PSAM 16 - Paper Overview

PSAM 16 Conference Paper Overview

Welcome to the PSAM 16 Conference paper and speaker overview page.

Lead Author: Andrei Gribok Co-author(s): Arman Seuylemezian arman.seuylemezian@jpl.nasa.gov

Performance of Shrinkage Estimators for Bioburden Density Calculations in Planetary Protection Probabilistic Risk Assessment
Planetary protection (PP) is a discipline that focuses on minimizing the biological contamination of spacecraft to ensure compliance with international policy. The National Aeronautics and Space Administration (NASA) has developed a set of requirements (NPR 8715.24) based on recommendations from the Committee on Space Research (COSPAR) that each mission must comply with regarding both forward and backward planetary protection. Biological cleanliness requirements to target bodies, such as Mars, include spacecraft assembly control and direct testing of the microbial bioburden to comply with planetary protection requirements. Constraints include spore requirement limits for Mars missions of 5105 spores at launch with a maximum limit of 300 spores/m2 bioburden density on flight hardware surfaces, while preventing recontamination by utilizing International Organization for Standardization (ISO) 8 or better cleanroom environments. The data for each component are collected using either swabs or wipes. For each component, a number of samples are collected on one given date or on several different dates along the course of the part assembly. A swab data collection covers the area of 0.0025 m2 with a single swab, while a wipe covered area varies typically between 0.1 and 1.0 m2 depending on the geometric complexity and size of the sampled component. Having been processed in the microbiology laboratory, the samples were deposited in petri dishes, covered with tryptic soy agar and incubated for 72 hours to observe for growth. Given the clean spacecraft, on the average 93% of the swabs and 63% of the wipes have 0 colony forming units (CFU) count at 72 hours resulting in ~85% of the 39,379 petri dishes yielding 0 CFU. Due to low CFU counts and small sampling areas, given Poisson distributional model, the bioburden density estimates have inflated variance and wide confidence intervals. Shrinkage estimators are standard tools to deal with large variance and inconsistency of the estimates. This study presents results on performance of different James-Stein-type (JS) estimators to evaluate the microbial bioburden using data collected for the InSight mission. Different types of shrinkage estimators are considered, and their performance is compared and contrasted with maximum likelihood estimator (MLE) and each other. The regions of domination for different shrinkage estimators are analyzed and estimators’ admissibility is considered. The results show that shrinkage estimators dominate MLE uniformly and JS estimator has the lowest risk among other shrinkage estimators.

Performance of Shrinkage Estimators for Bioburden Density Calculations in Planetary Protection Probabilistic Risk Assessment

Planetary protection (PP) is a discipline that focuses on minimizing the biological contamination of spacecraft to ensure compliance with international policy. The National Aeronautics and Space Administration (NASA) has developed a set of requirements (NPR 8715.24) based on recommendations from the Committee on Space Research (COSPAR) that each mission must comply with regarding both forward and backward planetary protection. Biological cleanliness requirements to target bodies, such as Mars, include spacecraft assembly control and direct testing of the microbial bioburden to comply with planetary protection requirements. Constraints include spore requirement limits for Mars missions of 5105 spores at launch with a maximum limit of 300 spores/m2 bioburden density on flight hardware surfaces, while preventing recontamination by utilizing International Organization for Standardization (ISO) 8 or better cleanroom environments. The data for each component are collected using either swabs or wipes. For each component, a number of samples are collected on one given date or on several different dates along the course of the part assembly. A swab data collection covers the area of 0.0025 m2 with a single swab, while a wipe covered area varies typically between 0.1 and 1.0 m2 depending on the geometric complexity and size of the sampled component. Having been processed in the microbiology laboratory, the samples were deposited in petri dishes, covered with tryptic soy agar and incubated for 72 hours to observe for growth. Given the clean spacecraft, on the average 93% of the swabs and 63% of the wipes have 0 colony forming units (CFU) count at 72 hours resulting in ~85% of the 39,379 petri dishes yielding 0 CFU. Due to low CFU counts and small sampling areas, given Poisson distributional model, the bioburden density estimates have inflated variance and wide confidence intervals. Shrinkage estimators are standard tools to deal with large variance and inconsistency of the estimates. This study presents results on performance of different James-Stein-type (JS) estimators to evaluate the microbial bioburden using data collected for the InSight mission. Different types of shrinkage estimators are considered, and their performance is compared and contrasted with maximum likelihood estimator (MLE) and each other. The regions of domination for different shrinkage estimators are analyzed and estimators’ admissibility is considered. The results show that shrinkage estimators dominate MLE uniformly and JS estimator has the lowest risk among other shrinkage estimators.

Paper AG47 Preview

Author and Presentation Info

Lead Author Name: Andrei Gribok (agribok@utk.edu)

Bio:

Country: United States of America
Company: Idaho National Laboratory
Job Title: Distinguished Researcher

PSAM 16 Conference Paper Overview

Welcome to the PSAM 16 Conference paper and speaker overview page.

Paper AG47 Preview

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