Spectacle or speck?
OK, I stole that phrase. But it’s just too good not to use.
Comet ISON will very soon be upon us. The image above was taken with the Hubble Space Telescope when ISON was a little over four times as far away from the sun as is the earth. Starting in mid-November, it will get ever lower in the eastern sky just before dawn. If it survives its passage very near the sun, we could see it again in early December. But will it be the “comet of the century” promised earlier this year? Or will it join the infamous Comet Kohoutek and numerous other over-hyped comets as a disappointment for the general public? At this point no one really knows, although prospects for the former are—ahem—fading.
The main reason for the uncertainty is that ISON is a “new” comet, one that is on its first trip into the inner solar system. Comets that have passed around the sun multiple times are more predictable–not only because there may be historical records, but for a less obvious reason. They are pristine and thought to be coated with very volatile icy material. Originating from a cloud of icy objects almost a light year away from the sun, these comets have material that never gets warm enough to vaporize. Once something sends them plunging toward the sun (it doesn’t take much since they are so weakly bound gravitationally), that outer layer warms, vaporizes, reflects sunlight and causes the comet’s brightness to sharply increase. Once that layer is gone however, what lies beneath may or may not be all that volatile and all that bright.
The classic description of a comet is a “dirty snowball”. The icy volatile material should not be thought of as water ice only, although that is certainly the major component. Astronomical “ices” include solid carbon dioxide, carbon monoxide, methane and ammonia. The “dirt” component includes dust grains of the sort seen throughout the solar system, and complex organic compounds as well. The relative abundances of ice and dirt mean that some comets might more accurately be described as “icy dirtballs”. Which is ISON? We don’t really know yet.
Most really spectacular comets are not particularly predictable. Their orbits are, once we have tracked them over several weeks. But exactly how bright they will be just reflects how little we understand about the composition and structure of comets. The ones we have observed close up with spacecraft show a good bit of variety. My personal favorite of the last few years is Hale-Bopp, which appeared in early 1997. It was quite bright and beautiful, illustrating very well the two separate tails of a comet: the dust tail (white) and the ion tail (blue). (See this link on how comets work.)
More recently, southern hemisphere observers were treated to the spectacular Comet McNaught in early 2007, while those of us farther north had to be content with images like this one.
So what DO we know about ISON? Here are some images from several angles and scales showing its orbit next to those of the planets. The comet’s orbital path is blue; the viewer from which these images were taken is here. You can manipulate the image in any of several ways. This is one of my favorite web sites!
The times shown here are Eastern Standard.
When and where do you look? When: in the morning about an hour before sunrise, starting in mid-November. The comet should brighten as it drops lower in the sky and gets ever closer to the sun. Where: in the east, above where the sun will rise.
IF the comet emerges intact from its close encounter with the sun, it will re-emerge into our view in the first few days of December, reversing its November behavior and fading as it moves higher in the sky and away from the sun. An hour before dawn is still when and east is still where.