Comet ISON's demise a boon to science

NASA's Solar Dynamics Observatory monitored ISON's path by the sun, but the comet's nucleus was not seen. In this image, the small cross marks where the comet should have been at the time the picture was captured.

During the final hours of its one and only trip into the inner solar system, Comet ISON, born in frozen anonymity 4.5 billion years ago, apparently disintegrated in the hellish glare and crushing grip of the sun, a Thanksgiving Day disappointment to many who had hoped for a spectacular sky show as it headed back into deep space. 

Instead, the torn remnants of the comet -- a thinning cloud of dust and perhaps a few charred boulders or maybe even the burned-out husk of its once-bright nucleus -- almost certainly will be too dim to be seen by the unaided eye. 

But for scientists around the world who spent the past year studying Comet C/2012 S1, aka ISON, "it was rich with science, and that met expectations. It was ridiculously unpredictable," said Karl Battams, an astrophysicist and comet expert with the U.S. Naval Research Laboratory in Washington. 

"This is where the hard work really starts," he said in an interview Wednesday. "We've done all the data collection and now it's a matter of pulling all that data together ... and trying to piece together, no pun intended, the comet and its history." 

In this time-lapse composite from a coronagraph aboard the Solar and Heliospheric Observatory spacecraft, Comet ISON can be seen approaching the sun from lower right, disappearing behind an occulting disk used to block the sun's glare and emerging after a close flyby. The comet's nucleus is believed to have disintegrated during the approach, leaving an expanding cloud of debris in its wake.
Of particular interest, he said, is "what caused the fragmentation to occur, exactly when did it occur, how big was the distribution of the particles, was it a mixture of big, sort of boulder-size chunks, all the way down to tiny fine dust? Or was it mostly just fine dust, and did that happen at different rates? 

"You get the picture," he said. "We're going to try to model this event and figure out what happened. We'll tie it into the size of the nucleus and then refine our models of how and when comets near the sun might break up." 

Matthew Knight, an astronomer at the Lowell Observatory in Flagstaff, Ariz., wrote in an internet question-and-answer session Wednesday that "we've never had a comet be so comprehensively observed as it broke up, and I think that will yield some really cool insight into what was going on inside Comet ISON in its last days/weeks leading up to perihelion." 

Two Russian astronomers discovered Comet ISON in September 2012. An analysis of the trajectory showed the comet would be a "sun grazer," passing less than a million miles from the star on Thanksgiving Day after a three-million-year plunge into the inner solar system. 

The comet originated in the Oort Cloud, a vast sphere of icy debris beyond the outermost planets left over from the birth of the solar system. As such, ISON was a time capsule of sorts, a frozen sample of the original solar nebula that coalesced to form the sun and its retinue of planets 4.5 billion years ago. 

The early discovery gave excited astronomers a full year to plan a coordinated observing campaign incorporating ground-based telescopes and more than a dozen spacecraft that tracked ISON's approach in unprecedented detail. 

Initial observations with the Hubble Space Telescope suggested the nucleus could have a diameter of up to two-and-a-half miles, fueling speculation the "comet of the century" might survive its flight through the sun's outer atmosphere and put on a spectacular show in Earth's sky as it headed back into deep space. 

As it turned out, ISON was no match for the extreme temperatures and crushing gravity it experienced flying through the sun's outer atmosphere. Sun-watching spacecraft saw no signs of an intact nucleus during or after the flyby and while an initially bright cloud could be seen moving away from the sun along ISON's trajectory, it quickly thinned and faded. 

ISON's path carried it through the sun's million-degree corona, or outer atmosphere, "but it's not valid to say the comet, then, must have been millions of degrees," Battams said. "Clearly, it wasn't at millions of degrees (or) we would have actually seen it in the SDO (Solar Dynamics Observatory) field of view." 

That's because the SDO spacecraft is sensitive to oxygen heated to extreme temperatures. The spacecraft closely monitored the comet's trajectory, but it did not detect any signs of a super-heated nucleus. 

"So clearly the comet never got that hot," Battams said. "One of the questions is why didn't it? We'll dig into all that." While the upper limit to the temperatures experienced by ISON is not yet known, it likely was at least several thousand degrees at the surface of the nucleus, hot enough to vaporize exposed rock. 

"And that's something that most comets don't normally undergo," Battams said. "Comets are kind of used to having their ices melt away, they sort of cope with that, but what comets don't expect to happen is their rocks will also get vaporized. That's one of the unique things about sun grazers." 

Regardless of the details, the outcome seems clear. ISON will not grace Earth's skies as a brilliant naked-eye comet. Observers will need relatively large telescopes to see whatever is left. "Could there be a burned out husk of a nucleus that's a few meters across, or tens of meters? It's conceivable, I guess. It could be a loose rubble pile." In a tongue-in-cheek obituary posted on the Comet ISON Observing Campaign web site, Battams hailed the ill-fated comet, "Born 4.5 Billion BC, Fragmented Nov 28, 2013." 

"Could there be a burned out husk of a nucleus that's a few meters across, or tens of meters? It's conceivable, I guess. It could be a loose rubble pile," Battams said.  

  • William Harwood

    Bill Harwood has been covering the U.S. space program full-time since 1984, first as Cape Canaveral bureau chief for United Press International and now as a consultant for CBS News. He covered 129 space shuttle missions, every interplanetary flight since Voyager 2's flyby of Neptune and scores of commercial and military launches. Based at the Kennedy Space Center in Florida, Harwood is a devoted amateur astronomer and co-author of "Comm Check: The Final Flight of Shuttle Columbia."