Introduction

The Near Earth Asteroid Thermal Model (NEATM) is a widely-used approach in thermal infrared (TIR) observations of near-Earth asteroids (NEOs). However, it comes with certain assumptions and limitations, particularly regarding nighttime thermal emission. This article delves into the limitations of the NEATM, its performance under different solar phase angles and thermal inertias, and the methods to correct its estimations.

Limitations of the NEATM

A significant disadvantage of the NEATM is the assumption of zero thermal emission from the night side of the asteroid. Without knowing the asteroid's spin vector and thermal inertia, it becomes challenging to accurately estimate the thermal energy emitted from the night side. This assumption becomes problematic especially at low solar phase angles, where the telescope primarily receives thermal radiation from the day side. However, at large phase angles, thermal emission from the night side can skew the size estimations provided by the NEATM.

Performance Analysis of NEATM

Mommert et al. conducted a comprehensive study involving one million synthetic thermophysically generated NEOs with random physical and observational properties. They found that the NEATM offers statistically more robust diameter estimates for solar phase angles below approximately 65 degrees. This stands true unless the thermal inertia and solar phase angles are significant, in which case the Fast Rotating Model (FRM) performs better.

Model Comparison and Corrections

The research visually articulated the relative performances of the Standard Thermal Model (STM), NEATM, and FRM in terms of thermal inertia. The plots illustrated that for lower solar phase angles (e.g., 20 and 50 degrees), the NEATM outperformed the other models. However, at higher solar phase angles, the performance advantage of the FRM over the NEATM is reduced when realistic non-zero subsolar latitudes are considered. This highlights the importance of considering geographical factors when analyzing the thermal behavior of asteroids.

Mommert et al. also provided statistical functions to correct NEATM- and FRM-derived diameters and albedos for the variation due to solar phase angles. Harris et al. (2011) further investigated the NEATM in fixed-η mode, finding that for Spitzer observations at 3.6 μm and 4.5 μm bands, the root-mean-square errors in diameter were ±20% and in albedo were ±50%. These high uncertainties are particularly relevant when dealing with broader thermal emission bands like those from Warm Spitzer.

Improving Estimations

To address the limitations of the NEATM, researchers like Trilling et al. (2016) acknowledged the significant uncertainty in the η parameter. They used the thermal modeling approach with a full distribution of previously measured η values to derive diameter and albedo uncertainties. This approach led to typical diameter uncertainties of 40% and albedo uncertainties of 70%, demonstrating the critical nature of obtaining thermal-infrared observations at multiple wavelengths to minimize these uncertainties.

Conclusion

In summary, the NEATM is a powerful tool in NEO thermal infrared studies, but its reliance on idealized assumptions can lead to inaccuracies. By understanding these limitations and employing correction methods, we can enhance the accuracy of our asteroid diameter and albedo estimations.

Key Takeaways

The NEATM assumes zero thermal emission from the night side, limiting its accuracy. The FRM is preferred at high solar phase angles or with large values of thermal inertia. Statistical corrections are available to improve NEATM estimations under variable solar phase angles. Multiphase angle thermal observations significantly reduce uncertainties in diameter and albedo estimations. References Mommert, M., et al. “The Performance of the NEATM and FRM Models in the Measurements of Synthetic NEA Particles.” Astronomy and Astrophysics 539 (2012). Harris, A., et al. “The Surface Properties of Small Asteroids from Thermal-Infrared Observations.” In Proceedings IAU Symposium 229: Asteroids Comets and Meteors, vol. 29, pp. 449-463. Cambridge University Press, 2006. Trilling, D., et al. “Spitzer Space Telescope Observations of Near-Earth Objects (NEOs): Diameter and Albedo Determinations.” Proceedings of the International Astronomical Union 6(S293) (2010).