Don't bet on Comet ISON becoming "the comet of the century." But it could still put on a fine naked-eye display in the dawn of December 2013.

Comet ISON (C/2012 S1), discovered last year as a tiny, distant speck, has raised a lot of hopes among skywatchers. It's on track to swing very close around the Sun — missing it by less than one solar diameter! — in late November and early December 2013. Here are charts, pictures, and the light curve as predicted and as observed so far. But how bright will it actually become?

It's early, and scientific opinions differ.

First, don't believe the hype that it will dazzle the world as a night-sky spectacle brighter than the full Moon. It is conceivable that the comet will indeed reach magnitude –12, but only when it'is just a couple degrees from the Sun in the daytime sky! Under such a circumstance you should be able to detect it as a fuzzy point by masking out the Sun, which is one million times brighter, but that's hardly the picture that such headlines call up.

Do, however, plan for a fascinating dawn-sky visitor that will be all the better in binoculars and wide-field telescopes.

We covered it in the January 2013 Sky & Telescope; click to read A Great Comet Coming? by John Bortle. Since then there have been new developments.

UPDATES:

April 23: Hubble photo and another size estimate. Today NASA released the Hubble Space Telescope photo at right, taken April 10th when the comet was still 4.15 a.u. from the Sun. The researchers involved write,

"Preliminary measurements from the Hubble images suggest that the nucleus of ISON is no larger than 3 or 4 miles (5 to 6.5 km) across. This is remarkably small considering the high level of activity observed in the comet so far, said researchers. Astronomers are using these images to measure the activity level of this comet and constrain the size of the nucleus, in order to predict the comet's activity when it skims 700,000 miles above the Sun's roiling surface on November 28."

Press release with links to larger images.

April 8: Good news from Swift. NASA's Swift spacecraft measured the comet in ultraviolet on January 30th and in February, and found that its nucleus was spraying 850 tons of dust per second even when still 2.7 a.u. from the Sun. Program scientists estimate that this means the comet's nucleus is roughly 5 km wide. That's about 10 times larger than another Sun-grazer, Comet Lovejoy (C/2011 W3) in 2011, which became a long-tailed spectacle. However, ISON's water-vapor production was very low; carbon dioxide and/or carbon monoxide vaporization still seems to be the driving force, and no one knows how long these more volatile substances will last. NASA press release.

Here's a 35-minute video of the comet barely creeping across the stars on April 7th, still at magnitude 16.1, made by two observers at the University of Narino in Colombia (courtesy SpaceWeather.com).

March 19: Back to some pessimism. Clay Sherrod reports, "Unfortunately we are seeing a dip in total brightness and size of the comet; we have logged it dropping from a maximum magnitude of 15.7 two weeks ago to this morning's measurement of 16.1 and 16.2. The size has likewise diminished.... The overall intensity and size are decreasing at this time."

February, March, and April are predicted to be a time when ISON is brightening only a little, but not actually dimming.

Feb. 28: And now on the optimistic side.... After my February 21st post below gave David Seargent's pessimistic view, Daniel Fischer calls attention to an analysis by Ignacio Ferrín, comet expert in Colombia, who finds a 75% chance that ISON has already undergone its brightening slowdown, meaning it should be on track for that magnitude minus-12 perihelion peak after all. Ferrín writes,

"The reason for this is that the current power law describing the brightness is rather steep, increasing at a rate R^4.35....

"We have calculated the temperature of the object at perihelion and find T = 2900º K, sufficient to melt lead and iron.

"Additionally the comet will penetrate the forbidden region defined as Roche's Limit. Any object within Roche's limit has a large probability of desintegrating due to differential gravitational forces from the Sun. The combinations of Roche's Limit, plus solar radiation plus very high temperature, suggest that the comet may not survive its encounter with the Sun, desintegrating into several pieces. Or it may survive, if its internal cohesion is sufficient to endure those conditions."

References:
   http://arxiv.org/ftp/arxiv/papers/1302/1302.4621.pdf
   http://arxiv.org/ftp/arxiv/papers/0909/0909.3498.pdf

On the other hand, Ferrin's way of interpreting visual brightness estimates is being criticized.

Feb. 21: Again, a "great comet" to weaken? From Australia, longtime comet expert David Seargent posts to the comets-ml mailing list:

"All recent upgrades of the orbit of this comet give weakly hyperbolic eccentricities, suggesting that it is coming in from Oort Cloud distances and is probably making its first (and only?) close approach of the Sun." That's a formula for early false promise followed by a slowdown of brightening. Right now, says Seargent, "there must be something more volatile than water ice at the surface of the nucleus, which seems unlikely if the comet had experienced previous episodes of fierce heating at sunskirting distances."

Accordingly, says Seargent, ISON's future brightness development will probably be like that of Comet PanSTARRS (C/2011 L4): fairly steep brightening until it's within about 1.7 astronomical units from the Sun — meaning late September 2013 — "followed by a very slow increase until perihelion," its closest approach to the Sun on November 28th.

Fitting this model to recent observations "has the comet barely brighter than magnitude 5 in mid-November, about –4 or –5 near perihelion [when it's just a couple degrees from the Sun in the daytime sky], and magnitude +1 or better in early December. Northern observers should see a moderately bright comet, most probably sporting a very long and intense tail. For us in [the Southern Hemisphere], however, it seems as though it will be marginally naked-eye at best."

Also, its much-speculated connection to the Great Comet of 1680 now seems ruled out.

P.S. from Alan MacRobert: As an example of the stuff that's causing friends and relatives to ask me about the "monster comet coming," this appeared on the popular science site Red Orbit:

"...Viewers can expect to see a rapid whirling of the comet as it engages in a near hairpin-like turn around the sun.... The comet will become dazzlingly bright, rivaling the brightness of a full moon, as it streaks across our sky."

Read for yourself and see the ridiculous graphic.

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