Bright comets are rare, but 2013 might prove exceptionally plentiful in that regard. No less than three of these celestial visitors are expected in the Sun’s vicinity over the coming months — all potentially visible to the naked eye. But comets are capricious, and while these seem promising, they could wind up disappointing…
The word "comet" evokes images of ghostly interlopers displaying their diaphanous tails across millions of kilometers of interplanetary space. At the heart of each of these gossamer objects there lies, none-the-less, a solid core the size of a mountain. This core, while small on a cosmic scale, consists of a mixture of dust and a multitude of volatile compounds (water, ammonia, methane, carbon dioxide) frozen by the extreme cold of space. Upon approaching the Sun, these ices vaporise creating an immense gaseous cloud that envelopes the nucleus: This is the coma. The gaseous coma glows because of ultra-violet radiation from the Sun, while dust, liberated from the icy nucleus, trails behind the comet forming its characteristic tail.
PanSTARRS in the March sky
PanSTARRS (C/2011 L4), the first comet to pay us a visit this year, was discovered in June 2011. Back then it was beyond Jupiter, but it will pass within 45 million kilometers of the Sun on March 10, 2013. Before then the comet will only be visible from the southern hemisphere. But on the evenings following that date, it will make an appearance in northern-hemisphere skies: Look for it low on the western horizon as night falls, about 40 minutes after sunset.
March 12 – a date to remember: At twilight, PanSTARRS will be just five degrees (a binocular field-of-view) left of the crescent Moon. By the following evening, the comet will have moved slightly to the right, and the Moon will be higher in the sky.
Will PanSTARRS be easily visible at nightfall? The brightest part, the coma, might be bright enough to see with naked eye, in the dying glow of twilight. However, the tail will most likely be visible only in binoculars or a small telescope. Of course, a cloudless, unobstructed view of the western horizon will be essential.
From evening to evening, the comet will gradually distance itself from the Sun while it slips slowly northward, providing a chance to observe it against darker skies before it disappears below the horizon. However, as it gains altitude it also loses brightness. After ten days or so, PanSTARRS will have faded substantially, and the nearly-full Moon will impede observations.
A comet’s activity depends on the size of its nucleus and how close to the Sun its orbit carries it. But another factor sometimes enters the equation: the number of times it has passed close to the Sun. PanSTARRS is on its first passage; it is arriving directly from the distant Oort cloud — a vast region surrounding the solar system that’s home to billions of comet nuclei. We often observe that “fresh” comets have a tendency to liberate volatile compounds that cover their nuclei while they are still far from the Sun, the moment they encounter the slightest solar heat. This premature outgassing can lead to an overestimation of the comet’s potential visibility. Unfortunately, the volatility and brightness of “first-time” comets have a tendency to reach a plateau rather than continue to brighten as they near the Sun.
Recent observations indicate that this is happening: PanSTARRS will not be as bright as previously hoped. But surprises are always possible. The comet will get closer to the Sun than the planet Mercury, and no one can predict what will happen when its nucleus is subjected to the intense radiation of our star.
Comet Lemmon: A pleasant surprise?
A second comet discovered in March 2012, named C/2012 F6 (Lemmon), will pass our way in the coming weeks. Comet Lemmon will be closest to the Sun on March 24, but only observers in the southern hemisphere have the opportunity to see it at maximum brightness. It should be visible from northern hemisphere, during the third week in April, low in the east at dawn. But by then it will have faded considerably...
Waiting for ISON
If PanSTARRS and Lemmon don’t meet expectations, at least they’ll have served as a warm-up for the comet show of 2013, happening at the end of the year. Arriving from the Oort cloud as well, comet C/2012 S1 ISON will pass within a million kilometers of the Sun’s scorching surface on November 28. If its nucleus survives the encounter, it could remain easily visible through the evenings of December, unfurling a beautiful tail across the sky. Astronomers around the globe are anticipating a spectacular show. Unless...
As astronomer David Levy (who has 22 comet discoveries under his belt) likes to say: "Comets are like cats: They have a tail, and they do precisely what they want."