The Geminids is the king of the meteor showers. It is considered by many to be the best shower in the heavens, producing up to 120 multicolored meteors per hour at its peak.
An asteroid known as 3200 Phaethon is responsible for the Geminids meteor shower. This differs from most meteor showers which are caused by comets, not asteroids. What’s the difference between a comet and an asteroid?
A comet is defined as a dirty snowball, with a solid nucleus covered by a layer of ice which sublimates (turns from a solid to a gas) as the comet nears the sun. Comets are typically lightweight, with a density slightly heavier than water. They revolve around the sun in elongated orbits, going close to the sun, then going far from the sun. Seen through a telescope, a comet will show a coma, or head of the comet, as a nebulous patch of light around the nucleus, when it gets close to the sun. But when seen far from the sun, most comets appear star-like, because only the nucleus is observed.
An asteroid is a rock. Typically, an asteroid’s orbit is more circular than that of a comet. Through a telescope an asteroid appears star-like.
These definitions worked well until a few decades ago. Larger telescopes began discovering asteroids far from the sun, and some of these objects, as they approached the sun, grew comas and tails, requiring the change of designation from asteroid to comet. An odd object named Chiron, considered an asteroid when discovered in 1977, was reclassified as a comet in 1989 when it showed a coma. It orbits the sun every 50 years and travels from just inside the orbit of Saturn to the orbit of Uranus.
So, an object initially considered an asteroid can be reclassified as a comet. Can the opposite occur: a comet be reclassified as an asteroid? Yes, it can. It is possible that a comet can shut down when its volatile materials become trapped beneath the nucleus’ surface. This is known as a dormant comet. When the comet loses all of its volatile materials, it is known as an extinct comet. The asteroid 3200 Phaethon seems to be an example of either a dormant or an extinct comet.
The shower runs annually from December 7-17. It peaks this year on the night of the 13th and morning of the 14th. This should be an great year for the Geminids. The nearly new moon means dark skies for what should be an excellent show. Best viewing will be from a dark location after midnight. Meteors will radiate from the constellation Gemini, but can appear anywhere in the sky.
Irene W. Pennington Planetarium 