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Space Portable SpectroReflectometer

SPSR Principal Investigator
Melvin R. Carruth, NASA

SPSR Co-Investigator
James M. Zwiener¹, NASA

SPSR Co-Investigator
Dr. Stanislov Naumov, RSC Energia

SPSR Co-Investigator
Don Wilkes², AZ Technology

SPSR Project Manager
John Owens, NASA

SPSR Engineering Manager
Glenn James¹, AZ Technology

¹Currently employed at AZ Technology: James Zwiener, Coatings and Materials Manager; Glenn James, Instruments.
²The late Don Wilkes was founder of AZ Technology

Overview:
The stability of materials used in the space environment continues to be a limiting technology for space missions. This technology is important to all users of space -- NASA, Department of Defense (DoD), industries, and universities. The Space Portable SpectroReflectometer (SPSR) offers a space research program for studying the effects of the space environment (both natural and induced) on optical, thermal, space power, and other materials.

The SPSR provides an in-space inspection instrument for non-destructive, quantitative engineering evaluation of spacecraft exterior surfaces. The SPSR measures total hemispherical reflectance as an indication of the effects of the space environment on materials such as thermal coatings, viewing windows, reflectors, solar power systems, etc. The SPSR provided valuable data for determining how materials degrade when exposed to the space environment within the Shuttle/Mir implementation framework.

Science Background:
It has been demonstrated that the natural and induced space environment can cause optical, mechanical, and thermal damage to exposed surfaces. This materials damage can seriously affect the performance of critical spacecraft systems including thermal control systems, solar arrays, and optical instruments. The space environment is a complex combination of mostly independent constituents including Atomic Oxygen (AO), particulate radiation (electrons, protons, etc.), thermal vacuum, micrometeoroid/debris bombardment, and contamination. These constituents vary in composition and quantity with orbital parameters, solar activity, season, and time of day.

The damaging effects of the space environment are critically important for spacecraft thermal control. The increasing size and complexity of spacecraft and the longer-duration mission requirements increase the difficulty of maintaining spacecraft thermal control. Even in an era of highly complex active thermal control systems utilizing fluid loops and heat pipes, the ultimate regulation of absorbed solar energy and radiated thermal energy remains dependent on the optical and thermal properties of thermal control surfaces.

The stability of materials in the space environment is not well understood. To compensate for this uncertainty, spacecraft and instrument designers frequently over design systems at greater cost and weight--sometimes with reduced performance. For the large, long-duration missions of today and the future such as the International Space Station (ISS), over design of systems is extremely undesirable and, in some cases, impossible.

Most materials in use today will not provide adequate performance for the duration of long-term missions such as the Hubble Space Telescope and the ISS. On-orbit servicing and maintenance of spacecraft systems will be required to complete these missions. One area of concern is the external surfaces including thermal control surfaces. The thermal surfaces are critically important to mission performance and their optical/thermal properties must be maintained within design limits. Because of the complexity of spacecraft thermal systems, the value of these optical/thermal properties cannot be determined accurately from operational measurements such as temperature, coolant flow rates, etc. To determine the condition of these external surfaces and to obtain an indication of when to perform maintenance, the optical properties of the surfaces must be measured. This continuing need to inspect the optical properties is particularly important for the Space Station where maintenance and repair are major issues for the operational life.

Mission Objectives:
The SPSR provides an in-space inspection instrument for quantitative engineering evaluation of spacecraft exterior surfaces utilized for the thermal control, viewing windows, reflecting mirrors, or solar power systems. Spacecraft exterior surfaces may suffer degradation during exposure to the combined space environment including the induced spacecraft contamination effects. Extended missions result in severe degradation of materials as demonstrated by the Long Duration Exposure Facility (LDEF). The scope of the mission was to take direct in-flight measurements using the SPSR to document any degradation over time of the Mir hardware surfaces. Specific objectives were: 1) To determine effects and damage mechanisms of the Mir space environment on materials. Mir was at the timeof the mission the only opportunity to study the environment around a large space platform and its effects on materials and systems. The high inclination orbit of the Mir results in a different mix of environmental constituents than observed on other long duration missions such as LDEF. The SPSR EVA activities on Mir offer the opportunity to study this environment's effect on operational surfaces. The SPSR was designed to collect this in-situ data on the performance of materials in the space environment. 2) Provide data to validate ground test facilities and prediction models. The current generation of laboratory space simulation facilities is extremely complex and well-designed. However, because of the inability to simulate the space environment exactly, these facilities provide only relative performance of test materials in those limited conditions. Past flight measurements show significant disagreement between flight and laboratory data. An important objective of the SPSR was to provide a "calibration" for ground test facilities.
3) Develop and test a reusable flight instrument for the in-situ measurement of operational spacecraft surfaces. There is a need to test many different operational materials in space and under different conditions of environment, orientation, temperature, duration, etc. Ground-based studies and limited flight data allow prediction of on-orbit degradation. The SPSR is able to directly measure actual thermal control surfaces. This data will allow better predictions of degradation and performance. The SPSR was designed to be reflown with minimum refurbishment to meet varied mission requirements.

Mission Summary Overview:
The SPSR was transported to the Russian Mir Space Station during the STS-86 mission. The SPSR, battery housing, and flight battery were stowed in McDonnell-Douglas SpaceHab lockers during transport by the Space Shuttle. After the shuttle docked with Mir, the SPSR was transferred to Mir and stowed during one of the Intravehicular Activities (IVA). In early 1998, the SPSR was utilized during various Extravehicular Activity (EVA) operations to check spacecraft surfaces for optical performance. Reflectance data gathered by the SPSR was stored in the unit's internal memory and transferred to a Mir Interface to Payloads Systems (MIPS) computer for storage on optical disk and down linking by means of the Mir telemetry system. The SPSR was returned to earth at the conclusion of the STS-89 mission. The intent is for the SPSR to be flown on subsequent missions and used on the ISS.

Note: Spectroreflectometer, spectro-reflectometer and spectral reflectometer are often used interchangeably.