It's time again for the Geminids, one of the year's best meteor showers. There'll be interference from moonlight this year, but you'll still probably see plenty of these "shooting stars."
In mid-December every year Earth passes through the Geminid meteoroid stream. The resulting meteor shower is unusual in several ways. It's often the richest reliable shower of the year, and it seems to be strengthening over the decades. It becomes nicely active as early as mid-evening, so you don't have to wait until after midnight (as is the case with many other showers). Its meteors are relatively slow, arriving at 36 km (22 miles) per second, hardly more than half the speed of the Perseids, Orionids, and Leonids. Also, these meteors often appear yellowish, and their deep plunges into the atmosphere betray them as dense bits of rock rather than fluffy dust clods like most cometary meteors.
And in fact they come not from a comet but a tiny asteroid, 3200 Phaethon, that is itself an oddball. It's in a tight, highly elliptical orbit that, every 1.43-years, brings it three times closer to the Sun than Mercury's average distance. So the Sun repeatedly shines on it 50 times brighter than sunlight on Earth.
This year the Geminid shower should peak around 6h Universal Time (1 a.m. Eastern Standard Time) on Saturday, December 14th — ideal for North Americans, especially easterners. But the waxing gibbous Moon will be bright, three days from full and shining most of the night. The Moon sets one hour before the first glimmer of dawn on the morning of the 14th, affording a nice dark window for serious meteor observers.
The shower is also active for a few days before then, and the Moon sets about an hour earlier for each day before the 14th. The Geminids drop off more steeply after maximum, as seen in last year's activity profile at right.
Bundle up as warmly as you can. Find a spot with an open view of the sky and no lights to get in your eye. Lie back in a lawn chair facing away from the Moon, gaze into the stars, and be patient. You might see a Geminid every minute or two on average depending on the brightness of your sky. The shower's radiant point is in Gemini near Castor, and the higher that is the better, but the meteors themselves will appear anywhere in the sky. So watch wherever it's darkest.
What exactly creates the Geminid meteoroid stream? Small-body researchers David Jewett and Jing Li (UCLA) say they've figured it out. Astronomers have thought that the stream's parent body, 3200 Phaethon - discovered in 1983 and only 5 km (3 miles) wide — must be a dead comet nucleus. During its loops around the Sun every 17 months its surface roasts to more than 700°C (1300°F), and all water and other volatiles must have been baked out ages ago. So then, how can it shed any particles at all?
Jewett and Li found proof that it still does. NASA's two STEREO spacecraft, designed to observe the Sun, are also able to see Phaethon when it's nearest the Sun despite its tiny size. STEREO images show that Phaethon brightens and sheds a dust tail when it's in the most intense heat. With no volatiles left to vaporize and carry dust away, the two astronomers propose that Phaethon's bare rock cracks and crumbles by thermal fracturing. Jewett calls Phaethon a "rock comet."
"While this is the first time that thermal disintegration has been found to play an important role in the solar system," the two write, "astronomers have detected unexpected amounts of hot dust around some nearby stars that might have been similarly produced." So count it as another way the Geminids are unusual.
Become a Meteor Counter
Have you ever done a scientific meteor count? The International Meteor Organization always wants more observers around the globe. You'll need to follow the IMO's standardized instructions so that your counts can be meaningfully compared with others. See the IMO's website and also our site's introduction to advanced meteor observing.