  Interstellar comets penetrating through the Solar System had been anticipated
for decades. The discovery of asteroidal-looking 'Oumuamua was thus a huge
surprise and a puzzle. Furthermore, the physical properties of the 'first
scout' turned out to be impossible to reconcile with Solar System objects,
challenging our view of interstellar minor bodies. Here, we report the
identification and early characterization of a new interstellar object, which
has an evidently cometary appearance. The body was discovered by Gennady
Borisov on 30 August 2019 UT and subsequently identified as hyperbolic by our
data mining code in publicly available astrometric data. The initial orbital
solution implies a very high hyperbolic excess speed of ~32 km/s, consistent
with 'Oumuamua and theoretical predictions. Images taken on 10 and 13 September
2019 UT with the William Herschel Telescope and Gemini North Telescope show an
extended coma and a faint, broad tail. We measure a slightly reddish colour
with a g'-r' colour index of 0.66 +/- 0.01 mag, compatible with Solar System
comets. The observed morphology is also unremarkable and best explained by dust
with a power-law size-distribution index of -3.7 +/- 1.8 and a low ejection
speed (44 +/- 14 m/s for $\beta$ = 1 particles, where $\beta$ is the ratio of
the solar gravitational attraction to the solar radiation pressure). The
nucleus is probably ~1 km in radius, again a common value among Solar System
comets, and has a negligible chance of experiencing rotational disruption.
Based on these early characteristics, and putting its hyperbolic orbit aside,
2I/Borisov appears indistinguishable from the native Solar System comets.
