4.2.1 How clean is clean?
The ISO standard (ISO 14644) specifies how a cleanroom should be designed, built and operated and classifies cleanrooms from ISO1 (cleanest) to ISO9 (least clean). When the Beagle 2 spacecraft you met in Section 4.1.1 was built, the cleanroom standards were described differently as Class 1, 10, 100, 1000, 10000 or 100000, with Class 1 the cleanest; Beagle 2’s Aseptic Assembly Facility on the Open University campus was a Class 1.
The Class number (and today’s ISO classification) was determined by the number of airborne particles between 0.1 to 10 μm in one m3 of air (Table 6). These particles could be aerosols, dust, cells, or spores.
| Table 6 Maximum concentration of particles per m3 of varying sizes | ||||||
|---|---|---|---|---|---|---|
| ISO classification number | ≥0.1 μm | ≥0.2 μm | ≥0.3 μm | ≥0.5 μm | ≥1 μm | ≥5 μm |
| ISO class 1 | 10 | – | – | – | – | – |
| ISO class 2 | 100 | 24 | 10 | – | – | – |
| ISO class 3 | 1,000 | 237 | 100 | 35 | – | – |
| ISO class 4 | 10,000 | 2,370 | 1020 | 352 | 83 | – |
| ISO class 5 | 100,000 | 23,700 | 10,200 | 3,520 | 832 | – |
| ISO class 6 | 1,000,000 | 237,000 | 102,000 | 35,200 | 8,320 | 293 |
| ISO class 7 | – | – | – | 352,000 | 83,200 | 2,930 |
| ISO class 8 | – | – | – | 3,520,000 | 832,000 | 29,300 |
| ISO class 9 | – | – | 35,200,000 | 8,320,000 | 293,000 | |
The choice of cleanroom class per mission depends on the category of the mission, as defined on Table 5. The COSPAR recommendation is for a minimum of ISO8 for the build and testing of any spacecraft. If parts of the spacecraft will come into contact with a planetary surface, then an ISO5 cleanroom (or better) is required. However, the specific requirements depend on the details of each mission and a single spacecraft might have components built within different classifications of cleanroom.
To maintain ‘cleanliness’, the cleanroom environment is controlled by being constructed from materials unlikely to shred or outgas, adding filters to the airflow and applying a positive pressure to the room. The surfaces are regularly cleaned using sterile wipes and a cleaning product such as ethanol or isopropyl alcohol.
Since humans are estimated to bring up to 80% of particles into a cleanroom, users may need to use an air shower before entering/exiting and are required to wear cleanroom garments (Figure 29). This reduces the risk of microorganisms on human skin, hair or clothes being introduced into the clean room environment and settling onto the spacecraft.
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What term describes microbes that settle onto surfaces?
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We describe these as bioburden.
