Calculation of Outgassing Contamination
for the Genesis Solar Wind Collectors

Eileen Stansbery¹ and Frank Jarossy²

The contamination associated with return flux self scattering of outgassing during the 2 year Genesis collection period is assessed using the SPACE II contamination model developed by Martin Marietta for JSC and MSFC. The analysis includes a conservative estimate of 1 months (rather than the planned 3 month) outgassing period before the science canister is opend for solar wind collection. The model uses a symmetrical hemisphere, a default contaminant molecular weight of 100, a default contaminant diameter of 10 angstroms, and a mach number for outgassing of 0.67. Inputs to the model include the outgassing surface temperature, the vent mass loss rate, vent diameter, and receiver location. I have assumed an outgassing surface temperature of 300K. A description of the other input parameters follow.

Analysis Geometry

To simplify the basic analysis we have assumed that the vent dimensions are defined by the gap between the Science Canister and the SRC to determine an "equivalent vent area." The diameter of the science canister = 0.85 m and the diameter of the SRC = 1.28 m.

Gap Area = pi/4(1.28² - 0.85²) = 0.719 m² = 7190 cm²

Gap Width = (1.28 - 0.85)/2 = 0.215 m = 21.5 cm

A_vent = equivalent vent area = (21.5 cm)² = 462 cm² for each vent

Since the model, in its present form, calculates for individual vents rather than a continuous vent area we need to divide the vent annulus into a sequence of indivual vents. The number of vents is found by

N_vents = Gap Area/equivalent Vent Area = 7190 cm²/462 cm² per vent = 16 vents

By dividing the annulus into 16 sectors and calculating a circular vent area equivalent to the annulus sector area we have an "equivalent vent diameter" of

d_vent = 24 cm

Vent Mass Loss Rate:

Flight data from SCATHA and other spacecraft has demonstrated long term diffusion limited outgassing rates characterized by an exponential (~SQRT(time)) function. Therefore, the mass loss rate function over the 2 year mission period is assumed to be dominated by early diffusion.

The total mass loss over the mission is assumed to be 1% of the organic mass.

Since outgassing is assumed to primarily occur in the annulus between the outside wall of the science canister and the SRC, the mass loss rate from outgassing per unit area is defined as:

Analysis

In this analysis we assume canister lid remains closed for 1 month after outgassing begins.

The deposition rate due to return flux self-scattering is computed from the SPACE II contamination model.

Inputs to the model include an outgassing surface temperature of 300K, the vent mass loss rate, and vent diameter determined above and a position for deposition to occur (receiver location in cm from vent axis).

In this figure, the x-axis (short axis) is the time from 0 to 24 months, the y-axis (long axis going up and to the right) is the distance from 0 to 45 cm from the vent, and the z-axis (vertical) is the deposition rate, on a log scale, from 1e-18 to 1e-15 g/cm²/sec.

The integrated deposition rate of the vents and their contribution to contamination at a particular distance allows us to calculate the depth of contamination as a function of distance from the vent.

The maximum contamination of <1Å is acceptable.

1. NASA - Johnson Space Center, Houston, TX 77058
2. Lockheed Martin Astronautics, Denver, CO 80201

Other Contamination Documents:

• Phase A Contamination Control Plan
• Phase A Contamination Presentation
• Micrometeorite Background
• Hydrazine Thruster Contamination Analysis