STS-75 Day 3 Highlights

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http://science.ksc.nasa.gov/shuttle/missions/sts-75/sts-75-day-03-highlights.html

On Saturday, February 24, 1996, 8 a.m. CST, STS-75 MCC Status Report # 04 reports:

Troubleshooting a balky experiment data relay box was the focus of work aboard Columbia throughout Friday night and into this morning as the astronauts and flight controllers attempted to track down the problem and preserve their options for deploying the tethered satellite at 2:37 this afternoon.

Early this morning, in a methodical step-by-step process, the crew and flight control team slowly activated several experiments individually at first and then together trying to isolate the problem. At about 2 a.m., all experiments were activated and data was being transferred through the relay box to a laptop computer on the flight deck of the orbiter and data was transmitted successfully to the ground. The computer relay is continuing to perform well and is not evidencing any of the communications problems previously observed. Mission Managers are meeting this morning to review the system's status and make a determination on whether to deploy TSS on time or to delay 24 hours.

Called a Smart Flexible Multiplexer Demultiplexer, or "Smart Flex" for short, the relay box reset itself several times and even switched to an alternate system on occasion when it perceived it was being overloaded by the checkout work associated with ensuring the experiments were ready to support Tethered Satellite System operations.

The relay box is used as a way station for obtaining and distributing data collected from four science experiments designed to take measurements during the deployment, on- station, and retrieval operations of the satellite. The "Smartflex" must be operating for a deploy of the tethered satellite since it provides data and commands to the satellite-related experiments and equipment in the cargo bay.

Once deployed, the tethered satellite will slowly fly away from Columbia, eventually reaching a distance of 12 miles (20 kilometers). Communications and commanding of the satellite itself during deploy and retrieve is done by a radio link to the Space Shuttle by ground commands from NASA's Marshall Spaceflight Center in Huntsville, which has overall program management for the TSS project.

The tethered satellite will slowly be deployed to a 12 mile distance over a five and a half hour period. The satellite will remain "on station" for science operations for about 24 hours. The retrieval process would follow with the satellite being reeled back in during another five and half hour period.

On Saturday, February 24, 1996, 6 a.m. CST, STS-75 Payload Status Report # 03 reports: (1/15:42 MET)

After TSS-1R Payload Commander Franklin Chang-Diaz finished replacing a defective cable connecting the Data Display and Control System (DDCS) laptop computers and the smart flexible multiplexer/demultiplexer (smartflex) computer in Columbia's cargo bay, Mission Specialist Claude Nicollier reactivated the Tethered Satellite Deployer Core Equipment, the Shuttle Potential and Return Electron Experiment (SPREE) and the Shuttle Electrodynamic Tether System (SETS). Science teams at the Marshall Space Flight Center in Huntsville then readied all three experiments for performing preliminary measurements. Currently, the satellite-carried experiments are all operating well in preparation for Saturday's afternoon deployment of the Tethered Satellite System (TSS- 1R).

Earlier in the day, the smartflex computer, which sends crew commands to the tethered satellite and collects science data, experienced some timing errors, causing it to automatically switch to its backup system. The DDCS laptop computer in Columbia's cabin that displays TSS data had also been operating sluggishly. Ground controllers and engineers at Marshall did intensive troubleshooting with computers on the ground to simulate and diagnose the problem. After determining that a defective cable had caused overloads in both computers, they then requested Chang-Diaz to replace the faulty connection with a spare cable.

After complete reactivation of the Core, SETS and SPREE experiments, the smartflex computer relay again experienced errors and automatically switched over from its backup system, in which it had been operating since the earlier timing errors, back to its primary system. The crew then worked to reconfigure the laptop computer to restore communications and command ability to the experiments. Chang-Diaz again disconnected the laptop cable and successfully rebooted the computer.

While reestablishing the link between the laptop and the smartflex, Chang-Diaz reported that the smartflex, which acts like a data switching box and checks the status of each experiment, again entered a "warm start" or reset mode because it found that the SPREE experiment had been deactivated. Then, he again rebooted the laptop after Nicollier had reactivated SPREE.

After a fourth crash of the smartflex computer, Chang-Diaz then allowed it to operate with the three experiments turned off and only the data acquisition and control assembly (DACA) computer activated. Ground controllers then monitored the smartflex relay box as Chang-Diaz activated and deactivated the Core, SPREE and SETS experiments one at a time. After encountering no problems with any of these experiments, he reactivated the laptop DDSC computer and observed how it operated in conjunction with the smartflex while the three experiments were inactive. The smartflex, laptop and the three experiments were then successfully operated together and continue to function normally in preparation for the tethered satellite deployment later this afternoon.

The SETS investigation, by Brian Gilchrist of the University of Michigan, is designed to study the ability of the tethered satellite to collect electrons by determining the current and voltage of the tethered system and measuring the resistance to current flow in the tether itself. The Italian Space Agency's Deployer Core Equipment, developed by Dr. Carlo Bonifazi, will demonstrate the capability of a tether system to produce electrical energy and allow studies to be made of the tether's interaction with the ionosphere. It does this by controlling the current flowing through the tether between the satellite and the orbiter, and by making a number of basic electrical and physical measurements of the Tethered Satellite System. Finally, the SPREE experiment, conducted by David Hardy of the Department of the Air Force, will measure the charged particle quantities around Columbia before and during active tethered satellite operations.

Meanwhile, Nicollier took video of Earth's horizon using the Tether Optical Phenomena (TOP) equipment. This experiment, conducted by Stephen Mende of Lockheed Martin, involves the use of a camera system with image intensifiers and special filters and will provide visual data that will allow scientists to answer a variety of questions concerning tether motions and optical effects generated during the Tethered Satellite's deployment. In particular, this experiment will examine the high voltage sheath of electrically charged, or "ionized" gas that will surround the satellite as it flies through Earth's ionosphere.

