Comet PanSTARRS, the first of two bright comets expected in 2013, will waltz across our evening sky in March. If it lives up to early predictions, it may be visible to the naked eye low in the western evening twilight.
Comets are remnants from the formation of the solar system about 4.6 billion years ago. These "dirty snowballs" travel around the sun in elliptical orbits and can sprout tails from their nucleus and coma that extend millions of miles during their closest approach to the sun.
The nucleus of a comet is an irregularly shaped ball of minerals, rock and ice typically the size of a small town. When the nucleus approaches the sun, sunlight warms the surface and the ice turns to vapor. The resulting cloud of water vapor and carbon dioxide surrounding the nucleus is called the coma. Most comets come from the most distant reaches of the solar system. By the time they reach the inner solar system, their coma can be larger than Jupiter. Together, the coma and the nucleus form the head of the comet.
As the comet nears the sun, it develops two tails. The dust tail is composed of small dust particles jettisoned off the nucleus by escaping gases. A longer blue-colored ion tail is made of electrically charged atoms and molecules blown away from the coma by the solar wind. The gaseous ion tail is pushed straight away from the sun by the solar wind, while the brighter dust tail traces the comet's curved orbit. Usually the two tails point in slightly different directions.
Experts predict that Comet PanSTARRS will glow between +1 and +3 magnitude. They also predict the best week to view the comet will be March 12-18. Keep in mind, however, comets are very unpredictable and the current brightness predictions for Comet PanSTARRS could change before its appearance in our evening sky.