C/2014 UN271 (Bernardinelli-Bernstein) is a comet incoming from the Oort cloud which is remarkable in having the brightest (and presumably largest) nucleus of any well-measured comet, and having been discovered at heliocentric distance rh≈29 au farther than any Oort-cloud member. We describe the properties that can be inferred from images recorded until the first reports of activity in June 2021. The orbit has i=95∘,
Nearby in the stack
with perihelion of 10.97 au to be reached in 2031, and previous aphelion at
40,400±260
au. Backwards integration of the orbit under a standard Galactic tidal model and known stellar encounters suggests this is a pristine new comet, with a perihelion of
q≈18
au on its previous perihelion passage 3.5 Myr ago. The photometric data show an unresolved nucleus with absolute magnitude
Hr=8.0,
colors that are typical of comet nuclei or Damocloids, and no secular trend as it traversed the range 34--23 au. For
r
-band geometric albedo
pr,
this implies a diameter of
150(pr/0.04)−0.5
km. There is strong evidence of brightness fluctuations at
±0.2
mag level, but no rotation period can be discerned. A coma consistent with a ``stationary'
1/ρ
surface-brightness distribution grew in scattering cross-section at an exponential rate from
Afρ≈1
m to
≈150
m as the comet approached from 28 to 20 au. The activity is consistent with a simple model of sublimation of a surface species in radiative equilibrium with the Sun. The inferred enthalpy of sublimation matches those of
CO2
and
NH3
. More-volatile species --
N2,
CH4,
and
CO
-- must be far less abundant on the sublimating surfaces.