The asteroid (3200) Phaethon is notable for several reasons. Phaethon is a B-type asteroid with a high-eccentricity orbit that is more comet-like than most asteroids, it is considered the parent body of the Geminids meteor shower in December, and it has been observed at times to have a dust tail as well as unexpected brightening [1,2]. Phaethon is categorized as a potentially hazardous asteroid and is the target of the JAXA DESTINY+ mission .
Given wide interest in Phaethon, we conducted observations with SpeX at NASA’s Infrared Telescope Facility (IRTF) in order to measure the asteroid’s near-infrared spectrum on several dates in December 2017. These spectra constrain thermal models of Phaethon that utilize a shape model based on lightcurves and Arecibo radar observations, enabling us to investigate the physical properties of Phaethon’s surface. We are also using NEOWISE observations to provide additional information to the modeling process, which is carried out using our thermophysical code SHERMAN [4,5].
Although any single asteroid spectrum can be fit equally well with either simple or detailed thermal models and different combinations of parameters, more robust solutions can be derived when multiple observations, particularly over a wide range of wavelengths, and a detailed shape are available. Across our four nights there are also differences in observational geometry and illumination across the surface. By considering all of these factors, we are therefore able to investigate the physical parameters more consistently and account for some of the variations that are “rolled up” in the beaming parameter and geometry assumptions of simpler models. The resulting thermophysical models are providing a detailed picture of the surface properties of Phaethon, including the need for heterogeneity in those properties in order to fit all the observations. We will present our results during the meeting.
Parts of this study were funded by the National Science Foundation (AST-1856411).
References:  Jewitt D. & Li J. (2010) AJ 140, 1519.  Battams K. & Watson A. (2009) IAU Circ. No. 9054, #3.  Arai T. et al. (2018) LPSC XLIX, Abstract #2570.  Magri C. et al. (2018) Icarus 303, 203.  Howell E. S. et al. (2018) Icarus 303, 220.