Just prior to its closest approach to the sun on November 28, Comet ISON went through a major heating event, and likely suffered a major disruption. At this time, scientists are not sure how much of the comet survived intact. We may be seeing emission from rubble and debris in the comet's trail, along its orbit, or we may be seeing the resumption of cometary activity from a sizable nucleus-sized chunk of ISON.
Most agree that ISON was destroyed (with greater than 90% probability of this having occurred), leaving behind small (< 10 m radius) pieces of rubble, but perhaps, with maybe 10% probability of occurring, also leaving behind some important fragments 100m radius or larger, big enough to study. If previous sungrazing comets are any guides, there may be a sizeable piece of comet nucleus left. At this point, though, scientists are waiting for a variety of telescopes to make observations before the status of Comet ISON can be confirmed.
What remains of Comet ISON appears to brighten and spread out, then fade. The disappearance of a strong central brightness condensation after perihelion is telling, the comet is clearly fainter and more diffuse going out than going in, but it continues to shine. The spread out light is likely due to dust emitted in the few hours before perihelion going around the sun.
We have seen comets come close to the sun and disrupt and disappear before, like Comets Lovejoy and Elenin in 2012. We have also seen them survive in much smaller pieces, like the Kreutz sungrazer family progenitor and Comet Kohoutek in 1973.
Twelve NASA spacecraft and the International Space Station have observed and detected Comet ISON on its multi-million year journey from the Oort Cloud to the solar corona.
What NASA will be doing in the next days to clarify ISON's status:
If a fragment that acts like a comet is detected, but at a much reduced level, it may be hard to see it from the Earth at the time of its closest approach on December 26, 2013.
NASA's Comet ISON Observing Campaign is holding a post-perihelion workshop to discuss the status of Comet ISON, to be held at the Johns Hopkins Applied Physics Laboratory on Dec. 6, 2013. The morning sessions, from approximately 8:30 am EST to 11:30 am EST, will be available for viewing at: http://www.livestream.com/cometison . More details on the meeting are available at http://www.isoncampaign.org/ison-followup-meeting .