The effect of aspect changes on Near-Earth Asteroid phase curves

Abstract

Phase curves of asteroids are typically considered to depend solely on the scattering properties of airless particulate surfaces and the size of the object being studied. In this study, we demonstrate the additional dependence of phase curves on object shape, rotation pole orientation, and viewing geometry over an apparition. Variations in the phase curve of near-Earth asteroid (159402) 1999 AP10 over its apparition from July 2020 - January 2021 are verified to be due to aspect changes over the apparition. This is achieved through shape modelling of the asteroid and simulation of the phase curve over the apparition. We present simulations of asteroid phase curves over a range of geometries to understand the potential magnitude of this aspect effect, and under which circumstances it can begin to dominate in the phase curves. This dependence on aspect may introduce significant additional uncertainty in the properties derived from phase curve data. We provide and demonstrate software code to estimate the aspect-related uncertainty in near-Earth asteroid phase curves through simulation and model fitting of a randomly generated sample of ellipsoidal asteroid models over the observed viewing geometry. We demonstrate how ignoring this effect may lead to misleading interpretations of the data and underestimation of uncertainties in further studies, such as those in the infrared that use phase curve derived parameters when fitting physical properties of an asteroid.

Physical modelling of YORP-evolved NEA (23187) 2000 PN9 from optical and radar observations

“…Using the absolute magnitude and the radar-derived diameter D eq = 1.71 ± 0.07 km, the optical albedo is calculated as 0.25 ± 0.02, consistent with the polarimetrically-derived albedo from Belskaya et al (2009). Caution must be used when inferring physical properties from phase curve-derived parameters of NEAs, however, due to the potential for changing aspect to introduce additional brightness modulations in the phase curve that are unrelated to the scattering behaviour of the surface material (Jackson et al 2022).…”

Photometry of the Didymos System across the DART Impact Apparition

“…These issues could be related to differences in the times and viewing geometries of our data relative to those used to derive the model parameters. Disentangling the effects of shape and viewing geometry on photometric phase curves can be challenging for near-Earth asteroids (e.g., Jackson et al 2022). Further work is needed to address whether discrepancies with the mutual event model (e.g., Figure 7) could be attributed to these potentially unusual photometric properties of the Didymos system.…”

Pre-impact Thermophysical Properties and the Yarkovsky Effect of NASA DART Target (65803) Didymos

“…As demonstrated in Jackson et al (2022), aspect effects can also influence the observed phase-curve behavior and affect the derived H and G values. To investigate any potential systematics in the Didymos phase curves, we implemented a shape-based Hapke model (Hapke 2012;Jackson et al 2022) utilizing the shape models of Didymos and Dimorphos derived from DART images taken prior to impact (Barnouin et al 2023;Daly et al 2024). In this model, the bidirectional reflectance, r BI , of each facet is calculated using…”

“…Finally, the Bond albedo is typically derived from the asteroid absolute magnitude H and phase parameter G in combination with the asteroid diameter (Bowell et al 1989;Fowler & Chillemi 1992). However, such quantities are susceptible to catalog biases (Pravec et al 2012) and aspect effects (Jackson et al 2022), and therefore well-constructed phase curves are required to derive an accurate value for the Bond albedo. Furthermore, phase curves can also provide additional diagnostic information on the composition of the asteroid surface (Oszkiewicz et al 2012;Penttilä et al 2016).…”