Figure 6.
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.
the brief visibility window 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 sub-
mitted. Figure 6 displays the resulting com-
posite color image; an extended coma and
cometary tail are clearly visible. This makes
2I/Borisov the first known interstellar comet.
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
photometric color and its derived dust par-
ticle 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 nor-
mal cometary range. In light of ‘Oumuamua’s
anomalies, the apparent banality of 2I/Bor-
isov is in itself remarkable.
32
GeminiFocus
The observational study
of this second interstellar
interloper has only just
begun. Additional Gem-
ini observations have
already been obtained,
and more are currently
scheduled in the queue.
2I/Borisov is entering
the Solar System from
“above,” and its visibility
will gradually improve
as it crosses the celestial
equator in mid-Novem-
ber and moves towards a
perihelion distance of 2.0
AU, near the inner edge
of the Asteroid Belt, on
December 8th. It reaches
a minimum distance of
1.9 AU from the Earth in late December, and
will continue to be visible from the South-
ern Hemisphere for much of next year. Thus,
Gemini’s access to the entire sky will enable
detailed study of 2I/Borisov throughout the
entire course of its visit — we are sure to
have more highlights on this first interstel-
lar comet before it leaves our corner of the
Galaxy forever.
Probing for Patterns in Io's
Volcanoes Using Adaptive
Optics
Ever since the Voyager spacecrafts revealed
the rampant volcanism on Jupiter’s inner-
most large moon Io, planetary scientists
have been puzzling over the variations in
the timing and intensities of the splotchy
satellite’s many eruptions. Intense tidal heat-
ing, the stretching and squeezing of Io’s
crust as it follows its 1.8-day elliptical orbit
around the giant planet, supplies the energy
to melt interior silicates and produce mag-
ma, which eventually erupts to the surface.
However, the variations in the volcanic activ-
January 2020 / 2019 Year in Review