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.
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.