A.

Verification of reflection model by experiment [4]

Several kinds of reflection model exist, such as specular and diffuse reflection models, Maxwell model, etc. A specular reflection model is one in which the incident- and reflection angles of molecules converge. Conversely, in the diffuse reflection model, incident molecules are reflected isotropically from the surface, independent of the incident angle. The Maxwell model is defined as a mixture of specular and diffuse reflection models and J-SPICE has applied the diffuse reflection model among various others. JAXA establishes an apparatus to investigate the reflection behavior of molecules, the facility diagram of which is shown in Figure 1. The apparatus comprises several parts such as a vacuum chamber, cryogenic shroud, effusion cell with an orifice, mirror, and two CQCMs (Cryogenic Quartz Crystal Microbalances.) One of the purposes of this equipment is to correlate the reflected molecular flux with incident molecular flux to the mirror with several reflection angles. Moreover, we also investigate the validity of the diffusion reflection model by comparing the analytical result of the experiment calculated by J-SPICE with the data of the experiment.

Fig. 1.

Diagram of the facility.

In conclusion, it seems the accuracy of the diffuse reflection model suffices for practical contamination analysis (determining whether the amount of contaminants on orbit is within the acceptable limit.)

B.

Outgassing rate measurement technique

J-SPICE basically using uses the power model for the outgassing model and JAXA has maintained an outgassing rate measurement facility at the Tsukuba Space Center. This facility is made in accordance with ASTM E 1559 and includes 4 CQCMs, an Effusion Cell and a Shroud at Liquid Nitrogen temperature. Figure 2 depicts the facility scheme.

Fig. 2.

Outgassing rate measurement equipment in JAXA.

We tested the outgassing rate test using approx. 40 samples per year and compared some of the results to data obtained by another test facility. Some parameters which affect the outgassing rate data, such as the baking condition, are included in the experiment.

A.

Evaluation of the bakeout process [5]

Bakeout is commonly applied to the components and materials used for satellites before launch, since such components and materials include volatile low molecular elements emitted in a space environment. Moreover, this molecular contamination is known as one of the most serious problems for satellites, typically including optical components and solar panels that are sensitive to contamination which impairs their performance. However, the effectiveness of bakeout may also depend on conditions such as temperature, duration, and configuration. It is believed that bakeout should be conducted at high temperatures, for extended periods, and in configurations conducive to emission. Moreover, bakeout under such ideal conditions is unrealistic, given the temperature limits imposed by the specifications of components and facilities, limited durations typically imposed by cost or system timeline considerations, and configurations limited by the development phase. Consequently, the bakeout conditions vary in line with the limitations regarding each object, resulting in different bakeouts on a case-by-case basis.

In this experiment, the outgassing source is heated to 80°C. The effectiveness of bakeout in terms of directly measuring optical properties could be shown by an In-situ Contamination Spectroscopic Analysis Chamber. The “In-situ Contamination Spectroscopic Analysis Chamber”, developed by JAXA, is located at the Tsukuba Space Center and features heating and cooling parts located within a single vacuum volume. The sample (i.e. outgassing source) is set at the heating part and the gold-coated mirror at the cooling part. During protracted heating of the sample, volatile materials from the outgassing source are deposited on the gold-coated mirror, while the IR beam from FT-IR traverses an IR transmissive window and is reflected on the mirror, thereby allowing the IR detector to measure the spectral reflectance. When the mirror is contaminated by the absorption of contaminants, the spectral reflectance changes. This design allows us to observe changes in the optical properties of a critical surface contaminated by the outgassing source in real time. One TQCM sensor unit is also placed in a position allowing a view factor equivalent to that of the mirror from the outgassing source under a regulated temperature to measure the deposition of mass on a critical surface at specific temperatures. The TQCM can thus measure the correlation between deposition thickness and optical properties. Figure 3 shows a full photo of this equipment.

Fig. 3.

In-situ Contamination Spectroscopic Analysis Chamber.

Four cured samples were prepared, three (in a vacuum oven at temperatures of 60, 80, and 125°C, respectively) of which were subject to bakeout under the various conditions. Figure 4 shows the FT-IR spectral absorbance for only samples of baked-out at 125°C, when the sample is heated to 80°C respectively. The spectral absorbance is overlaid with the spectrum of each elapsed time period. The horizontal axis denotes the wave number, and the vertical axis indicates absorbance.

Fig. 4.

FT-IR spectral absorbance overlaid with the spectrum of each time elapsed (Bakeout at 125°C)

This experiment attempted to evaluate the effectiveness of bakeout for RTV-S 691 by employing quantitative and optical methods using the “In-Situ Contamination Spectroscopic Analysis Chamber.” The quantitative measurement results confirmed that the thickness of contaminants existing on the surface at the same time was reduced by bakeout and that the reduced thickness then increased with bakeout temperature. The optical measurement results, however, revealed certain cases in which bakeout increased the absorption coefficient, which suggests that relying on the deposition thickness on an optical surface alone is insufficient.