By Peter Jenniskens
Meteor Astronomer, Carl Sagan Center, SETI Institute
Around the start of our year count, 2000 years ago, comet Kiess passed the sun and ejected a cloud of dust. Kiess completed one orbit in 1911 when it was discovered by Lick Observatory post-doc Carl Kiess. The dust took longer to return, and formed a continuous stream of dust particles that has been passing just outside Earth orbit ever since.
On Sept. 1, 2007, that trail of dust from Roman times will wander in the Earth's path again, causing an extremely rare meteor shower during the short time it takes the Earth to travel through the stream of dust. The meteors radiate from the constellation of Auriga, and are called Aurigids. Only three people alive today are known to have seen this shower before in 1935, 1986, and 1994. After the 2007 encounter, the Aurigids will not be seen again in our lifetimes.
The Aurigid shower will last only an hour and a half, with a bright Moon in the sky. The Moon is not expected to dim the spectacle much, however, because most Aurigids seen in the past were relatively bright -2 to +3 magnitude meteors. My colleague, Jeremie Vaubaillon of the California Institute of Technology, Pasadena, California, and I have performed detailed predictions of the dust stream's orbital evolution. The August 7 issue of the journal EOS, Transactions of the AGU, gives details of the encounter.
We predict that the shower will be visible by the naked eye from the western United States, especially in California, Hawaii, Alaska, other western states and from Mexico and the western provinces of Canada. Prime viewing time will be on Saturday, Sept. 1, 2007, a half hour on either side of 4:36 a.m. PDT. The whole event will last no longer than one-and-a-half hours. Twenty-five minutes long rates will be above half the peak rate.
Sunlight has pushed the comet's ejected particles into wider orbits around the sun in a thin stream just outside of Earth's orbit. On occasion, the combined gravity of the solar system's planets moves this dust trail into Earth's path. Only when Earth and dust trail collide do we see this meteor shower.
As they collide with the atmosphere, the dust grains of Kiess begin to vaporize at around 80 miles (130 kilometers) altitude, with the bigger ones penetrating down to as low as 50 miles (80 kilometers), before they are completely stopped. This fiery process creates a meteor.
Not only is the shower rarely seen, the Aurigid meteors also may be very unusual. Some could be bits of the comet's pristine crust. Comet Kiess returned from the Oort cloud of comets on the outskirts of the solar system only in recent history. Before that, Kiess spent 4.5 billion years in the Oort cloud, where cosmic rays baked its crust over the age of the solar system. Kiess could have shed some of this pristine crust 2000 years ago. Comets that return more frequently to the sun have long lost this pristine crust.
If so, the meteors are expected to penetrate 5 km deeper than normal in the atmosphere and lack a very specific color of yellow light from the element sodium. Such unusual meteors were seen once before during the alpha-Monocerotid meteor shower in 1995, caused by an unknown long-period comet.
RMStringer
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Let teachers and priests and philosophers brood
over questions of reality and illusion.
I know this: if life is an illusion, then I
am no less an illusion, and being thus, the
illusion is real to me. I live, I burn with life, I
love, I slay, and I am content.
(Robert E. Howard, Queen of the Black Coast, Weird Tales, May 1934)
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