Simultaneous Multicolor Photometry of the DESTINY$^{+}$ target asteroid (3200) Phaethon

TL;DR

This study provides precise photometric measurements of asteroid (3200) Phaethon, estimating its brightness, size, and surface uniformity, which are crucial for the DESTINY+ mission planning and understanding its physical properties.

Contribution

It offers new optical photometry data, refined brightness and size estimates, and surface homogeneity analysis of Phaethon at low phase angles, enhancing prior knowledge for mission support.

Findings

Estimated absolute magnitude H_V=14.23 with low uncertainty

Derived diameter range of 5.22 to 6.74 km based on optical and polarimetric data

Detected no significant surface color variations larger than 0.018-0.020 mag

Abstract

Accurate estimation of brightness of (3200) Phaethon up to lower phase angles are essential for planning of the on-board camera of the DESTINY$^{+}$ mission. We have carried out intensive observations of Phaethon in the optical wavelength ($g$, $r$, and $i$) with the TriCCS camera on the Seimei 3.8 m telescope in October and November, 2021. We derived the absolute magnitude $H_\mathrm{V}$ and the slope parameter $G$ of Phaethon as $H_\mathrm{V}=14.23\pm0.02$ and $G=0.040\pm0.008$ from multiple photometric observations including lower phase angles down to $\sim$9$^{\circ}$ with the $H$-$G$ model. Using the $H_\mathrm{V}$ value and the geometric albedo of Phaethon derived in previous polarimetric studies, we estimated that the Phaethon's diameter is within a range of 5.22 to 6.74 km, which is consistent with radar and occultation observations. With the linear model, we derived $H_\mathrm{V}=14.65\pm0.02$, which corresponds to a diameter range of 4.30 to 5.56 km. Our simultaneous tricolor lightcurves of Phaethon indicate that no rotational spectral variations larger than 0.018 and 0.020 mag in the g-r and r-i colors, possibly related to inhomogeneity of the surface material and/or structure, are seen at the 2021 apparition.