These balls of ice and rock abound in our solar system. There are more than 4000 known comets, including 500 that are short-term, meaning they orbit the sun in periods of less than 200 years. The number of unknown comets is immeasurable. Short-term comets likely come from the Kuiper Belt beyond Neptune's orbit, a field of icy objects that includes Pluto, while long-term comets are believed to originate from the solar system's Oort Cloud, a mysterious band that is 50,000 times further from the sun than the Earth is.
But great comets?the spectacular celestial bodies that shine so brightly they catch the eyes of people not even looking for them?are much rarer. Some, like Haley's Comet, might be seen twice in a lifetime if you're lucky. Others, such as Comet Ikeya-Seki, also known as the Great Comet of 1965, don't complete their solar orbits for thousands of years.
"There have been only 32 great comets in the past 1000 years," says Dennis Young, a Sedona, Arizona-based professional field astronomer with more than 35 years or experience, "indicating that we might expect an exceptional comet on average only three times per century." That's why the possibility of seeing two great comets in 2013?Comet PANSTARRS and Comet ISON?is such a big deal.
For a comet to be truly visible to the naked eye, three things have to happen. First, the comet has to come within close proximity to the earth?ISON is expected to come within 40 million miles. Second, it must have a large nucleus that ejects lots of gas and dust for us to see. The more it emits, the brighter the comet. Third, it must get as close as possible to the sun, because the closer it comes, the brighter it will be. With so many prerequisites it's no wonder great comets are few and far between.
While there's no guarantee of a spectacular sighting in either case?comets have a way of fizzling out at the last minute?both of the great comets slated for our celestial hemisphere in 2013 look extremely promising. The first great comet to appear this year will be Comet PANSTARRS, a nonperiodic comet?meaning this is a once-in-a-lifetime event. The comet was first discovered in June 2011 by Hawaii's Panoramic Survey Telescope and Rapid Response (PANSTARRS) project from which it draws its name. At first the astronomers assigned it an apparent magnitude of 19 (the higher the number, the dimmer it appears.) A magnitude of 19 is too dim to be seen without a telescope, but scientists revised their rating of PANSTARRS to an apparent magnitude of 13.5 by May 2012. It has grown in brilliance since then and should be clearly visible to the naked eye in March. Astronomers originally predicted that it may even reach an apparent magnitude of 0, which is as bright as Vega, the second brightest star in the northern celestial hemisphere, though they've since adjusted their estimates to a magnitude of 1.
The second comet, C/2012 S1 (ISON), was discovered this past September by two amateur astronomers in Russia. It's what's called a sungrazing comet, meaning it will approach so close to the sun that smaller comets would evaporate at the distance. "That this could be the brightest comet anyone alive has ever seen is a serious statement," says Young.
At the end of November, ISON will reach perihelion, or its closest point to the sun. It is expected to come within less than 750,000 miles of our star's surface?pretty much brushing against it. At this point it might also be visible during the day, and some astronomers predict that it may even outshine the moon.
Yet comets, even great ones, are unpredictable. They can brighten, fade, or often break up into fragments for reasons astronomers don't entirely understand. This year's night sky may indeed put on a show for the ages. Or it may simply fold into history. We're hoping for the former.
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