STS-75 Day 7 Highlights

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On Wednesday, February 28, 1996, 7 a.m. CST, STS-75 MCC Status Report # 13 reports:

With the focus of work aboard Columbia turning to weightless investigations of the United States Microgravity Payload, flight controllers in Houston and Huntsville continue to gather as much data from the tethered satellite as possible before its batteries run out.

USMP experiments are collecting data on various materials science investigations including crystal growth, materials solidification and fluid dynamics.

Half of the crew enjoyed a half day of free time as is customary on two-week long shuttle flights. During the break, Pilot Scott Horowitz, Mission Specialist Maurizio Cheli and Payload Specialist Umberto Guidoni discussed the flight with the C-SPAN network and answered questions from viewers.

The ground commanding to the tethered satellite began yesterday and is expected to continue today as it passes within range of the ground station at the Johnson Space Center. Payload officials will again take snapshots of data from the satellite to determine its health before its batteries completely discharge; this is expected within the next couple of days.

On Wednesday, February 28, 1996, 6 a.m. CST, STS-75 Payload Status Report # 11 reports: (5/15:42 MET)

As the third United States Microgravity Payload (USMP-3) performs its early primary science operations aboard the STS-75 Shuttle mission, the Tethered Satellite System (TSS) science teams at Spacelab Mission Operations Control continue to receive data from the free-flying tethered satellite.

Overnight, Commander Andrew Allen maneuvered Columbia through a series of orbiter calibrations for NASA Marshall Space Flight Center's Advanced Automated Directional Solidification Furnace (AADSF) which performed a "dress rehearsal" for their first experiment sample processing, scheduled for early Thursday morning. The Shuttle's on-board acceleration sensors made measurements that gave the AADSF science team valuable information about how they will make adjustments to enhance science collection during this first experiment run. The AADSF will grow three cylinder-shaped crystals using samples of lead-tin-telluride, a material used to make infrared radiation detectors and lasers. This process is expected to take a number of days to complete. It is important to conduct this kind of experiment in microgravity in order to provide insight into the influence of gravity-driven fluid flows on crystal formation.

Scientists hope that information gathered from this experiment will lead to the production of better, faster semiconductors for components used in products ranging from automobiles to computers. The speed and amount of information that can be stored and sent by computers and high-tech electronics, using sophisticated semiconductor materials, may be increased by better control of how the semiconductors' structures form.

AADSF findings will help researchers develop improved processes for semiconductor materials that cost less to produce. "The ages of humanity are known for the type of materials humans used, such as 'Stone Age,' 'Bronze Age,' etcetera," observed AADSF Principal Investigator Dr. Archie Fripp of NASA's Langley Research Center. "If we can contribute in any way to the development of the 'Semiconductor Age,' then we have really achieved something."

Rensselaer Polytechnic Institute's Isothermal Dendritic Growth Experiment (IDGE), has completed its "science of opportunity" phase and, now that USMP-3 is the primary mission payload, has entered its main science gathering mode. The investigation team members are studying the three-dimensional shape of their tiny "pine tree-shaped" dendrite crystals. The device consists of a thermostat containing a growth chamber filled with a transparent substance that imitates the behavior of metals and solidifies as it cools to form these dendrites.

IDGE scientists, led by Dr. Martin Glicksman, used the preliminary acceleration data gathered by NASA Lewis Research Center's Space Acceleration Measurement System (SAMS) and Orbiter Acceleration Research Experiment (OARE) to evaluate their opportunity science data and to relate the size and speed of the dendrite growths with accelerations in the microgravity environment. Also, scientists looked for relationships between the angle of the dendrites' direction and the direction from which microgravity influences their growth. The experiment's science team at Rensselaer will use their telescience capabilities to remote-command and receive data from their IDGE instrument.

Early in the evening, the Critical Fluid Light Scattering Experiment, or ZENO, science team, led by Dr. Robert Gammon of the University of Maryland, completed the collection of dynamic light scattering data just above their xenon sample's "critical point." The critical point is the exact temperature and pressure at which the sample is simultaneously in both the liquid and vapor phase. Understanding how matter behaves at the critical point can provide insight into a variety of physics problems, ranging from state changes in fluids to changes in the magnetic properties of solids. This knowledge will prove valuable in a wide variety of applications, including liquid crystals and superconductors.

Meanwhile, science teams for the Tethered Satellite System demonstrated their own brand of telescience throughout the night, commanding and receiving data from their reactivated instruments on the satellite. The Marshall Space Flight Center's Research on Orbital Plasma Electrodynamics (ROPE), the Italian Space Agency's Research on Electrodynamic Tether Effects (RETE) and the Second University of Rome's Magnetic Field (TEMAG) experiments continue to collect what TSS Mission Scientist Dr. Nobie Stone called "very good data" about the satellite's interaction with its surrounding region of charged particles and magnetic fields.

Scientists report that they can measure a sunlight-induced electrical charge on the satellite as it moves through the daylight and night portions of its orbit around the Earth. Later today, the satellite's ROPE and RETE instruments will measure the effects of electron gun firings from the Shuttle Electrodynamic Tether System (SETS) on the satellite and its environment.

