Comet ISON is a ball of rock and ice a few miles across that was first spotted over a year ago beyond the orbit of Jupiter. It was thought that it would be a bright comet, and indeed it has recently been visible in the early-morning sky. Today – in a couple of hours in fact – it makes its closest approach to the Sun as it dives into the centre of the Solar System before swooping back out and (hopefully) being visible in the morning sky once again.
There is a chance that ISON will be destroyed as it sweeps around the Sun, because it is passing very close by it. Many astronomers around the world have been obsessively watching it as it makes this perilous descent. You can follow along live on the SOHO website at http://soho.esac.esa.int/ and at this dedicated Comet ISON Tracking site. Here are the current images, which show ISON racing toward the Sun at almost 800,000 mph!
If ISON survives, look for it just after sunset and shortly before sunrise over the next few days: it may be very bright.