Winter's Fireworks

December 12, 8:58 AMSt. Louis Astronomy ExaminerGary Kronk

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The final major meteor shower of the year is just around the corner. The Geminids have been a strong and consistent meteor shower during the last few decades. But they are not like other major meteor showers.

First of all, the Geminids move around the sun with a period of 1.65 years. This considerably contrasts with the 33 years for the Leonid meteor shower, the 76 years for the Orionids and Eta Aquarids, the 135 years for the Perseids, and the possible 400 years for the Lyrids.

Second, it appears that the Geminids are a fairly new meteor shower for our planet. It was first observed during December 10-12, 1862. Hourly rates have steadily increased since the first observation, reaching 10 during the 1880s, 20 during the 1900s, 50 during the 1930s, 60 during the 1940s and 1950s, and 70-80 from the 1970s to the present. Several astronomers have concluded that the orbit of the Geminids is slowly being shifted by the strong gravitational pull of Jupiter. If the earliest display occurred in 1860s, there is a chance the meteor shower will cease near the end of this century.

Third, of the meteor showers mentioned, as well as numerous others not mentioned, the Geminids seem to be the only one that has an asteroid moving in its orbit instead of a comet! At first the presence of an asteroid baffled astronomers, but after a very short time, they became excited, as this might be the missing link or transition object between asteroids and comets that had been theorized for years.

Comets shed gases and dust when they are closest to the sun, with the dust remaining in their orbits. As the dust slowly spreads throughout the orbit of each comet, a meteor shower is possible if Earth crosses the orbit of the comet.

Typical asteroids are nothing more than large chunks of rock, the majority of which exclusively stay between the orbits of Mars and Jupiter. But of the several hundred thousand asteroids that we know of, a few hundred have orbits that are surprisingly comet-like…meaning they are very elliptical and can move from near Jupiter to within the orbit of Earth. The theory has been put forth that these are extinct comets, but there has been no conclusive data supporting this idea…until now.

The asteroid moving within the orbit of the Geminids is known as Phaethon and was discovered in 1983. Now the only way for an asteroid to have dust moving in its orbit is for it to suffer an impact from another object. But for Phaethon to have produced so much dust to create such a strong meteor shower would require it to have suffered hundreds, if not thousands of impacts. If such were the case, Phaethon would probably not even exist.

It is very likely that Phaethon is the transition object that astronomers have been waiting to discover. The fact that it sits in the midst of a dense meteor stream means it must have been a comet. The fact that the orbital period is only 1.65 years, means it would probably have remained active all the time, thus shedding its dust and gas faster than typical comets. Since the discovery of the Geminids, Phaethon has probably circled the sun 90 times, but the fact that the meteors are so evenly spread around the orbit indicates it has circled the sun many more times than that. That is important, because theories about the evolution of comets indicate that they may eject all of their dust and gas after circling the sun 100-150 times. Phaethon no doubt comes close to that and might even far exceed this!

The Geminid meteor shower produces hourly rates of 70-80 and appears to radiant from the northern portion of the constellation Gemini. Unlike most of the other major showers, the Geminids can be seen throughout the night, although the hourly rates really pick up after 9:00 p.m. This will be a great year for the Geminids as the moon will not be a factor. This will also be a particularly good year for the United States, especially the eastern half, because maximum will occur when the radiant point is high above the horizon. Although the Geminids typically produce a lot of bright meteors, it is still important to observe them in an area with as few lights, trees, and buildings as possible. Get a reclining lawn chair, lie flat so that you are gazing straight up into the sky, and remember to dress warmly!