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,
September 1st: 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 September 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 BC visit will
run smack into Earth's path when our planet passes by on September 1.
Far-western states favored
On Saturday morning, September 1st, 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. Pacific Daylight Time) 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 kps), 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.
Joe Rao serves as an instructor
and guest lecturer at New York's
Hayden Planetarium. He writes about astronomy for The New York Times and other
publications, and he is also an on-camera meteorologist for News 12 Westchester, New
York.
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