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Astronomers on the ground and more spacecraft than ever before in history have been keeping a watchful eye on Comet ISON as it makes its way perilously close to the sun.
The 3- to 4-mile-diameter chunk of ice and rock has spent more than 4 billion years in the frozen depths of space, and on Thanksgiving, it will get so close to the sun that it will reach 5,000 degrees hot enough to melt iron, said Seth Jarvis, director of Clark Planetarium.
In fact, it will get so close to the sun that it will be subject to the powerful tidal forces caused by the star, and the comet's nucleus could be torn apart.
ISON began brightening significantly late last week, and if that trend continues, it could make for a sight visible with the naked eye in the low east-southeast horizon in the pre-dawn sky and could last through mid-January. If the comet is torn apart, though, a best-case scenario would be a bright flash visible to Earth, and then nothing more.
"It's rare that we see a sun grazer that's significant enough in size that it has a good probability it will survive," Jarvis said. "But that's the cool stuff, that we're not sure what's going to happen."
Patrick Wiggins, NASA/JPL solar system ambassador to Utah, said he has become more encouraged in the past few days.
"I'm keeping my fingers crossed that with a little bit of luck we'll have a nice comet to see around the Thanksgiving time frame, but I'm not putting any money on it," Wiggins said.
Jarvis shares Wiggins' cautious optimism.
"Astronomy is always a great way to teach humility, and with comets, that's especially true," said Jarvis, who remembers with a pang of heartache Comet Kohoutek, which ended up costing him a girlfriend back in 1973. That one astronomers touted as the Comet of the Century, and when it fizzled in the wee hours of a particularly cold December night, a certain girl who was woken up and dragged to the top of East Canyon for nothing never really wanted anything to do with Jarvis again.
Worse, though, was the fact that just three years later, Comet West came through spectacularly but few people saw it. The media didn't want to get burned again (Time had put Kohoutek on its cover) and little buzz surrounded the event, Jarvis said.
ISON is a somewhat risky bet to tout as something that will be worth watching because of its relatively small size Halley's Comet is 7 miles wide and Comet Hale-Bopp is about 20 miles wide.What makes ISON promising, though, is that it will get much closer to the sun than either of those comets. ISON will get about 700,000 miles above the sun's surface, while most comets orbit about as far away as Mercury, which is 30 million miles away from the sun.
The nucleus of a comet is usually a mix of rock and ice, and is relatively small. But as the nucleus warms, gases begin to vaporize, creating the coma, or head, of the comet, which can measureabout 10,000 miles in diameter, making it observable from Earth. Solar wind and radiation push those gases backward to form the comet's tail.
It is believed that ISON came from the Oort Cloud, a spherical shell filled with icy bodies that is nearly a light-year away from the sun. ISON is named after the place that discovered it in September 2012 Russia's International Scientific Optical Network.
A total of nine spacecraft are observing the comet, setting a new record. The Chandra X-Ray Observatory has determined for the first time that a comet does emit X-rays due to a combination of solar winds and ionized gases coming off the comet.
ISON is also losing water at a tremendous rate, Jarvis said. Daily, the water being vaporized could fill 16 Olympic-sized swimming pools.
For now, astronomers will keep their eyes to the early-morning sky, hoping to see some spectacular sights as the comet rockets toward the sun and survives its long journey.