Asteroid (3200) Phaethon: Colors, Phase Curve, Limits on Cometary Activity, and Fragmentation

Abstract

We report on a multiobservatory campaign to examine asteroid 3200 Phaethon during its 2017 December close approach to Earth, in order to improve our measurements of its fundamental parameters, and to search for surface variations, cometary activity, and fragmentation. The mean colors of Phaethon are B – V = 0.702 ± 0.004, V – R = 0.309 ± 0.003, and R – I = 0.266 ± 0.004, neutral to slightly blue, consistent with previous classifications of Phaethon as a F-type or B-type asteroid. Variations in Phaethon's B – V colors (but not V – R or R – I) with observer sublatitude are seen and may be associated with craters observed by the Arecibo radar. High-cadence photometry over phases from 20° to 100° allows a fit to the values of the HG photometric parameters; H = 14.57 ± 0.02, 13.63 ± 0.02, 13.28 ± 0.02, 13.07 ± 0.02; G = 0.00 ± 0.01, −0.09 ± 0.01, −0.10 ± 0.01, −0.08 ± 0.01 in the BVRI filters respectively; the negative G values are consistent with other observations of F-type asteroids. Light-curve variations were seen that are also consistent with concavities reported by Arecibo, indicative of large craters on Phaethon's surface whose ejecta may be the source of the Geminid meteoroid stream. A search for gas/dust production sets an upper limit of 0.06 ± 0.02 kg s^(−1) when Phaethon was 1.449 au from the Sun, and 0.2 ± 0.1 kg s^(−1) at 1.067 au. A search for meter-class fragments accompanying Phaethon did not find any whose on-sky motion was not also consistent with background main-belt asteroids.