Like Kohoutek (orbit shown here), comet ISON is a first-time Oort cloud visitor. What's more, ISON will pass much closer to the sun that Kohoutek did and so astronomers may be able to see its layers chemically come apart like peeling open an onion. Unlike in 1974, today we have a powerful and diverse collection of space and ground based telescopes to following the far visitor, and the Internet to share information at the speed of light.
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A comet like this only comes along once or twice a century say experts. "Sungrazing comets are common. Fresh-from-the-Oort-cloud comets are common. Comets detected more than six times farther out than the Earth's orbit? Not so much. The combination of all three is very rare," said comet veteran Mike A'Hearn of the University of Maryland.
At present the comet is no longer rising in intrinsic brightness and perhaps dipped in brightness a little. It could have a shallow reserve of super-volatiles ices (not water) that has been mostly exhausted, with more waiting to be uncovered by continued activity as the comet approaches the sun. As the comet is blowtorched it may simply disintegrate and never reappear after passing within a million miles of the sun in late November.
Mike Mumma of NASA's Goddard Space Flight Center predicted that if the comet is weakly bound together, the sun's gravity will pull it apart and cook off the volatiles before the remaining dust could re-coalesce. "ISON is a great experiment to see all the factors that influence sungrazing survival," said Matthew Knight of the Lowell Observatory.
Regardless of how the comet behaves, this is becoming sort of a Woodstock of space astronomy. Unlike Kohoutek, there is an armada of spacecraft position near the sun or at other planets that will be ready to follow the comet along every step of its sunward plunge. Add to that a powerful new generation of ground base observatories, and an army of dedicated amateur astronomers with sophisticated instruments for monitoring the comet.
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The comet will be dissected across the electromagnetic spectrum, from radio to X-rays. "There's a lot of potential in studying ISON, said Karen Meech of the Institute for Astronomy in Hawaii. "We'll have a huge amount of data to learn from."