The Unusual Apparition of Comet 252P/2000 G1 (LINEAR) and Comparison with Comet P/2016 BA14 (PanSTARRS)

We imaged Comet 252P/2000 G1 (LINEAR; hereafter 252P) with the Hubble Space Telescope and both 252P and P/2016 BA₁₄ (PanSTARRS; hereafter BA₁₄) with the Discovery Channel Telescope in 2016 March and April, surrounding its close encounter to Earth. The r‧-band Afρ of 252P in a 0.″2-radius aperture were 16.8 ± 0.3 and 57 ± 1 cm on March 14 and April 4, respectively, and its gas production rates were Q(OH) = (5.8 ± 0.1) × 10²⁷ s^-1, and Q(CN) = (1.25 ± 0.01) × 10²⁵ s^-1 on April 17. The r‧-band upper limit Afρ of BA₁₄ was 0.19 ± 0.01 cm in a 19.″2-radius aperture, and Q(CN) = (1.4 ± 0.1) × 10²² s^-1 on 2017 April 17. 252P shows a bright and narrow jet of a few hundred kilometers long in the sunward direction, changing its projected position angle in the sky with a periodicity consistent with 7.24 hr. However, its photometric light curve is consistent with a periodicity of 5.41 hr. We suggest that the nucleus of 252P is likely in a non-principal axis rotation. The nucleus radius of 252P is estimated to be about 0.3 ± 0.03 km, indicating an active fraction of 40% to >100% in its 2016 apparition. Evidence implies a possible cloud of slow-moving grains surrounding the nucleus. The activity level of 252P in the 2016 apparition increased by two orders of magnitude from its previous apparitions, making this apparition unusual. On the other hand, the activity level of BA₁₄ appears to be at least three orders of magnitude lower than that of 252P, despite its 10 times or larger surface area.