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 km² (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 \overline{a} = 100 μm, 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 × 10⁷ 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.