In the weeks ahead, some of your friends and relatives may be asking you where they should look for “the comet of the century.” After you explain to them that the Sun knocked the stuffing out of the comet when it passed closest to our star on November 28 and that there’s nothing left — certainly nothing visible in binoculars or a backyard telescope — you can expect the next question: “But I thought it was supposed to be as bright as the full Moon!”
This widely disseminated “fact” was initially published in the London (U.K.) newspaper The Telegraph shortly after the discovery of ISON, on September 21, 2012, and was quickly propagated by newswire to media outlets worldwide. As recently as mid-November, the same newspaper was still proclaiming: “The comet will become dazzlingly bright, rivalling the brightness of a full Moon, as it streaks across our sky.”
Everything in that last prediction is either false or an exaggeration. The “bright as a full Moon” part was one astronomer’s estimate of the comet’s possible brightness at its closest approach to the Sun on November 28, 2013. But what the news stories didn’t mention was that on that maximum brightness date, the comet would be invisible to viewers on Earth because it was so close to the Sun that only spacecraft could see it. In any case, astronomers already had revised the brightness estimate downwards months before.
Even so, ISON might still have been an impressive sight, but like many sungrazing comets that have come before, it succumbed to the blowtorch blast of the Sun’s intense heat as it passed within one solar diameter of the Sun’s surface during perihelion. Although hopes were high that the comet would survive the hairpin solar turn, no one really knew for sure what its fate would be. As a “fresh” comet on its first trip to the inner solar system from the Oort cloud (the cloud of cometary objects that extends to about one light-year from the Sun), fragmentation and rapid vaporization of the comet’s one-kilometre-wide icy nucleus were always a possibility.
To the disappointment of observers watching spacecraft images, the last remnant of ISON on December 1 was a fan-shaped debris cloud of small fragments dispersing into interplanetary space. The video below shows the complete transformation:
ISON seemed to completely disappear just before its closest approach to the Sun, then reappeared in spacecraft images, zombie-like. Astronomers attribute this to the release of gas and dust as the icy nucleus fragmented and dispersed. A fuller picture of what happened will emerge when cometary scientists examine the spacecraft data in the coming weeks and issue a coroner’s report on how this potentially great comet died.
Part of the difficulty in predicting a comet’s brightness stems from how little we know about these visitors from the frigid, distant Oort cloud. Most comets we have seen have swept through our part of the solar system before. Take Comet Hale-Bopp, for instance. When it was first picked up on its 1997 return, it was about the same distance as ISON was when it was discovered. Yet because Hale-Bopp was a returning comet, it followed early predictions well. But ISON had two trapdoors for astronomers: It was a first-timer and a sungrazer. It turned out to be an unpredictable visitor from the deep freeze of the Oort cloud.