Comet ISON will pass within 40 million mile of earth on Christmas of this year and be in decent position for viewing by eye or small scope throughout the fall of 2013. Predicting the brightness of a comet is real dicey, it depends on what the comet is made of and how many times it has been close enough to the sun to vaporize some of those volatile substances. But the scant evidence we have is that ISON could be relatively new to the inner solar system and may be made off the traditional ices found in many comets.

Astronomers believe comets are lumps of exotic and water ices, some with rocky or metal compound inclusions left over from the formation of the solar system. They are, typically between several meters and several hundred kilometers in diameter. The average really big comet we see during a normal lifetime is in the five to 20 mile diameter range.. Periodic comets like Halleys observed up close also show a thin layer of black, tarry coating, possibly produced by repeated solar encounters acting on organic ices over time.

ISON was imaged by NASA’s Deep Impact spacecraft a few weeks ago producing the video above, and already indicated a faint tail 40,000 miles long. It’s still as far away as the planet Jupiter, beyond what solar astronomers call the frost line, where water and carbon dioxide are solid rocks, even methane is at least a slushy liquid. But other frozen gases, nitrogen and oxygen for example, are now well within their respective melting and boiling ranges and if those low boiling point substances exist in enough quantity on the surface to produce a faint tail at that distance in those frigid reaches of deep space, it bodes well for viewing later this year. It suggests this particular object may have never visited the inner system since its creation almost 5 billion years ago, or that it has visited only a few times over millennia.

That would mean the tiny nucleus could be chock full of pristine CO2, water, methane, and ammonia. If so, ISON could develop an enormous tail over a million miles long or longer as it passes Mars on the way to earth’s orbit, making it a once in a lifetime celestial sight!

While it won’t come real close to earth by some standards, it should be in a fair position relative to earth and sun for several months, meaning we’ll have a good long look at the full length of any tail that develops. It will be moving toward the sun when the earth is about 60 to 70 millions away and will appear high in the night sky during the course of a clear November evening later this year. ISON is a sun grazing comet, it will be heated to many hundreds of degrees Celsius for several days as it whips around Sol, enough that it might calve into smaller bodies or disintegrate. But if it survives, ISON will then swing back, tail leading the way, making its closest approach to earth on December 26, 2013, when it will be about 39 million miles away as shown by the blue portion of the cometary orbit above.

There is zero chance ISON or any sizable fragments of it will strike earth. But in the event an object similar in size and composition does hit us one day the results will be catastrophic. I made modest assumptions, assuming you were 1000 miles way from the impact site, that the object is mostly low density ice, that long period comets like ISON hit at a leisurely 20 miles per second. The fireball as it streaked over head and struck the earth would be so bright it would instantly blind anyone unfortunate enough to see it. The final crater would be almost a 100 miles wide and 20 miles deep.

The seismic wave would arrive within a few minutes, magnitude 10.2, greater than any recorded earthquake in recorded history. Even 1000 miles away ejecta would rain down like artillery for hours at near super sonic speeds. An hour and a half after impact the atmospheric shockwave would arrive to the tune of 400 mph windblasts. In short, it would be as devastating as the K-T impact, the world would catch on fire, followed by a nuclear winter and massive climatic shifts that would permanently alter weather patterns.

The good news is that won;t happen. The somewhat bad news is we have lousy experience with comets living up to their potential over the last 50 years. But after the letdown of Comet Kohoutek in 1974, and Halley’s Comet being in just about the least favorable position for viewing on record in 1986, it seems like we’re about due for a beauty. Regardless, with all the scopes on earth, in space, and space craft like Deep Impact trainined on ISON, this will be the most studied and well photographed comet in history.