Although just spotted a few days ago, Comet ISON (C/2012 S1) has the potential to become a very bright object that will be well placed for viewing in late 2013.
Faint, distant comets get discovered all the time, usually by robotic telescopes that sweep up huge swaths of sky every clear night. Most come and go quietly. But a new find made on September 21st by a pair of amateur sky sleuths has the astronomy world atwitter (in this word's traditional and modern connotations) with the prospect that it could become very bright late next year.
It was first spotted as a faint, 18.8-magnitude object in images taken by Vitali Nevski (in Belarus) and Artyom Novichonok (in Russia) using a 16-inch (0.4-m) reflector that's part of the worldwide International Scientific Optical Network (ISON). "We could not be certain that it was a comet," Novichonok explains, "because the scale of our images is quite small [2 arcseconds per pixel], and the object was very compact."
The next night they confirmed its cometary nature using the larger reflector at Majdanak Observatory in Uzbekistan, but by then other astronomers had done likewise. According to naming conventions established by the International Astronomical Union, that one day of uncertainty led to the comet being generically named "ISON" instead of "Nevski-Novichonok". Its formal designation is C/2012 S1.
Naming uncertainties aside, we already know a lot about this object, thanks to two sets of prediscovery images dating to last December and January. The comet is inbound from the Oort Cloud and will pass very close to the Sun — just 725,000 miles (1.2 million km) from its white-hot photosphere — on November 28, 2013. Before then and thereafter (if it survives perihelion), Comet ISON could put on a spectacular show.
In fact, the geometry could make C/2012 S1 a "dream comet," as one eager skywatcher has commented, because it will swing just 40 million miles (0.4 astronomical unit) from Earth a few weeks after perihelion, when it will be high in moonless, northern skies after sunset. Initial predictions by the IAU's Minor Planet Center suggest that Comet ISON could peak at magnitude –10 or brighter at perihelion (when it will be just 1° from the Sun), and that it could remain visible to the unaided eye from early November to the first weeks of 2014.
An added bonus is that the comet passes very close to Mars in early October 2013 and could potentially be observed by the sensitive Mast Cameras on the rover Curiosity.
Of course, a lot can happen in the coming year. I'm old enough to recall how the much-heralded Comet Kohoutek (C/1973 E1), whose apparition was over-hyped, ultimately proved very disappointing (read: "complete dud"). And yet, Comet ISON has "turned on" very early — it's still 6¼ a.u. from the Sun, well beyond Jupiter's orbit. Moreover, its orbit bears striking similarity to that of the Great Comet of 1680, a dazzler with a very long tail so bright that that reportedly could be seen in daylight. Dynamicists are wondering whether that object and Comet ISON are fragments of the same parent body.
Regardless, it's been a long dry spell since Comet Hale-Bopp (C/1995 O1) put on its long-running show in 1997 — though Comet 17P/Holmes briefly caught our eye in 2007). Now prospects are good for beautiful appearance from not one but two celestial visitors next year. Comet Pan-STARRS could provide the warm-up act in March, followed by headliner Comet ISON eight months later.