"The object was slow and had a unique movement. But we could not be certain that it was a comet, because the scale of our images are quite small and the object was very compact," Artyom Novichonok, wrote on a comets mailing list hosted on Yahoo.
Follow-up observations as well as a search of archived images of the area confirmed the discovery, which was officially reported on Sept. 24, three days after Novichonok and Vitali Nevski found the object far beyond Jupiter's orbit.
The International Astronomical Union's Minor Planet Center predicts Comet ISON could be visible without binoculars or telescopes to skywatchers on Earth from early November through the first few weeks of January 2014.
NASA's Mars Curiosity rover also may get a look when the comet sails past the red planet in early October.
The comet's journey likely started in the Oort Cloud, a cluster of icy rocks that circle the sun about 50,000 times farther away than Earth's orbit. Comet ISON is expected to pass as close as 700,000 miles, or 1.1 million kilometers, from the sun on Nov. 28.
If it survives, the comet could be the brightest to appear in Earth's skies since 1965 and could even be visible in daylight.
Image: Color-enhanced view of Comet ISON photographed at the RAS Observatory near Mayhill, NM on Sept. 22, 2012, by amateur astronomers Ernesto Guido. Credit: Remanzacco Observatory/Ernesto Guido, Giovanni Sostero and Nick Howes