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You see many pictures of spacecraft being handled by people with masks and hairnets. What's the reason for this?

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    $\begingroup$ The sad list of missions killed by "foreign objects" is just way too long. $\endgroup$ – Deer Hunter Aug 19 '13 at 8:42
  • $\begingroup$ Much of the FOD issues don't necessarily need a clean room ... I've heard stories of people leaving tools on spacecraft (fell out, possibly damaging things when the spacecraft was rotated). NASA even had an order that some badge holders were considered a FOD risk. $\endgroup$ – Joe Aug 19 '13 at 14:27
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    $\begingroup$ FOD is an issue, but doesn't require clean-rooms. Airplanes aren't assembled in clean rooms, they have an even worse FOD problem. @Everyone concerning the bacteria: I've seen earth-orbiting satellites assembled in clean-rooms. Contamination of extraterrestrial ecosystems isn't an issue, there. Also, a single bacteria surviving would be enough to contaminate a planet, and even the best of cleanrooms will hardly get rid of every single bacteria. Sterilizing the spacecraft could do the trick, but then why a clean-room assembly beforehand if sterilization happens anyway? $\endgroup$ – yippy_yay Aug 20 '13 at 8:54
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You're right about clean rooms not effectively protecting against bacteria. So why are they used?

First, there are different levels of clean rooms. Typically the level is specified as a maximum number of particles per cubic meter (with different levels for different size particles). For instance, under the ISO standard, a class 6 clean room must not have more than 100,000 particles ≥0.1 µm, 23,700 particles ≥0.2 µm, and so on (for quick reference, see Wikipedia).

The "cleanest" clean rooms (lowest number of particles per unit volume) are used when the satellite has sensitive optics or other instruments on board. Cryogenically-cooled instruments are particularly susceptible to particulate contamination. Obviously if you have components that are very sensitive to contamination you want to keep your entire spacecraft free from possible contaminants, thus the need for your entire environment to be as clean as you can get it.

The "less clean" clean rooms (higher number of particles per unit volume) are for when you don't have components that are highly sensitive to contamination, and you can afford a few particles per square cm of surface area. While instrument performance may not be adversely affected by the presence of contaminants, you still want to keep the amount of dust and debris to a minimum to protect against possible electrical shorts or electrostatic discharge (ESD).

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    $\begingroup$ Another common use of clean rooms is to prevent particulate contamination from getting inside of closed systems, for example propellant feed lines, which can clog filters, valves, etc. Those systems are built up and sealed in very clean rooms, at which point higher levels of assembly can occur in less clean rooms. +1 for optics being a major reasons to keep things clean. Solar cells also work better when not covered by a fine layer of dust, and don't like being handled for dusting. $\endgroup$ – Adam Wuerl Sep 11 '13 at 4:36
  • $\begingroup$ @Adam's comment by far was he most concise, I would have accepted this as the answer $\endgroup$ – killjoy May 2 '17 at 0:15
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No specialist I, but I would argue the cleanliness becomes essential for at-least two reasons as follows -

  • Contamination: Even the air we breathe is filled with bacteria, and what-not. Spacecraft sent on scientific missions could potentially return a false positive at the least. Worse, assuming life existed on the target body, Earth bacteria could easily do something they ought not.
  • Risk: Dust may be susceptible to charge. In gravity, such dust would settle at the point of largest attraction. In a vacuum, dust would be free to potentially short out a micro-chip, or a PCB track ... whatever. Thus posing a hazard to the mission.

Ergo, we err on the side of caution by clean-room assembly.

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    $\begingroup$ Bacteria will hardly be avoided with clean rooms. $\endgroup$ – yippy_yay Aug 19 '13 at 8:00
  • $\begingroup$ 'What-not' may be avoided (+: Bacteria are just a part of a bigger composition. Dessicated matter, skin, dandruff, motes that live on the skin ... to name a few $\endgroup$ – Everyone Aug 19 '13 at 8:03
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    $\begingroup$ @SebastianHenckel - actually, it depends on the kind of the clean room. $\endgroup$ – Deer Hunter Aug 19 '13 at 8:46
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    $\begingroup$ I would not consider bacterial nor static build-up / discharge the main or even relatively important reasons for building spacecraft in cleanrooms. Most spacecraft are not looking for life or at risk of contaminating another body with Earth life. Tin whiskers are a much greater risk for shorts in a spacecraft than dust. $\endgroup$ – Adam Wuerl Sep 11 '13 at 4:34

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