Last year, astronomers reported that two recently discovered comets, Comet C/2011 L4 (PanSTARRS) and Comet C/2012 S1 (ISON), would become visible to observers in the Northern Hemisphere in 2013. PanSTARRS would be visible in March and ISON would be visible in November and December. But how bright would they be? Could they be seen with the naked eye? Would city dwellers be able to see them?
The last comet widely seen in North America was Comet C/1995 O1 (Hale-Bopp), which put on a spectacular display for months in spring 1997. I remember the excitement and wonder I felt when I saw Hale-Bopp back in 1997, and with that same sense of anticipation, I set out to look for Comet PanSTARRS this past March. Early reports indicated that the comet had not brightened as much as anticipated, so it would be a challenge to spot with the naked eye. In March 2013, the comet would be at its brightest, but it would also be quite close to the sun. Because of that, it would be very low in the sky after sunset, close to the horizon and caught within the glow of the twilight (and the city lights).
On March 13, conditions were as good as they were going to get for spotting the comet. According to a finder chart from Sky and Telescope magazine, the comet would be directly below the crescent moon. I walked to a nearby park with a large open field, giving me as close to a clear western horizon as I could expect in the city. Armed with my binoculars, I went out and waited until the sky darkened.
Unfortunately, I went home disappointed. The comet was coy, and my attempts to spot it were in vain. Comet PanSTARRS was just not bright enough to punch through the glow of the twilight and the city lights. By the time it got dark enough, I believe it had already set. As I scanned the sky with my binoculars, not only did I fail to see the comet, I couldn’t even see any stars—just a flat gray light washing out everything.
The brightness of comets is notoriously hard to predict. Most of the time, comets are dark objects, small clumps of rock, dust, water ice, and other frozen gases. This “dirty snowball” or “rocky iceberg” is called the nucleus of the comet. The extremely elliptical shape of their orbits keeps them far away from the sun for most of their lives, where they are almost undetectable. Some comets originate in an area beyond the orbit of Neptune known as the Kuiper Belt (extending about 30 to 50 times the distance between the Sun and Earth) and some in the as-yet unobserved Oort Cloud, a thousand times farther away than the Kuiper Belt.
As a comet approaches the sun, pockets of ice in the nucleus vaporize, giving the comet a temporary atmosphere called a coma. Radiation from the sun ionizes some of the gas, causing it to glow. The solar wind then pushes the glowing gas away, creating an ion tail. The expanding gases also loosen up the rock and dust which stream away to form a dust tail. The coma and tails make comets visible and give comets their distinct appearance.
This process is what makes comets so unpredictable in brightness. It is impossible to know exactly how much ice will vaporize and to predict exactly how a nucleus will react to solar radiation and increased temperatures. There are patterns based on the behavior of past comets, but there is a great deal of variation. Adding to the difficulty is the fact that most comets take thousands of years to orbit the sun once. Thus, upon discovering a comet, it is uncertain exactly how many times it has visited the inner solar system. The more times a comet has visited the inner solar system, the less material would remain to form the coma and tails.
Bright comets—those that can be seen even by city dwellers with the naked eye—are rare indeed. The International Comet Quarterly lists just 43 comets that have reached a magnitude of 0 (that is, as bright as the star Vega, easily seen in even the most urban areas) since 1800. Eleven of these were visible primarily in the Southern Hemisphere only, leaving just 32 for observers in North America. As the chart below illustrates, there have been no more than 4 in a single decade, and most decades have had zero bright comets.
Although Comet PanSTARRS did not pan out as a bright comet, I still have high hopes for Comet ISON. ISON could be the first bright comet of the twenty-first century for Northern Hemisphere observers. We only have to wait until November to find out.