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Comet ISON could be spectacle . . . or celestial dud

Sunday, November 24, 2013
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Star Talk


This graphic shows when and where to view Comet ISON. Note that the visibility of the comet is probably exaggerated.  Binoculars may be needed to see the comet or improve the view. Courtesy of SkyandTelescope.com.
This graphic shows when and where to view Comet ISON. Note that the visibility of the comet is probably exaggerated. Binoculars may be needed to see the comet or improve the view. Courtesy of SkyandTelescope.com.

A potentially bright comet is about to arrive in the innermost regions of the solar system. Astronomers found the comet more than a year ago while using a telescope operated by the International Scientific Optical Network, hence the comet has been named ISON.

What was remarkable about Comet ISON when it was discovered was its brightness for its distance of 460 million miles from the sun. Since then, astronomers have wondered if it will be “the comet of the century,” one of the most visually stunning comets to cross the sky in the past hundred years or an overestimated disappointment.

Two Russian astronomers, Vitali Nevski and Artyom Novichonok, discovered Comet ISON on Sept. 21, 2012 as part of a research program that uses a large telescope of the ISON network to find asteroids.

What caught their attention was how evident Comet ISON was in their discovery images. When it was found, the comet was more than 10,000 times fainter than the faintest stars that can be seen by the human eye. If it continued to brighten as it moved farther into the solar system, it would become a light fantastic, outshining such recent showpieces as 1997’s Comet Hale-Bopp, Comet McNaught in 2007 and the 2011 Southern Hemisphere spectacle, Comet Lovejoy.

Icy conglomerate

During the 1950s, Fred Whipple, an astronomer at the Harvard College Observatory, developed a conceptual model for the structure of a comet. He described it as an “icy conglomerate,” icy material left over from the beginning of the solar system with rocky dust and some complex chemicals mixed in. Once the scientific community accepted the description, it became known as the “dirty snowball” model.

The actual snowball is the core or nucleus of the comet and is typically only a few miles across. As a comet moves toward the sun, it starts to be warmed by sunlight at approximately the distance of Jupiter. Escaping gases drag dust particles from the warmed nucleus and create a huge, dusty atmosphere around its icy core — commonly many thousands of miles across — called the coma. Closer to the sun, solar particles and radiation push the gas and dust released by the comet outward, forming the tail.

Whipple’s explanation was not truly confirmed until 1986 when the European Space Agency flew a spacecraft through the coma and by the nucleus of Comet Halley.

Comet ISON developed its coma early, leading some in the astronomy community to infer that it would be very bright when it reached the inner solar system. However, after weeks of observing the comet in order to establish its orbit, it was found that Comet ISON was heading toward the sun for the first, and only, time. Consequently, this would affect its initial brightness.

The past few decades of comet research have shown that when a comet is inbound for the first time, its icy surface dissipates quickly and the comet initially undergoes a surge in brightness. This fast rise, however, soon drops off and the comet’s new rate of brightening is slow. This has been the situation with Comet ISON.

After it was discovered last year, it was tracked and monitored through the spring of this year. During most of the summer, the sun blocked our view of the comet and when it was recovered in mid-August, it had fallen short of its brightness predictions. In early November though, sky watchers detected an upturn in the overall brightness of Comet ISON as well as a lengthening of its tail.

Where to look

Comet ISON is in the east-southeastern sky before sunrise. As this week progresses, it will appear lower each morning and farther into the brightening dawn as it plummets sunward for a Nov. 28 astoundingly close passage by the sun.

At 2 p.m. on Thanksgiving Day, Comet ISON will pass less than one million miles above the sun’s surface, becoming what astronomers call a sun-grazing comet. It will be at its brightest at this time but unobservable.

By next Sunday, Comet ISON will be back in the east-southeast before sunrise, gaining altitude each morning but losing brightness as it moves away from the sun. Be sure to start looking for the comet no later than 30 minutes before sunrise and bring binoculars just in case the fuzzy ice ball doesn’t make it to naked-eye visibility.

Comet memories

Astronomers lost their “comet credibility” for several decades when the debacle of 1973 occurred. The advance press on Comet Kohoutek that year was touting its appearance as the brightest comet of our lives. When it arrived, it turned out to be a garden-variety comet — a nice sight in binoculars but far from the cosmic spectacle that news reports anticipated.

Sadly, that was ill-timed because in 1976 the show-stopping Comet West appeared in the morning sky as “a fantastic fountain of light” but by then astronomers were perceived as “crying wolf” when it came to comets.

The anticipated 76-year return of Comet Halley didn’t help either. After its awe-inspiring 1910 appearance — its second-best in its history — it returned for its second-worst showing in 1986 and became another disappointment for the public. (I can remember showing someone a view of Comet Halley through binoculars during a cold January night in 1986 and receiving the frosty reply, “That’s it?”)

Since then, astronomers have been cautious about forecasting comets. So it might have been a good thing that another 10 years elapsed before two back-to-back, naked-eye comets showed up — Comet Hyakutake in 1996 followed the next year by Comet Hale-Bopp — and this time there were no letdowns for the public.

Comet ISON now seems unlikely to put on a spectacular naked-eye display in the dawn sky during early December, but no one can be sure. As comet hunter David Levy, co-discoverer of the one that slammed into Jupiter in 1994 likes to say, “Comets are like cats, both have tails and do what they want.”

Richard Monda is an astronomer living in the Capital Region.

 
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