Comet of the Century? - A Series of Articles about Comet ISON
By Pecier Decierdo
Because Comet ISON is a sungrazer that is larger than the lovely Comet Lovejoy, some scientists project that it will be one of the brighter comets of the last few years. This led some writers to prematurely hail it as a possible “comet of the century”. Some even claimed that it might rise to become brighter than the full moon come late December, a claim that is almost certainly false.
On the opposite end of the spectrum of optimism, there are people who proclaimed that ISON has already “fizzled”, that it is going to be a failure. They compare it with Comet Kohoutek of 1973, a comet that was also hailed as a “comet of the century” but failed to live up to the hype. Like ISON, Kohoutek is a first-time visitor bringing virgin ice and dust into the inner Solar System. And some claim that ISON, like Kohoutek, will be a dud, another failure.
Comet experts, however, advise caution to both daydreamers and naysayers. While the universal law of gravitation strictly dictates the paths of comets, the great variability in comet composition and structure means that it is difficult to predict the brightness, size of coma, length of tail, color of ion clouds, and other properties of a newly observed comet. This fact about comets lead astronomer David Levy, co-discoverer of the famous Comet Shoemaker-Levy 9, to say, “Comets are like cats: they have tails, and they do precisely what they want.”
What makes the behavior of comets hard to predict? To answer this question, we first need to take a closer look at the heart of a comet, the nucleus. Comets have often been described as dirty snowballs, although some astronomers prefer calling them snowy dirtballs. This is especially true when they are in the outskirts of the Solar System. As they approach the Sun, especially after they cross what is known as the frost line, the Sun blowtorches the outer layer of these snowy dirtballs making them release gas, dust, and streams of ions. It is when a comet is blowtorched that it becomes a marvel of the heavens. The frost line is an imaginary boundary around the Sun within which the Sun’s heat is enough to cause ice to sublimate efficiently.
The amount of gas and dust released by a comet, and therefore its brightness, depend on many things. One factor already mentioned is the comet’s distance from the Sun; the closer a planet is to the Sun, the more dust it will potentially release, and the brighter it may become.
Another important factor is the size of the comet’s nucleus. Not all nuclei are created equal. Some nuclei can be a dozen kilometers wide while others can be less than a kilometer in size. Halley’s Comet is around 11 km wide while Comet Lovejoy is only around 200 meters across. The Comet Hale-Bopp, with a nucleus estimated to be 60 km from end to end, is considered a giant.
With a nucleus having a diameter of more than 4.5 km, Comet ISON is significantly bigger than Comet Lovejoy. This means that Comet ISON has potentially more gas and dust to release.
Another important factor in Comet ISON’s chances of being a spectacle is its survival. As already mentioned, not all sungrazing comets survive their solar swing-by; some disintegrate before they even get the chance to swing past the Sun. Comet Ikeya-Seki, another spectacular sungrazer, survived perihelion, but not in one piece; it broke into three pieces around the time of its perihelion.
Comet ISON is very likely to survive its swing-by. Comet Lovejoy, a comet much smaller than Comet ISON, survived perihelion. Not only that, Comet Lovejoy also came much closer to the Sun than ISON will. More than two times closer, in fact.
A final factor affecting a comet’s brightness is the structure and composition of its nucleus. This is where the element of unpredictability comes in. Scientists are just beginning to realize the amazing diversity of chemical composition of the bodies in the Kuiper Belt and Oort Cloud. But as we learn more and more about the composition of comets, we realize that the kinds of substances they contain dramatically alter their brightness and color. Especially important to the brightness of comets is the amount of volatiles they contain. Volatile substances are chemicals that easily evaporate into their gaseous form. The more volatiles a comet’s nucleus has, the brighter it can get as it approaches the Sun.
Given our current knowledge and technology, science has little in the way of predicting how bright a comet can get. Perhaps in the future when our technology will allow us to determine the amount and kinds of volatiles in a comet while it is still far away from the Sun, then we can predict with precision how spectacular a comet can get. In the meantime, the best we can make is give forecasts that increase in precision as the comet approaches the Sun.
When it comes to Comet ISON, the current forecast shows that the optimists and pessimists were both wrong. Comet ISON will be a bright comet, and it may even be visible via the naked eye. But it won’t nearly be as bright as a full moon, either.
Nonetheless, Comet ISON will still be the most observed comet in history thanks to the hundreds of thousands amateur and professional astronomers who watched its progress ever since its discovery. And because of its other genuine claims to fame, the close study of Comet ISON will dazzle the astronomical community far more than mere visible brightness ever will.