) profile at millimeter wavelengths. A simple thermal model similar to the Near-Earth Asteroid Thermal Model (NEATM) of near-Earth asteroids is applied to the ϵ ring to determine a ring particle temperature of 77.3 ± 1.8 K. The observed temperature is higher than expected for fast-rotating ring particles viewed at our observing geometry, meaning that the data favor a model in which the thermal inertia of the ring particles is low and/or their rotation rate is slow. The ϵ ring displays a factor of 2–3 brightness difference between periapsis and apoapsis, with 49.1% ± 2.2% of sightlines through the ring striking a particle. These observations are consistent with optical and near-infrared reflected light observations, confirming the hypothesis that micron-sized dust is not present in the ring system.

Constraints on asteroid magnetic field evolution and the radii of meteorite parent bodies from thermal modelling

Paleomagnetic measurements of ancient terrestrial and extraterrestrial samples indicate that numerous planetary bodies generated magnetic fields through core dynamo activity during the early solar system. The existence, timing, intensity and stability of these fields are governed by the internal transfer of heat throughout their parent bodies. Thus, paleomagnetic records preserved in natural samples can contain key information regarding the accretion and thermochemical history of the rocky bodies in our solar system. However, models capable of predicting these field properties across the entire active lifetime of a planetary core that could relate the processes occurring within these bodies to features in these records and provide such information are limited. Here, we perform asteroid thermal evolution models across suites of radii, accretion times and thermal diffusivities with the aim of predicting when fully and partially differentiated asteroids generated magnetic fields. We find that dynamo activity in both types of asteroid is delayed until ∼4.5-5.5 Myr after calcium-aluminium-rich inclusion formation due to the partitioning of 26Al into the silicate portion of the body during differentiation and large early surface heat fluxes, followed by a brief period (<12.5 Myr for bodies with radii <500 km) of thermally-driven dynamo activity as heat is convected from the core across a partially-molten magma ocean. We also expect that gradual core solidification produced compositionally-driven dynamo activity in these bodies, the timing of which could vary by tens to hundreds of millions of years depending on the S concentration of the core and the radius of the body. There was likely a pause in core cooling and dynamo activity following the cessation of convection in the magma ocean. Our predicted periods of magnetic field generation and quiescence match eras of high and low paleointensities in the asteroid magnetic field record compiled from paleomagnetic measurements of multiple meteorites, providing the possible origins of the remanent magnetisations carried by these samples. We also compare our predictions to paleomagnetic results from different meteorite groups to constrain the radii of the angrite, CV chondrite, H chondrite, IIE iron meteorite and Bjürbole (L/LL chondrite) parent bodies and identify a likely nebula origin for the remanent magnetisation carried by the CM chondrites.

The AKARI IRC asteroid flux catalogue: updated diameters and albedos

The AKARI IRC all-sky survey provided more than twenty thousand thermal infrared observations of over five thousand asteroids. Diameters and albedos were obtained by fitting an empirically calibrated version of the standard thermal model to these data. After the publication of the flux catalogue in October 2016, our aim here is to present the AKARI IRC all-sky survey data and discuss valuable scientific applications in the field of small body physical properties studies. As an example, we update the catalogue of asteroid diameters and albedos based on AKARI using the near-Earth asteroid thermal model (NEATM). We fit the NEATM to derive asteroid diameters and, whenever possible, infrared beaming parameters. We fit groups of observations taken for the same object at different epochs of the survey separately, so we compute more than one diameter for approximately half of the catalogue. We obtained a total of 8097 diameters and albedos for 5170 asteroids, and we fitted the beaming parameter for almost two thousand of them. When it was not possible to fit the beaming parameter, we used a straight line fit to our sample’s beaming parameter-versus-phase angle plot to set the default value for each fit individually instead of using a single average value. Our diameters agree with stellar-occultation-based diameters well within the accuracy expected for the model. They also match the previous AKARI-based catalogue at phase angles lower than 50°, but we find a systematic deviation at higher phase angles, at which near-Earth and Mars-crossing asteroids were observed. The AKARI IRC All-sky survey is an essential source of information about asteroids, especially the large ones, since, it provides observations at different observation geometries, rotational coverages and aspect angles. For example, by comparing in more detail a few asteroids for which dimensions were derived from occultations, we discuss how the multiple observations per object may already provide three-dimensional information about elongated objects even based on an idealised model like the NEATM. Finally, we enumerate additional expected applications for more complex models, especially in combination with other catalogues.

