Investigation of Systematic Bias in Radiometric Diameter Determination of Near-Earth Asteroids: the Night Emission Simulated Thermal Model (NESTM)

TL;DR

This paper examines biases in the Near-Earth Asteroid Thermal Model (NEATM) caused by assumptions about night-side emission and introduces the NESTM model to improve diameter estimates, especially at large phase angles.

Contribution

The paper introduces the Night Emission Simulated Thermal Model (NESTM), which accounts for night-side emission based on thermal inertia, reducing systematic biases in asteroid diameter estimation.

Findings

NEATM overestimates diameters at large phase angles.

NESTM reduces bias in diameter estimates compared to NEATM.

Adopting NESTM with thermal inertia=200 improves accuracy.

Abstract

The Near-Earth Asteroid Thermal Model (NEATM, Harris, 1998) has proven to be a reliable simple thermal model for radiometric diameter determination. However NEATM assumes zero thermal emission on the night side of an asteroid. We investigate how this assumption affects the best-fit beaming parameter, overestimates the effective diameter and underestimates the albedo at large phase angles, by testing NEATM on thermal IR fluxes generated from simulated asteroid surfaces with different thermal inertia. We compare NEATM to radar diameters and find that NEATM overestimates the diameter when the beaming parameter is fitted to multi-wavelength observations and underestimates the diameter when the default beaming parameter is used. The Night Emission Simulated Thermal Model (NESTM) is introduced. NESTM models the night side temperature as an iso-latitudinal fraction (f) of the maximum day side temperature (Maximum temperature calculated for NEATM with beaming parameter = 1). A range of f is found for different thermal parameters, which depend on the thermal inertia. NESTM diameters are compared with NEATM and radar diameters, and it is shown that NESTM may reduce the systematic bias in overestimating diameters. It is suggested that a version of the NESTM which assumes the thermal inertia = 200 S.I. units is adopted as a default model when the solar phase angle is greater than 45 degrees.