So far, Comet ISON has been a bit of a puzzle. Discovered in September 2012 by two Russian astronomers, ISON (an acronym for the International Scientific Optical Network near Kislovodsk, Russia, which made the find) was extremely bright, given its distance at the time far beyond the orbit of Jupiter. That led some scientists to predict that Comet ISON might be so bright by the time it got to Earth that it could be visible even in daylight.
Over the past few months, however, Comet ISON proved once again just how variable comets can be. It hasn't brightened as much as original predicted, perhaps because it has fewer volatiles to be vaporized by the sun, or perhaps because it is smaller than expected, or made of different materials.
This week, Comet ISON passed just 6.5 million miles from Mars. It was too dim to be seen by NASA's Mars rover Curiosity, but the agency's sharp-eyed Mars Reconnaissance Orbiter managed to take a shot, MRO project scientist Richard Zurek told Discovery News.
ANALYSIS: Mars Orbiter Spies Lackluster Comet ISON