The comet gets brighter as the outgassing
increases and pushes more dust from the
surface of the comet. Scientists are using
the comet’s brightness, along with information about the size of the nucleus and
measurements of the production of gas and
dust, to understand the composition of the
ices that control the activity. Most comets
brighten significantly and develop a noticeable tail at about the distance of the asteroid
belt (about 3 times the Earth-Sun distance
— between the orbits of Mars and Jupiter)
because this is when the warming rays of
the Sun can convert the water ice inside the
comet into a gas. This comet was bright and
active outside the orbit of Jupiter — when
it was twice as far from the Sun. This meant
that some gas other than water was controlling the activity.
Meech concludes that Comet ISON “could
still become spectacularly bright as it gets
very close to the Sun,” but she cautions, “I’d
be remiss, if I didn’t add that it’s still too early
to predict what’s going to happen with ISON
since comets are notoriously unpredictable.”
A Close Encounter
On November 28, 2013, Comet ISON will
make one of the closest passes ever recorded as the comet grazes the Sun, penetrating
our star’s million-degree outer atmosphere,
called the corona, and moving to within
800,000 miles (1.3 million km) of the Sun’s
surface. Shortly before that critical passage,
the comet may appear bright enough for
expert observers using proper care to see it
close to the Sun in daylight.
What happens after that no one knows for
sure. But if Comet ISON survives that close
encounter, the comet may appear in our
morning sky before dawn in early December
and become one of the greatest comets in
the last 50 years or more. Even if the comet completely disintegrates, skywatchers
shouldn’t lose hope. When Comet C/2011
W3 (Lovejoy) plunged into the Sun’s corona
in December 2011, its nucleus totally disintegrated into tiny bits of ice and dust, yet it
still put on a glorious show after that event.
The question remains, are we in for such a
show? Stay tuned…
Comet ISON: The View from the North and South
Regardless of whether Comet ISON becomes the “Comet of the Century,” as some speculate, it will likely be a nice naked-eye and/or
binocular wonder from both the Northern and Southern Hemispheres in the weeks leading up to its close approach with the Sun.
By late October, the comet should be visible through binoculars as a fuzzy glow in the eastern sky before sunrise, in the far
southeastern part of the constellation of Leo. By early November, the comet should be a much finer binocular object. It will steadily
brighten as it drifts ever faster, night by night, through southern Virgo, passing close to the bright star Spica. It is during the last
half of the month that observations will be most important, as the comet edges into Libra and the dawn, where it will brighten to
naked-eye visibility and perhaps sport an obvious tail.
The comet reaches perihelion (the closest point in its orbit to the Sun) on November 28th, when it will also attain its maximum
brightness, and perhaps be visible in the daytime. If Comet ISON survives perihelion, it will swing around the Sun and appear as
both an early morning and early evening object from the Northern Hemisphere. The situation is less favorable from the Southern
Hemisphere, as the comet will set before the Sun in the evening and rise with the Sun in the morning.
By December 10th, and given that everything goes well, Comet ISON may be a fine spectacle in the early morning sky as viewed
from the Northern Hemisphere. Under dark skies, it may sport a long tail stretching straight up from the eastern horizon, from
the constellations of Ophiuchus to Ursa Major. The comet will also be visible in the evening sky during this time but with its tail
appearing angled and closer to the horizon.
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GeminiFocus
July2013