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 r _ { H } = 25.8 au .
Two outburst events around 2009 and 2017 were observed .
The coma and tail of the comet consist of dust grains of \sim 10 \micron in radius , ejected protractedly at speeds \lesssim 50 m s ^ { -1 } near the subsolar point , and are subjected to the Lorentz force , solar gravitation and radiation pressure force altogether .
The prolonged activity indicates that sublimation of supervolatiles ( e.g. , CO , CO _ { 2 } ) is at play , causing a net mass-loss rate \gtrsim 1 kg s ^ { -1 } .
To sustain the mass loss , the nucleus radius has to be \gtrsim 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 < r _ { H } < 15 au , concurrent with the onset of crystallisation 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 has visited the planetary region 1.96 \pm 0.04 Myr ago , with barycentric perihelion distance q = 8.364 \pm 0.004 au .
Thus , C/2010 U3 ( Boattini ) is almost certainly a dynamically old comet from the Oort cloud , and the observed activity can not be caused by retained heat from the previous apparition .