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ISON is currently speeding through the cosmos at roughly 67,000 mph and is expected to accelerate to about 844,000 mph as it swings around the sun. But because ISON and other comets are so far away from Earth, unlike meteors, they appear relatively static in the sky.
Comets are huge chunks of ice, frozen gases, dust and rock. As comets fall closer to the sun, heat vaporizes the ice, releasing plumes of gas and dust. Sunlight reflects off that debris, illuminating the comet.
Particularly bright comets are dubbed “great comets” and have typically appeared about once every 10 years.
The last truly great Northern Hemisphere comet, according to many astronomers, was Hale-Bopp, which was first spotted in 1995 and eventually remained visible for a record 18 months.
“A comet’s brightness depends on how close it passes to Earth and the sun. But it also comes down to size and composition, making predictions tricky. In general, the more ice and gases that can be vaporized, the more stunning the comet, Hammergren said.
In the meantime, Hammergren continues to check on ISON’s arrival and brightness at least twice a week, remaining cautiously optimistic in spite of himself.