Comets are a fascinating area of astronomy that holds a special place in the hearts of the public, not just astronomers. This fact mainly holds due to the potential for a new comet to become visible to the naked eye, a rather uncommon event. Maybe the same mechanism that keeps this fascination going in the public is the same one that makes gamblers keep going back to the poker table. Usually, you will lose at gambling, but every now and then you might win big. In a way, the same holds for comets.
Most comets that become well known do so because they come back at regular intervals and have been seen many times throughout recorded history, like Halley’s comet. But most of these frequent visitors have been around the block so many times that they’ve lost much of the ices that can make them spectacular. As a comet approaches the Sun, it heats up, and the volatile ices it contains start to “boil away.” The closer it gets to the Sun, the more active the comet becomes. Because the Sun is always blowing outward via its solar wind, these “boiling” ices are blown outward, away from the Sun, and form a tail.
Many people don’t realize that there are thousands of known comets. On a given night, a good telescope has a fair chance of finding one of these comets, which look more like faint smudges against a black sky. In the mid-90s, famous comets like comet Hale-Bopp and comet Hyakutake rekindled public excitement as they both became obvious naked-eye objects. Comet Hale-Bopp was a newcomer to the inner solar system and drew attention early on due to its apparent hyperactivity even when it was beyond Jupiter. Excitement grew as this activity was maintained, and the comet became a media favorite because of its highly photogenic nature.
Comet Hyakutake, on the other hand, was only impressive because it came relatively close to Earth, causing a rapid increase in brightness and an apparently long tail, reported to be up to 80 degrees long in darker night skies!
These two exceptional comets seemed to finally break the losing streak of comets that had been going on since 1973, when comet Kohoutek “promised” to be a great comet in the media and became a spectacular dud when it finally arrived in the inner solar system. After this, the media seemed much more hesitant to publish early predictions out of fear of getting a pie in the face. There’s a reason famous amateur astronomer and comet hunter David Levy was quoted as saying, “Comets are like cats: they have tails, and they do precisely what they want.”
Well, here we are in 2012, and recent media reports are starting to focus on C/2012 S1 (ISON). Why the obscure name? Comets rarely have simple names. Simple names are ambiguous, since comets are often found by automated surveys or astronomers that have discovered many. Naming a comet after its discoverer can lead to an ambiguous reference. When a comet has an ambiguous name, a reference to it using that designation usually refers to the most recent/brightest/etc. comet, but it’s hard to say. Astronomers want to be more precise. “Comet ISON” is named after the survey that discovered it, the International Scientific Optical Network. The “C” at the beginning of the name means “non-periodic” and is part of why it’s getting early media attention. It may be a first-time visitor to the inner solar system. It has a highly eccentric orbit, perhaps carrying it out to the hypothetical Oort cloud. It could be loaded with volatile ices!
But it’s difficult to infer very much from the name. The orbit of this new comet is what has attracted early attention. Not only is the orbit highly eccentric, but it will come amazingly close to the Sun, on the order of only one million miles away! Keep in mind that Mercury, on average, is 37 million miles from the Sun. When a comet gets that close to the Sun, it is often referred to as a Sun grazer and can lead to spectacular results. Its ices “boil” away furiously and can lead to a breakup of the comet. At this point we only know that the comet will get close to the Sun, which raises the odds of a good performance. It could break up well before its closest approach to the Sun and fizzle before it has had a chance to perform. I’m not putting my chips down on the table yet, but it’s always fun when astronomy makes it into the headlines. Though you have to be careful reading media reports, since they can often have misleading headlines and contain “bad astronomy,” all for the sake of a good story.
How does comet ISON compare to famous comets of the past? One interesting comparison is to the “Great comet of 1680.” Both comets have a similarly shaped orbit and come from roughly the same direction:
One hypothesis is that comet ISON may have broken off of the Great comet of 1680 and taken a slightly different path. Hard to prove this, but if so, maybe comet ISON will perform as well as the Great comet of 1680. Artists’ renditions of the Great comet of 1680 might help raise your hopes.
Will comet ISON be the next “Great” comet? Or the next “Kohoutek”? Place your bets.