Spacecraft Sterilization Research Articles

Spaceflight Virology: What Do We Know about Viral Threats in the Spaceflight Environment?

Viruses constitute a significant part of the human microbiome, so wherever humans go, viruses are brought with them, even on space missions. In this mini review, we focus on the International Space Station (ISS) as the only current human habitat in space that has a diverse range of viral genera that infect microorganisms from bacteria to eukaryotes. Thus, we have reviewed the literature on the physical conditions of space habitats that have an impact on both virus transmissibility and interaction with their host, which include UV radiation, ionizing radiation, humidity, and microgravity. Also, we briefly comment on the practices used on space missions that reduce virus spread, that is, use of antimicrobial surfaces, spacecraft sterilization practices, and air filtration. Finally, we turn our attention to the health threats that viruses pose to space travel. Overall, even though efforts are taken to ensure safe conditions during human space travel, for example, preflight quarantines of astronauts, we reflect on the potential risks humans might be exposed to and how those risks might be aggravated in extraterrestrial habitats.

Modern aspects of planetary protection and requirements to sterilization of space hardware

The viewpoint of working group of Russian experts on the problem of planetary protection for future manned and unmanned Mars mission is presented. Recent data of Martian environment and on survival of terrestrial microorganisms in extreme conditions were used for detailed analysis and overview of planetary protection measures in regard to all possible flight situations including accidental landing. The special emphasis on “Mars-94” mission was done. This analysis resulted in revised formulation of spacecraft sterilization requirements and possible measures for their best inplementation. New general combined aproach to spacecraft sterilization was proposed. It includes penetrating radiation and heat treatment of spacecraft parts and components which is to be carried out before the final assembly of spacecraft and gaseous radiation sterilization of the whole spacecraft during the flight to Mars (or from Mars for return mission).

Revised planetary protection policy for solar system exploration

In order to control contamination of planets by terrestrial microorganisms and organic constituents, U.S. planetary missions have been governed by a planetary protection (or planetary quarantine) policy which has changed little since 1972. This policy has recently been reviewed in light of new information obtained from planetary exploration during the past decade and because of changes to, or uncertainties in, some parameters used in the existing quantitative approach. On the basis of this analysis, a revised planetary protection policy with the following key features is proposed: deemphasizing the use of mathematical models and quantitative analyses; establishing requirements for target planet/mission type (i.e., orbiter, lander, etc.) combinations; considering sample return missions a separate category; simplifying documentation; and imposing implementing procedures (i.e., trajectory biasing, cleanroom assembly, spacecraft sterilization, etc.) by exception, i.e., only if the planet/mission combination warrants such controls.

U.S. planetary protection program: Implementation highlights

The implementation of planetary protection in the United States space program has reflected the trend in policy from an absolute to a probabilistic prohibition of the contamination of the celestial bodies of the solar system. The early emphasis on spacecraft sterilization (e.g. Ranger) was replaced by the imposition of contamination control procedures on later missions such as Pioneer, Viking, and Voyager. Similarly, analytical and laboratory techniques were developed to demonstrate compliance with probabilistic requirements. Microbial burden reduction methods that are not hazardous for spacecraft reliability supplanted the abstract concept of sterilization. The United States implementation of planetary protection has been completely successful. In an exploration program that has included Mercury, Venus, Mars, the Jovian system, and the Saturnian system, there have been no accidental impacts or detection of false positives (terrestrial microbes). Further, the contamination control and microbial burden procedures have proved beneficial to spacecraft systems and on-board science instruments. We review in this paper the implementation of planetary protection procedures by the Pioneer (10 and 11), Viking and Voyager projects.

A proposed new policy for planetary protection

In order to control contamination of planets by terrestrial microorganisms and organic constituents, U.S. planetary missions have been governed by a planetary protection (or planetary quarantine) policy which has changed little since 1972. This policy has recently been reviewed in light of new information obtained by planetary exploration during the past decade and because of changes to, or uncertainties in, some parameters used in the existing quantitative approach. On the basis of this analysis, a new planetary protection policy, with the following key features, is proposed: deemphasizing the use of mathematical models and quantitative analyses; establishing requirements for target planet/mission type (i.e., orbiter, lander, etc.) combinations; considering sample return missions a separate category; simplifying documentation; and imposing implementing procedures (i.e., trajectory biasing, cleanroom assembly, spacecraft sterilization, etc.) by exception, i.e., only if the planet/mission combination warrants such controls. Interpretation of the new policy for missions like Galileo, Mars Surface Sample Return, Saturn Orbiter with Twin Probes, and missions to comets are considered. In general, the new policy proposes elimination of all but documentation requirements for most planetary missions and simplification of the remaining compliance procedures.

An observation about the relative hardiness of bacterial spores and planetary quarantine

Currently, planetary quarantine spacecraft sterilization requirements are based on the ‘logarithmic’ model of bacterial death. This model is a single parameter exponential model in which the sole parameter is the decimal reduction time (D-value). The D-value associated with the sterilization of a population of bacteria is only a relative measure of the rate of bacterial inactivation: It is known to depend upon many factors, but generally these dependencies are not thoroughly understood. The D-values chosen for spacecraft sterilization have been selected conservatively relative to defined experimental procedures and bacterial spore stocks. New data indicate that these D-values are not conservative when compared with those of naturally occurring organisms. The possible implications of these new data for planetary quarantine are analyzed here.

