The Nucleus of Interstellar Comet 2I/Borisov

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 rn ≤ 0.5 km (geometric albedo 0.04 assumed). We obtain an independent constraint based on the nongravitational acceleration of the nucleus, finding rn > 0.2 km (nucleus density ρ = 500 kg m−3 assumed). The profile and the nongravitational constraints cannot be simultaneously satisfied if ρ ≤ 25 kg m−3; 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 flyby. 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.