Comet ISON appears to have evaporated after plunging near the Sun on Thanksgiving Day. Sun-watching satellites saw the comet approach the Sun on November 28, but initially, it did not reappear after passing about 700,000 miles above the Sun's surface. It was rediscovered by a Sun-watching spacecraft on November 29, suggesting that while the comet may have lost significant chunks of material, at least some of it remained. Continuing observations, though, show that it has fallen apart, leaving only a rapidly dissipating cloud of dust.
An automated asteroid-hunting telescope, part of the International Scientific Optical Network (ISON) in Russia, discovered Comet C/2012 S1 (ISON) on September 21, 2012. Some comet-watchers quickly suggested that it could become as bright as a full Moon late this year.
ISON probably came from the Oort Cloud, a vast shell of icy bodies that extends up to one light-year from the Sun. These bodies are leftover “building blocks” from the birth of the solar system, so they contain the same mixture of materials that gave birth to Earth and the other planets. Studying them helps scientists understand how Earth and the other planets took shape.
The comet probably was making its first pass by the Sun. As it neared the Sun, solar heat vaporized some of the ice at its surface, surrounding the comet’s nucleus, which appeared to be about 1 to 2 miles in diameter, with a big ball of gas and dust. Sunlight and the solar wind pushed some of this material away from ISON to form a tail. ISON couldn't withstand the heat and gravitational stresses of its close encounter with the Sun, though, so it disintegrated.
ISON's remnant nucleus had beenexpected to pass closest to Earth on December 26, at a distance of about 40 million miles.