An extraordinarily bright comet might share our skies this winter. Comet ISON, discovered in 2012 by Russian astronomers, is currently speeding toward the sun at several miles per second. It is expected to significantly brighten as it nears the sun, and hopeful predictions estimate that it may rival the moon in terms of brightness.
Astronomers believe that comet ISON originated from the Oort Cloud – a spherical cloud of comets that surrounds the solar system at a distance 1,000 times farther than the orbit of Pluto. Comets are occasionally perturbed in the Oort Cloud due to gravity and are sent on orbits that bring them into the solar system.
Comet ISON will make its closest approach to the sun Nov. 28, at a distance of approximately 800,000 miles above the sun’s surface – several times closer than the orbit of Mercury. As it approaches the sun, the water, ice and dust that make up the comet will turn into vapor and create the characteristic comet tail. It is this tail that is readily viewable – comet tails can reach lengths approaching the sun-Earth distance (93 million miles). The nucleus of comet ISON – the solid part of the comet similar to a dirty snowball in composition – is much too small to be visible (only a few miles across).
While many well-known comets make periodic trips to our solar system – Halley’s Comet, for instance, which will next return to the inner solar system in 2061 – comet ISON is a nonperiodic comet and will permanently leave the solar system after its flyby this winter. Comet ISON can be found in the constellation Virgo the Maiden in November. The best time to observe it is shortly before sunrise.
While comet ISON will hopefully prove to be a bright sight even under city skies, it is important to remember that estimating the brightness of comets is notoriously difficult.
“Predicting the behavior of comets is like predicting the behavior of cats – can’t really be done,” said Don Yeomans, manager of NASA’s Near-Earth Object Program Office.
Katherine Kornei grew up in Los Altos and earned a doctorate in astronomy from UCLA in 2012. She works as a science educator and writer in Portland, Ore.