A stochastic bioburden model for spacecraft sterilization.

An important factor in the calculation of sterilization cycles for spacecraft is the bioburden on the surface of the spacecraft at the start of the cycle. This bioburden must be predicted by the use of models. This paper presents a stochastic model for the prediction of the bioburden on the surfaces of spacecraft. This model has many desirable properties as well as being consistent with observations.

A kinetic analysis of spore inactivation in a composite heat and gamma radiation environment

This paper presents the analysis of a combined environment of dry heat and gamma radiation when applied for the purpose of spacecraft sterilization. The nature of the synergistic inactivation effect of dry heat and radiation on bacterial spores is explained using a semiempirical mathematical model, and the dependence of the inactivation rate upon a temperature dependent, nonlinear function of radiation dose rate is presented. An analysis of the temperature required for a defined population reduction with a defined upper limit on radiation dose and time is described. Also discussed is the dependency of the dose required for a defined population reduction on the radiation dose rate at any selected temperature.

Optimizing thermal and radiation effects for bacterial inactivation.

The temperatures required for dry-heat spacecraft sterilization have been known to degrade heat-sensitive components. Thermoradiation, the simultaneous application of dry heat and gamma radiation, can provide the same degree of microbial inactivation as dry heat alone while substantially reducing component degradation. This is made possible by the synergistic effects produced when relatively low levels of these agents (e.g., 90 to 350 krads and 60° to 105°C) are applied simultaneously, thus permitting the use of lower temperatures and a reduced duration of heat exposure. The effects of temperature, radiation dose rate, and relative humidity on microbial inactivation during thermoradiation exposure have been established.

Potential effects of recent findings on spacecraft sterilization requirements.

An important task related to the formulation of planetary quarantine standards is the achievement of an acceptable compromise between (1) the prevention of planetary contamination and (2) the impact of quarantine requirements on the conduct of planetary missions. Such a task is a continuing effort, which must take all pertinent new information into account as it becomes available. This paper provides an analytical framework for the assessment of data which have become available during the past year or which are currently being evolved. In particular an evaluation is made of the probability of release of viable organisms from the spacecraft as a function of: (1) impact velocity magnitudes and the probability of their occurrence; (2) the degree of equipment fracturing at impact velocities; and (3) the number of viable organisms in spacecraft materials. Work being done to quantify each of three types of contamination, i.e. that on open surfaces, mated surfaces and buried contamination, is described in the context of seeking an approach to spacecraft sterilization that would be most compatible with the implementation of planetary missions. It is concluded that the results of work now in progress on spacecraft-material fracturing, on the estimation of buried contamination loads, and on microbial resistance on mated surfaces, may lead to less severe dry-heat sterilization of planetary spacecraft than had been considered necessary in the past.

Release of microbial contamination from fractured solids.

Estimation of the probability of release of microbial contamination from the interior of solids upon fracture due to impact is essential to the formulation of planetary quarantine and spacecraft sterilization requirements. A model system was designed in which known concentrations of bacterial spores were incorporated in methyl methacrylate plastic. Pieces of plastic were fractured in a uniform manner exposing interior surface areas of consistent and measurable size. Known surface areas were incubated in sets of 20 culture tubes containing liquid growth medium. The subsequent occurrence of visible growth expressed as percent of tubes positive was interpreted as an estimate of the probability of release of at least one viable micro-organism.

Reliability and sterilization.

Relationships between the electronic reliability requirements and spacecraft sterilization requirements have been studied. An optimistic view on the compatibility of the reliability and sterilization requirements for present electronic systems is projected. The demands for reliability and long life have created parts immune to temperature damage well above the time temperature dose requirements for sterilization. Exceptions exist such as certain classes of capacitors where the sterilization time temperature dose is clearly damaging. Step stress data identifying the damage threshold over a broad range of time temperature dosage is used to illustrate the wide margin above the sterilization requirement for certain part types. The possibility is examined of more effectively utilizing the resources available for heat sterilization compatibility verification by broadening the objective to better identify the damage thresholds and the relationships of environments and failure mechanisms. Decontamination with ethylene oxide gas is examined. The few problems that exist appear to be primarily caused by the water vapor used with ethylene oxide rather than the ethylene oxide itself.

A system engineering approach to spacecraft sterilization requirements.

Recommendations are presented for quantitatively interpreting spacecraft sterilization requirements. The recommendations resulted from a brief preliminary exploration of the use of system engineering techniques in the control of spacecraft sterilization. Several specific applications of system engineering techniques potentially useful in formulating the over-all sterility assurance and planetary quarantine management system are suggested. These applications have been framed in terms of relative allocations of effort for performing failure analysis and for replicate testing. Examination of the sterilization requirements through the system engineering approach will lead to a realistic apportionment of the estimated contamination contributions by each of the various parts, processes, subsystems, and maneuvers, so that spacecraft sterility can be achieved and maintained.

NASA: Ranger Misfortunes Attract Attention of Congress to Problems of Spacecraft Sterilization

NASA: Ranger Misfortunes Attract Attention of Congress to Problems of Spacecraft Sterilization