Figure 1.
GMOS-North g,r
composite color image
of the interstellar comet
2I/Borisov, obtained in
morning twilight on
September 10, 2019,
at a mean elevation of
less than 30° from the
eastern horizon. The
alternating red-blue
streaks are background
stars that appear trailed
because the telescope
was tracking the comet,
which was moving non-
sidereally at a rate of 75
arcseconds per hour. The
comet was 3.4 au from
the Earth at the time of
these observations.
Gemini Observatory was first alerted to 2I/
Borisov by a Director’s Discretionary Time
(DDT) proposal received on the evening of
September 9th, when the object was in the
northern sky at a distance of 3.4 astronomi-
cal units (au) from the Earth and within 43
degrees of the Sun. Following careful re-
view, the proposal was found to be com-
pelling, with Gemini's large aperture being
well suited for investigating possible com-
etary activity during the brief visibility win-
dow at the end of the night. Consequently,
multi-band imaging observations with the
Gemini Multi-Object Spectrograph (GMOS)
on Gemini North were obtained during
morning twilight, less than 12 hours after
the proposal was submitted. Figure 1 dis-
plays the resulting composite color image;
an extended coma and cometary tail are
clearly visible. This makes 2I/Borisov the first
known interstellar comet.
14
GeminiFocus
A study based on these Gemini North ob-
servations, with supplementary data from
the William Herschel Telescope on La Palma,
finds that 2I/Borisov appears quite similar to
typical Solar System comets in terms of pho-
tometric color and its derived dust particle
properties. Graduate student Piotr Guzik
of Jagiellonian University in Poland led the
study, which is currently in press at Nature
Astronomy (a preprint is available online).
The object’s g-r color is only slightly redder
than average for comets, and the estimated
diameter of 2 km for the comet’s nucelus,
while highly uncertain, is well within the
normal cometary range. In light of ‘Oumua-
mua’s anomalies, the apparent banality of
2I/Borisov is in itself remarkable.
The observational study of this second in-
terstellar interloper has only just begun. Ad-
ditional Gemini observations have already
been obtained, and more are currently
October 2019