[AZ-Observing] John Bortle's Outlook for Comet ISON

  • From: Tom Polakis <tpolakis@xxxxxxx>
  • To: AZ-Observing@xxxxxxxxxxxxx
  • Date: Fri, 31 May 2013 1:18:44 -0400

Today, comet expert John Bortle posted a good prediction for the future 
behavior of the "comet of the century" on the comets-ml mailing list  Bortle is 
typically pessimistic in his predictions, but not without good reasons in this 


In looking at the most recent photometric data, it would appear that
C/ISON's absolute magnitude (assuming a change in heliocentric brightness in
step with the typical 8.3 log r) is close to magnitude 7.5 currently.
Under normal circumstances this would make ISON similar to many modestly
bright (i.e. "average") comets seen in the past. However, with this comet's
exceedingly small perihelion distance the ultimate situation is less clear.

Comet Ikeya-Seki of 1965, with an absolute magnitude of 6.5 , appears to
represent the faintest major sungrazer/sun-skirting comet to have survived
perihelion essentially intact. The non-surviving sungrazing comets of 1880
and 1887 were "thought" to have absolute magnitudes of around 7.0-7.5 ,
although this might be open to question. Then there is Lovejoy's recent comet
that violated all the rules and was intrinsically quite faint, ultimately
classified as a member of the non-survival group. So...where will C/ISON fall?
This is really difficult to predict at the moment. However, I would like
to offer the following tentative prognostication.

Comet ISON will develop more slowly in the autumn morning sky than had been
initially hoped for. It will not actually attain naked eye brightness
until just a week, or two, before perihelion passage. By then it will already
be descending into the morning twilight. On perihelion day the comet may
attain -6 very briefly (hours) and be visually detected near the Sun using
great caution, then immediately begin to fade rapidly.

As the comet retreats from the Sun its head will be brighter than
magnitude +2 or +3 for just a few days, but it will be beginning to unfurl an
extraordinary long straight tail of considerable surface brightness, at least
initially. This tail could reach 30 or perhaps even 45 degrees in length a
week post-T, but its visibility with the naked eye will rapidly wane. The
tail's impressive visual show will last perhaps no more than a week to ten
days in total. Thereafter, like C/Lovejoy, ISON's photographic tail will
continue to lengthen and fade for some time. Concurrent to this the comet's
will likely dissipate more-or-less in the same manner as did Lovejoy's,
with Comet ISON becoming a "Headless Wonder" by mid December, or very soon

I would anticipate that any further critical predictions of this comet's
future behavior will have to wait until at least early September.


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