Small Bodies Near and Far (SBNAF): A benchmark study on physical and thermal properties of small bodies in the Solar System

et al. 2021

A&A

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Context. Recent results for asteroid rotation periods from the TESS mission showed how strongly previous studies have underestimated the number of slow rotators, revealing the importance of studying those targets. For most slowly rotating asteroids (those with P > 12 h), no spin and shape model is available because of observation selection effects. This hampers determination of their thermal parameters and accurate sizes. Also, it is still unclear whether signatures of different surface material properties can be seen in thermal inertia determined from mid-infrared thermal flux fitting. Aims. We continue our campaign in minimising selection effects among main belt asteroids. Our targets are slow rotators with low light-curve amplitudes. Our goal is to provide their scaled spin and shape models together with thermal inertia, albedo, and surface roughness to complete the statistics. Methods. Rich multi-apparition datasets of dense light curves are supplemented with data from Kepler and TESS spacecrafts. In addition to data in the visible range, we also use thermal data from infrared space observatories (mainly IRAS, Akari and WISE) in a combined optimisation process using the Convex Inversion Thermophysical Model. This novel method has so far been applied to only a few targets, and therefore in this work we further validate the method itself. Results. We present the models of 16 slow rotators, including two updated models. All provide good fits to both thermal and visible data.The obtained sizes are on average accurate at the 5% precision level, with diameters found to be in the range from 25 to 145 km. The rotation periods of our targets range from 11 to 59 h, and the thermal inertia covers a wide range of values, from 2 to <400 J m−2 s−1∕2 K−1, not showing any correlation with the period. Conclusions. With this work we increase the sample of slow rotators with reliable spin and shape models and known thermal inertia by 40%. The thermal inertia values of our sample do not display a previously suggested increasing trend with rotation period, which mightbe due to their small skin depth.

Section: Visible and Infrared Datamentioning

confidence: 99%

Properties of slowly rotating asteroids from the Convex Inversion Thermophysical Model

Marciniak

Ďurech

et al. 2021

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“…It was extensively tested and validated against objects and object properties that are known from direct measurements (i.e., occultations, radar, or direct imaging) or from interplanetary mission rendezvous and flybys (e.g., Müller et al 2014a,b). Between 2016 and 2019, extensive efforts were undertaken to compare quantities derived from TPM techniques with other methods and to constrain the accuracy of TPM properties (Müller et al 2018). Overall, the TPM produced very high-quality diameters and albedos and consistent thermal properties in cases where the available thermal data have good quality, cover wide spans of time, wavelengths, and phase angles, which is why the new lunar HIRS measurements are so valuable for our purposes.…”

Section: Tpmmentioning

confidence: 99%

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

Müller

Burgdorf

et al. 2021

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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-based M-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.

“…This work was possible thanks to (i) the availability of a set of new shape models derived using the All-Data Asteroid Modelling (ADAM; Viikinkoski et al 2015) and the Shaping Asteroids with Genetic Evolution (SAGE; Bartczak et al 2014; Bartczak & Dudziński 2018) algorithms and (ii) the expert-reduced data products submitted to the Herschel Science Archive, which include some targets for which a non-standard data reduction approach was required 2 . Such data reduction and the creation of a large database featuring IRAS, AKARI, and WISE data (Szakáts et al 2020) have been carried out in the framework of the "Small Bodies: Near and Far" (SBNAF) project (Müller et al 2018), funded by the European Commission. SBNAF aimed at exploiting synergies between different small-body modelling techniques that use a wide range of data types from ground and space observatories (stellar occultations, visible and thermal infrared photometry, etc.)…”

Section: Introductionmentioning

confidence: 99%

Thermal properties of large main-belt asteroids observed byHerschelPACS

Müller

Kiss

et al. 2020

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Non-resolved thermal infrared observations enable studies of thermal and physical properties of asteroids via thermo-physical models provided the shape and rotational properties of the target are well determined. We used calibration-programme Herschel PACS data (70, 100, 160 μm) and state-of-the-art shape models derived from adaptive-optics observations and/or optical light curves to constrain for the first time the thermal inertia of twelve large main-belt asteroids. We also modelled previously well-characterised targets such as (1) Ceres or (4) Vesta as they constitute important benchmarks. Using the scale as a free parameter, most targets required a re-scaling ~5% consistent with what would be expected given the absolute calibration error bars. This constitutes a good cross-validation of the scaled shape models, although some targets required larger re-scaling to reproduce the IR data. We obtained low thermal inertias typical of large main belt asteroids studied before, which continues to give support to the notion that these surfaces are covered by fine-grained insulating regolith. Although the wavelengths at which PACS observed are longwards of the emission peak for main-belt asteroids, they proved to be extremely valuable to constrain size and thermal inertia and not too sensitive to surface roughness. Finally, we also propose a graphical approach to help examine how different values of the exponent used for scaling the thermal inertia as a function of heliocentric distance (i.e. temperature) affect our interpretation of the results.