Houston, We Have a Solution
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New research aboard the space station aims to adapt a tried-and-true repair tool to weightlessness
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November
10, 2003: Even on the advanced spacecraft of the future,
sooner or later things are bound to break. Space travelers on an extended
mission to Mars, for example, would need to be ready to do small repairs
just in case, soldering this, welding that.
In the unforgiving environment of space, a successful solder-repair
to, say, navigation hardware or an oxygen pump could mean the difference
between life and death.
Right:
Astronaut James Voss uses a soldering iron onboard the International
Space Station.
"Surprisingly, relatively little is known about the unique physics
of soldering in microgravity," says Richard Grugel, a scientist at
NASA's Marshall Space Flight Center who specializes in the solidification
of metals.
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Melted solder behaves differently in space. In the microgravity aboard a spacecraft, molten solder doesn't "feel" a downward pull as it does on Earth, so surface tension becomes more prominent and causes blobs of liquid solder to flow differently. In other words, soldering techniques that work on the ground may not form proper joints in orbit.
Below: A scanning electron micrograph of solidified solder on a wire. The precise angle at which the solder meets the wire is driven by forces such as surface tension, gravity, and wetting of the wire. Removing the effects of gravity lets physicists better understand these other influences. Image courtesy NASA.
Even
worse, those soldered joints could be much weaker in orbit. Gas bubbles
in the molten solder that often rise to the surface and disappear
on Earth can remain suspended within the liquid in orbit. That's because
the pull of gravity is the force behind buoyancy, so in microgravity
bubbles don't necessarily rise. These tiny bubbles become trapped
within the solder as it solidifies, making the joint less effective
and more prone to breakage--not a good thing when you're in the mission-critical
environment of space!
"How prevalent are these gas bubbles
in joints soldered in space? We need to find out, and perhaps find
some way to get those gas bubbles out so that we can make good joints
that won't break," Grugel says.
The experiment is straightforward: metal wires will be soldered together
in various arrangements, reflecting common geometries important for
making repairs. Space station crewmembers will use a soldering iron
to join these wires with lead-tin solder.
While the shuttle fleet is grounded, room for sending research equipment
and materials up to the space station is severely limited. The ISSI
experiment was selected in part because it requires no "up-mass" sent
to the station from the ground. The space station already has a soldering
kit onboard and coils of silver-coated copper wire.
Right: The experiment will be performed with reshaped wire (top frame) and a soldering kit (bottom frame) aboard the space station. Image courtesy NASA.
The
Expedition 7 crew, which recently returned to Earth, fashioned the
wire into L-shapes, loops, parallel threads, and other shapes in preparation
for the experiment. The researchers plan to have the current space
station crew (Expedition 8) perform the experiment during their stay,
which is scheduled to last until April 2004.
While the experiment happens, the researchers will be able to watch it unfold live at NASA's Telescience Center at the Marshall Space Flight Center in Huntsville, Alabama. They'll be watching to see the differences in how the solder flows and the shapes it assumes in space, which will offer clues to the fundamental physics of molten solder.
If
they see something unexpected, the live interaction will allow them to
suggest procedure changes to the station crew members in real-time.
After the samples are returned to Earth, the scientists will cut open
the joints to see how many gas bubbles were trapped during the process,
and they'll test how strong the joints are.
"We should be able to gain a window into how surface tension effects
soldering in microgravity," Grugel says. "This will help lay a
foundation for the future of in-space fabrication and repair."
more information
In Space Soldering Investigation (ISSI) -- information on the experiment from NASA's Office of Biological & Physical Research
In Space Soldering Investigation fact sheet -- from the Marshall Space Flight Center
Soldering
here on Earth: In addition to its importance for future space
missions, the ISSI experiment will benefit the terrestrial electronics
industry, according to Grugel. The industry is currently moving away
from lead-tin solder because of lead's negative health effects, but
as it does it's also losing much of the 50 years of experience it
has with lead-based solders.
"A
better theoretical understanding of the physics of molten solder will
help close the book on lead-based solder as well as open the door
for industry's newer work with lead-free solders," Grugel says. "Microgravity
research is a good way to explore that physics."
Above: Hardened solder on a wire. Notice how the solder has been pulled downward by gravity, and the gold-colored flux has accumulated at the bottom of the drop. Image courtesy NASA.
Other soldering in microgravity research: by Imperial College, London; by the Ursa Astronomical Association in Finland; experiments at the 2.2 second drop tower at NASA's Glenn Research Center.
A New Look at Molten Metals in Space -- (Science@NASA) Scientists discuss ways to peer inside solid materials
Hoolder electronics -- downloadable instructional videos, from NASA's Goddard Space Flight Center
Research on the ISS -- information about other science experiments currently aboard the International Space Station
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