For professional astronomers, amateur stargazers, and anyone who has ever been captivated by a particularly bright shooting star, 2013 is shaping up to be one for the record books.
Two separate incoming objects have the potential to become so-called ‘Great Comets’ this year, a phenomenon that tends to occur only once per decade. Comet Pan-STARRS will cross the skies of the northern hemisphere in mid-March, and Comet ISON in late November. Many astronomers expect ISON to outshine 1997’s Hale-Bopp, and some say it could even become the brightest and grandest of all ‘Great Comets’ in history.
While no official definition exists for what qualifies as a ‘Great Comet’, there have been several historical instances when a celestial object shined so brightly and for so long that viewers across the planet were sufficiently impressed to record the event for posterity– including the Great Comets of 1680 and 1811.
In more recent memory, 1996’s Comet Hyakutake and 1997’s Hale-Bopp lit up the darkness above the northern hemisphere, and in 2007 Comet McNaught dazzled stargazers in the southern hemisphere. None of these met the ‘know it when you see it’ requirements of a true ‘Great Comet’, however.
Now two inbound objects have the potential to achieve that coveted status, both in the span of a single year, and both in the skies of the northern hemisphere.
Comet Pan-STARRS, the first candidate, was discovered in 2011 by the Panoramic Survey Telescope in Hawaii. The comet should first be visible in southern skies by January of 2013 as it nears the sun and begins to reflect its light. By mid-March it will be visible from the north as it reaches its brightest point and makes its closest approach to the Earth– within a distance of 102 million miles.
Comet ISON, the second candidate, is even more favored by astronomers to become a ‘Great Comet’ for the history books. Discovered on September 21, 2012 by two amateur Russian astronomers, ISON is expected to make a close shave past the sun at only 750,000 miles, before approaching the Earth at an almost-threating 40 million miles. The comet should be visible before passing the sun on November 28, and then in both the morning and evening sky on November 29.
When first discovered using a 15.7-inch reflecting telescope from Russia’s International Scientific Optical Network (ISON- hence the name), the object was 625 million miles from Earth and 584 million miles from the sun, situated (from Earth’s perspective) within the zodiacal constellation of Cancer, according to Fox News.
At the moment, ISON appears to be mingling with the stars of Gemini, while it is in fact nearing the orbit of Jupiter at a distance of 474 million miles and closing. On October 1 the comet will pass just 6.5 million miles from Mars, which will allow NASA’s Curiosity rover to snap some pictures from the surface of our planetary neighbor.
By the end of October, ISON should be visible from your backyard using a pair of binoculars. As November brings the comet closer to the sun, it will become significantly brighter as its tail begins to form. Gareth Williams of the Smithsonian Astrophysical Observatory has calculated that on November 28 the comet will hurtle around the sun at a speed of 425,000 mph, coming within just 732,000 miles of the solar surface and experiencing temperatures up to 2 million degrees Fahrenheit.
Comets consist of a nucleus of ice and dust, and begin to develop a ‘tail’ of gas and dust particles as they near the sun. The major factors influencing how brightly they shine from Earth’s perspective are the distance of their approach to the sun (generating the tail), and their distance from our planet. ISON is on track to make ideal passes in both regards– if our star doesn’t consume it first.
Popular astronomy author Guy Ottewell of the UK is enthusiastic about ISON’s prospects. In his 2013 Astronomical Calendar, Ottewell envisions the comet as resembling “…a lighted match at the sun’s edge,” during the day. “Using what formulas we can for magnitude, we have it reaching -12.6, the brightness of the full moon!” Since the late 17th century, there have been only nine other occasions when a comet has shone brightly enough to be viewed in broad daylight.
To keep our own expectations reasonable, one must consider that comets often prove notoriously unpredictable regarding their tail size and brightness. Many factors can affect their orbits, and depending on their composition and ultimate proximity to the sun they run the risk of disintegrating before becoming visible from Earth.
Calculations improve astronomers’ certainty as we get closer to the actual transit dates, however, so check back here for updates as March and November draw closer. And keep your fingers crossed for what could be the show of a lifetime.