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Leonids Sample Return Mission Update


Leonids Sample Return Mission Update
Marshall Space Flight Center
http://science.nasa.gov/newhome/headlines/ast01apr99_1.htm

NASA scientists will describe initial results from a program to catch
meteoroids in flight at the NASA/Ames Leonids Workshop April 12-15, 1999

Apr. 1, 1999: Later this month NASA scientists will present initial results from an
innovative program to catch meteoroids in flight through Earth's atmosphere.
The paper, to be presented at the NASA/Ames Leonid Workshop on April 12-15,
will describe tiny particles captured 20 km above Earth's surface during
last year's Leonids meteor shower.

During the peak of the 1998 Leonids, Dr. David Noever and colleagues from
the NASA/Marshall Space Flight Center launched a 10m weather balloon into
the stratosphere where it hovered for nearly 2 hours above 98% of Earth's
atmosphere. The payload included a digital video camera and an xerogel
micrometeorite collector, similar in some respects to the cosmic dust
collector on board the Stardust spacecraft now in route to comet Wild-2.

"The 1998 balloon flight was really intended as an engineering test," says
Noever. "We wanted to see if it was possible to sample meteoroids -- or any
kind of dust -- in the stratosphere and to evaluate the hardware in our
payload before the 1999 Leonids."

As it turned out, the 1998 flight was more successful than anyone expected.
The video camera recorded a number of fireballs that were broadcast live on
the web to nearly 100,000 people. Over 1,000,000 people saw the replay the
next day.

That's not all. After the balloon payload was recovered, Noever and his
colleagues used an Environmental Scanning Electron Microscope at NASA's
Marshall Space Flight Center to examine the xerogel dust collector. They
found 8 tiny craters created by impacts from particles measuring 20 to 50
microns in diameter.

"Are they Leonid meteoroids?" asks Noever. "We really don't know at this
point. The size of the particles is about what we would expect for
meteoroids in the stratosphere, but they may also be terrestrial."

The answer a least partly lies in the chemical composition of the impactors.
Particles like meteoroids that have entered the atmosphere at high speeds
tend to be enriched in elements that are not easily vaporized by the extreme
heat of atmospheric friction. The mass ratios Mg/Si, Al/Si and Fe/Si along
with isotopic abundances can usually be used to distinguish terrestrial from
extraterrestrial particles.

"The way the impactors look is important, too" continued Noever. "Meteroids
that have been partially melted by atmospheric friction often have a
translucent rim surrounding an unmelted, opaque core. Right now we're
studying both the appearance of the impactors and their chemical makeup.
Hopefully we'll have an answer soon."

Meanwhile, Noever and his colleagues plan to launch an identical balloon
this month, on April 10, to sample the dust environment of the stratosphere
when the Earth is not passing through a major meteoroid debris stream like
the Leonids. "During the first quarter of every year there's a minimum of
meteor activity," explains Dr. Tony Phillips, a NASA astronomer. "Earth
doesn't pass through any major cometary debris fields from about Jan 15
until late April of each year. There's nothing special about those months.
The low meteor flux during that time is just a result of the random
distribution of comet debris trails in the solar system."

The scientists working on this project hope that the balloon flight
scheduled for April 10 will provide valuable information about the
background environment of dust and meteoroids in the stratosphere for
comparison with their Leonids sample. Next week Science@NASA will provide
more details about the upcoming launch and offer readers an opportunity to
participate in this research by counting visual meteors on the night of the
flight.

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