Presentation #301.07 in the session “Distant Activity in Comets: C/2017 K2 & C/2014 UN271”.
We report an identification of long-period comet C/2010 U3 (Boattini) active at a record inbound heliocentric distance of rH ≈ 26 au. The dust morphology of the coma and tail cannot be explained unless the Lorentz force is taken into account in addition to the solar gravitation and solar radiation pressure force. Optically dominant dust grains have radii of ~10 μm and are ejected protractedly at speeds ≤50 m s−1 near the subsolar point. The prolonged activity indicates that sublimation of supervolatiles (e.g., CO, CO2) is at play. Two outburst events around 2009 and 2017 were seen. The color of the cometary dust is unsurprisingly redder than the solar colors. We observed potential color variations when the comet was at 10 < rH < 15 au, concurrent with the onset of crystallization of amorphous water ice, if any. Using publicly available and our refined astrometric measurements, we estimated the precise trajectory of the comet, propagated it backward to its previous perihelion, and found that the comet visited the planetary region ~2 Myr ago at perihelion distance q ≈ 8 au. Thus, C/2010 U3 (Boattini) is almost certainly a dynamically old comet from the Oort cloud, and the observed activity cannot be caused by retained heat from the previous apparition. The study is detailed in Hui et al. (2019, AJ, 157, 162).