The Geminids are coming!
Earlier this week, an object 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 object, it is the parent body of December’s annual Geminid meteor shower.
Meteor showers occur when Earth passes through streams of comet debris as we orbit the Sun. Their names are related to the constellation from which they appear to emanate. The Perseid meteors appear to be coming out of Perseus, the Leonids from Leo and so on. Many meteor showers have been known about 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 body first thought to be an 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 is typical of cometary debris. But, the density was still lower than most asteroids. Analysis of 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 object that has a ‘comet-like’ orbit. Most scientists now think that 3200 Phaethon is an extinct comet with a rocky core.
The Geminid shower is one of the most reliable and observer friendly of the annual meteor showers. But, is it is less well-known than the summertime favorite Perseids because it appears during the cold nights of December. 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 to catch the show
The best times to watch will be Sunday the 13th starting at 9:30-10PM and overnight from midnight to dawn. Pick an observing spot away from city sky glow or ambient light and bring along a reclining chair, blankets or extra clothes to keep warm. Geminid meteors can appear anywhere in the sky. The brilliant red beacon of Mars is an easy catch lying lower and to the east 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!
