About 10,000 years ago, Comet ISON left our solar system's distant shell, a region known as the Oort cloud, and began streaking toward the sun. This November, the icy wanderer will reach the climax of its journey, potentially providing a stunning skywatching show here on Earth.
Comet ISON was discovered just last September by two Russian amateur astronomers. Scientists have since recognized ISON as a possible "comet of the century," but to live up to its promise, it will have to survive its dangerous perihelion, or closest approach to the sun.
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ISON is what's known as a sungrazing comet. These suicidal objects have orbits that bring them within 850,000 miles (1.4 million kilometers) of the sun, and scientists estimate that ISON's closest pass will be about 730,000 miles (1.2 million km) above the surface of Earth's star.
Sometime this month or perhaps in August, ISON is set to cross what's called the frost line. At this boundary, which lies some 230 million to 280 million miles (370 to 450 million km) from the sun, our star's radiation will start taking its toll on the comet, driving off more of its water and making ISON appear brighter.