An Investigation of the Ranges of Validity of Asteroid Thermal Models for Near-Earth Asteroid Observations

Abstract The majority of known asteroid diameters are derived from thermal-infrared observations. Diameters are derived using asteroid thermal models that approximate their surface temperature distributions and compare the measured thermal-infrared flux with model-dependent predictions. The most commonly used thermal model is the Near-Earth Asteroid Thermal Model (NEATM), which is usually perceived as superior to other models like the Fast-Rotating Model (FRM). We investigate the applicability of the NEATM and the FRM to thermal-infrared observations of Near-Earth Objects using synthetic asteroids with properties based on the real Near-Earth Asteroid (NEA) population. We find the NEATM to provide more accurate diameters and albedos than the FRM in most cases, with a few exceptions. The modeling results are barely affected by the physical properties of the objects, but we find a large impact of the solar phase angle on the modeling results. We conclude that the NEATM provides statistically more robust diameter estimates for NEAs observed at solar phase angles less than ∼65°, while the FRM provides more robust diameter estimates for solar phase angles greater than ∼65°. We estimate that &lt;5% of all NEA diameters and albedos derived up to date are affected by systematic effects that are of the same order of magnitude as the typical thermal model uncertainties. We provide statistical correction functions for diameters and albedos derived using the NEATM and FRM as a function of solar phase angle.

Asteroid thermal modeling in the presence of reflected sunlight

A new derivation of simple asteroid thermal models is presented, investigating the need to account correctly for Kirchhoff's law of thermal radiation when IR observations contain substantial reflected sunlight. The framework applies to both the NEATM and related thermal models. A new parameterization of these models eliminates the dependence of thermal modeling on visible absolute magnitude H, which is not always available. Monte Carlo simulations are used to assess the potential impact of violating Kirchhoff's law on estimates of physical parameters such as diameter and IR albedo, with an emphasis on NEOWISE results. The NEOWISE papers use ten different models, applied to 12 different combinations of WISE data bands, in 47 different combinations. The most prevalent combinations are simulated and the accuracy of diameter estimates is found to be depend critically on the model and data band combination. In the best case of full thermal modeling of all four band the errors in an idealized model the 1σ (68.27%) confidence interval is −5% to +6%, but this combination is just 1.9% of NEOWISE results. Other combinations representing 42% of the NEOWISE results have about twice the CI at −10% to +12%, before accounting for errors due to irregular shape or other real world effects that are not simulated. The model and data band combinations found for the majority of NEOWISE results have much larger systematic and random errors. Kirchhoff's law violation by NEOWISE models leads to errors in estimation accuracy that are strongest for asteroids with W1, W2 band emissivity ϵ12 in both the lowest (0.605≤ϵ12≤0.780), and highest decile (0.969≤ϵ12≤0.988), corresponding to the highest and lowest deciles of near-IR albedo pIR. Systematic accuracy error between deciles ranges from a low of 5% to as much as 45%, and there are also differences in the random errors. Kirchhoff's law effects also produce large errors in NEOWISE estimates of pIR, particularly for high values. IR observations of asteroids in bands that have substantial reflected sunlight can largely avoid these problems by adopting the Kirchhoff law compliant modeling framework presented here, which is conceptually straightforward and comes without computational cost.

Sizes and albedos of Mars-crossing asteroids from WISE/NEOWISE data

Context.Mars-crossing asteroids (MCs) are a dynamically unstable group between the main belt and the near-Earth populations. Characterising the physical properties of a large sample of MCs can help to understand the original sources of many near-Earth asteroids, some of which may produce meteorites on Earth.Aims.Our aim is to provide diameters and albedos of MCs with available WISE/NEOWISE data.Methods.We used the near-Earth asteroid thermal model to find the best-fitting values of equivalent diameter and, whenever possible, the infrared beaming parameter. With the diameter and tabulated asteroid absolute magnitudes we also computed the visible geometric albedos.Results.We determined the diameters and beaming parameters of 404 objects observed during the fully cryogenic phase of the WISE mission, most of which have not been published elsewhere. We also obtained 1572 diameters from data from the 3-Band and posterior non-cryogenic phases using a default value of beaming parameter. The average beaming parameter is 1.2 ± 0.2 for objects smaller than 10 km, which constitute most of our sample. This is higher than the typical value of 1.0 found for the whole main belt and is possibly related to the fact that WISE is able to observe many more small objects at shorter heliocentric distances, i.e. at higher phase angles. We argue that this is a better default value for modelling Mars-crossing asteroids from the WISE/NEOWISE catalogue and discuss the effects of this choice on the diameter and albedo distributions. We find a double-peaked distribution for the visible geometric albedos, which is expected since this population is compositionally diverse and includes objects in the major spectral complexes. However, the distribution of beaming parameters is homogeneous for both low- and high-albedo objects.

