Comet ISON now an ex-comet, says NASA (+video)
After months of anticipation, Comet ISON grazed by the sun on Thanksgiving Day (Nov. 28). Something emerged on the far side, but NASA astronomers can now confirm: ISON is no longer a comet.
Comet ISON sprang into public awareness shortly after its discovery, when its early brightening inspired hopes that it would blaze like the full moon. Quickly dubbed the "Comet of the Century," ISON continued its plunge from the Oort Cloud to the sun, but despite predictions, it failed to brighten much. It buzzed by Mars, where NASA's HiRISE observed it, then it passed through the orbits of all the inner planets before skimming the sun on Nov. 28.Skip to next paragraph
Subscribe Today to the Monitor
Scientists were riveted. Would ISON's ices melt completely in the sun's heat? Would it get drawn in by the sun's gravity? Or – as amateur astronomers everywhere hoped – would it survive, achieve its early promise, and light up the sky?
RECOMMENDED: Are you a space whiz? Take our quiz!
ISON got lost in the sun's glare, ultimately passing less than 1.2 million miles from the sun's surface. Even sun-observing instruments couldn't keep track of it at the most critical moment, since they block out the brightest part of the sun to protect their instruments.
Within hours, astronomers saw something faint emerge from the other side of the sun-blocking disk. Maybe ISON hadn't broken up completely? A glowing cloud continued ISON's parabolic path before dissipating so completely that it's now indistinguishable from interstellar dust, says NASA.
"We see the comet going in, and the object formerly known as ISON emerging from the other side," joked astrophysicist Karl Battams at a meeting of astronomers last week.
The Comet ISON post-mortem
"The comet essentially eroded to nothing," explains Zdenek Sekanina, a senior research scientist at NASA's Jet Propulsion Laboratory.
In fact, says Dr. Sekanina, ISON had ceased to exist as a comet even before it reached "perihelion," the moment of closest approach to the sun. So what happened to ISON, known to scientists as C/2012 S1?
Sekanina has monitored the comet closely for months, comparing it to past comets to try to understand its behavior..
As ISON swept through our solar system, it passed through four or five "cycles" of brightening and dimming, he found. Each cycle corresponded to a source of some ice (or a mixture of ices) sublimating into gas. First was probably carbon monoxide and/or carbon dioxide, he says, converting from dry ice to gas. Then came other ices in turn, until the most stable, water ice, began to turn to water vapor around the time ISON crossed the orbit of Mars.
By mid-November, the nucleus – the "dirty snowball" of ices, dust, and rock that make up the core of the comet's glowing head – had started to fragment severely, says Sekanina. "Parts of the interior became exposed to sunlight," he says, accelerating the rate of ISON's demise. By Nov. 21, one week before perihelion, scientists recorded a huge increase in water vapor production. "At that point," he says, "the nucleus was fragmented into multiple pieces."