Asteroid Photometric Phase Functions From Bayesian Lightcurve Inversion

Abstract

Photometry is an important tool for characterizing the physical properties of asteroids. An asteroid’s photometric lightcurve and phase curve refer to the variation of the asteroid’s disk-integrated brightness in time and in phase angle (the Sun-asteroid-observer angle), respectively. They depend on the asteroid’s shape, rotation, and surface scattering properties, and the geometry of illumination and observation. We present Bayesian lightcurve inversion methods for the retrieval of the asteroid’s phase function, the unambiguous phase curve of a spherical object with surface scattering properties equal to those of the asteroid. A collection of such phase functions can give rise to a photometric taxonomy for asteroids. In the inverse problem, first, there are four classes of lightcurves that require individual error models. The photometric observations can be absolute or relative and they can have dense or sparse cadence in comparison to the rotation period of the asteroid. Second, the observations extend over varying phase angle ranges, requiring different phase function models. Asteroid photometry from the European Space Agency Gaia space mission extends, typically, over a range of phase angles, where the phase curve tends to be linear on the magnitude scale. Photometry from ground-based observing programs can reach small phase angles, where the asteroids show an opposition effect, a nonlinear increase of brightness on the magnitude scale towards zero phase angle. We provide error models for all four classes of lightcurves and make use of linear or linear-exponential phase functions for phase angles below 50°. We apply the inverse methods to sparse absolute Gaia and dense relative ground-based lightcurves and obtain absolute magnitudes and phase functions, with uncertainties, for ∼500 asteroids. Finally, we assess the lightcurve inversion problem for dense absolute photometry with the help of a numerical simulation for a Gaussian-random-sphere asteroid.

Continued PSP/WISPR Observations of a Phaethon-related Dust Trail

“…The results presented in Table 2, based upon 495 individual data points across seven of the nine encounters, present a consistent picture of the trail mass on the order of 10 11 kg for 0.5 mm dust grains under the assumption of Phaethon-like dust grains, per Paper I. However, asteroidal phase functions can show significant responses across the ∼20°-120°phase angle range observed by WISPR (Muinonen et al 2022;Ansdell et al 2014) that should manifest clearly in photometry. We find no significant variation in trail brightness as a function of phase angle, implying a limitation in our mini-Phaethon assumption.…”

Surface Heterogeneity, Physical, and Shape Model of Near-Earth Asteroid (52768) 1998 OR2

“…As was pointed out by many authors (e.g., Mahlke et al 2021;Muinonen et al 2022), observations obtained during different oppositions can result in different H magnitudes. This is due to the fact that an asteroid observed pole-on (asteroid subobserver latitude of ±90°) will appear larger than an asteroid observed equator-on (asteroid subobserver latitude of 0°).…”

The Unusual Brightness Phase Curve of (65803) Didymos

“…One particular model, the linear-by-parts, enables a tentative comparison with meteorite samples and places Didymos among L chondrites. In total, our analysis made use of four varied models in order to provide parameters that can be used for comparison with other asteroid surveys (Muinonen et al 2022). In addition, such results will be compared with the observations of the upcoming ESA/ HERA mission (Michel et al 2022) that will reach the Didymos-Dimorphos system and study the outcome of the DART experiment at very high spatial resolution.…”