An October 8 view of the comet from the Mount Lemmon SkyCenter near Tucson.
"Comet On Target for Thanksgiving Roasting"
November 1st, 2013
A comet that’s caused a lot of excitement is racing toward a close encounter with the Sun on Thanksgiving Day. It will pass about 700,000 miles above the Sun before whipping around and heading back toward deep space. That could make the comet easily visible to the unaided eye for a few weeks after the encounter.
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.
Continued observations, however, show that it’s not brightening as much as those optimistic projections indicated. However, the comet appears to be holding together as it approaches the Sun, suggesting that it could survive its encounter and brighten enough to see without aid in early December.
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 is making its first pass by the Sun. As it nears the Sun, solar heat vaporizes some of the ice at its surface, surrounding the comet’s nucleus, which appears to be about three miles in diameter, with a big ball of gas and dust. Sunlight and the solar wind push some of this material away from ISON to form a tail.
The comet will get brighter as it approaches the Sun, but more difficult to see through the Sun’s glare. It will shine at its brightest as it passes the Sun, although it will be too close to the Sun to view safely. In early December, though, it will move into the morning sky, where it could put on a reasonable display for a few weeks.
ISON will pass closest to Earth on December 26, at a distance of about 40 million miles.