I will always
remember the Aurigid shower, which put up a strong show for us scientists
deployed at 47,000 ft in two aircraft over California. I also cherrish the many
enthusiastic reports that poured in from observers on the ground, who had no
difficulty seeing the rare display of bright meteors. For some, there could
have been more....
If you,
too, have pictures of the September 1 shower, please let us know.
The
event was caused by the fortuitous collision of Earth with the
ever-wandering dust trail of comet C/1911 N1 (Kiess), a potential Earth
impactor and a source of debris that might harm satellites in orbit. In other
years, that stream of dust particles passes by undetected, just outside or
inside Earth's orbit, but in 1935, 1986, and 1994 the stream wandered into
Earth's path and created a brief shower of bright meteors.
The shower
was seen by only two observers in 1994 and one observer in 1986. Thanks to the
power of foresight, this year thousands were able to enjoy the spectacle.
"This
was my daughter's (age 9) first shower and she and her friend were absolutely
delighted, asking: 'When's the next one?' Hopefully we have a new astronomer in
the making!" wrote
Doug Blasco of California, one of many people who witnessed the meteor
outburst.
This is the
first time that the encounter with the dust trail of a long-period comet has
been predicted
through modeling. Each meteor was a small time capsule from the year 4 A.D.,
give or take 40 years, when the comet rounded the sun. Comet and dust moved in
a 2000-year orbit far beyond Neptune, all the way into the Oort cloud, before
falling back again. The comet returned in 1911 and was discovered by then Lick
Observatory post-doc Carl Kiess. The dust took longer to return, pushed into a
wider orbit by solar radiation, and has been coming back ever since in a continuous,
but very thin, stream of dust.
As
predicted, Earth was
hosed by that dust in the early morning of September 1, 2007, when, during
a short period of time, meteors radiated from the constellation of Auriga, the
Charioteer. The shower was seen in most states west of the Mississippi,
including Hawaii and Alaska, and from the western provinces of Canada. Some eyewitnesses watched the shower in a jacuzzi, or from their bedroom window.
Each observed between a handful and several tens of bright and colorful
meteors, some with flares and brief trains. More than a hundred observers wrote
us about their experiences and so far 53 contributed images of the Aurigids,
captured with digital cameras. Some of the images are quite spectacular, also
because the 4-day past full moon nicely lit the landscape that framed the
bright meteors in the sky.
Daytime
observers in Europe, the eastern USA, and Brazil detected the presence of the
shower from reflected radio signals while listening for distant radio stations.
The shower was very obvious, because there were many long-lasting overdense
echoes during a 1.5-hour period.
Earlier
that night, an international team of 24 researchers had departed in two Gulfstream V aircraft from NASA
Ames at Moffett Field, a little nervous about whether the predictions would
hold. But as soon as the planes climbed into the sky, the first Aurigids were
seen and rates continued to climb during the night. As the shower peaked, there
were enthusiastic cries whenever another meteor spectrum was recorded.
The
airborne observers measured meteor rates and magnitudes, and collected
spectroscopic data with a wide variety of instruments. They were able to monitor
a large surface area near the horizon, where extinction is low at 47,000 feet.
Each observer saw around 120 Aurigids during the outburst in a 40 x 30 degree
field of view. Results of our counts were called down and were immediately
posted on our mission website, to the delight of those clouded out or living in
parts of the world where the shower could not be seen.
The shower
peaked at 04:15 +/- 5 minutes PDT, earlier than our predicted 04:36 PDT +/- 20
minutes, reported in the August 7 issue of EOS, Transactions of the AGU. At the peak, meteors were detected
at a Zenith Hourly Rate of about 100/hr, better than the August Perseids, and
within a factor of two from the anticipated rate. The details of the shower
profile and particle size distribution will help improve our prediction model.
Comet
Kiess, on account of being in a long-period orbit, has returned from the Oort
cloud relatively recently. Whether or not some of the meteors may have been caused
by the remains of a proposed primordial comet crust awaits a further analysis
of the many meteor trajectories and spectra that were recorded by airborne and
ground-based observers.
The Aurigid
Multi-Instrument Aircraft Campaign (Aurigid MAC) was the first deployment of
the two Gulfstream V aircraft in a research mission. We received tremendous
support, and thank all that made this mission possible. Sadly for Doug's
daughter, the Aurigid shower will not return in her lifetime, but there will be
other opportunities to study unusual meteor showers. At which time, we all hope
to do this again.
The first
impressions, images, and predictions of the Aurigid shower (and instructions on
how to submit your Aurigid images) are posted at: http://aurigid.seti.org
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