STS-75 Day 15 Highlights

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On Thursday, March 7, 1996, 6 a.m. CST, STS-75 Payload Status Report # 20 reports: (13/15:42 MET)

As the STS-75 mission nears the finish line, the prize for the third United States Microgravity Payload (USMP-3) research teams is data as diverse as their experiments. "USMP-3 has given scientists a richness of information about the space environment, plus real-time interaction that now extends from a university environment into space," said USMP-3 Mission Scientist Dr. Peter Curreri. Referring both to overall mission success and the first remote commanding of a Space Shuttle microgravity science instrument from a U.S. college campus, he added, "these accomplishments give us the sense that orbital space is ours to manipulate to further our knowledge."

The Critical Fluid Light Scattering Experiment, known as ZENO, is providing data about today's final approach to the xenon sample's critical point. A fluid's critical point is the precise temperature and pressure at which it exists as both a liquid and a gas. At "supercritical temperatures," fluids make remarkable solvents and have other applications, such as the use of carbon dioxide at its critical point to extract caffeine from coffee beans. Near the critical point, the normally clear xenon sample undergoes some dramatic physical changes, going from clear to cloudy as the fluid fluctuates between vapor and liquid states.

To determine the precise point where this phenomenon occurs, the Zeno team relies on over 100 channels of data from their experiment, updated every second, to plot statistical curves that point to the sample's critical temperature. These plots, known as correlograms, show the relationship between temperature and pressure to give researchers clues as to exactly when the critical point is reached.

"When you get way up near the top of Mount ZENO, everything has to move very slowly, like climbing through the low-oxygen environment of a mountain at high altitudes," explained Principal Investigator Dr. Robert Gammon of the University of Maryland, referring to his desired slow approach to the critical point. As the sample's temperature is nudged downward through ever-smaller steps, the ZENO light-scattering equipment measures the xenon's cloudiness, caused by variations in density near the critical point. In addition to these real-time measurements, the ZENO team will further analyze the set of correlograms, along with light- scattering measurements, to gain insight into this unusual state of matter.

Early this morning, the joint U.S./French MEPHISTO experiment began solidifying its final crystal samples to be brought back to Earth for post-flight analysis. Using a high-temperature furnace and three samples of a tin-bismuth alloy, representative of mixtures used for semiconductors and metal alloys on Earth, MEPHISTO studies the point at which materials change from the melted to solid state. This is done by selectively heating and cooling the material, while making noninvasive electronic measurements -- readings that do not interfere with the actual crystallization process.

With help from the Space Acceleration Measurement System, the MEPHISTO team again adjusted their sample's growth rate in response to Shuttle maneuvers. This knowledge could lead to the development of furnaces that compensate for unavoidable acceleration effects in future space experiments. Today, the team will "flash-cool" a portion of one of their final samples to preserve the crystal's shape for microscopic analysis.

Yesterday, crew members prepared the Middeck Glovebox, a versatile facility for hands-on lab work in space, for landing. They also ran procedures to gather engineering data comparing checkout steps performed in low gravity with similar procedures conducted on the ground. This gives better assurance that such procedures will work when this facility flies later this year on the Russian space station Mir and in the future on the International Space Station.

Using the Glovebox over the course of the mission, crew members burned 65 samples, ranging from paper to Teflonª, while researchers closely observed how these samples reacted in low gravity, free from the effects of rising hot air and falling cool air found on Earth. The combustion researchers gathered more than 125 percent of planned scientific data, in the form of video and photographs, in order to better understand how to control and prevent fires. Other data from the Glovebox include soot and ashes, as well as particles trapped in the facility's filters.

Having completed primary science collection for the Rensselaer Polytechnic Institute's (RPI) Isothermal Dendritic Growth Experiment (IDGE), scientists at RPI in Troy, N.Y., and at Spacelab Mission Operations Control in Huntsville, continue analyzing the dozens of video images they have been collecting from on board Columbia. The images will reveal, in greater detail than ever before possible, how microscopic dendrites grow free from the sedimentation and convection effects experienced on Earth. As most molten materials solidify, they form tiny crystals, or dendrites, that look like frost on a cold window. The size, shape and direction of these crystals dictate the final properties of the resulting solid materials.

"Now, we're using our 'telescience' capabilities to probe the effect of our sample container on solidification," explains IDGE Research Scientist Dr. Matthew Koss. "This is important because, eventually, every metal product is affected by the container it's in." After landing, the IDGE team will use a series of photographs, taken from two 90-degree angles, to compose three-dimensional images of these dendrites.

Over the next 18 hours, the MEPHISTO, ZENO and IDGE experiments will be deactivated in preparation for Columbia's landing.

On Thursday, March 7, 1996, 4 p.m. CST, STS-75 MCC Status Report # 28 reports:

Columbia's astronauts have readied their ship to return to Earth after a journey of more than 6 million miles. Landing at the Kennedy Space Center is targeted for 7:52 a.m. central time, the first of two opportunities to KSC tomorrow.

Early this morning, during the standard pre-landing check of Columbia's flight control system, one of four data channels failed. The command path involved is one of four that commands an actuator to move flight control surfaces during reentry. The command path is considered "failed", but the three remaining command paths remain healthy and in position to support tomorrow's planned landing.

Throughout the night, the reentry flight control team will continue to review weather conditions at the primary landing site at KSC, as well as the alternate landing site at Edwards Air Force Base in California.

Flight controllers opted to forego an earlier KSC landing opportunity at 6:16 a.m. central time, because of forecast low clouds and the possibility of rain and gusty winds. Weather conditions are expected to improve later in the morning to support the two KSC landing opportunities.

The first landing opportunity would see a deorbit firing of Columbia's engines at 6:49 a.m. central time, with a landing on Runway 33 at 7:52 a.m. The second opportunity, one orbit later, has a deorbit burn at 8:25 a.m., followed by landing at 9:27 a.m.

If weather precludes a landing at KSC, there are two opportunities for a Friday landing at Edwards Air Force Base. The first would see a deorbit burn of Columbia's engines at 8:15 a.m. central time, with landing on Runway 22 at 9:27 a.m. The second opportunity, has a deorbit burn at 9:54 a.m. central time with landing at 10:54 a.m. Weather in California is predicted to be acceptable to support landing.

If Columbia remains in orbit an additional day due to weather conditions, there are three landing opportunities available on Saturday at KSC and three at Edwards.

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