[meteorite-list] Comet ISON Leaves A Mystery Behind As It Goes Around The Sun

[Ron Baalke]

http://www.nbcnews.com/science/comet-ison-vanishes-puff-mystery-it-goes-around-sun-2D11670914

Comet ISON leaves a mystery behind as it goes around the sun
Alan Boyle, NBC News
November 28, 2013

Comet ISON - once touted as the "comet of the century" - fizzled out during 
its swing around the sun, leaving behind what scientists said was a trail 
of dust that continued rolling through space.

"It does seem that Comet ISON probably hasn't survived this journey," 
Karl Battams, an astrophysicist at the Naval Research Laboratory, acknowledged 
at the end of a NASA-sponsored Google+ Hangout SaveFrom.net that attracted 
more than 27,000 viewers at its peak.

Battams' assessment dashed the hopes of millions who were looking forward 
to a celestial Yuletide treat. Satellite images appeared to show ISON's 
remnants spreading out in an arc around the sun - a phenomenon known as 
a "headless tail."

It's still possible that the initial reports of ISON's demise were exaggerated. 
"It is now clear that Comet ISON either survived or did not survive, or... 
maybe both," Bruce Betts, director of projects for the Planetary Society, 
said in a Twitter update. "Hope that clarifies things."

In a follow-up tweet, Battams said he and his colleagues have observed 
a couple of thousand sungrazer comets, but "we've never seen one behave 
like ISON."

Highs and lows

Over the past few days, ISON's condition had sparked waves of up-and-down 
speculation: Was it brightening? Fading? Resurging? On Thursday morning, 
astronomers saw clear signs that the sungrazing comet was getting dimmer 
as it headed toward peak heating, at an expected minimum distance of 730,000 
miles (1.2 million kilometers) and maximum velocity of 850,000 mph (380 
kilometers per second).

That suggested that ISON's nucleus, estimated to have a radius of roughly 
a kilometer (half a mile), was rapidly shedding ice and dust to feed its 
multimillion-mile-long tail. Scientists hoped there would still be something 
left after its closest approach to the sun, known as perihelion - but 
nothing was detected by NASA's Solar Dynamics Observatory.

"I'd like to know what happened to our half a mile of material that was 
going around the sun," SDO project scientist W. Dean Pesnell of NASA's 
Goddard Space Flight Center, said during Thursday's Hangout. "Now it's 
broken up, and I didn't see anything."

It was an inglorious and inconclusive end for a "dirty snowball" that 
scientists say was a fossil relic of the solar system's formation 4.5 
billion years ago. ISON spent much of that time on the solar system's 
farthest reaches, in a haze of comets known as the Oort Cloud. A passing 
star probably perturbed the comet's orbit enough to send it on a 5.5 million-year 
journey toward the sun.

Russian astronomers detected Comet ISON (C/2012 S1) in September 2012, 
and observers sparked a worldwide buzz when they calculated that the comet 
would come so close to the sun. Some hoped that it would rate as the comet 
of the century - perhaps shining as bright as the full moon just after 
it passed around the sun.

As the months wore on, astronomers downplayed those expectations - but 
still held out hope that the sungrazer could make as big an impression 
as Comet Lovejoy did for Southern Hemisphere observers in 2011. In the 
end, however, ISON was too small to weather such a close encounter with 
the sun.

Astronomers are still keeping at least one hope alive: that the voluminous 
data collected during ISON's trip will shed light on how comets fall apart. 
Scientists could use those insights to "run the film backward" and reveal 
how the earliest material surrounding the sun coalesced into comets and 
planets, billions of years ago. That should keep astronomers busy until 
the next "comet of the century" comes around. 

"It's been an amazing journey," Battams said. "It's been the busiest year 
of my career. ... We're going to learn so much more about the comet."

Update for 4:10 p.m. ET Nov. 28: The comet's nucleus may have fizzled, 
but scientists say sun-watching satellites are still seeing the dust left 
over from ISON swing around in a gravitational arc as it dissipates. "Dust 
continues to move around the orbit, just as it should," Pesnell told NBC 
News. The arc is visible in imagery from NASA's STEREO and SOHO satellites.

"Yes, something came out from behind the occulter," Battams said in a 
Twitter update. "Pretty certain there's no nucleus, though."

That accounts for at least some of the half-mile-deep pile of material 
that scientists thought the comet contained. However, the fact that ISON's 
staying power didn't match astronomers' expectations suggests that something 
may be out of whack with their models for comet composition and dynamics. 


"The story isn't over yet," Pesnell said, "because now we have an even 
bigger mystery."