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 \sim 100 days prior to the observations .
Cometary activity thus started in 2019 June , at which time C/Borisov was at \sim 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 \pm 0.05 , V-R = 0.47 \pm 0.03 and R-I = 0.49 \pm 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 \mu 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 \times 10 ^ { 7 } kg and the current dust mass loss rate \sim 2 kg s ^ { -1 } .
We set an upper limit to the nucleus radius using photometry at r _ { n } \leq 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 .