UPDATE, Nov 13, 15:00 UT: Today, TRAPPIST scientists reported a x2 increase of all gas production rates in just 24 hours. This is the strongest rise in activity the comet has experienced so far, and definitively an indication that ISON, now inside the orbit of Venus, is starting to feel the sun's heat. Simultaneously, french observer Francois Kugel reports the inner coma brightness has jumped one full magnitude. (Note the magnitude shown in Kugel's graph refers to the inner coma only, and is therefore smaller than the total magnitude of comet ISON, which is reported to be around 8mag or slightly brighter).
Whether these are signs that the comet is now finally ramping up performance or breaking apart remains to be seen. Yesterday, rumours on Twitter suggested ISON's core is only one kilometer in diameter, maybe less (contrary to the 2-4 km reported earlier):
.@NickAstronomer Sage advise, and we now think #ISON is smaller than that. Upper limit ~1km, maybe half that. Not a big comet!
— Sungrazing Comets (@SungrazerComets) November 12, 2013
If this holds true, survival at perihelion seems highly unlikely.
ISON is still not a naked-eye object, but accessible with binoculars. Images show an interesting tail structure, changing virtually every day. Perihelion ist at Nov. 28, so the comet is approaching the sun rapidly, making it harder to see at dawn - especially with the waxing moon lighting up the sky. If you have clear skies, observe now!
UPDATE Nov. 7, 09:50 UT: bifurcation confirmed by this beautiful picture, might be the first glimpses of an ion tail!)
C/2012 S1 ISON has slowed down producing dust and hence did not brighten very much during the second half of October, but there are indications that this is changing now. Today, a Belgian team operating the TRAPPIST robotical telescope at La Silla Observatory in Chile, reported:
"While the activity was stable for about a month (we have started our Monitoring on October 12), the gas production rates have finally increased rapidly the last days! They were multiplied by a factor 2 since about a week, and mostly in the last 3 nights."
At the same time, there are reports of a possible tail bifurcation (hard to see in this picture of Nov. 6), indicating some development of the tail stucture of comet ISON.
There is no telling now if these are early signs of a core disintegration (as suggested - not only - by Ignacio Ferrín) or of a brightening due to fresh layers of ice being sublimated. Since neither TRAPPIST nor any visual observer noticed anything strange happening in the coma so far, there is no indication of the core breaking apart. ISON is still looking healthy at the moment. In fact observers report a visual brightening trend, the comet has hence reached 8th magnitude.
As things are going, we have no choice but to monitor ISON's behaviour on a daily basis. The mailing lists CometObs, comets-ml and the CIOC-homepage provide good sources of information. I will update this blog and tweet about new observations coming in. By the way, I haven't given up hope yet to see the comet myself some time, despite very discouraging weather prospects in central Europe. And I'm still optimistic that there will be something to see once ISON has made it's swing around the sun. If you want to see ISON (and three other fairly bright comets currently visible) yourself, these maps may help you!