The Geminid Meteor Shower
December 9, 2008
This weekend will see one of the year’s better meteor showers. In most years, the Geminids would rank as the year’s #1 or #2 best shower. Unfortunately, this year the Full Moon will put a damper on the show resulting in much lower rates.
From a dark, moon-less sky, the Geminids have been known to consistently produce rates of over 100 meteors per hour at their peak. Due to the bright Moon, the observed rate of Geminids will be much lower, probably in the 10-30 meteors per hour range. Still, if you are willing to brave the cold, the Geminids are one of the easier showers to observe. Unlike most showers that can only be observed in the early hours of the morning, the Geminids can be seen in good numbers as early as 10 pm and are great anytime after midnight. The Geminids will appear to radiate from the constellation of Gemini near the bright star Castor.
According to analysis of meteor video data by Sirko Molau, the Geminids are active for almost an entire month between the dates of November 23 and December 21. Though high rates are only possible within a few days of the peak. This year the peak is predicted for the nights of December 13/14.
Most meteor showers are produced by comets with orbits that extend out to the orbit of Jupiter or beyond. The Geminids are different. They are on a very short uncomet-like orbit that extends from a very close 0.14 AU from the Sun to a not so far 2.40 AU.
In 1983, the parent body of the Geminids was discovered and surprisingly it looked like an asteroid. Since its discovery, the Geminid parent (3200) Phaethon has not shown any cometary activity. So what is it?
1) Phaethon could be a comet whose original orbit evolved into its current one after many millennia of close approaches with the inner planets. Some models of the formation of the Geminids require the shower particles to be released over many centuries to millennia. This is consistent with the behavior of a comet.
2) Phaethon may be a Main-Belt comet. Main-Belt comets are objects that originate in the outer Asteroid, or Main, Belt. Since they contain a sizable fraction of volatile material (water, carbon monoxide, etc.), they can occasionally exibit cometary activity. Four of these objects have been observed to display cometary activity in the Main Belt. Since they start on asteroid orbits, it is not too difficult for one of them to find itself on an orbit similar to Phaethon.
3) Phaethon is an asteroid that broke up in the past. There is evidence to suggest that Phaethon is just the largest piece of a past break-up. In fact, two additional asteroids that may once have been a part of Phaethon have been found, (155140) 2005 UD and 1999 YC. According to Peter Jennisken’s book “Meteor Showers and Their Parent Comets”, the Geminids can be explained by the break-up of Phaethon just after perihelion many orbits ago. Since Phaethon gets to within 0.14 AU (14% of the Earth-Sun distance), perhaps it split under the intense solar heat. BTW, this scenario does not rule out Phaethon as a ice-rich Main-Belt comet.