Comet ISON mostly fizzled out during its swing around the sun on Thursday, but remnants of the "comet of the century" continued their outward journey — and continued to confound astronomers.
"We are seeing something beginning to gradually brighten up again," Karl Battams, an astrophysicist at the Naval Research Laboratory, said in a blog posting late Thursday. "One could almost be forgiven for thinking that there's a comet in the images!"
That assessment ran counter to what Battams and other scientists said earlier in the day, during a NASA-sponsored Google+ Hangout. When ISON didn't appear in images beamed back to Earth from NASA's Solar Dynamics Observatory, astronomers characterized it as an "ex-comet."
That seemed to sound the death knell for a comet that skywatchers hoped would turn into a celestial Yuletide treat. But later in the day, images from other sun-watching probes — including NASA's STEREO spacecraft and the Solar and Heliospheric Observatory, or SOHO — showed ISON's remnants spreading out in an arc around the sun. Such a phenomenon, known as a "headless tail," has been seen before with sungrazing comets.
The pictures suggested 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."
Battams said it's too early to write ISON off completely.
"Right now it does appear that a least some small fraction of ISON has remained in one piece and is actively releasing material," he wrote. "We have no idea how big this nucleus is, if there is indeed one. If there is a nucleus, it is still too soon to tell how long it will survive. If it does survive for more than a few days, it is too soon to tell if the comet will be visible in the night sky. If it is visible in the night sky, it is too soon to say how bright it will be. ... I think you get the picture, yes?"
A picture from the SOHO spacecraft shows the arc traced by the comet's tail, sweeping in from bottom right, and ending with a bright blob at upper left. In this image, the sun's glare is covered by an occulting disk.
Highs and lows
That's par for the course when it comes to Comet ISON. Over the past few days, the comet's condition 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) from the sun 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 seemed to be an inglorious 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.
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.
It could be a couple of days before ISON's fate becomes clear. Pesnell told NBC News that the bright glow seen after the comet's solar encounter might be nothing more than a leftover dust cloud. "Dust continues to move around the orbit, just as it should," he said.
But he also acknowledged that ISON's behavior was challenging the conventional wisdom about comets — and that's not necessarily a bad thing.
"The story isn't over yet," Pesnell said, "because now we have an even bigger mystery."
More about Comet ISON:
An earlier version of this report included an incorrect reference to the solar system's age. Hat tip to Brittney Gougeon for the pointer to post-perihelion imagery.
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.
First published November 28 2013, 11:12 PM