Mid-infrared spectra of comet nuclei

Comet nuclei and D-type asteroids have several similarities at optical and near-IR wavelengths, including near-featureless red reflectance spectra, and low albedos. Mineral identifications based on these characteristics are fraught with degeneracies, although some general trends can be identified. In contrast, spectral emissivity features in the mid-infrared provide important compositional information that might not otherwise be achievable. Jovian Trojan D-type asteroids have emissivity features strikingly similar to comet comae, suggesting that they have the same compositions and that the surfaces of the Trojans are highly porous. However, a direct comparison between a comet and asteroid surface has not been possible due to the paucity of spectra of comet nuclei at mid-infrared wavelengths. We present 5–35 µm thermal emission spectra of comets 10P/Tempel 2, and 49P/Arend-Rigaux observed with the Infrared Spectrograph on the Spitzer Space Telescope. Our analysis reveals no evidence for a coma or tail at the time of observation, suggesting the spectra are dominated by the comet nucleus. We fit each spectrum with the near-Earth asteroid thermal model (NEATM) and find sizes in agreement with previous values. However, the NEATM beaming parameters of the nuclei, 0.74–0.83, are systematically lower than the Jupiter-family comet population mean of 1.03 ± 0.11, derived from 16- and 22-µm photometry. We suggest this may be either an artifact of the spectral reduction, or the consequence of an emissivity low near 16 µm. When the spectra are normalized by the NEATM model, a weak 10-µm silicate plateau is evident, with a shape similar to those seen in mid-infrared spectra of D-type asteroids. A silicate plateau is also evident in previously published Spitzer spectra of the nucleus of comet 9P/Tempel 1. We compare, in detail, these comet nucleus emission features to those seen in spectra of the Jovian Trojan D-types (624) Hektor, (911) Agamemnon, and (1172) Aneas, as well as those seen in the spectra of seven comet comae. The comet comae present silicate features with two distinct shapes, either trapezoidal, or more rounded, the latter apparently due to enhanced emission near 8 to 8.5 µm. The surfaces of Tempel 2, Arend-Rigaux, and Hektor best agree with the comae that present trapezoidal features, furthering the hypothesis that the surfaces of these targets must have high porosities in order to exhibit a spectrum similar to a comet coma. An emissivity minimum at 15 µm, present in the spectra of Tempel 2, Arend-Rigaux, Hektor, and Agamemnon, is also described, the origin of which remains unidentified. The compositional similarity between D-type asteroids and comets is discussed, and our data supports the hypothesis that they have similar origins in the early Solar System.

Near-infrared thermal emission from near-Earth asteroids: Aspect-dependent variability

Here we explore a technique for constraining physical properties of near-Earth asteroids (NEAs) based on variability in thermal emission as a function of viewing aspect. We present case studies of the low albedo, near-Earth asteroids (285263) 1998 QE2 and (175706) 1996 FG3. The Near-Earth Asteroid Thermal Model (NEATM) is used to fit signatures of thermal emission in near-infrared (0.8 - 2.5 µm) spectral data. This analysis represents a systematic study of thermal variability in the near-IR as a function of phase angle. The observations of QE2 imply that carefully timed observations from multiple viewing geometries can be used to constrain physical properties like retrograde versus prograde pole orientation and thermal inertia. The FG3 results are more ambiguous with detected thermal variability possibly due to systematic issues with NEATM, an unexpected prograde rotation state, or a surface that is spectrally and thermally heterogenous. This study highlights the potential diagnostic importance of high phase angle thermal measurements on both sides of opposition. We find that the NEATM thermal beaming parameters derived from our near-IR data tend to be of order10’s of percent higher than parameters from ensemble analyses of longer wavelength data sets. However, a systematic comparison of NEATM applied to data in different wavelength regimes is needed to understand whether this offset is simply a reflection of small number statistics or an intrinsic limitation of NEATM when applied to near-IR data. With the small sample presented here, it remains unclear whether NEATM modeling at near-IR wavelengths can robustly determine physical properties like pole orientation and thermal inertia.

