  We present high resolution imaging observations of interstellar comet
2I/Borisov (formerly C/2019 Q4) obtained using the Hubble Space Telescope.
Scattering from the comet is dominated by a coma of large particles
(characteristic size 0.1 mm) ejected anisotropically. Convolution modeling of
the coma surface brightness profile sets a robust limit to the
spherical-equivalent nucleus radius r_n < 0.5 km (geometric albedo 0.04
assumed). We obtain an independent constraint based on the non-gravitational
acceleration of the nucleus, finding r_n > 0.2 km (nucleus density 500 kg/m3
assumed). The profile and the non-gravitational constraints cannot be
simultaneously satisfied if density < 25 kg/m3; the nucleus of comet Borisov
cannot be a low density fractal assemblage of the type proposed elsewhere for
the nucleus of 1I/'Oumuamua. We show that the spin-up timescale to outgassing
torques, even at the measured low production rates, is comparable to or shorter
than the residence time in the Sun's water sublimation zone. The spin angular
momentum of the nucleus should be changed significantly during the current
solar fly-by. Lastly, we find that the differential interstellar size
distribution in the 0.5 mm to 100 m size range can be represented by power laws
with indices < 4 and that interstellar bodies of 100 m size scale strike Earth
every one to two hundred million years.
