Sources of contamination

Spacecraft generally acquire their largest load of contaminating organisms from the personnel involved with the assembly and testing of the spacecraft. The organisms transferred through such forms of contact therefore belong predominantly to the species found on and within people. Human beings are host to around 200 species of microbial organisms, with bacteria the most common skin-borne organisms, followed by fungi and their spores. The most prevalent bacteria types are those associated with the human gut, skin, hair, mouth and nose. Using wash-and-strip techniques it has been shown that operators display a near constant load of organisms which is weakly affected by climate or season. Some individuals have an intrinsically higher loading of viable organisms in their skin effluvia, and in measurements involving full-environment chambers, people (wearing a sterile scrub suit, socks and cap) have been shown to shed cultivatable skin flakes at a rate of several thousand particles per minute in still air (Riemensnider, 1968). This flux of organic debris from the assembly personnel can be minimized with particular hygiene protocols, but most of the contamination control comes from the use of air filtration systems, careful planning of assembly areas, appropriate garb, and clean handling and working procedures. The main interfaces between people and spacecraft hardware are the hands, and gloves of either latex or polythene are commonly used along with standard cleanroom donning and doffing procedures. Table 14.1 shows the degree of contamination incurred during assembly of spacecraft-representative fasteners.

Clearly, the assembly of hardware in an absolutely sterile state cannot be easily achieved. If the presence of biological contamination is a given, then methods of removing or killing the organisms may have to be employed. As there is no unique sequence of steps by which a spacecraft can be produced from a collection of parts, there are many alternative schedules for the cleaning procedures necessary. These can rely on building with non-sterile parts and sterilizing the final assembly, or using sterilized components from the beginning and ensuring rigorous bio-load monitoring and process control.

Table 14.1. The degree of contamination incurred during assembly of spacecraft-representative fasteners (adapted from Vesley et al., 1966)

Process

Minimum, mean and maximum microbial colony count per average assembly process

No hand care

4

122.6 380.8

2 min ordinary soap wash

2

13.3 56

2 min hexachlorophene wash

0

1.3 7.4

2 min hexachlorophene wash,

0

0.2 2

and gloves

Figure 14.2. The die-off rates at 22 °C for skin-carried organisms on two metallic and non-metallic spacecraft construction materials.

Storage duration (days)

Figure 14.2. The die-off rates at 22 °C for skin-carried organisms on two metallic and non-metallic spacecraft construction materials.

Given that humans are the prime source for the biota deposited on spacecraft materials, the simplest way to place such organisms under stress is to subject the contaminated hardware to conditions drastically different from those found on and in the human body. The moist ecosphere of the human body means that the easiest way to stress a human-borne microbe is to put it in a dessicating environment. The data in Figure 14.2, taken from Vesley et al. (1966), shows the survival rates for spores on plates of aerospace materials that had been handled by a group of laboratory workers.

Having outlined the problem of spacecraft hardware contamination, it is useful to describe the present regulations, and then the techniques that can be employed to achieve these requirements.

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