Observations of comet 252P/LINEAR during its historically close approach to Earth in 2016 from Lowell Observatory

We report on imaging and photometry of comet 252P/LINEAR acquired at Lowell Observatory during 2016 February-April. 252P passed 0.036 AU from Earth on March 21, among the closest passages on record. Its southern declination and the full moon made observations during the close encounter impractical from Lowell Observatory, but we observed 252P on one night each in February and early March, and on 13 nights from April 2-21 using the 4.3-m Discovery Channel Telescope, Hall 1.1-m, and 0.8-m. According to reports by other observers, the comet brightened significantly beginning in late-February and, coupled with the extreme close approach, was highly extended during our April observations. Narrowband photometry revealed a typical gas composition and an extremely low dust-to-gas ratio. The ratios remained essentially unchanged between late-February and April, e.g., during and after the extended "outburst" reported by other observers. 252P exhibited distinctly different coma morphology between dust and gas species. Enhanced images revealed a short sunward dust feature and the dust tail. Enhanced CN and C3 images exhibited a tilted spiral that was seen partially edge on approximately in the north-south directions. Enhanced OH images were also brightest along this direction but with considerably more material in the tailward hemisphere, potentially implying icy grains subject to radiation pressure. The CN coma morphology varied smoothly during a night and repeated every ~22 hr, implying a period of ~22 hr or a sub-multiple. There was also a repetition of features after ~95.5 hr, implying that the actual period is 7.35 +/- 0.05 hr. The repetition of features was most consistent April 2-7; the morphology diverged during later later nights, with the apparent spiral seen earlier separating into two or more distinct jet features. We will discuss these results as well as the results of our ongoing analyses. These studies were supported by NASA Planetary Astronomy grant NNX14AG81G.