(This 1145th Buffalo Sunday News column was first published on March 3, 2013.)
Missing from the astronomical calendar I wrote about earlier this year was the possible appearance of two comets, one of which may represent a spectacular event. Several readers called my attention to that omission which I address in this column.
First, let's agree on what a comet is. It is a nucleus of ice and rock that ranges in diameter from a few hundred meters to several kilometers. When they come within sight their local atmosphere (called a coma) appears white and they also often sport a white tail. It is this coma and tail that differentiate comets from asteroids.
I used to think that the tail followed the comet as it sped through space but that is not true: the comet's tail points directly away from the Sun. It can stretch for hundreds of millions of miles. Although we usually notice only one tail, there are actually two: the most apparent is a dust tail, the other is an ion tail made up of gasses.
Comets are attracted by the Sun's gravity and most, like Earth, follow elliptical paths around the Sun. The comet's path, however, stretches far out into outer space and thus nears the Sun rarely. The most famous, Haley's comet, does so only every 75 years.
Mark Twain was born the same month as the passing of Halley's comet in November 1835. He vowed in his autobiography that he would "go out" with its next pass. Halley's comet returned in April 1910 and Twain did indeed die that month, the day after it passed closest to the Sun. The next return of Haley's comet in 1986 was a disappointment, however: it was barely visible.
Comets are not rare. In fact over 4000 have been identified and millions more are estimated to be in the outer reaches of our Solar System. But only about one a year is visible to the naked eye and those only dimly. The most recent widely observed comet was Hale-Bopp, which was seen for 18 months, coming closest to the Sun in April 1997.
This year two already identified comets could not only be visible but might be the brightest in many years. Notice that I said "could" as David Levy tells us, "Comets are like cats; they have tails and they do precisely what they want." Often as they approach the Sun their visibility is markedly reduced as they are broken up by the force of solar gravity.
The first comet this year is variously known as Comet C/2011 L4 to astronomers, Comet Pan-STARRS for the discovering team (Panoramic Survey Telescope and Rapid Response System) or Comet Wainstock for Richard Wainstock, the Pan-STARRS member who first confirmed its existence. Naming comets is overseen by the Committee on Small Body Nomenclature of the International Astronomical Union. This comet was first seen in 2011 and has been followed with powerful telescopes since then.
Comet Wainstock is already visible in the Southern Hemisphere. A recent report from observers there says that it shines at about magnitude 4.5 and it might reach 1st or 2nd magnitude. (Magnitude 4.5 is the brightness of the dimmest visible stars; 1st or 2nd magnitude is a little brighter than the Big Dipper stars.)
On March 10 this comet will pass the Sun and, assuming it continues to be visible, it will appear in the evening sky near where the sun has set. It will be best seen then, but it will continue to be visible through mid-April.
The second comet is named Comet C/2012 S1, Comet ISON (for the International Scientific Optical Network) and Comet Nevski-Novichonok for its Russian discoverers, Vitali Nevski and Artyom Novichonok.
Comet Nevski-Novichonok could prove truly spectacular. It is scheduled to be first visible to the naked eye in early November in both the Northern and Southern Hemispheres and will pass very close to the Sun on November 28. If it survives this encounter, it could be brighter than the full moon and even be visible in the daytime sky. It could continue to be visible until January 2014. This comet will also be seen first near where the Sun has set.
This could be an exciting year not for only astronomers but for the rest of us as well.