Also today, science data collection by the USMP-3 experiments will continue, and the crew will perform a Forced Flow Flamespreading Test (FFFT), an investigation led by Dr. Kurt Sacksteder of NASA's Lewis Research Center, the first in a series of hands-on combustion experiments conducted in the Middeck Glovebox.

On Wednesday, February 28, 1996, 5 p.m. CST, STS-75 MCC Status Report # 14 reports:

NASA managers decided today not to return Columbia to the Tethered Satellite for either a close inspection or a possible retrieval after concluding that propellant margins would not be adequate to support the operations.

Officials were asked to investigate the possibility of a rendezvous with TSS. After reviewing propellant margins, spacewalking considerations, and radar requirements, a decision was made not to fire Columbia's engines to begin what would have been a six-day phased approach to meet up with the satellite.

The Tethered Satellite is currently 7,100 nautical miles ahead of Columbia with the distance between the two spacecraft closing at the rate of 340 nm with every revolution of the Earth. The two spacecraft will pass within 50 nm of one another about 11:48 p.m. Thursday, at a Mission Elapsed Time of 7/09:30.

Earlier today, Mission Specialist Jeff Hoffman set a record for the most hours flown on a Space Shuttle, breaking the previous record of 975 hours 18 minutes held by Kathy Thornton. Hoffman will break the 1,000 hour mark aboard the Shuttle on Thursday.

Work with the United States Microgravity Payload experiments continues with the astronauts supporting a variety of investigations including crystal growth, materials solidification and fluid dynamics.

Columbia is functioning normally, with no problems being tracked by the flight control team as the Shuttle orbits the Earth every 90 minutes at an altitude of 180 statute miles.

On Wednesday, February 28, 1996, 6 p.m. CST, STS-75 Payload Status Report # 12 reports: (06/3:42 MET)

With the shift to the third United States Microgravity Payload (USMP-3) as the primary science on STS-75, the Tethered Satellite System (TSS) is in a "science of opportunity" mode. The spirit of cooperation was evident at Spacelab Mission Operations Control as the USMP-3 team worked with their TSS colleagues to accommodate post-tether-break science plans.

After ground controllers successfully commanded the free-flying satellite's on-board instruments to turn on yesterday, data began to flow to TSS science teams in Huntsville. TSS Mission Manager Robert McBrayer emphasized that "a tremendous amount of cooperation" has made this possible. The satellite is now operating on battery power, with enough energy to last another day, depending on how commanding and data resources are handled.

TSS Mission Scientist Dr. Nobie Stone confirmed that "we have a very interesting and high-quality data set. We are still taking data and already are seeing evidence that some very significant things will come out of this mission." The science of opportunity being formulated by the entire TSS Investigators Working Group involves firing electron accelerators, known as "electron guns," located in Columbia's cargo bay, to see if the satellite's instruments detect plasma disturbances as they spread out through the ionosphere. This experiment has been executed once, and there are opportunities to repeat it several times as the Shuttle and satellite move toward a closer approach of about 56 nautical miles.

One of the TSS primary mission objective may be met in spite of the overall situation. Dr. Marino Dobrowolny, the Italian Space Agency TSS mission scientist, noted that a main goal was to characterize the relationship between current collected and voltage across the tether. During tether deployment, current and voltage levels were observed under conditions that cannot be duplicated in the laboratory. He said "we have shed light on plasma physics questions raised in the 1920s that have never before been understood and are too difficult to duplicate in the laboratories or model with computers."

Data gathered while the satellite was deployed is better than scientists anticipated. There are cases where twice as much current was collected than was predicted by the best computer models available. This may indicate some degree of ionization around the satellite even during the period when the satellite thrusters were not on when no enhanced gas cloud was present. In fact, when satellite thruster operations were performed, the current went even higher, up to 580 milliamps, compared to the 270 milliamps predicted by the models.

In addition, after commanding current to flow in the tether, energetic electrons were observed on the satellite with energies ranging up to 10 kiloelectronvolts -- ten times the energy of the electron beams that were emitted. This provides valuable clues into the physical processes by which electrons become energized.

The acceleration that should be seen with a 12.2 mile (19.7 kilometer) tether hanging below the satellite was predicted theoretically to be 0.9 milli-g's, or less than one-thousandth of Earth's gravity. The satellite's linear accelerometer data measured accelerations within this range, indicating the tether to be intact. Based on this, it may be possible to tell if the tether length changes by more than 4 kilometers due to orbital debris or micro-meteoroid hits over the next day or so. Dr. Stone noted that "this would be an interesting and useful data point for future applications. If it doesn't sever, that's even better information." The accelerometer also is providing tether dynamics data that indicates a possible swinging back and forth motion known as libration.

During the post-break, free-flight phase, the Research on Orbital Plasma Electrodynamics (ROPE) equipment, located on one of the satellite's booms, is collecting information about the plasma sheath created by applying a voltage to the boom tip. Understanding this effect that may be significant to geosynchronous satellites, such as those that transmit television and telephone signals, where differential charging occurs on the order of kilovolts.

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