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We identified a new ultra-distant comet C/2019 E3 (ATLAS) exhibitingpreperihelion cometary activity at heliocentric distances $\gtrsim\!20$ au,making it the fourth member of this population after C/2010 U3 (Boattini),C/2014 UN$_{271}$ (Bernardinelli-Bernstein), and C/2017 K2 (PANSTARRS). Fromserendipitous archival data, we conducted analyses of the comet, finding thatthe activity was consistent with steady-state behaviour, suggestive ofsublimation of supervolatiles, that the cross-section of dust increasedgradually on the inbound leg of the orbit, varying with heliocentric distancesas $r_{\rm H}^{-1.5 \pm 0.4}$, and that the dust was produced at a rate of$\gtrsim\!10^2$ kg s$^{-1}$ within the observed timespan. Our modelling of thelargely symmetric morphology of the comet suggests that the dust environmentwas likely dominated by mm-scale dust grains ejected at speeds $\lesssim\!0.4$m s$^{-1}$ from the sunlit hemisphere of the nucleus. Assuming a typicalgeometric albedo of 0.05 and adopting several simplistic thermophysical models,we estimated the nucleus to be at least $\sim\!3$ km across. We also measuredthe colour of the comet to be consistent with other long-period comets, exceptbeing slightly bluer in $g-r$. With our astrometric measurements, we determinedan improved orbit of the comet, based upon which we derived that the comet isdynamically new and that its perihelion distance will further shrink due to theGalactic tide. We conclude the paper by comparing the known characteristics ofthe known ultra-distant comets.

Serendipitous Archival Observations of A New Ultra-distant Comet C/2019 E3 (ATLAS)