Only mostly dead: Astronomers puzzle over zombie Comet ISON


Like the hero in "The Princess Bride," Comet ISON appears to be only mostly dead, raising a myriad of questions for scientists and skywatchers.

Images from sun-watching satellites show that something is still kicking after ISON's fateful close encounter with the sun on Thanksgiving Day — but what is it? A dust cloud, or the battered remnant of the "dirty snowball" of a nucleus? What kind of weird tail (or tails) is it sprouting? Why wasn't it spotted by a NASA probe during its closest approach to the sun? And will we be able to see anything in the night sky, as long hoped?

It's "a tad too early still" to say exactly what's going on, said Karl Battams, an astrophysicist from the Naval Research Laboratory who has been monitoring ISON's mostly-deadness from Arizona's Kitt Peak National Observatory.

"Seems maybe something is still producing dust," he wrote Friday in a Twitter update, "but whether it's a coherent nucleus or a dust ball, we don't know."

Just a day earlier, Battams and other experts said ISON was looking like an "ex-comet," partly based on the fact that nothing showed up in imagery from NASA's sharpest-eyed sun-watching satellite, the Solar Dynamics Observatory. But within hours, other satellites — including NASA's STEREO spacecraft and the Solar and Heliospheric Observatory, or SOHO — were tracking the movement of a bright spot heading away from the sun.

On its own Twitter account, the team behind the Solar Dynamics Observatory mission said "scientists are still looking at the data to figure out why ISON was not visible" during Thursday's closest approach to the sun, known as perihelion.

ISON's reappearance was only the latest head-turning twist in the comet's history. When Russian astronomers with the International Scientific Optical Network discovered the comet in September 2012, some observers said there was a chance it could get so bright in the night sky that it'd rate as "the comet of the century." An analysis of its orbit showed that ISON was a pristine comet, coming in from the farthest edge of the solar system for the first time, perhaps the only time. Never before had a comet from that far away been tracked on a course coming so close to the sun.

Astronomers eventually tempered their predictions, suggesting that ISON could get as bright for Northern Hemisphere observers as Comet Lovejoy did for Southern Hemisphere observers in 2011. If, that is, ISON survived its swing around the sun.

So did enough of ISON survive to put on a show? What will skywatchers see in pre-dawn skies next week? Battams and other experts say they'll need more observations before they can even venture a guess. In the meantime, one thing's for sure: ISON may not look like a normal comet, but it's still at least slightly alive.

Astronomers at Germany's Max Planck Institute for Solar System Research said they matched up the latest observations of ISON with computer models of comet behavior — and concluded that the nucleus was still spewing out dust during perihelion. That would explain the current "two-tailed" or fan-tailed appearance of the mostly-dead blob that's now being tracked.

"From the assessments, it is not clear whether the nucleus still exists or whether it partially fragmented on its fiery swing around the sun," the institute said in a news release. Additional imagery, due to be collected on Saturday, may tell the tail ... er, tale.

More about Comet ISON:

Tip o' the Log to The New York Times' John Schwartz for the "Princess Bride" reference, and to Daniel Fischer for the pointer to the Max Planck Institute.

Alan Boyle is'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'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.