Comet ISON Part II: What To Expect

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Saturday, November 02, 2013

Dave Huestis, GoLocalProv Astronomy Expert

In Part I, I provided a comet primer to prepare the reader for the arrival of Comet ISON from the depths of the solar system. If you missed Part I, it can be found online on the Skyscrapers website.

Today’s Part II will serve as an ISON observing guide, noting dates, times and locations of where to look in the sky. Should Comet ISON become a very bright comet, and if it survives its very close approach to the Sun, Skyscrapers will hopefully be planning observing opportunities. (Unfortunately due to our tree-obscured northwestern horizon we will not be able to offer comet observing with our telescopes at Seagrave Memorial Observatory in North Scituate.) Keep looking for updates in this news media regarding the location(s) and observing dates and times.

The Comet ISON story began in Russia on September 21, 2012. A faint spec was discovered on images taken with a 16-inch reflector telescope as part of an asteroid search conducted by the International Scientific Optical Network, hence the name of the comet. ISON was also given an official designation of C/2012 S1.

When the orbit and distance were confirmed, it was determined to be approximately 623,000,000 miles from the Earth. Though the comet was still quite faint, original estimates suggested this dirty snowball was up to six miles across. And because it was discovered so far out, beyond Jupiter’s orbit, some astronomers started to use such descriptions like “comet of the century” and will be “brighter than a Full Moon” and “even visible in broad daylight.”

As I mentioned in my comet primer last month, comets do pretty much what they want to do. In fact, the only thing predictable about their behavior is that they are unpredictable.

Comet ISON continues its journey towards the inner solar system and a close encounter with the Sun on November 28. Calculations indicate the comet will pass within about 700,000 miles of the solar surface. More about that prospect later.

ISON was initially very bright at its discovery distance because it is believed that this is the comet’s first trip towards the Sun. That means there was a lot of loose material on ISON’s surface. Solar radiation blew this material off into space, making the comet brighten.

However, after a period of time, the comet did not continue to brighten as predicted. The brightening stalled because the surface was most likely blown clean of loose material. By the time ISON got to the H²O turn-on point (where water and other volatiles react to the Sun’s heat and escape through cracks on the comet’s surface), the comet was about two magnitudes (6.3 times) fainter than forecast.

At the end of May, while Comet ISON was still out beyond the orbit of Mars, it was lost to view because it was in the direction of the Sun. When the comet finally moved out of the solar glare and was imaged on August 12, it was still fainter than what was originally forecast. At the time, some comet experts thought it was still too early to make a reliable call on what we could expect to see as ISON neared the Sun. Others simply didn’t believe it would become visible to the naked-eye. And there are those experts who do not believe Comet ISON will actually survive its close passage to the Sun.

Even as I write this column in late-October, I find it difficult to make any definitive predictions for the visibility of Comet ISON. (Remember, comets are quite unpredictable.) ISON has been visible to stargazers with telescopes using star charts or computerized pointing systems to locate the comet in the sky. In time, binoculars may eventually provide a small but recognizable image. Naked-eye views may not be afforded. Only time will tell. For now I will simply relate when and where in the sky you will have the best opportunity to catch a glimpse of Comet ISON.

While advanced amateur astronomers have been imaging ISON for some months, it is now more easily accessible to a casual stargazer with modest telescope equipment. On November 1, you can locate the comet about 22 degrees above the eastern horizon at 5:00 a.m. Don’t expect to see it with the naked-eye. The comet will be about as faint as the planet Neptune. To verify you are in the general area of the sky, you will easily see the red planet Mars residing within the constellation of Leo. ISON will be about seven degrees below and to the left of Mars. Use a wide field eyepiece and slowly scan this region. Comet ISON should display the typical comet shape with a tail of unknown length.

Each morning the comet will move closer and closer to the horizon and the Sun. Don’t forget that we set our clocks back one hour on Sunday, November 3 at 2:00 am as we “fall back” from Daylight Saving Time to Eastern Standard Time. Even though this practice will have no affect on the comet itself, the area of sky where you observed the comet on the first at 5:00 am will now be seen at 4:00 am On the 5^th Comet ISON will move into the constellation of Virgo. By the 10^th, ISON will only be 17 degrees above the horizon. To observe the comet at a higher position in the sky you’ll need to observe later in the morning, closer to dawn’s early light.

ISON will quicken its movement each morning as it plunges towards the horizon and a rendezvous with the Sun on November 28. At this time it is very hard to predict when the comet will be lost to view, especially since more recent brightness forecasts have ISON much fainter than originally thought. We may lose sight of it a day or two before perihelion (closest point to the Sun).

If ISON survives its close approach to the Sun (within about 700,000 miles of the solar surface), it may briefly brighten as it rises back into the morning sky to begin its journey back into the depths of the solar system. First it will appear just before sunrise very low in the east-south-east. Each morning it will climb higher into the sky and away from the Sun’s glare. Soon ISON might be seen in morning twilight, and then will rise into a darker sky. It’s merely guesswork at this point what may be seen.

Also after perihelion ISON will become visible in the evening sky right after sunset. However, during the first couple of weeks in December ISON will hang very low above the west to north-west horizon. You’ll need an unobstructed view to observe the comet. Originally astronomers were saying the tail would extend many tens of degrees into the sky. That scenario is not likely to happen. I do hope the now pessimistic predictions are wrong. If ISON ends up being a nice, not great, comet visible to the naked-eye, then we’ll all be happy.

As the weeks progress, ISON will begin to rise above the north-western horizon. By December 25, it will still be visible in telescopes as it climbs towards the north. On January 7, it will pass within two degrees (four Full Moon diameters) of Polaris, which shines at magnitude +2. The comet will be about +7 magnitude and not visible to the naked-eye. Binoculars may still show it, but it is mere speculation whether a tail will still be detectable.

After that the comet will even more quickly fade from view and memory.

Please keep in mind the above forecasts are extrapolation from known ISON facts and past comet behavior. Anything can happen.

Should new information come to light during the next few months I’ll be sure to pass them along to the various news outlets so you will have the best opportunity to view Comet ISON.

Since ISON received so much hype after it was discovered, I hope the national news services will keep everyone posted on Comet ISON updates. They can more quickly keep the general public informed should ISON’s viewing prospects change.

Keep your eyes to the skies.

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