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| Four Geminid meteors slice through the sky above weak northern lights, viewed over Lake McDonald, in Glacier Park early Monday morning. The orange glow (bottom right) is light pollution from the lodge. |
Our perennial winter clouds parted for one evening on Sunday night and Monday morning, just past the peak of our annual Geminid meteor shower. A good number of meteors still rained slowly down throughout the night, joined late by a little bit of green aurora glow. As comet-caused meteor showers go, the Geminids' source is far stranger than any comet we currently know of.
When the Geminid shower first appeared in 1862, it was weak with few "shooting stars." But it has grown in intensity ever since and is now one of our most prolific storms. The amount of debris left in the Geminid stream outweighs other meteor streams by 5X to 500X. Yet the source managed to evade our eyes and instruments for another 121 years.
Finally in 1983, the storm's brooding source was discovered and named "Phaethon" (after the underachieving son of "Helios," the Greek sun god). Phaethon is an asteroid-like object whose orbit takes it closer to the Sun than any other named asteroid - less than half the distance between Mars and the Sun. At its closest point Phaethon's surface reaches 1,400 degrees Fahrenheit (blazing when compared to Halley's maximum temperature of 135 degrees).
It was originally thought that, at such extreme temperatures, solar heating would blow and shake dusty debris off the asteroid's surface. Its passes near the Sun (every 17.5 months) have been carefully observed and analyzed since 2009, and indeed Phaethon doubles in brightness like a comet. But, surprisingly, the amount of debris blown off during each orbit only adds 0.01% to the mass of Geminid's debris stream. Phaethon does not kick up enough dust and gravel to keep the Geminid shower stocked with meteors.
Now we need new categories. Perhaps Phaethon is the remains of a nearly-dead comet. Its elliptical orbit and dark surface color are both comet-like. Maybe it's made enough trips around the Sun that it's been left parched and gravelly - at my age, I understand.
So what does all this ambiguity leave us? Introducing, "rock comet." As the lines blur between asteroids and comets, the idea of desiccated comets of rock and gravel is gaining ground. Rock on, Geminid mother, whatever you are.