potential. However, it’s anyone’s guess if the
comet has the “right stuff” to survive its extremely close brush with the Sun at the end
of November and become an early morning
spectacle from Earth in early December 2013.
When Gemini obtained this time sequence,
the comet ranged between roughly 455360 million miles (730-580 million kilometers; or 4.9-3.9 astronomical units) from the
Sun, or just inside the orbital distance of
Jupiter. Each image in the series, taken with
the Gemini Multi-Object Spectrograph at
the Gemini North telescope on Mauna Kea,
Hawai‘i, shows the comet in the far red part
of the optical spectrum, which emphasizes
the comet’s dusty material already escaping from what astronomers describe as a
“dirty snowball.” Note: The final image in the
sequence, obtained in early May, consists
of three images, including data from other
parts of the optical spectrum, to produce a
color composite image.”
The images show the comet sporting a welldefined parabolic hood in the sunward direction that tapers into a short and stubby
tail pointing away from the Sun. These features form when dust and gas escape from
the comet’s icy nucleus and surround that
main body to form a relatively extensive
atmosphere called a coma. Solar wind and
radiation pressure push the coma’s material
away from the Sun to form the comet’s tail,
which we see here at a slight angle (thus its
stubby appearance).
Discovered in September 2012 by two Russian amateur astronomers, Comet ISON is
likely making its first passage into the inner
Solar System from what is called the Oort
Cloud, a region deep in the recesses of our
Solar System, where comets and icy bodies dwell. Historically, comets making a first
go-around the Sun exhibit strong activity as
they near the inner Solar System, but they
often fizzle as they get closer to the Sun.
July2013
Sizing up Comet ISON
(Previous page)
Astronomer Karen Meech, at the University
of Hawai‘i’s Institute for Astronomy (IfA) in
Honolulu, is currently working on a preliminary analysis of the new Gemini data
(as well as other observations from around
the world) and notes that the comet’s activity has been decreasing somewhat over the
past month.
“Early analysis of our models shows that
ISON’s brightness through April can be reproduced by outgassing from either carbon
monoxide or carbon dioxide. The current
decrease may be because this comet is coming close to the Sun for the first time, and a
“volatile frosting” of ice may be coming off
revealing a less active layer beneath. It is just
no w getting close enough to the Sun where
water will erupt from the nucleus revealing
ISON’s inner secrets,” says Meech.
“Comets may not be completely uniform in
their makeup and there may be outbursts
of activity as fresh material is uncovered,”
adds IfA astronomer Jacqueline Keane. “Our
team, as well as astronomers from around
the world, will be anxiously observing the
development of this comet into next year,
especially if it gets torn asunder, and reveals
its icy interior during its exceptionally close
passage to the Sun in late November.”
NASA’s Swift satellite and the Hubble Space
Telescope (HST) have also imaged Comet
ISON recently in this region of space. Swift’s
ultraviolet observations determined that the
comet’s main body was spewing some 850
tons of dust per second at the beginning of
the year, leading astronomers to estimate
the comet’s nucleus diameter is some 3-4
miles (5-6 kilometers). HST scientists concurred with that size estimate, adding that
the comet’s coma measures about 3100
miles (5000 km) across.
GeminiFocus
Images of Comet ISON
obtained using the
Gemini Multi-Object
Spectrograph at Gemini
North on February 4,
March 4, April 3, and
May 4, 2013 (left to right,
respectively; Comet ISON
at center in all images).
Technical Specifications:
The three images on
left are through an
r-band filter only, and
the color composite on
right includes g, i, and r
bands. All are integrated
for 2 x 45 seconds with
the February 4 image
integrated for 2 x 75
seconds (increasing
the comet’s apparent
brightness). During the
period of this sequence,
the comet shined at about
magnitude 15.5-16.5
in visible light. In these
images north is up, east is
left, and the field-of-view
is about 2.5 arcminutes
across, which corresponds
to about 270,000-290,000
miles (435,000-470,000
kilometers) at the distance
of the comet.
Color composite produced
by Travis Rector, University
of Alaska Anchorage.
Credit: Gemini
Observatory/AURA
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