[meteorite-list] Shooting Marbles at 16, 000 mph to Simulate Meteoroid Impacts

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 14 Mar 2007 15:33:56 -0700 (PDT)
Message-ID: <200703142233.l2EMXu418105_at_zagami.jpl.nasa.gov>


Shooting Marbles at 16,000 mph
NASA Science News

March 14, 2007: NASA scientist Bill Cooke is shooting marbles and he's
playing "keepsies." The prize won't be another player's marbles, but
knowledge that will help keep astronauts safe when America returns to
the Moon in the next decade.

Cooke is firing quarter-inch diameter clear shooters - Pyrex glass,
to be exact - at soil rather than at other marbles. And he has to use a
new one on each round because every 16,000 mph (7 km/s) shot destroys his

"We are simulating meteoroid impacts with the lunar surface," he
explains. Cooke and others in the Space Environments Group at NASA's
Marshall Space Flight Center have recorded the real thing
<http://science.nasa.gov/headlines/y2006/13jun_lunarsporadic.htm> many
times. Their telescopes routinely detect explosions on the Moon when
meteoroids slam into the lunar surface.

A typical flash involves "a meteoroid the size of a softball hitting the
Moon at 27 km/s and exploding with as much energy as 70 kg of TNT."

"Mind you," says Cooke, "these are estimates based on a flash of light
seen 400,000 km away. There's a lot of uncertainty in our calculations
of speed, mass and energy. We'd like to firm up these numbers."

That's where the marbles come in....

Cooke is using the Ames Vertical Gun Range at NASA's Ames Research
Center in Mountain View, CA, to shoot marbles into simulated lunar soil.
The firings allow him to calibrate what he sees on the Moon. His work is
funded by NASA's Office of Safety and Mission Assurance.

"We measure the flash so we can figure out how much of the kinetic
energy goes into light," he explained. "Once we know this luminous
efficiency, as we call it, we can apply it to real meteoroids when they
strike the Moon." High-speed cameras and a photometer (light meter)
record the results.

The Ames Vertical Gun Range was built in the 1960s to support Project
Apollo, America's first human missions to the lunar surface. The Ames
gun can fire a variety of shapes and materials, even clusters of
particles, at speeds from 0.5 to 7 km/s. The target chamber usually is
pumped down to a vacuum, and can be partially refilled to simulate
atmospheres on other worlds or comets.

Equally important, the gun's barrel can be tilted to simulate impacts at
seven different angles from vertical to horizontal since meteors rarely
fly straight into the ground. Black powder propels the marble, and
special valves capture the exhaust gases so they don't blow away the
impact crater.

Cooke's experiments are being run in two rounds. The first set of 12
shots in October 2006 fired Pyrex glass balls into dust made from
pumice, a volcanic rock, at up to 7 km/s. Follow-up experiments will use
JSC-1a lunar simulant, one of the "true fakes
<http://science.nasa.gov/headlines/y2006/28dec_truefake.htm>" developed
from terrestrial ingredients to mimic the qualities of moon soil.

Knowing the speed and mass of the projectile will let Cooke to scale the
flash and estimate the energies of the softball-size meteoroids that hit
the Moon at up to 72 km/s, more than six times the speed of the Ames
gun. But luminous efficiency is just part of the question. A lot of the
impact energy goes into shattering and melting the projectile -- the
main reason for using glass rather than metal -- and then spraying
debris everywhere.

"The ejecta kicked out from an impact can travel hundreds of miles,"
Cooke continued. "We need to know more about that if we are going to
live on the lunar surface for months at a time." Because the moon has
virtually no atmosphere to slow down flying debris, particles land with
the same speed that launched them from the impact site.

So you might dodge a bullet but still get caught by its shrapnel. And
the question is, Are you more likely to get cut off at the ankles by
debris spattered along the horizon, or hit from above by material on
high, ballistic trajectories?

To gauge that danger, Cooke will measure the speed and direction of
secondary particles by the sheet-laser technique. Lenses and mirrors
spread a laser beam into paper-thin sheets of light so flying particles
are briefly illuminated several times. The light traces then tell the
size, direction, and speed of debris particles leaving an impact.

The technique requires a lot of image analysis, but it is cleaner and
more accurate than the older way of hanging aluminum sheets in the
chamber and counting holes.

The answers will help determine the kinds of shielding needed on
exploration vehicles protecting humans where every day is for "keepsies."
Received on Wed 14 Mar 2007 06:33:56 PM PDT

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