A Comet Kerfuffle

On September 24th a very faint comet was discovered in the constellation of Cancer at a distance from Earth of about one billion kilometers.  Now named C/2012 S1, or colloquially “Comet ISON” it has picked up quite a bit of press in the past weeks.  Looking at the orbit of this comet it certainly looks like it has the potential to be “spectacular” as seen from Earth.  It will pass close (within 1.2 million kilometers) to the Sun in late November 2013, and then pass by the Earth at about 40% of the Earth-Sun distance (~ 65 million kilometers).  The comet already seems to be active, that is showing some fuzziness as a result of gas and dust being liberated from its surface, even though it is still out beyond the orbit of Jupiter.  These two things have a whole lot of folks talking about this being a great comet, one that will put on a heck of a visible show in the night sky.  The idea being that this comet will produce a tremendous cloud and tail of gas and dust as it passes by the Sun, and will shine brightly in the night sky.  Some have even proclaimed that Comet ISON could put on a show similar to that of the Great Comet of 1680. Or, you know, it could be a total dud, viewable by almost none <cough Kohoutek cough>

The Great Comet of 1680 by Lieve Verschuier

Now, how can people on one had be comparing this comet to very visible, spectacular looking comets like the Great Comet of 1680, or even Comet Hale-Bopp, yet at almost the same time caution that we may end up seeing nothing?  It all has to do with how a comet behaves, and how unpredictable that behavior actually is.

What we see of the comet in the sky is not the object itself: The actual comet is a small, dark, potato-shaped object made up of rocks, carbon, and ices.  In 1950 Fred Whipple described comets as “Dirty Snowballs,” and that’s how we still see them, although recent observations, and fly-bys of several comets have me leaning toward calling them“Icy-Dirtballs” to better describe them.  As these objects (the comet nucleus) nears the sun, many of the ices near the surface will vaporize, producing a “coma” or cloud of gas and dust around the comet.  This is what we see when we look at the head of a comet.  This gas and dust interacts with the solar wind and gravity and eventually produces tails for the comet, completing our mental image of what a comet is.

That said, the exact processes and amounts of gas and dust to be liberated are unknown quantities on a comet-by-comet basis.  How much ice is left near the surface from previous passages through the solar system?  How deep can the Sun actually warm the comet?  Will the comet crack, or break apart under the gravitational and thermal stresses as it passes by the Sun?  Will a large amount of dust be liberated with the vaporizing gasses?  All of these questions play a vital role in determining how a comet will look in the sky.  A very gassy, dusty comet that dredges up material from deep within could put on quite a show.  At the same time, one that only vaporizes a thin surface layer of ice could certainly be a “dud” for the eyes (although it could still have quite a bit of scientific value!).

So what will happen in late November 2013 when Comet C/2012 S1 comes by?  No one has any clue, but we may get lucky and have a really nice comet hanging in the sky at the end of that year.  Or not.