Comet ISON's demise a boon to science

CBS News

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.

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 enigmatic cloud of debris in its wake. (Credit: ESA/NASA/SOHO)

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."

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, Az, 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.

But as it passed by Mars, observations by the Mars Reconnaissance Orbiter's high-resolution camera allowed astronomers to refine the size estimate to an upper limit of three-quarters to two-thirds of a mile across.

As ISON neared the sun, scientists were circumspect about its chances for survival. Carey Lisse, a senior research scientist at Johns Hopkins Applied Physics Laboratory in Laurel, Md., put the odds of a break up at 60 percent to 70 percent. But even the realists held out hope ISON would survive in some form after its brush with the sun.

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.

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 mark's where the comet should have been at the time the picture was captured. (Credit: NASA/SDO)

Battams said the presumed breakup was the result of a combination of extreme temperatures and gravity-driven tidal forces that subjected the nucleus to enormous stress.

"Either one of those by themselves is problematic, but when you've got the combination of them and the radiation is causing all the furious outgassing, and the gravity induces stretching and tugging ... and opens up a fracture, then the radiation's going to penetrate there, and that's just going to exacerbate the situation," he said. "It snowballs, basically, no pun intended. But it snowballs."

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.

"There's not going to be anything to see, certainly it's way below naked-eye visibility, already far, far below that, and by the time it gets into darker skies ground observers might be able to pick it up with a telescope (but) I'm not even sure if they're going to see anything," Battams said.

"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."

"Never one to follow convention, ISON lived a dynamic and unpredictable life, alternating between periods of quiet reflection and violent outburst," he wrote. "In late 2013, Comet ISON demonstrated not only its true beauty but a surprising turn of speed as it reached its career defining moment in the inner solar system. Tragically, on November 28, 2013, ISON's tenacious ambition outweighed its ability, and our shining green candle in the solar wind began to burn out.

"Survived by approximately several trillion siblings, Comet ISON leaves behind an unprecedented legacy for astronomers, and the eternal gratitude of an enthralled global audience."