Night Sky: December's annual Geminid meteor shower starts tonight

Peter Lipscomb | For The New Mexican

12/12/2007 -

On Monday, an asteroid named 3200 Phaethon passed by Earth at a distance of about 11 million miles. Ordinarily, such an event draws little attention with the regular traffic of planets, comets and other small bodies moving about the solar system. But, 3200 Phaethon is not just any asteroid, it is the parent body of December's annual Geminid meteor shower.

Meteor showers occur when Earth passes through streams of comet debris in our orbit around the sun. Their names are related to the constellation from which they appear to emanate. Persied meteors appear to be coming out of Perseus, the Leonids from Leo, and so on.

Many meteor showers have been seen for centuries. But when a sudden occurrence of meteors began to appear mid-December in the 1860s, observers were surprised and began working to identify the source of the activity.

In the following decades, the pattern of activity gradually strengthened. Scientists noticed that as the Geminid hourly rates increased, estimated meteor size grew. Larger meteor size nearing peak hourly rates is characteristic of older streams of debris, but a review of known comet orbits failed to turn up any likely candidates. The parent body of the Geminids was a mystery that persisted well into the 20th century.

In early 1983, a satellite survey of objects moving through the solar system discovered a previously unknown asteroid. Its orbit was a close match for the Geminid stream. The link was further strengthened through photographic study of Geminid fireballs. Density measurements showed that the meteors were heavier than typical of cometary debris, but the density was still lower than most asteroids. Analysis of the spectral data, or colors, of the meteor flashes revealed a composition consistent with asteroids. So, while 3200 Phaethon is almost certainly the source of the Geminids, it is a very strange asteroid that has a "comet-like" orbit.

The Geminid shower is one of the most reliable and observer-friendly of the annual meteor showers. This time of year, the constellation Gemini is well placed in the eastern sky in the late evening, so you don't have to give up a good night's sleep.

The best times to watch will be tonight starting at 9:30 and on Friday and Saturday from midnight to dawn.The brilliant red beacon of Mars is an easy catch southwest of Gemini's brightest star Castor. If you can trace a meteor's path back to near Castor, you can count it as a Geminid.