Comet ISON On Uncertain Course Towards The Sun
May 31, 2013

Astronomers Track Comet ISON From Gemini Observatory

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Comet C/2012 S1 (ISON) is racing toward an uncomfortably close rendezvous with the Sun, according to a new series of images from the Gemini Earth Observatory. Comet ISON might present a stunning sight in the twilight sky of late November that will remain easily visible, even brilliant, into early December.

Gemini´s time-sequence images span from early February through May 2013, showing the comet´s remarkable activity despite its vast distances from the Sun and Earth. Data accumulated from the image series provides vital clues as to the comet´s overall behavior and the potential to present a spectacular show this fall. What scientists don´t know is if Comet ISON will survive its close brush with the Sun to become the early morning spectacle in late 2013.

During the image collection, the comet ranged between roughly 455-360 million miles (or 4.9-3.9 astronomical units) from the Sun. This put it just inside the orbital distance of Jupiter. The images, taken with the Gemini Multi-Object Spectrograph at the Gemini North telescope on Mauna Kea, Hawaii, reveal the comet in the far red part of the optical spectrum. This emphasizes the comet´s dusty material already escaping from what the astronomers call a “dirty snowball.”

The comet sports a well-defined parabolic hood in the sunward direction that tapers into a short and stubby tail pointing away from the Sun. Such features form when dust and gas escape from the comet´s icy nucleus and surround the main body to form a relatively extensive atmosphere called a coma. The coma´s material is pushed away from the Sun by solar wind and radiation pressure to form the comet´s tail.

Comet ISON, discovered in September 2012 by two Russian amateur astronomers, is likely making its first passage into the inner Solar System from the Oort Cloud, which is a region deep in the recesses of our Solar System where comets and icy bodies dwell. Most comets making a first orbit around the Sun exhibit strong activity as they near the inner Solar System, but fizzle as they get closer to the Sun.

The preliminary analysis of the Gemini data is being performed by astronomer Karen Meech at the University of Hawaii´s Institute for Astronomy (IfA). Meech notes that the comet´s activity has been decreasing somewhat over the last 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 now 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.”

Comet ISON has also been captured by NASA´s Swift satellite and the Hubble Space Telescope (HST) recently. The ultraviolet observations from Swift determined that the comet´s main body was spewing approximately 850 tons of dust per second at the beginning of the year. This led astronomers to estimate the comet´s nucleus diameter is some 3-4 miles. HST scientists concurred with this size estimate, adding that the comet´s coma measures about 3100 miles across.

As the outgassing increases and pushes more dust from the surface, the comet becomes brighter. That brightness, along with information about the size of the nucleus and measurements of the production of gas and dust, is allowing scientists to understand the composition of the ices controlling the activity. A majority of comets brighten significantly and develop a noticeable tail at about the distance of the asteroid belt — between the orbits of Mars and Jupiter. This is where the warming rays of the Sun can convert the water ice inside the comet into a gas. Comet ISON was bright and active outside the orbit of Jupiter, when it was twice as far from the Sun, indicating that some gas other than water was controlling the activity.

According to Meech, 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.”