Hubble Space Telescope Detection of the Nucleus of Comet C/2014 UN${}_{271}$ (Bernardinelli-Bernstein)

We present a high-resolution observation of distant comet C/2014 UN${}_{271}$ (Bernardinelli-Bernstein) using the {\it Hubble Space Telescope} on 2022 January 8. The signal of the nucleus was successfully isolated by means of the nucleus extraction technique, with an apparent $V$-band magnitude measured to be $21.64\pm 0.11$, corresponding to an absolute magnitude of $8.62\pm 0.11$. The product of the visual geometric albedo with the effective radius squared is $p_V{R}_n^2$ = 159$\pm$16 km${}^2$. If the ALMA observation by Lellouch et al. (2022) refers to a bare nucleus, we derive a visual geometric albedo of $0.034 \pm 0.008$andaneffectivediameterof$137 \pm 15$ km. If dust contamination of the ALMA signal is present at the maximum allowed level (24%), we find nucleus diameter $119\pm 13$ km and albedo of $0.044\pm 0.011$. In either case, we confirm that C/2014 UN${}_{271}$ is the largest long-period comet ever detected. Judging from the measured surface brightness profile of the coma, whose logarithmic gradient varies azimuthally between $\sim$1 and 1.7 in consequence of solar radiation pressure, the mass production is consistent with steady-state production but not with impulsive ejection, as would be produced by an outburst. Using aperture photometry we estimated an enormous (albeit uncertain) mass-loss rate of $\sim$10${}^3$ kg s${}^{-1}$ at a heliocentric distance of $\sim$20 au.