Cleaning Genesis Mission Payload for Flight with Ultra-Pure Water and Assembly in ISO Class 4 Environment

Introduction: Genesis mission to capture and return to Earth solar wind samples had very stringent contamination control requirements in order to distinguish the solar atoms from terrestrial ones. Genesis mission goals were to measure solar composition for most of the periodic table, so great care was taken to avoid particulate contamination. Since the number 1 and 2 science goals were to determine the oxygen and nitrogen isotopic composition, organic contamination was minimized by tightly controlling offgassing. The total amount of solar material captured in two years is about 400 micrograms spread across one m 2 . The contamination limit requirement for each of C, N, and O was <10 15 atoms/cm 2 [1]. For carbon, this is equivalent to 10 ng/cm 2 . Extreme vigilance was used in preparing Genesis collectors and cleaning hardware for flight. Surface contamination on polished silicon wafers, measured in Genesis laboratory is approximately 10 ng/cm 2 [2]. “Start Clean – Stay Clean” a whole mission approach: Contamination control was integrated into the Genesis mission from the very beginning: in mission design, in spacecraft design, in spacecraft cleaning and assembly, and in sample curation. This abstract focuses on cleaning and assembly of the payload, a science canister containing 300 solar wind collectors. The contamination sensitive solar collectors were isolated inside of a science canister in which organic materials (including lubricants and seals) were severely restricted and which was equipped with a molecular sorbent filter for pressure equalization upon re-entry. The mechanisms of the few moving parts were isolated and vented outside of the canister. The canister outer structure was fabricated from aluminum 7075 and interior structure from 6061. All interior aluminum surfaces were not anodized, to avoid the higher surface area and contamination introduced by the anodization process. Cleaning and assembly environment: Canister components were cleaned and assembled inside of an ISO Class 4 laminar flow cleanroom [3]. Assembly was done by individuals totally enclosed in HEPAfiltered Teflon suits (Fig. 1). Gloved hands never touched cleaned hardware, to preclude transfer of any nitrile glove residue. Even smallest screws were installed using stainless steel tweezers. Cleaning process rationale: The hardware was cleaned using ultrapure water (UPW) to avoid leaving any organic residue. [Note for future cleaning: UPWmay be especially useful if certain cleaning solvents are banned because of ozone depleting or greenhouse gas properties.]