As it flares out of the distant Oort Cloud, the newly discovered comet C/2012 S1 (ISON) appears to be heading on a trajectory that could make for one of the most spectacular night-sky events in living memory. Why is this comet expected to be so unique? Two reasons:
Astronomers predict that the comet will pass just 1.16 million miles from the Sun as it swings around its perihelion, or closest approach. (This may seem like a lot, but remember—the Sun is big. If we were to scale the Sun down to the size of Earth, the comet would pass well within the orbits of dozens of satellites.) The close approach will melt enormous amounts of the comet's ice, releasing dust and gas and forming what should be a magnificent tail.
After it loops around the Sun and forms this tail, the comet should then pass relatively close to Earth—not near enough to cause any worry, but close enough to put on a great show. Viewers in the Northern Hemisphere will get the best view as the comet blooms in the weeks approaching Christmas 2013. The comet could grow as bright as the full moon.
Of course, comets have a habit of not living up to expectations. This one could be sucked into the Sun during its close approach, or not grow as much of a tail as astronomers hope.
But that hasn't dampened enthusiasm for what Astronomy Now is awkwardly calling "a once-in-a-civilisation's-lifetime" event. The comet expert John E. Bortle is already comparing ISON with the Great Comet of 1680, which, according to contemporary accounts, caused the people of New York's Manhattan Island to be "overcome with terror at a sight in the heavens such as has seldom greeted human eyes.... In the province of New York a day of fasting and humiliation was appointed, in order that the wrath of God might be assuaged."
We can only hope for such a show.
Image of comet Hartley 2 courtesy NASA/JPL-Caltech/UMD