USMP-2 status report #4 

USMP-2 Public Affairs Status Report #4
7:00 a.m. CST, March 08, 1994
3/23:07 MET
Spacelab Mission Operations Control
Marshall Space Flight Center
Huntsville, Ala.

Another successful day of microgravity science has been completed by the second
United States Microgravity Payload (USMP-2), managed for NASA by the Marshall
Space Flight Center.

The Critical Fluid Light Scattering Experiment, or Zeno, team reported
overnight that they are seeing behavior in the fluid xenon unlike any they have
seen on Earth. They believe this may mean the experiment has passed through the
xenon sample's critical point.  Meanwhile the team is continuing their delicate
temperature manipulations in order to verify what they have seen.  Once the
team is certain they have located the critical point, they will conduct a
series of precise measurements in the area surrounding it using laser light
scattering.  When xenon is at or extremely near its critical point -- the point
where it is simultaneously a liquid and a gas -- patches of the otherwise clear
substance briefly take on a "milky" irridescence.  Closer to the critical
point, the milky-white areas are larger and exist for longer periods.  When a
laser light is passed through the sample in these areas, fluctuations in the
sample's density cause the light to be scattered.

Team members for the MEPHISTO furnace are running a series of metal
solidification studies and are again receiving analyzable data.  On Monday, the
team made much progress in overcoming some difficulty they had been
experiencing with one of the experiment's electronic measurements and
successfully completed a Seebeck run.  The Seebeck measurement is an electrical
signal which measures temperature variations during crystal growth at the
boundary where liquid becomes solid -- the solidification front.  MEPHISTO is
used to conduct a series of melting and solidification cycles on three
identical rod-shaped samples of a bismuth-tin alloy.  During these runs,
temperature, velocity and shape of the solidification front are measured in
order to study the behavior of metals and semiconductors as they solidify.

Team members of the Isothermal Dendritic Growth Experiment (IDGE), say they are
well pleased with the performance of their apparatus and the data they are
acquiring during USMP-2. While dendrite growth is taking place, two 35mm
cameras are taking photographs for post-mission analysis.  When a dendrite
growth cycle is completed, the tiny crystalline structure is re-melted and
another grown at a different "supercooling" temperature.  Dendrites are being
grown at 20 different levels of supercooling ranging up to approximately 1.3
degrees C. Supercooling is the term used to describe the condition in which a
liquid is slowly cooled to below its normal freezing point, but due to its
purity, does not solidify.  The level of supercooling refers to the difference
between the temperature of the liquid and its normal freezing point.  IDGE is a
fundamental materials science experiment performed in the microgravity
environment of space in order to increase understanding of the solidification
processes.  This knowledge should be useful in improving industrial production
of a wide range of metals used in applications from aluminum foil to jet
engines.

The Advanced Automated Directional Solidification Furnace (AADSF) continues to
operate smoothly, growing a single cylinder-shaped crystal of mercury cadmium
telluride, an exotic material used as an infrared radiation detector.  The
AADSF provides scientists with a unique apparatus in which to test theories of
semiconductor crystal growth without the effects and limitations caused by
Earth's gravity.  The information gained by growing crystals of a semiconductor
material in microgravity can be used to study the physical and chemical
processes of many materials and systems.  A greater understanding in these
areas could aid researchers in the discovery of processes and materials that
perform better and cost less to produce.

All continues to go well for the Space Acceleration Measurement System (SAMS)
as it measures onboard accelerations and vibrations experienced during the
STS-62 Shuttle flight.  This information is transmitted to scientists on the
ground who can make adjustments to their investigations to improve their
results if the disturbances are significant.