  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 sq. km (assuming geometric albedo 0.1) and increases by about 1 percent 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 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 1.3e7 kg and the current dust mass loss rate 2 kg/s. We set an upper
limit to the nucleus radius using photometry of <3.8 km (again for albedo 0.1).
However, we use a statistical argument to show that the nucleus must be much
smaller, likely a few hundred meters in radius.
