The Perseid meteor shower has come and gone, but that doesn't mean that there aren't any more good meteor displays to look forward to. In fact, another potentially good shower is just around the corner, scheduled to reach its peak during the early hours of Saturday, Sept. 1: the Aurigid meteors.
The Aurigids get their name from the constellation of Auriga the Charioteer. The meteors appear to emanate from a spot in the sky near the bright yellowish star Capella in Auriga. The Aurigids, however, are rarely mentioned in most astronomy guidebooks because they are hardly worth mentioning in any given year.
Then why bother about a shower that almost nobody has heard of and that's due this year just days after a bright full moon?
Because in 2007, these unheralded Aurigids are this summer's wild card.
Unlike the steady and reliable Perseids, little or no activity is usually seen from the Aurigids. Yet, every once in a great while a short-lived outburst of bright meteors has been reported from the Aurigid shower. Peter Jenniskens of the SETI Institute's Carl Sagan Center has recently shown that these rare outbursts of Aurigid activity were caused by a comet which ejected a trail of dust that very occasionally wanders into the Earth's path.
Outburst in 2007?
Aurigid outbursts are very infrequent, having been definitively seen on only three occasions: in 1935, 1986 and 1994. There were very few witnesses on each occasion, because these sudden outbursts were totally unexpected.
But now, for the first time, an alert for an impending Aurigid outburst has been issued.
At the 26th General Assembly of the International Astronomical Union Conference held last August in Prague, Czech Republic, Jenniskens announced that the Aurigids would likely produce another round of unusual meteor activity on Sept. 1 of this year. This forecast is based on calculations made by Jenniskens and colleague Jérémie Vaubaillon of Caltech, and on earlier work in collaboration with amateur astronomer Esko Lyytinen of Finland.
The comet responsible for the Aurigids is well known: Comet Kiess, which last appeared in 1911. In carefully examining its movements, the meteor scientists have determined that prior to 1911, this comet previously swept passed the sun sometime around the year 82 BC (when Julius Caesar was alive). Jenniskens and Vaubaillon calculated that a trail of dust released by the comet at that 82 B.C. visit will run smack into Earth's path when our planet passes by on September 1.
Western states favored
On Sept. 1, in the hour centered on about 11:37 UT, Earth should encounter the trail of dust released from Comet Kiess.
That moment (4:37 a.m. PT) favors the far western parts of the United States, where the constellation Auriga will be about two-thirds up in the northeast sky around the time morning twilight begins. If you live in Portland, San Francisco or Los Angeles, you'll be in perfect position to view the meteors.
Hawaii (1:37 a.m.) and Alaska (2:37 a.m.) will also be in darkness when the Aurigids arrive, although Auriga will appear much closer to the northeast horizon. Like the previous outbursts, the upcoming display is expected to be short-lived, probably lasting no more than an hour or so at most.
Unfortunately, most of the rest of North America will be out of luck, either bathed in bright twilight or daylight when the dust trail arrives.
Jenniskens and Vaubaillon are forecasting a ZHR (zenithal hourly rate – the number of meteors seen by a single observer under a clear, dark sky with the meteors' emanation point directly overhead) of 200. Their outlook is very similar to Esko Lyytinen, who predicts a ZHR of 300, but with an added uncertainty of 1 to 3. Notes Lyytinen, "This would mean something between about 100 and 1,000. Let's hope this will be around 1,000!"
There is, however, a significant drawback: A bright gibbous moon will be lighting up the sky on the morning of the shower.
But because the Aurigids ram through our atmosphere at exceptionally high speeds of 41 miles per second (66 kilometers per second), and since these particles are predicted to be rather large, the display is expected to be rich in bright meteors, with many appearing as bright as the brightest stars and a few perhaps approaching or even rivaling Jupiter and Venus. "So," says Jenniskens, "the moon probably won't dim much of the display."
Probably the safest bet is to play it conservative and forecast a rate of perhaps only 20 to 30 per hour, which is what was observed during the three previous Aurigid outbursts. But if those meteors are as bright as the calculations suggest they will be, it could still be a memorable, albeit brief, show.