  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 $r_h\approx29$ 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 $i=95^\circ,$ with perihelion of 10.97 au
to be reached in 2031, and previous aphelion at $40,400\pm260$ 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
$q\approx18$ au on its previous perihelion passage 3.5 Myr ago. The photometric
data show an unresolved nucleus with absolute magnitude $H_r=8.0,$ 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 $p_r,$ this implies a
diameter of $150 (p_r/0.04)^{-0.5}$ km. There is strong evidence of brightness
fluctuations at $\pm0.2$ mag level, but no rotation period can be discerned. A
coma consistent with a ``stationary' $1/\rho$ surface-brightness distribution
grew in scattering cross-section at an exponential rate from $A f \rho\approx1$
m to $\approx150$ 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
$CO_2$ and $NH_3$. More-volatile species -- $N_2,$ $CH_4,$ and $CO$ -- must be
far less abundant on the sublimating surfaces.
