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

As the STS-62 Space Shuttle mission nears the end of its third day,
the second United States Microgravity Payload (USMP-2) experiments,
located in the cargo bay of Columbia, continue to produce a wealth
of data for scientists on the ground.  The USMP-2 part of
Columbia's mission is managed for NASA by the Marshall Space Flight
Center.

The Critical Fluid Light Scattering Experiment, or Zeno, science
team reports that they expect to locate the critical temperature of
xenon at "any time."  Team members are closely watching computer
data traces which indicate their experiment is very near the
critical temperature -- the goal of a lengthy, methodical
"sensitive" search process.  This is a more precise search for the
critical temperature after its location has been determined within
a narrow band.  Once the temperature is located, the team will
spend nearly 24 hours taking a good look at the phenomenon they've
waited years to see.  They will study the properties of xenon at
its critical point, taking subtle optical measurements in the
region surrounding it. A fluid's "critical point" occurs at a
condition of temperature and pressure where the fluid is
simultaneously a gas and a liquid.  By understanding how matter
behaves at the critical point, scientists hope to gain a better
insight into a variety of physics problems ranging from phase
changes in fluids to changes in the composition and magnetic
properties of solids. 

The Space Acceleration Measurement System (SAMS) continues to
measure the microgravity environment on the USMP-2 carrier in
support of the four other experiments onboard.  The SAMS team has
begun sending results of their data collection during various
orbiter activities to STS-62 crew members.  The crew is interested
in how they can minimize their influence on the microgravity
environment.  Measurements are made with the system at specific
times when microgravity disturbances may be caused by events such
as crew exercise and movement of the Shuttle's Ku-band antenna.
Such observations also collect "signatures" which the team will be
able to easily identify in future data.

A related system, the Orbital Acceleration Research Experiment
(OARE), is managed by NASA's Johnson Space Center.  It is useful on
missions such as USMP-2 where it is important to accurately
characterize a wide variety of disturbances in the microgravity
environment.  Working closely with SAMS, the OARE records any
low-frequency activity such as the Shuttle's friction with the
rarefied upper atmosphere.  SAMS is most suitable for recording
higher-frequency activity such as crew exercise.  The OARE        
instrument continues to process data in support of the USMP-2
experiments, and team members say all is going well.

The Isothermal Dendritic Growth Experiment (IDGE) is continuing to
assemble data to test theories concerning the effect of
gravity-driven fluid flows on dendritic solidification of molten
materials.  When the USMP-2 mission is over, the IDGE team will
study hundreds of photographs taken of the dendrites grown in
microgravity.  Learning more about how dendrites grow is one
valuable key to developing better metal products and improving our
industrial competitiveness.

Upon completion of its first phase of pre-programmed operations
last night, the dendritic experiment entered its second phase of
crystal growth when team members began sending commands to their
experiment from the ground using a unique set of capabilities known
as "telescience."  This allows them to get the best possible data
from their investigation.  

The Advanced Automated Directional Solidification Furnace (AADSF)
studies the directional solidification of semiconductor materials
in microgravity.  Downlinked experiment data indicates that
solidification of a crystal of mercury cadmium telluride is taking
place, and the AADSF science team is constantly monitoring this
slow but steady progress.  Testing the AADSF in microgravity is
beneficial because on Earth, gravity causes fluids to rise or fall
within the melted portion; a warm liquid is less dense than a cool
one and will rise to the top of the melt.  These convective
movements of molten material contribute to physical flaws in the
internal structure of the growing crystal.  Such flaws affect a
crystal's overall electrical characteristics, and consequently, its
usefulness in electronic devices.

The MEPHISTO team reports that they have gathered good data with
their directional solidification furnace.  Currently, however, the
team is still troubleshooting a problem discovered on Saturday
night with a troublesome "Seebeck measurement."  This electronic
signal measures changes in the microstructure of a solidifying
metal, and is conducted on one of three experiment samples of
bismuth-tin.  Other measurement techniques will be used on the two
remaining samples later in the mission; both these samples are
operating nominally.  Measurement data from the three samples will
give scientists insight into the precise nature of solidification
in reduced gravity.