Comparing NEO Search Telescopes

Multiple terrestrial and space-based telescopes have been proposed for detecting and tracking near-Earth objects (NEOs). Detailed simulations of the search performance of these systems have used complex computer codes that are not widely available, which hinders accurate cross-comparison of the proposals and obscures whether they have consistent assumptions. Moreover, some proposed instruments would survey infrared (IR) bands, whereas others would operate in the visible band, and differences among asteroid thermal and visible-light models used in the simulations further complicate like-to-like comparisons. I use simple physical principles to estimate basic performance metrics for the ground-based Large Synoptic Survey Telescope and three space-based instruments—Sentinel, NEOCam, and a Cubesat constellation. The performance is measured against two different NEO distributions, the Bottke et al. distribution of general NEOs, and the Veres et al. distribution of Earth-impacting NEO. The results of the comparison show simplified relative performance metrics, including the expected number of NEOs visible in the search volumes and the initial detection rates expected for each system. Although these simplified comparisons do not capture all of the details, they give considerable insight into the physical factors limiting performance. Multiple asteroid thermal models are considered, including FRM, NEATM, and a new generalized form of FRM. I describe issues with how IR albedo and emissivity have been estimated in previous studies, which may render them inaccurate. A thermal model for tumbling asteroids is also developed and suggests that tumbling asteroids may be surprisingly difficult for IR telescopes to observe.

Size and albedo distributions of asteroids in cometary orbits using WISE data

We study the distributions of effective diameter ($D$), beaming parameter ($\eta$), and visible geometric albedo ($p_V$) of asteroids in cometry orbits (ACOs) populations, derived from NASA's Wide-field Infrared Explorer (WISE) observations, and compare these with the same, independently determined properties of the comets. The near-Earth asteroid thermal model (NEATM) is used to compute the $D$, $p_V$ and $\eta$. We obtained $D$ and $p_V$ for 49 ACOs in Jupiter family cometary orbits (JF-ACOs) and 16 ACOs in Halley-type orbits (Damocloids). We also obtained $\eta$ for 45 of them. All but three JF-ACOs (95% of the sample) present a low albedo compatible with a cometary origin. The $p_V$ and $\eta$ distributions of both ACO populations are very similar. For the entire sample of ACOs, the mean geometric albedo is $\bar{p_V} = 0.05 \pm 0.02$, ($\bar{p_V} = 0.05 \pm 0.01$ and $\bar{p_V} =0.05 \pm 0.02$ for JF-ACOs and Damocloids, respectively) compatible with a narrow albedo distribution similar to that of the Jupiter family comets (JFCs), with a $\bar{p_V} \sim 0.04$. The $\bar{\eta} =1.0 \pm 0.2$. We find no correlations between $D$, $p_V$ , or $\eta$. We compare the cumulative size distribution (CSD) of ACOs, Centaurs, and JFCs. Although the Centaur sample contains larger objects, the linear parts in their log-log plot of the CSDs presents a similar cumulative exponent ($\beta = 1.85 \pm 0.30$ and $1.76 \pm 0.35$, respectively). The CSD for Damocloids presents a much shallower exponent $\beta = 0.89 \pm 0.17$. The CSD for JF-ACOs is shallower and shifted towards larger diameters with respect to the CSD of active JFCs, which suggests that the mantling process has a size dependency whereby large comets tend to reach an inactive stage faster than small ones. Finally, the population of JF-ACOs is comparable in number that of JFCs, although there are more tens-km JF-ACOs than JFCs.

A new analysis of Spitzer observations of Comet 29P/Schwassmann–Wachmann 1

We present a new analysis of Spitzer observations of Comet 29P/Schwassmann–Wachmann 1 taken on UT 2003 November 21, 23, and 24, similar to a previous investigation of the observations (Stansberry et al., 2004), but using the most recent Spitzer data pipeline products and intensive image processing techniques. Analysis of images from the IRAC 5.8 and 8.0μm bands and the MIPS 24.0 and 70.0μm bands resulted in photometry measurements of the nucleus after a suite of coma modeling and removal processes were implemented. SW1 was not identified in the 5.8μm image from the previous work so its incorporation into this analysis is entirely new. Using the Near Earth Asteroid Thermal Model (Harris, 1998) resulted in a nucleus radius measurement of R=30.2-2.9+3.7km and an infrared beaming parameter value of η=0.99-0.19+0.26. We also measured an infrared geometric albedo, p5.8=0.5±0.5. Extrapolating a 0.04V-band albedo and using a normalized reflectivity gradient S′=14.94±1.09 [% (1000Å)−1] (Duffard, R., et al. [2014]. Astron. Astrophys. 564, A92) we recover an infrared albedo of p5.8=0.31 in the near infrared consistent with the value recovered from thermal modeling. The dust composition extracted from IRS spectra are very comet-like, containing mainly amorphous ferromagnesian silicates (but with a minority of crystalline silicates as well), water ice, and metal sulfides.