We report on a multi-observatory campaign to examine asteroid 3200 Phaethon during its December 2017 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 \pm 0.004 , V-R = 0.309 \pm 0.003 , R-I = 0.266 \pm 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 sub-latitude are seen and may be associated with craters observed by the Arecibo radar ( ) .
High cadence photometry over phases from 20 to 100 degrees allows a fit to the values of the HG photometric parameters ; H = 14.57 \pm 0.02 , 13.63 \pm 0.02 , 13.28 \pm 0.02 , 13.07 \pm 0.02 ; G = 0.00 \pm 0.01 , -0.09 \pm 0.01 , -0.10 \pm 0.01 , -0.08 \pm 0.01 in the BVRI filters respectively ; the negative G values are consistent with other observations of F type asteroids e.g ( ) .
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 set an upper limit of 0.06 \pm 0.02 kg/s when Phaethon was 1.449 AU from the Sun , and 0.2 \pm 0.1 kg/s 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 .