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August
8, 2007: Will they come, or will they not? That is
the question.
On
Sept. 1, 2007, a flurry of bright and oddly-colored meteors
might—emphasis on might--come streaming out of the
constellation Auriga, putting on a beautiful early morning
show for sky watchers in western North America: sky
map.
 Right:
An early morning meteor over Italy in Nov. 2006. This is what
an Aurigid might look like. Photo credit: Antonio Finazzi.
The
source of the putative shower is Comet Kiess (C/1911 N1),
a mysterious "long-period comet" that has visited
the inner solar system only twice in the past two thousand
years. In 83 BC, give or take a few centuries, Comet Kiess
swung by the sun and laid down a trail of dusty debris that
has been drifting toward Earth's orbit ever since. On Sept.
1, 2007, the dusty trail and Earth will meet.
But
will a shower actually materialize? The answer lies in the
unknown contents of the debris stream.
"We
have so little experience with ancient debris from long-period
comets," notes Bill Cooke of NASA's Meteoroid Environment
Office (MEO) at the Marshall Space Flight Center. "Almost
anything could happen—from a fizzle to a beautiful meteor
shower."
Astronomers
were first alerted seven years ago to the possibility of a
shower by Finnish astronomer Esko Lyytinen and Peter Jenniskens
of the SETI Institute in Mountain View, CA—both are accomplished
meteor forecasters who study the dynamics of cometary dust
trails. Jenniskens has since teamed with Jeremie Vaubaillon
of Caltech to refine the forecast using a debris stream model
developed by Vaubaillon.
"We
expect the outburst to peak at 11:36 UT (4:36 a.m. PDT) +/-
20 minutes on Sept. 1st," says Jenniskens. "The
whole event should last about 2 hours and be visible from
California, Oregon, Hawaii and the eastern Pacific Ocean."
Above:
Earth at the time of the shower's predicted peak. Darkness
favors sky watchers west of the Rocky Mountains. [Larger
image]
An
independent model of the debris stream calculated by Danielle
Moser, a colleague of Cooke at the MEO, predicts a peak time
of 11:26 UT. "That's in good agreement with Jenniskens
and Vaubaillon," says Cooke. "However, our model
predicts a mostly empty stream and a very weak shower."
"Personally,
I think the meteor rate will reach 100 per hour at best,"
notes Vaubaillon, "but some colleagues are more optimistic,
so, suspense, suspense!!!"
Earth
has had at least three encounters with the debris stream in
the past century—in 1935, 1986, and 1994. Unfortunately, few
people were outdoors paying attention. The best observed encounter
was in 1994 when veteran meteor watchers Bob Lunsford and
George Zay of southern California witnessed a number of bright
blue-green meteors emerging from Auriga. The brief shower
was remarkable both for its conspicuous lack of faint meteors
and for the vivid colors--characteristics that may be repeated
on Sept. 1st.
Meteors
from long-period comets are of special interest for two reasons:
#1
-- Long period comets almost always take us by surprise.
They linger in the outer solar system, hiding in the dark
for thousands or millions of years, until their slow orbits
turn them sunward and--in they plunge! Because of this surprise
factor, long period comets pose a unique impact threat. Jenniskens
and others are keen to study meteor showers from long period
comets because the showers could be a "tell" that
a comet is out there, and the orbit of the meteoroids can
reveal where.
#2
-- Meteors from long period comets may be very primitive.
Consider the following: Most meteor showers (e.g., the Perseids
and Leonids) are caused by short period comets, which pass
through the inner solar system every few decades or, at most,
centuries. Their icy surfaces are frequently heated and vaporized
by intense sunlight, and the comet dust they produce is correspondingly
fresh. Long period comets, on the other hand, are rarely sun-blasted,
and their surfaces may retain ancient substances formed by
billions of years of cosmic ray exposure in the outer solar
system. Flakes from this "pristine crust" may produce
odd colors when they hit Earth's atmosphere.
Is
that why the Aurigid meteors of 1994 were blue-green? Were
they bits of pristine crust from Comet Kiess? Again, no one
knows.
 Jenniskens
notes that another meteor outburst, the alpha Monocerotids
of 1995, also thought to hail from an unknown long-period
comet, was strange: "The alpha-Monocerotids penetrated
5 km deeper in the atmosphere than other meteors of similar
size and speed and they had [an unusually] low content of
sodium."
Right:
Flight path of Jenniskens' airborne Aurigid observing campaign.
[More]
To
get to the bottom of some of these mysteries, Jenniskens and
colleagues from the NASA Ames Research Center, Utah State
University, the USAF Academy and elsewhere will board two
private jets to observe the Aurigids from the clear air of
45,000 feet. They'll use spectrometers, cameras and telescopes
to measure the velocity, penetration, and chemical composition
of incoming meteoroids.
Bill
Cooke of the MEO won't be on board, but he wishes the flyers
well. "If this shower actually happens, they data they
collect may tell us new things about an important population
of meteoroids in the solar system. Plus, it would be a good
show for people on the ground."
Sept.
1, 2007: The answers await.
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Author: Dr.
Tony Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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