Biases in TPM-derived Diameters for High-Eccentricity NEAs

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°, however became increasingly inaccurate for phase angles >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.