Like a thrill-seeking moth diving into a candle flame, Comet ISON is plunging toward a close encounter with the sun – close enough to be singed, vaporized or broken to bits.
But ISON also could swing around the sun and emerge intact on the other side, venting a little gas and dust, perhaps, but remaining otherwise unscathed.
“It's doing this death-defying plunge by the sun,” said astronomer Amy Mainzer of NASA's Jet Propulsion Laboratory in Pasadena. “The question is: What is going to happen when it makes that near miss? Anything could happen with this particular comet; we just don't know. Comets are funny things. They are notoriously difficult to predict. That's part of what makes them fun to look at.”
So as ISON approaches from the most distant, icy reaches of the outer solar system, tension is mounting for professional and amateur astronomers alike.
Orbital instruments as well as telescopes around the globe will be watching as the comet speeds through the inner solar system, becoming lost in the sun's glare as it makes its closest approach on Thanksgiving Day.
Then, in early December, it reveals its fate to the world.
If it survives the plunge and becomes bright enough, it could be visible to the naked eye; some even have speculated that it might be seen during the day.
Or it could wind up a big dud, hardly troubled at all by its solar visit as it packs its bags and hurtles back into the cold of deep space.
As it makes its exit, it will come no closer to Earth than a bit less than 40 million miles, on about Dec. 26.
It's the climax of a voyage that might have taken billions of years. ISON is believed to come from the Oort cloud, a hypothesized body of objects on the solar system's farthest outer edge.
The Oort cloud, Mainzer said, “is a shell of material that might extend even as far as halfway to the nearest star. This stuff is really old, and it's been in deep freeze. It's really cold out there, only a few degrees above absolute zero – a few tens of degrees at most.”
ISON could conceivably have begun its journey back when the first life-forms were getting a toehold on planet Earth, or even before, though just when it started out is unknown.
A gravitational jiggle from some passing object might have jarred it loose, allowing it to fall back toward the sun and return from its exile when the solar system formed more than 4.5 billion years ago.
“This is once not just in a lifetime for a human life, but a lifetime for the human species,” Mainzer said.
That's why scientists are riveted by such objects, known as “long-period” comets.
“That material is really pristine, left over from a long time ago,” she said. “It's really a time capsule, preserving conditions at the very beginning of our solar system.”
So the big question is, what's it made of? That ultimately will determine how it reacts to its grazing encounter with the sun, making a brilliant fireworks show or remaining locked up tight and keeping its secrets.
“It could be enough of a frozen chunk of ice that very little activity occurs,” she said. “There could be enough rock near the surface and not enough ice to really push a lot of particles off. There could be internal structures; there could be larger chunks – pieces better able to withstand solar radiation.”
Or, a looser structure and larger share of ice – frozen water or frozen carbon dioxide – could mean ammo for the big show.
“It could be as heat soaks into the surface of the comet, those frozen materials could start to evaporate,” she said. ISON “could burst off chunks of it. It's really hard to say, because we don't really know what's on the inside of it.”
Lately, as ISON moved within the orbit of Mars, it has shown some promising signs. Outgassing is already producing a fairly spectacular tail.
“Actually it has a real nice double tail structure that's developed,” she said. “There's a dust tail and gas tail essentially.”
Mainzer, a deputy project scientist with NASA's WISE space telescope, hopes the comet might pass in front of her instrument. But WISE, which measures the sky in infrared light, is now in the process of being awakened after it was placed in hibernation when its earlier mission ended; it is unknown whether it will be ready in time.
She'll be keeping watch, in any case.
“To me, it's kind of an astonishing moment, if you think about it, for this comet,” she said. “Here it's merrily bumping away, going along, doing nothing in particular, so cold and dark nothing has changed it for possibly billions of years.
“Now, all of a sudden, here it is, going into the most dramatically different environment imaginable for a comet.”
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