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Comet ISON: Will It Be a Great Comet?

Images of Comet ISON obtained on Gemini North on February 4, March 4, April 3, and May 4, 2013. The comet has a well-defined 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). Credit: Gemini Observatory/AURA.

On November 28, Comet ISON will come extremely close to the Sun, penetrating our star’s million-degree outer atmosphere (the corona) and moving to within 800,000 miles (1.3 million km) of its surface. If it 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.

IfA astronomer Karen Meech, who worked on the analysis of a series of images taken between February 4 and May 4 with the Gemini North telescope on Mauna Kea (as well as other observations from around the world), notes that the comet’s activity decreased somewhat during April. The Gemini time-sequence images, taken when the comet was just inside Jupiter’s orbit, show the comet’s remarkable activity despite its great distance from the Sun and Earth. “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,” Meech says.

“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.”

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. Comet ISON’s main body was spewing some 850 tons of dust per second when observed by NASA’s Swift satellite at the beginning of the year, leading astronomers to estimate that the diameter of the comet’s nucleus is 3–4 miles (5–6 km).

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. But this comet was bright and active outside the orbit of Jupiter, so 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.” But even if Comet ISON 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.