WNC Skies – January 2013
By Calvin L. Chrisman
There is currently much excitement in the astronomical community about two comets that are expected to put on a striking show during 2013. In March, our skies will be visited by a comet that has been named PanSTARRS (after an automated sky survey being conducted by a large telescope in Hawaii). Then, in December, a comet called ISON (after the International Scientific Optical Network that was involved in its discovery) might put on one of the most significant displays in recent history. Please note the “might”. The well-known comet hunter, David Levy, once said: “Comets are like cats; they have tails, and they do precisely what they want.” These comets could brighten and become spectacular sights as they approach the Sun or they could fizzle and fade as many have in the past. Much depends on the shape of their orbit and whether they have passed this way before. But first, what is a comet?
The simplest characterization of a comet is that it is a dirty snowball. Comets are made up of frozen gasses and rocky material and are usually a few miles in diameter. They generally originate in the Kuiper Belt (the location of the erstwhile planet, Pluto) or much farther out in the Oort cloud. If they approach the Sun on an elliptical orbit, they are said to be periodic comets, meaning they pass the Sun on a regular cycle that can range from a few years to hundreds of thousands of years. If they are on a hyperbolic orbit, they pass only once. This is what makes their brightness so hard to predict. As a comet approaches the Sun; its frozen gasses heat up and cook off producing a tail or coma. If it is the comet’s first visit to the Sun, it may be unstable and its components may disintegrate and disappear from the sky. This has happened on several occasions in recent years, including Comets Kohotek and Elenin. If, on the other hand, it has survived previous visits and is more stable, it can produce striking brightness as it passed by the Sun.
Comet PanSTARRS is on a slightly parabolic orbit, meaning this is its first dance with the Sun, and so its stability is unknown. If it does not fade and disintegrate as it approaches, it will pass within 0.30 astronomical units (a.u.) of the Sun. An astronomical unit is the distance from the Sun to the Earth. If PanSTARRS is fairly stable, predictions are that it could brighten to zero or greater magnitude through the middle of March. It will be visible low in the western sky during this time. Check back for more on this next month.
Comet ISON is still 6 a.u. from the Sun and is very faint at this time. It will likely be autumn before its orbit is calculated with accuracy but there is a possibility that it is on a highly elliptical orbit and so may have passed the Sun previously. There appear to be similarities between its orbit and that of the Great Comet of 1680. If they turn out to be related, ISON could be truly breath taking. Current speculation is that, during December, 2013, ISON could brighten to magnitude -1 or -2 (very bright) and its tail could reach as much as 40 to 60 degrees. This remains to be proven, but the anticipation is keen!