Comet ISON Spotted Again, Faintly

Since June, Comet ISON has been hidden behind the Sun. Now an amateur imager has just recovered it low in the dawn — and it hasn't been brightening as much as we hoped. Don't bet on a great naked-eye spectacle this December.

Comet ISON at 14th magnitude on August 12, 2013
Bruce Gary's recovery image of Comet ISON (C2012 S1), taken on the morning of August 12th. The comet looks hardly better than it did last spring. Click for larger, ½°-wide image.
Bruce Gary
A couple weeks earlier than we expected, amateur imager Bruce Gary in Arizona has become the first person to pick up Comet ISON again after its 2½-month intermission behind the glare of the Sun. Using an 11-inch scope pointing only 6° above the eastern dawn horizon, and by stacking images, he succeeded in recording a fuzzy point with an anti-sunward tail at Comet ISON's exact predicted position among stars that are as faint as magnitude 16. Measuring the image, Gary comes up with a total V magnitude of 14.3 ± 0.2 for the comet that is being so widely anticipated worldwide.

That's about 2 magnitudes fainter than the comet "should" be, compared to the formula that first led astronomers to predict it would become a grand naked-eye sight before dawn in early December. It's no improvement on the 2-magnitude deficit the comet was showing when astronomers last had good looks at it around the end of May.

Here is Gary's detailed and thorough report and analysis of his observation.

There too (at the bottom) are three new light-curve predictions for the coming months, based on three model formulas. The short version: the comet could still turn out to be fairly good, or it might never reach naked-eye visibility at all.

Sky & Telescope's longtime comet analyst John Bortle writes us:

ISON is currently about at the distance from the Sun where water ice sublimation would be expected to be taking over in the comet's photometric development. That the comet continues to appear as faint as it does implies that its intrinsic brightness (absolute magnitude) is low and that the nucleus is probably small and relatively inactive.

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 [as they approach, after appearing promising when farther out].

In the lightcurve prediction graph presented by Scarmato, Morales and Gary, their green line corresponds to typical "new" comet behavior. Note that this suggests the comet's brightness barely ever breaks the naked-eye barrier, even at perihelion!

Further, if one accepts anything like the green line's absolute magnitude of +9.73 for the comet, then ISON has no chance of surviving its perihelion, based on my paper "Post-Perihelion Survival of Comets with Small q" (International Comet Quarterly, Vol. 13, No. 3, July 1991).

So... things are looking ever more bleak for chances of any grand display to be put on by ISON come this December. Still, I wouldn't fully commit to such until I see some actual visual observations reported.

Many other observers will be looking and imaging on coming mornings as the comet moves higher into less difficult view. Watch for more news updates.

Update Aug. 13: A disagreement has emerged on discussion lists about whether Gary's measurement of magnitude 14.3 refers to the comet's total magnitude or just its nucleus. Gary tells us, "My photometry aperture circle has a diameter of 28 arcseconds, so I think this would include the entire coma and part of the tail." In other words, practically all of the comet's light. He goes into full detail about this here.

Bruce L. Gary's Hereford Arizona Observatory houses his Celestron-11 Schmidt-Cassegrain telescope. Gary is a retired radio astronomer and atmospheric scientist.
Gruce Gary

9 thoughts on “Comet ISON Spotted Again, Faintly

  1. chris

    Yup this is why I am not interested in comets much as part of my observing. They’re almost always overhyped. One thing I,ve noticed is if everyone’s calling it "the comet of the century" than odds are it’s gonna be a dud. Sorry to sound so jaded but I;ve been disappointed on to may comets.

  2. Mike

    Great report and it does not appear to be as bright as one may expect. However reading the report I see that his dome was blocking some of the aperture of his 11" scope so he states that is has an effective aperture of 8". I wonder how much additional light scatter this obstruction created and how that may affect a more diffuse object through his image processing and eventual magnitude estimate?

  3. B. Leitner

    Plotting a new lightcurve after one CCD-observation and making predictions about the comets brightness at perihelion seems rather speculative to me…

  4. Paul

    When they started calling it the Comet Of the Century,it already sounded funny, given that the century is not even 13 years old yet. But by the sounds of it, there have already been a good half-dozen comets that have already performed better than this one will in our fledgling century. The best ones are usually the unheralded last-minute ones: West in 1976, Hyakutake in 1996, McNaught in 2007. Hale-Bopp (1997) was a notable exception to that rule. But at the distance from the Sun that ISON currently is, Hale-Bopp was already easily naked eye.
    Better luck next comet!

  5. rocksnstarsTom Hoffelder

    West’s spectacular show resulted from nucleus breakup. ISON may experience the same since it passes so closely to the sun. If it doesn’t break, it doesn’t; we all know comets are more unpredictable than even New England weather, so mainstream media hype can simply be ignored. And comets don’t have to be super bright to be enjoyed. I’ve been pleased with all the ones I’ve seen and most of them were just dim fuzzballs. Seeing five in one night was rather exciting, while viewing two at once in binocs and another only 10 arc minutes away from Mercury was just plain fun. I’m looking forward to seeing ISON, regardless of performance, so that it can be added to my comet log as #124.

  6. Anthony Barreiro

    Predicting the visual magnitudes of comets making their first passes through the inner solar system is a fun game. But we shouldn’t take the predictions too seriously, nor should we judge the value of a comet solely by its brightness. A new comet is a visitor from the furthest reaches of the solar system, unchanged since the birth of the solar system billions of years ago. No matter how bright or dim, comets are fascinating objects. If you’re able to see a comet with naked eye, binoculars, or a telescope, it’s worth the effort.

  7. Justin

    We need to clarify what is meant by “of the century.” The 21st century is only 13 years old. So any bright comet has a fair shot at being the “comet of the century.” I would think that “of the century” would mean “of the past 100 years, not just the 21st century.

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