Tomorrow, Comet ISON is due to swing around the sun. This is the first trip the comet has made to the inner solar system and no one knows how well formed it is. It could break up into pieces or it could come through unfazed. Or even something in between.
Comets have been doing this for literally millions of years but we've got some new things in line that make this one special. The big thing is that we have satellites that do nothing but look at the sun all day to monitor the solar weather. They can see the comet approaching and will be able to tell how it comes through. You can go to the SOHO site tomorrow to follow along.
If it passes without too much damage, we could be in for some special sights over the next few weeks!