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803 y-axis (cm) Fig. 2 Return flux impacts for outgassing species-BGK modelfull surface accommodation.Fig. 3 Return flux impacts for outgassing species-full flow modelfull surface accommodation.return flux conditions, the MOLFLUX code was run for the CO 2 return flux conditions. These results are shown in Figs. 1 and 2 for both reduced and full surface accommodation conditions. The peak values for both surface conditions was approximately 3.2 x 10 s impacts/cm 2 /s. Unlike the ambient species results, the return flux distribution peaked at the center of the disk and decreased rapidly toward the edges. The CARLOS code, representing the full physics of the problem, was next run for the CO 2 return flux case with full surface accommodation. The results of this run are shown in Fig. 3. The peak value of the return flux contact is 7.0 x 10'° impacts/ cm 2 /s and occurs at the center of the disk with a decrease toward the disk edge not as severely pronounced as the BGK results. It is noted that the peak value of the full physics result is approximately 200 times that of the BGK result.It is expected that the recontact distribution should be axisymmetric over the surface of the disk. Our calculations (not shown herein) have shown that the freestream contact distribution is essentially uniform over the surface. This result is taken as another indication that these calculations represent a physically realistic description of the problem. ConclusionsIt is evident that the use of a BGK-based model for this outgassing problem underpredicts the return flux by roughly two orders of magnitudes compared to a DSMC-based model that used the largest molecular diameters found in the literature. However, the results obtained from either of these models have not yet been validated under actual flow conditions.The BGK-based method is a neutrals-only simulation scheme. It does not have the capability, as does the full flow simulation, of representing the remaining fully coupled effects (such as plasma flow, the electromagnetic field effects, and spacecraft charging) in addition to the neutral particle flow. The exclusion of all but the neutral flow, along with the BGK simplification, apparently makes the BGK method inadequate to represent the return flux problem properly.This study has also indicated that surface accommodation has a negligible effect on the computed return flux.No orbital experiment to date has had sensors of sufficient sensitivity to yield compelling data. The physical phenomena that require assumptions on the part of computational investigators remain a major restriction to this area of investigation. Specifically, the physical processes involved in the gas-gas interaction as well as gas-surface interaction require a better understanding. Upcoming NASA Shuttle missions should provide some of this data. AcknowledgmentsThe investigators would like to thank NASA/JSC for its support, especially Max Engert of the Engineering Directorate for making available the Engineering Computational Facility, and Rose Rodriguez of LESC...

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