At the end of next week, we are in for an annual holiday sky event - the Geminid meteor shower will peak next Thursday night. I have always considered the Geminids to be the "best" meteor shower of the year - as long as you can stand the cold while viewing. Because this is a December event, the chances are much better that skies will be clear in New England than for the better-known Perseid meteors that occur in mid-August. The Geminids are also particularly fascinating as they are composed of larger bits of rock than most other showers, therefore they tend to be brighter, and the speed at which they cross the sky is slower, giving observers a much better chance at seeing each of them.
This year promises to be a particularly good show of the Geminids, with projected rates over 100 meteors per hour, and with the moon setting shortly after sunset skies (if clear) will be very dark and crisp. Unlike most other showers where few meteors occur during evening hours, starting as early as 9pm Geminids will begin to appear, though peak activity will be between 1 and 3am on the morning of the 14th. The timing couldn't be better for me personally - my astronomy class meets that night, and I plan on staying up right through the shower's peak.
For best viewing, starting at about 9pm, face east, and settle back into a reclining chair. Meteors will come from the direction of the constellation Gemini, which at that hour will be about 30 degrees (three fist-widths held at arm's length) above the horizon. If, as is the case in my yard, this puts Gemini in the trees, don't fret - the meteors will streak far across the sky to nearly overhead even while the constellation is low in the sky. As the night progresses, viewing will steadily improve.
Beyond the event itself, the story of the Geminids turns out to be highly unusual. Unlike many of the other famous meteor showers which have been known for many centuries, the Geminids did not begin occuring until 1862, which, on the time scale of astronomy, is as if it started this morning. When first observed, this was a meager shower, with a rate of about 14 meteors per hour. When it occured again in 1863, the shower was recognized to be a new annual shower.
Over the years into the 20th century, the Geminids continually increased in intensity, reaching 20 per hour by 1900, 50 per hour in the 1930s, and rising to 80 per hour by the 1980s. Since the beginning of the 21st century, rates have again been rising steeply, with nearly 200 per hour reported in 2011.
Most annual meteor showers are caused by the intersection of Earth's orbit with the orbit of a comet - as the comet slowly vaporizes over millenia, the sand in the body of the comet fills its orbit. When that sand comes near Earth, our gravity pulls these particles into the atmosphere where they vaporize, creating a "shooting star". The onset of a new meteor shower therefore started a quest to identify the parent comet from which the Geminids were produced.
Early on in the observations of the Geminids, it was noted that this meteor shower contains an unusual number of bright meteors, that their rate of travel is slower than most showers, and the meteors display a broad range of colors as they vaporize in the atmosphere. Astronomers were able to calculate the orbit in which the meteors traveled, but were unable to find the parent comet.
In 1983, while surveying data taken using the Infrared Astronomy Satellite (IRAS), a very early space-based telescope, astronomers identified a small asteroid which later received the name 3200 Phaethon. Upon computing Phaethon's orbit, it was found to be virtually identical with the orbit computed for the Geminids. This orbit is very similar to a typical cometary orbit, approaching within 13 million miles of the sun (1/3rd the distance of Mercury to the sun), and traveling out beyond the orbit of Mars into the asteroid belt. Yet Phaethon has never shown a tail, and has been categorized as an asteroid, with the distinction of being the first asteroid discovered by an orbiting observatory.
And so the Geminid meteors are now known to have been created from Phaethon, though how exactly this occured remains an active area of research. It is possible that Phaethon was once a comet, of which now only a rocky core remains, along with a trail of sand. However, the structure of other known comets does not include a rocky center, but more resembles a "dirty snowball", with sand distributed throughout a ball of ice.
It is also possible that Phaethon and the Geminids formed from the collision of two other asteroids. In fact, there is some evidence for this theory. The orbit of Phaethon takes it near to the orbit of the second-largest asteroid, Pallas (330 miles in diameter). By observing the spectrum of the light reflected from Phaethon and from Pallas, researchers have found a very good match, indicating that the material on the surface of these objects is similar in composition. Phaethon is now considered to be one of about 10 objects that have orbits and apparent compositions consistent with having formed from an impact on Pallas.
Having established the orbit of Phaethon and the Geminids, it becomes possible to mathematically study the effect of the rest of the mass in our solar system on the objects in this orbit. It was found that the effect of the massive planet Jupiter is causing the orbit to gradually shift over a period of centuries. The nearest approach of the orbit to Earth's orbit was found to be over 10 million miles in 1700, before the meteor showers began, and to be within 1 million miles in 1900, when the shower was underway, but with lower meteor rates than we see currently. By 2100, the expected nearest approach increases to about 9 million miles, and we can expect the Geminid meteor showers to end.
With meteor rates now approaching 200 per hour, we are about at the center of the orbit of Phaethon and the Geminids. This year's display could therefore be truly amazing with the combination of rate and darkness. So, next Thursday night, get yourself layered up, fill up the thermos with hot chocolate, get outside after 9pm, sit back and enjoy the best fireworks display of the season.
Hey! Check out my astronomy class at www.turnerclasses.com!