One means to control the current developed by the Tethered Satellite System involves the use of electron accelerators which return electrons to the ionized gas, or plasma, surrounding the orbiter. The interaction between these electron beams and the plasma is not well understood. By using the TOP images to make measurements of the visible light radiated by the plasma, this process, and how it affects the spacecraft, can be better understood.

During the next twelve hours, experiment operations will be performed as mission managers and ground controllers meet to discuss their schedule for deploying the tethered satellite.

On Saturday, February 24, 1996, 10:30 a.m. CST, STS-75 MCC Status Report # 05 reports:

STS-75 mission managers Saturday morning decided to delay deployment of the Tethered Satellite by 24 hours to give flight controllers time to gain additional confidence in the operations of a data relay that provides satellite system information during deploy and science operations.

The Smart Flex data relay box has been stable since early this morning, but mission managers wanted additional time to understand its previously observed behavior. The relay box had experienced several unexpected "restarts". It has been performing without incident, however, since early this morning.

The 24-hour delay also gives flight controllers the opportunity to enhance any contingency procedures that might be used in the event the Smart Flex develops problems during the deploy operations.

Late this afternoon, the astronauts will begin a standard one-day-before-deploy timeline, allowing them to catch up on some activation steps which were omitted during the compressed timeline which would have been required if TSS were deployed as originally scheduled this afternoon.

On Saturday, February 24, 1996, 6 p.m. CST, STS-75 Payload Status Report # 04 reports: (02/03:42 MET)

With the decision to delay deployment of the joint NASA/Italian Space Agency Tethered Satellite System (TSS) by one day, researchers located at Spacelab Mission Operations Control are working around the clock, analyzing initial data and ensuring that their instruments are ready for full-up science operations. The initiation of satellite deployment, known as "flyaway," now is scheduled for around 2:45 p.m. CST Sunday.

TSS principal investigators and their science teams view this adjustment of the mission timeline as an opportunity to accomplish all their major science objectives. Specifically, Mission Scientist Nobie Stone, of the Marshall Space Flight Center (MSFC), pointed out that important baseline data collection would have been sacrificed by the rush to complete instrument activation and checkout if the satellite deployment had not been postponed by 24 hours.

Dr. Stone emphasized that gathering baseline measurements before deployment is "like learning to walk before you run." Part of this data set reveals how the sensitive TSS scientific instruments react to periodic thruster firings to maneuver the Shuttle and the satellite. Today's science activities included mapping Earth's charged particle upper atmosphere, which varies dramatically as Columbia orbits through periods of daylight and darkness every 45 minutes. Scientists will use these and other predeployment background readings when they analyze data collected during the more than 30 hours of planned TSS science operations.

One example of baseline data is the Joint Science Display produced earlier today by combining measurements gathered by several TSS instruments. This graphical profile of the intensity of charged particles around the Shuttle is giving scientists an indication of how instruments may respond when the tethered satellite collects such particles when deployed. This display showed intense electron and ion collection in the Shuttle's flight direction, or "ram," versus the trailing side, known as the "wake" direction.

Another example of baseline data is a "space weather map" produced today by the Theory and Modeling Support of Tether (TMST) Sundial investigation. The map provides a global view of the plasma density and temperature in the ionosphere at the altitude of the STS-75 orbit (160 nautical miles). This map is produced from a combination of theoretical ionospheric models and measurements obtained from 30 ground-based stations around the world.

Mission Manager Robert McBrayer, also of MSFC, said the deployment delay "makes sense," especially since the satellite will be operating on battery power once the umbilical that attaches it to the Shuttle's power system is removed shortly before flyaway. He emphasized the satellite and its deployer, as well as the TSS experiments, have been activated and are operating properly. McBrayer noted that ground- based flight controllers are using the extra time to better understand problems that arose and to develop enhanced contingency plans for satellite deployment.

Essentially turning back the clock 24 hours, the crew spent the day stepping through normal science and satellite predeployment activities that were interrupted yesterday while they performed troubleshooting for three on-board computers. As part of a dress rehearsal for satellite deployment, the crew maneuvered Columbia to the altitude for satellite flyaway. They also worked through a number of steps designed to simulate the same orbital conditions and environment for actual deployment the satellite, including lighting and lighting angles.

During some nighttime orbital passes, crew members worked with the Tether Optical Phenomena Experiment (TOP), which gave a stunning view of atmospheric air glow and auroras over the South Pole as viewed from the flight deck's overhead window. This is the window through which the crew later will watch the deployed satellite. TOP science team members viewed live video images and sent voice commands to vary the filters and exposure setting for better viewing. The heart of the TOP instrument is a hand-held low-light video camera with special filters whose primary purpose on TSS is to observe luminescence produced by electron beams and the interaction of the electrically charged satellite with the local charged-particle and neutral atmosphere. The TOP has many advantages over similar photographic recordings made on previous flights because it allows real-time observations of the images seen by the orbiter crew.

The crew also resumed calibration of on-board instruments and electron accelerators, including one test of the Shuttle Electrodynamic Tether System (SETS). SETS instruments, mounted in the cargo bay, will make measurements of tether voltage and current, as well as background magnetic fields and plasma characteristics. SETS also provides one electron accelerator, known as an electron gun, designed to propel charged particles back into the ionosphere after they travel from the satellite, down the conducting tether, and collect in the cargo bay. The ejection of electrons is one step in completing the circuit required for current to flow.

During the next 12 hours, science data will continue to be collected while the crew continues predeployment activities.

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