The objective of this research was to design, build, and test an experimental apparatus for studying the parameters of atmospheric entry heating, and the inactivation of temperature-resistant bacterial spores. The apparatus is capable of controlled, rapid heating of sample coupons to temperatures of 200 to 350 ºC and above. The vacuum chamber permits operation under vacuum or special atmospheric gas mixtures.

The Experimental Apparatus consists of a vacuum chamber (left) and the stand for the silicon chips (right).
A radiant heating system using tungsten- halogen lamps was chosen to heat the spores to the desired temperatures. This method of heating was preferred because there was no physical contact between the heater and the sample coupons, the radiant heat can be controlled more precisely than heating methods by conduction and convection, and halogen light bulbs are readily available. The design allowed for the bulbs to radiantly heat the backside of the sample coupons, avoiding possible sterilization of the spores by a method other than just heating, such as ultraviolet radiation.

The material chosen for the sample coupons was silicon, due to its favorable properties for this application. Silicon is chemically and biologically inert, and has very high thermal conductivity. Furthermore, silicon has high emissivity in the visible and near-infrared portion of the electromagnetic spectrum, and has a lower emissivity in the mid-infrared range. This means that the silicon coupons are able to absorb a significant portion of the radiation output by the halogen light bulbs, but not re-radiate much midinfrared radiation at the sample temperatures. This unique property of silicon allows for the sample coupons to be heated very quickly and accurately using the radiant heat from the halogen light bulbs. Furthermore, due to the widespread use of silicon in the microelectronics industry, silicon was available in very thin wafers. The low thermal mass of the thin wafers helped them heat up very quickly.

This work was done by Wayne W. Schubert and James A. Spry of Caltech; Paul D. Ronney and Nathan R. Pandian of the University of Southern California; and Eric Welder of Stanford University for NASA’s Jet Propulsion Laboratory. NPO-48091

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Bacteria Bio-Medical Biological sciences Conductivity Education and training Entry, descent and landing Entry, descent, and landing Gases Hazardous materials Hazards and emergency operations Materials properties Medical People and personalities Radiation Research and development Simulation and modeling Vacuum
This Brief includes a Technical Support Package (TSP).
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Experimental Modeling of Sterilization Effects for Atmospheric Entry Heating on Microorganisms

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This article first appeared in the August, 2012 issue of NASA Tech Briefs Magazine (Vol. 36 No. 8).

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Overview

The document titled "Technical Support Package for Experimental Modeling of Sterilization Effects for Atmospheric Entry Heating on Microorganisms" (NPO-48091) presents research conducted by the Jet Propulsion Laboratory (JPL) and the University of Southern California. The primary objective of this research was to develop proof-of-concept hardware and software tools to model the effects of atmospheric entry heating on microorganisms, specifically temperature-resistant bacterial spores, which could inadvertently attach to spacecraft surfaces.

The research involved designing and building an experimental apparatus capable of controlled rapid heating of sample coupons to temperatures ranging from 200 °C to 450 °C. This apparatus was innovative in its use of thin silicon chips as spore carriers, allowing for rapid heating and precise control of the heating profiles. The apparatus could operate under vacuum or in special atmospheric gas mixes, enhancing its versatility for various experimental conditions.

The document outlines several key milestones achieved during the research. These included the construction of two identical small experimental vacuum chambers, characterization of the heating profiles within these chambers, and biological evaluations to confirm the apparatus's effectiveness in studying spore inactivation. The experiments utilized bacterial spores of Bacillus atrophaeus, which were deposited onto silicon chips and subjected to various heating profiles to assess their inactivation.

Results indicated that the apparatus could closely follow desired heating profiles, demonstrating its capability to achieve complex temperature control. The findings also highlighted that minimal spore inactivation occurred at temperatures of 200 °C and below, necessitating further experiments to explore higher temperatures and their effects on spore inactivation.

The document emphasizes the significance of these results in the context of spacecraft sterilization, as understanding the thermal effects on microorganisms is crucial for planetary protection and preventing contamination of extraterrestrial environments. The research aligns with NASA's Strategic University Research Partnership goals, fostering collaborations that advance scientific knowledge and technological innovation.

Overall, this Technical Support Package provides valuable insights into the development of experimental methodologies for studying microbial inactivation through atmospheric entry heating, with implications for future space missions and sterilization protocols.