August 19, 2013

Dennis di Cicco, Senior Editor, Sky & Telescope
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Robert Naeye, Editor in Chief, Sky & Telescope
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More information and updates about Comet ISON are available at

Since the discovery of Comet ISON in September 2012, astronomers worldwide have eagerly anticipated its peak performance in late 2013. Many have high hopes that ISON will turn out to be a glorious spectacle, perhaps even “the comet of the century.”

Unfortunately, the latest images of Comet ISON suggest otherwise. After the comet spent 2.5 months hidden behind the Sun’s glare, Arizona amateur astronomer Bruce Gary successfully imaged ISON on the morning of August 12th. His images, and those of other astronomers since then, show that ISON has not brightened as much as expected.

“Comet ISON is about two magnitudes (six times) fainter than it should be compared to the calculations that first led astronomers to predict it would become a grand naked-eye sight before dawn in early December,” says Sky & Telescope senior editor Alan MacRobert. “The comet could still turn out to be fairly good, or it might never reach naked-eye visibility at all.”

MacRobert also notes that the comet was about two magnitudes fainter than predicted when it was last seen in late May before being hidden by the Sun. The combined light of the comet’s coma and short tail is currently glowing in the constellation Cancer about 1,500 times fainter than the dimmest stars we can see with the naked eye.

“Comets are notoriously fickle, unpredictable objects, so I’m not giving up hope just yet,” says S&T editor in chief Robert Naeye. “But these latest observations should temper expectations. We’ll simply have to wait and see how the comet develops in the months ahead as it ventures closer and closer to the searing heat of the Sun.”

A comet’s trajectory through the solar system is governed by the laws of gravity, so astronomers can predict many months in advance exactly where a comet will appear in the sky. But how bright a comet appears depends on more than just its precisely known distances from the Sun and the Earth; it also depends on the highly unpredictable amount of ice and dust burned off the nucleus by the Sun’s heat, which in turn depends on such factors as the comet’s size and age. Comets with a large nucleus and that have swung around the Sun many times usually give off large amounts of material, and are thus more likely to appear bright and sport long tails.

“The fact that Comet ISON continues to appear as faint as it does implies that its intrinsic brightness is low and that the nucleus is probably small and relatively inactive,” says S&T contributing editor John Bortle, an experienced comet observer and analyst. “Past performances by dynamically ‘new’ comets [newcomers to the inner solar system], as ISON has turned out to be, have typically been pretty lackluster. With very few exceptions, these comets brighten only very slowly.”

In contrast, some comets have performed better than expected, such as Comet Lovejoy. After many experts predicted that its nucleus would disintegrate during a very close pass to the Sun in mid-December 2011, it survived long enough to produce a long and spectacular tail easily visible for people living in the Southern Hemisphere.

“All of us at Sky & Telescope are hoping that Comet ISON will be a glorious spectacle in late November and early December,” says S&T senior editor Dennis di Cicco. “But we also remember past comet flops such as Comet Kohoutek in 1974, which was actually a fairly nice comet but unfortunately fell far short of the hype. It’s important for the media to emphasize the unpredictable nature of comets, and that it’s by no means guaranteed that ISON will turn into a show-stopping comet later this year, or even one that most people will see at all without good charts and optical aid.”

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