|Latest 30 images from the SOHO LASCO C3 instrument animated. Comet ISON skims past delta Scorpii.||Latest 30 low resolution beacon images from the STEREO COR2A instrument animated. The Big blob is Venus.|
We are just over 7 hours away from comet C/2012 S1 ISON reaching perihelion. What an amazing trip it has been, with the comet defying all expectations, stops and starts in brightness increase, dramatic outbursts and fades.
Now on the last leg of its long journey, which began around a million years ago in the Oort cloud at the edge of the solar system, in a few short hours it will skim around a solar diameter from the surface of the sun, being exposed to almost unimaginable heart wich will vaporize the very dust on its surface.
Will it survive is the question on everyone's lips? How bright will it get?
Currently it is outperforming C/2011 W3 Lovejoy, latest brightness estimates put it at around magnitude -3 (a bit dimmer than Venus ), quite a bit above Lovejoy at the same time, and ISON has a more complex tail. The twin tails you see in the LASCO C3 instrument are a broad dust tail and a synchronic dust streamer. No ion tail can be seen in the LASCO C3 image at the moment (info from the latest CBET via Jakub Cerny).
So, what happenes next? Will it disintegrate? For typical Kreutz sungrazer comets with radii under 100 meters, they tend to disintegrate at 10 solar radii from the Sun. ISON is much larger than 100 meteres (somewhere between 1 Km and 2.4 Km in radius), but the 10 Solar radii checkpoint is at around 12:00 UT in a half an hour from now as I type. If it passes this check point, then there is a good chance it will get to perihelion. After that, it's all still up in the air. Karl Battams of the STEREO mission is enthusiastic but cautious. Jakub Cerny has a detailed post here were he expects it not to survive.
How bright will it get? Jakub Cerny reports J. N. Marcus as saying it will not get too much brighter, as the heat from the Sun begins to vaporise the dust grains.
If it survives perihelion, it's grazing approach to the Sun, what happens then? Will it be like Lovejoy and disintegrate a few days later, leaving us with a pale and impressive tail? Perhaps the best analogs will be two sungrazers of around 1 Km radius, the great Southern Comet of 1880 and C/1970 K1, which survived and were reasonably bright with decently long tails for a short time.
So if ISON survies we might expect it to not be as bright as C/2006 P1 McNaught, but brighter than Lovejoy.
The best we can say is "don't count your comets before they are hatched". As the excitement mounts you can watch the comet in various spacecraft instruments (links here) . No matter what happens, this is the first sungrazing comet from the Oort cloud, so the show will be well worth watching.
So happy comet watching folks.