Saturday, October 19, 2013

A Spectacular New Comet Next Month? Possibly!

The Hubble Space Telescope captured this view of Comet ISON, C/2012 S1 (ISON), on May 8, 2013.
 Comet Ison heads sunward. Will it be spectacular? Don't make book on it yet.

Okay, there hasn't been a whole lot of publicity in the mainstream media concerning it, but that's with good reason. Astronomers were burned to a far-thee-well when Comet Halley arrived in 1986 and turned out to be a huge dud.  In Barbados, for example, after months of touting special Comet Halley tours, the Barbados Astronomical Society was burned when the visitors finally showed up in droves. Tourists coming to the Harry Bayley Observatory were disappointed and to quote some long time members, "Never again!" Predicted to arrive with a resounding brilliant  'bang', Halley actually appeared with a downer 'whimper'- and it bummed everyone out, astronomers and visitors alike.

Never again would a comet arrival be publicly hyped. Especially a comet heading sunward that might have most of its tail "burned off" as viewed from Earth, and so barely be seen with the naked eye.  So you can understand why astronomers aren't hyping Comet Ison (designated C/2012 SI) as it approaches for what may turn out to be a spectacular sight by the end of next month. How spectacular? That depends on whose POV you adopt, that of the optimists or pessimists. According to ASTRONOMY magazine ('Comet Ison Blazes Into Glory', November, p. 54):

"Optimists tout numbers that have  many amateur astronomers giddy with excitement. If they're right the comet could rival the full Moon when it passes closest to the Sun at perihelion November 28 and remain as bright as the planet Jupiter for several days around then."

Make no mistake that if the optimists are correct, the Comet WILL be spectacular - rivaling or exceeding Halley in its glorious 1910 arrival (and what many of us expected also in 1986).  The full Moon, with a magnitude of (-12.5) is the brightest object visible after the Sun, so if Comet Ison attains that level of brightness, no one will be able to ignore it. And that includes the "End of Time" folks who will undoubtedly be yelping again about "signs and wonders" and the approach of Armageddon.  But this is inevitable.

For astronomers, the comet will be a boon, and amateur astronomical societies around the world ought to be able to capitalize on it, to engender more wonder of the skies and in particular, solar system objects like comets, meteor showers, and also the planets.

But we haven't finished. What about the pessimists, which many of us became after Halley duped us in 1986. According to ASTRONOMY (ibid.):

"Pessimists peddle figures that would bring tears to the cheerleading squad of an undefeated football team during Homecoming. If they prove prophetic, Ison may not even reach naked eye visibility and will go down in the annals of astronomical history with Comet Kohoutek (C/1973 E1) which failed to live up to its billing as 'comet of the century' 40 years ago."

That last assessment is sobering to be sure. But as a realist, it seems to me the actual appearance is most likely to be somewhere between the two extremes: perhaps on a par with Comet West, see e.g.
Which was discovered in August, 1975, and reached a magnitude of (-3) from Feb. 25-27 and bright enough to be seen in daylight. The  magnitude of (-3) is some 9.5 magnitudes higher than for the full Moon brightness forecast by optimists,  which means it is about (2.5)9.5  =   6, 030 times dimmer than Ison is predicted to be. See e,g,

What got the optimists so revved up? Probably Ison's intensity from when it was first discovered, back on Sept. 12, 2012, beyond the orbit of planet Jupiter. It then beamed at magnitude +19, "an intensity almost unheard of for such a distant comet", according to the Astronomy piece. So, the optimists clearly have surmised that if it was that bright at such a distance, wow, it will be millions of times brighter by the time it's much closer to the Sun and within naked eye view of Earth.

The extraordinary brightness also made astronomers realize Ison's nucleus must be covered with ices sublimating directly into gas. by virtue of the solar heat, and if the rate of outgassing continued Earth viewers would be in for a real treat.

However, as the Astronomy article points out,  the projections for more rapid brightening didn't reach the expectations spurred by the earlier outgassing estimates. In particular, Ison appeared two magnitude dimmer (i.e. (2.5)2   =  6.25 times less bright)  than expected when it disappeared into the Sun's glare.  This has led a number of researchers to conjecture this dimming is a result of the fact less volatile gases are now in play. Thus, the extravagant brightening near the discovery date was due to really volatile gases like CO and CO2 - which preponderantly sublimate at lower temperatures. Now, high temp. gases are in play which don't translate into brightness intensity.  Once water ice is left in the nucleus, the comet will fall further behind the brightness curve- hence leading to the pessimists' prediction.

Will Ison soon appear bright in the early morning light? We have to wait and see. Brightness estimates put out by the IAU's Minor Planet Center, show Ison at 7th or 8th magnitude in early November. This will be enough to be seen in a good pair of binoculars. If it exceeds that magnitude threshold there's reason to be optimistic.

For those with binoculars (or even their naked eyes- if they're lucky) a good observational test comes up on November 1st. On that date Ison rises 4 hours before the Sun and reaches 30 degrees above the eastern horizon at morning twilight. For readers with a star atlas (like Norton's) it will be located  12 degrees southwest of Denebola, a (+2) magnitude star in the constellation Leo. Those observing at the time will also be able to see the planet Mars, some 7 degrees north-northwest of Ison, and at a magnitude +1.5.

I will observe on the date, and report back in my blog for November 1st with my magnitude estimate. Stay tuned!

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