Initial Characterization of Interstellar Comet 2I/2019 Q4 (Borisov)

We present initial observations of the interstellar body 2I/(2019 Q4) Borisov taken to determine its nature prior to the perihelion in 2019 December. Images from the Nordic Optical Telescope show a prominent, morphologically stable dust coma and tail. The dust cross-section within 15,000 km of the nucleus averages 130 km(2) (assuming geometric albedo 0.1) and increases by about 1% per day. If sustained, this rate indicates that the comet has been active for ?100 days prior to the observations. Cometary activity thus started in 2019 June, at which time C/Borisov was at ?4.5 au from the Sun, a typical distance for the onset of water ice sublimation in comets. The dust optical colors, B ? V;=;0.80;;0.05, V ? R;=;0.47;;0.03 and R? I;=;0.49;;0.05, are identical to those of a sample of (solar system) long-period comets. The colors are similar to those of 1I/(2017 U1) ?Oumuamua, indicating a lack of the ultrared matter that is common in the Kuiper Belt, on both interstellar objects. The effective size of the dust particles is estimated as ?m, based on the length of the dust tail and the 100 day lifetime. With this size, the ejected dust mass is of order 1.3;10(7) kg and the current dust mass loss rate ?2 kg s(?1). We set an upper limit to the nucleus radius using photometry at r(n) ?3.8 km (again for albedo 0.1) and we use a statistical argument to show that the nucleus must be much smaller, likely a few hundred meters in radius.