ATRM: Asteroid Thermal & Reflected light Model

Abstract

Over the last five decades, thermal modelling of airless minor planetary bodies such as asteroids have experienced significant improvements. However, at lower wavelengths of the mid-infrared range, thermal models such as the widely used Near-Earth Asteroid Model (NEATM) are still considered unreliable since reflected light starts contributing significantly towards the observed flux density. Through a controversy related to the Wide-field Infrared Survey Explorer (WISE) mission, which was an infrared survey telescope with four observational bands found at 3.4, 4.6, 12, and 22 microns, Nathan Myhrvold suggested that the thermal modelling carried out did not properly account for reflected light and that the results, especially derived from the first two observation bands, were compromised since Kirchhoff's law of thermal radiation was violated. To date, the WISE mission is considered the highest yielding mission with more than 158,000 asteroids detected, however Myhrvold's findings state that the result derived for about half of those detections are compromised. This controversy motivated this master thesis project to create a numerical code, which properly combines thermal and reflected light modelling, to further investigate the influence of the latter at the four WISE observational bands. The initial aim of this master thesis was the create an advanced thermal model, but due to the time scope of this master thesis, an intermediate thermal model named the Asteroid Thermal and Reflected light Model (ATRM) was achieved. On top of being able to model simple spherical and ellipsoidal shapes, the ATRM can model irregularly-shaped asteroids with precise orbital and rotational properties taken into account as do advanced thermal models, but assumes instantaneous thermal equilibrium as do simple thermal models such as the NEATM. Furthermore, the ATRM caters to mostly convex-shaped asteroids due to the simple shadowing algorithm implemented, and not taking into account multiple scattering. However, the ATRM is able to vary the surface albedo distribution pattern of an asteroid through an octant method, which is typically not the case for simple and advanced thermal models which all assume a homogeneous surface albedo. With the aforementioned capabilities of the ATRM, the percentage of reflected light in the total flux density at the four wavelength bands of WISE were estimated for different albedo values covering the majority of asteroids falling under the three broad Bus-DeMeo taxonomic classification system (C-, S-, X-types). Furthermore, the influence of the heliocentric distance, emissivity, and shape of the asteroid on the contribution of reflected light were investigated. Ultimately, this project is another step-wise progress in the field of physical characterisation of airless planetary bodies, especially asteroids, and has far-reaching consequences in terms of planetary formation, in-situ resource utilisation (ISRU), commercial asteroid mining, and planetary defence.