A quick update on Comet ISON: It brightened quite a bit last night, and then started dimming. It’s still going strong, but it’s unclear exactly what’s happening to the comet itself.
Here’s a shot taken by the SOHO satellite that shows ISON as of Thursday, Nov. 28 at 15:34 UTC (10:34 EST):
As you can see it’s still pretty bright, but it was far brighter a few hours earlier. Remember, comets are rocks, gravel, and dust held together by ice. When the ice warms, it turns into a gas, creating the fuzzy coma and long tail. That gas and dust reflect sunlight and make the comet bright.
Here's a wider field view, also from SOHO:
The tail is pretty impressive! But it's definitely dimmer than it was last night.
It’s possible that the brightening earlier was due to a pocket of gas erupting. It’s also possible the brightening was normal (it is getting closer to the Sun after all!) and for some reason the supply of ice turning into gas got choked off. It could also be that the solid nucleus of the comet disrupted, making it very bright for a short time, and then starting to fade.
There’s no real way to know right now. I’m seeing lots of speculative chatter in various places, but nothing definitive.
I’ll note the comet is close enough to the Sun that the rock on the surface is hot enough to vaporize, so the physical situation near the nucleus is complicated.
It reaches perihelion (closest point to the Sun) at about 18:37 UTC (1:37 EST) today, when it will be about 1.1 million km (700,000 miles) above the Sun’s surface, and moving at better than 360 kilometers per second (225 miles/second).
I’ll have more info soon. And don’t forget, I’ll be participating in the NASA live Google+ Hangout which starst at 18:00 UTC (1:00 p.m. EST). I’ll have the video feed embedded in a blog post here. Stay Tuned!