Comparisons of NEATM-like Models for NEAs with NASA IRTF and NEOWISE Data

Understanding the albedo distributions and regolith properties of near-Earth asteroids (NEAs) is a key part of investigations into meteorites, main-belt orbital dynamics, Earth impact probabilities, and asteroid surface processes. Because highly detailed observations of NEAs from spacecraft missions will only be available for a small number of objects, ground-based observations are important for gaining a full understanding of the heterogeneous NEA population. Two methods commonly used to obtain size estimates of NEAs are radar-derived shape models and simple thermal models. The Arecibo Telescope has imaged over 400 NEAs giving direct estimates of their size and shape. However, with the loss of the telescope, high-quality radar data will be more difficult to acquire. This heightens the existing need to compare these sizes to those derived from thermal data and thermal models to assess their reliability.

As part of our ongoing investigation into the physical properties of NEAs, we use a simple, NEATM-like model (Howell et. al., 2018) to model the physical properties of NEAs observed with the SpeX instrument at the NASA IRTF (0.7-5.5 microns). This model assumes a spherical shape and equatorial view. The user specifies the diameter, H-magnitude, orbital ephemerides, visible-to-near-IR reflectance ratio, and rotation period. The visual albedo, beaming parameter, and thermal inertia are allowed to vary as free parameters, and the subsequent model outputs are compared to the SpeX data to determine the best fit parameters. The consistency of the best fit parameters is checked by comparing model outputs to multiple nights of SpeX data at different viewing geometries. An additional check can be obtained by comparing the model total flux to absolute photometry taken by NEOWISE. This comparison, as well as comparison to thermophysical models using detailed radar-derived shape models (when available), will allow us to assess the effectiveness of simple thermal models when used with relative and absolute photometry, and to identify limitations of the models.

We will present such a comparison for NEAs with both SpeX and NEOWISE observations, including binary (285263) 1998 QE2 to test for consistency with the radar shape model (Springmann et al. 2014).