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Nov.
10, 2008: It started out as a normal day. NASA astronomer
and meteor expert Bill Cooke woke up, dressed, and went to
his office at the Marshall Space Flight Center. Colleagues
greeted him as usual, there was no hum of excitement.
And
then he checked his email.
"That's
how I found out—I'd slept through a meteor outburst!"
During
the dark hours before dawn on Sept. 9, 2008, a surprising
flurry of meteors had showered the skies above Huntsville,
Alabama. More than two dozen of them were fireballs brighter
than Jupiter or Venus; a few even cast shadows. Cooke like
everyone else he knew was sound asleep and saw nothing.
But
Cooke's all-sky Sentinel camera located on the grounds of
the Marshall Space Flight Center recorded the whole thing
and, when it was done, left him an email summarizing the outburst.
Above:
An outburst of bright meteors over the Marshall Space Flight
Center observed by the Sentinel system on Sept. 9, 2008. [movie]
"Our
Sentinel system consists of a computer-controlled camera,
fisheye lens and digital video recorder. It was developed
by researchers at the University of Western Ontario for studies
of meteors over Canada, and now we've adapted it for our purposes.
Every night Sentinel patrols the sky, looking for the unexpected,
and it never gets sleepy."
In
years past, sky watchers had occasionally noticed a small
number of dim meteors streaking out of the constellation Perseus
around Sept. 9th. The shower, hailing from an unknown comet,
was named "the September Perseids" and rarely monitored
because it was thought to be a feeble display.
"Now
we know better," says Cooke. "The Sept. Perseids
of 2008 were fantastic." Sometime in the past, the shower's
parent comet must have laid down a stream of dusty debris
which is now drifting across Earth's orbit. Apparently, the
stream contains clumps or filaments of dust that can produce
outbursts of meteors when Earth runs into one. "How often
this happens is anyone's guess."
Answering
the question how often? is one of the goals of the
Sentinel system. There could be many unknown streams of debris
"out there" crossing Earth's orbit, causing outbursts
that go unnoticed because, well, even astronomers need their
sleep. Using Sentinel, "we can discover new meteoroid
streams that could pose a threat to spacecraft and satellites—or
just put on a pretty show from time to time."
It
would have been nice to backtrack the fireballs of Sept. 9th
to their parent comet, solving the mystery of their origin,
but Sentinel couldn't do that. A single camera is not sufficient
to measure a meteoroid's 3D trajectory. To remedy the problem,
Cooke's team has since set up a second camera 100 miles away
in north Georgia at the Walker County Science Center.
"With
two cameras, we can gather the data we need to calculate orbits,"
he explains.
The
first successful test of the two-station Sentinel system came
on Oct. 1, 2008, when a centimeter-sized meteoroid hit Earth's
atmosphere over the southeastern United States with about
as much energy as 500 pounds of TNT. Both cameras recorded
the fireball:
Above:
A meteor explodes in the atmosphere above Huntsville, Alabama,
on Oct. 1, 2008. The full-length
movie shows the same fireball first over Huntsville, Alabama,
and then over Walker County, Georgia. [movies:
mov, avi,
m4v]
Using
Asgard software developed by Rob Weryk of the University
of Western Ontario, the Sentinel system automatically calculated
the orbit of the meteoroid and emailed the results to Cooke.
"It
came from the asteroid belt," he says: diagram.
Cooke
is especially interested in centimeter-class meteoroids because
he and his colleagues at the NASA Meteoroid Environment Office
frequently see them hitting the Moon. Since 2005, they've
recorded more than 100 lunar impacts. Unlike Earth, the Moon
has no atmosphere to cushion the blow of incoming meteoroids;
they simply hit the ground and explode. With NASA planning
to send people back to the Moon, the frequency and power of
lunar impacts has become a matter of considerable interest.
By studying the meteoroids at close range in the skies over
Alabama, he hopes to learn more about their properties, especially
their speeds, which is an important factor in luminous efficiency--i.e.,
how much of a meteoroid's kinetic energy is converted to light
when it disintegrates upon impact. This will help researchers
understand the distant flashes they see on the Moon.
Uncovering
new meteor showers on Earth is icing on the cake.
"Checking
my email," says Cooke, "has never been so much fun."
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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