JAXA is developing a contamination analysis tool “J-SPICE” (Japanese Spacecraft Induced Contamination analysis software). Generally speaking, contamination analysis tools predict based on various input data and mathematical models of contaminant behavior, which means prediction accuracy depends on the validity of mathematical models as well as of input data. We investigated the validity of a diffuse reflection model applied in J-SPICE by comparing the reflection flux of contaminant molecules measured by the ground experiment and the analytical result of the J-SPICE. The result showed that the diffuse reflection model of J-SPICE reasonably explains molecule distribution reflected by a flat surface.
Molecular contamination by outgassing can degrade the performance of optical components. In orbit, spacecraft are exposed to various environments. UV is one of the most critical. It may also have the potential to cut the chains of organic molecules in contaminants due to its high energy, degrading optical properties and even re-emission behavior. In the present study, using two kinds of UV sources with different wavelength ranges, we compare the effect of UV lights irradiated on an optical surface with silicone contaminants. The irradiated samples were evaluated in terms of their optical properties and re-emission behavior, i.e. transmittance, and thermal desorption.
Outgassing rate measurements are basically performed for fresh materials, e.g. just cured adhesives, paints, etc. and reveal a lot about how the material can behave as a contamination source. It is also important to determine the bakeout process sufficiently. In the present study, a typical silicone adhesive for use in space, RTV S-691, Wacker Chemie, was selected for the measurement. Two cured specimens, 40 × 40 mm in size, were applied for several isothermal tests under identical conditions: a specimen at 125 degrees C for 144 hours with CQCM at -193 degrees C to measure TML. Consequently, it was determined that the TML and TML rate could be reduced by bakeout as expected. It also emerged that a longer bakeout, i.e. a longer cumulative bakeout time, for the material would reduce the TML and TML rate more effectively. The results suggest that bakeout mainly affects the behavior in the “low-rate” phase, whereby the TML rate curve can be divided into two phases. The elapsed time for a specimen can also be considered the cumulative test time. Based on the cumulative elapsed time, the TML rate curve is replotted and a correlation emerges between the cumulative bakeout time and TML rate. The first measurement data of TML and the TML rate could be affected by the stored time from cure, which might result from the change in unreacted substances declining as the stored time elapsed.
The contamination control for the next-generation space infrared observatory SPICA is presented. The optical performance of instruments on space observatories are often degraded by particulate and/or molecular contamination. Therefore, the contamination control has a potential to produce a significant risk, and it should be investigated in the risk mitigation phase of the SPICA development. The requirements from contamination- sensitive components onborad SPICA, the telescope assembly and focal plane instruments, are summarized. Possible contamination sources inside and outside the SPICA spacecraft were investigated. Based on impact on the SPICA system design, the following contamination sources were extensively studied through simulation and measurement; (1) outgassing from the payload module surrounding the telescope mirror and focal plane instruments, (2) contamination due to the thruster plume, and (3) environmental contamination during the integration, storage and verification phases. Although the outgas from the payload module and the thruster plume were estimated to produce only a negligible influence, the environmental contamination was suggested to affect significantly the telescope and focal plane instruments. Reasonable countermeasures to reduce the environmental contamination were proposed, some of which were confirmed to be actually effective.
We attempted to evaluate the effectiveness of bakeout for certain materials by using the “In-Situ Contamination
Spectroscopic Analysis Chamber” newly developed by JAXA, in order to measure the optical properties of a surface
contaminated by condensed outgas. In the present case, the sample (RTV-S 691; subject to four bakeout conditions) is
heated at 125°C and a gold-coated mirror set opposite the sample is cooled at -10°C to collect outgassing from the
sample. FT-IR is set to measure the optical properties on the surface of the gold-coated mirror inside the chamber in-situ.
A thermoelectric quartz crystal microbalance (TQCM) is installed in the chamber where the view factor to the sample is
equivalent to that of the gold-coated mirror used to measure the thickness of deposited contaminants at the same
temperature as that of the mirror. The four bakeout conditions are no bakeout, bakeout at 60°C, at 80°C, and at 125°C for
72 hours, respectively. As a result, TQCM data showed an expected curve, revealing a lower deposition rate at higher
bakeout temperature. We then plotted the absorbance for obvious FT-IR spectra peaks against the optical path length, as
calculated from the deposition thickness measured using the TQCM. The absorption coefficient at certain wavenumbers
was found to vary under the four bakeout conditions. This suggests an insufficient deposition thickness on the optical
surface. It therefore follows that direct optical measurement should be performed to evaluate bakeout effectiveness as
pertaining to the essential purpose of bakeout.