C/2010 U3 (Boattini): A Bizarre Comet Active at Record Heliocentric Distance

We present a photometric and dynamical study of comet C/2010 U3 (Boattini), which was seen active in prediscovery data as early as 2005 November at a new inbound record heliocentric distance of rH = 25.8 au. Two outburst events around 2009 and 2017 were observed. The coma and tail of the comet consist of dust grains of ∼10 m in radius, ejected protractedly at speeds 50 m s−1 near the subsolar point, and are subjected to the Lorentz force, solar gravitation, and radiation pressure force all together. The prolonged activity indicates that sublimation of supervolatiles (e.g., CO, CO2) is at play, causing a net mass-loss rate 1 kg s−1. To sustain the mass loss, the nucleus radius has to be 0.1 km. The color of the cometary dust, similar to other long-period comets, is redder than the solar colors, but we also observed potential color variations when the comet was at 10 < rH < 15 au, concurrent with the onset of crystallization of amorphous water ice, if at all. Using publicly available and our refined astrometric measurements, we estimated the precise trajectory of the comet and propagated it backward to its previous perihelion. We found that the comet visited the planetary region 1.96 0.04 Myr ago, with barycentric perihelion distance q = 8.364 0.004 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.