Will it meet a fiery death — or survive — when it whips around the sun on Thursday?
The icy comet will be only about 1 million miles away from the sun's super-hot surface during its close encounter on Thanksgiving. On Monday, it looked like it was about to die even before it got there. On Tuesday, it appeared healthy again.
"We have never seen a comet like this," Naval Research Laboratory astrophysicist Karl Battams said during a NASA news conference Tuesday. "It has been behaving strangely."
Because it is so close to the sun, ISON (EYE'-sahn) will likely not be visible from Earth on Thursday — except via a fleet of NASA telescopes and spacecraft aimed at the comet as it gets closest to the sun at 1:37 p.m. EST, he said. And it will be a few hours before scientists know whether the comet survives.
But even if the comet dies, Johns Hopkins University scientist Carey Lisse said there's a good chance that people on Earth will get an interesting cosmic show. The comet's remnants could paint the sky with a wide swath of green in the Northern Hemisphere.
Lisse gives the comet a 30 percent chance of surviving, adding that it is just a gut-feeling that has little to do with logic. Logically, it should be 50-50, he said.
The comet — two-thirds of a mile wide — is made up of loosely packed ice and dirt, essentially a dirty snowball. It is a "dinosaur bone," from the formation of the solar system 4.5 billion years ago, Lisse said. It has been in "deep freeze" for billions of years in the Oort cloud, a vast area of comets and debris that never formed into planets that's between 450 billion and 9 trillion miles from the sun, he said.
The comet is racing around the sun, pulled close by our star's massive gravity, which can also break apart the dirty icy core.
Comet ISON was first spotted by a Russian telescope in September last year. While many comets come out of the Oort cloud and return after a long trip through the solar system and many comets graze by the sun, this is the first one that astronomers have watched that is from the Oort cloud and is skimming the sun.
Lisse said ISON could behave just like last year's comet Lovejoy, which fell apart a couple days after passing by the sun. Its remnants were visible like "a beautiful paintbrush swath in the sky" in the Southern Hemisphere, he said.
Unlike many blue comets, ISON has been more lime-green because it doesn't seem to have as much carbon monoxide as other comets, Lisse said.
If the comet does make it around the sun intact and returns past Earth, the Northern Hemisphere should get good naked-eye views just before sunrise and just after sunset in the first couple weeks of December, Lisse said. But this probably won't be as bright and visible as 1997's Comet Hale-Bopp, said Geoff Chester of the U.S. Naval Observatory in Washington.
Over the last few weeks, the reports weren't good for ISON's survival. On Monday, astronomers at Kitt Peak Observatory "were a little bit worried and depressed" because the comet produced a classic sign of flaming out, Lisse said from the Arizona telescope. Earlier, astronomers thought it may have broken up.
Then on Tuesday, a solar telescope spotted the comet and it was growing brighter just as it should have.
"It's a wild ride, this comet," Lisse said. "The reports of ISON's demise have been greatly exaggerated for the last month or two and ISON continues to surprise us."
NASA's Comet ISON page: http://www.nasa.gov/ison/
Seth Borenstein can be followed at http://twitter.com/borenbears
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