Hubble Space Telescope Detection of the Nucleus of Comet C/2014 UN271 (Bernardinelli-Bernstein)

We present a high-resolution observation of the distant comet C/2014 UN271 (Bernardinelli-Bernstein) using the Hubble Space Telescope on 2022 January 8. The signal of the nucleus was successfully isolated by means of a nucleus extraction technique, with an apparent V-band magnitude measured to be 21.65 +/- 0.11, corresponding to an absolute magnitude of 8.63 +/- 0.11. The product of the visual geometric albedo with the effective radius squared is p(V)R(n)(2) = 157 +/- 16 km(2). If the ALMA observation by Lellouch et al. refers to a bare nucleus, we derived a visual geometric albedo of 0.033 +/- 0.009. If dust contamination of the ALMA signal is present at the maximum allowed level (24%), we found an albedo of 0.044 +/- 0.012 for the nucleus having an effective diameter of 119 +/- 15 km. In either case, we confirm that C/2014 UN271 is the largest long-period comet ever detected. Judging from the measured surface brightness profile of the coma, whose logarithmic gradient varies azimuthally between similar to 1.0 and 1.7 as a 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 similar to 10(3) kg s(-1) at a heliocentric distance of similar to 20 au.