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Dust and small particles in microgravity environments are generally regarded as bad, and items prone to generating these tend to be discouraged:

Bread should be prepared quite differently, so that it does not crumble and lasts a long time. The Russian Institute of the Bakery Industry had to come up with solution for using the bread at the International Space Station without the risk of crumbs getting into the ventilation system. They came up with a miniature loaf - just big enough for one bite. In texture and taste, this bread is virtually indistinguishable from that which we eat on Earth. ["How Russian space food has evolved over the years" by Russia Beyond, found via this answer to the question "Why was there a miniature bread on the ISS?"]

Bread: Astronauts are not allowed to eat bread because their crumbs can go into machine and equipment, and into astronauts' eyes.

[...]

Salt and Pepper: Again similar to bread, salt and pepper can go into their eyes and damage equipment. Salt and Pepper are served on the ISS by being dissolved into liquids. However salt and pepper in their raw form are not allowed on the ISS. [This answer to the question "Are there foods that astronauts are explicitly never allowed to eat?"]

The biggest issue is that chalk notoriously produces dust, which will not settle and becomes a breathing hazard.

Pencils and grease pencils were phased out as they created particles and shavings, but at first they didn't have anything better.

The writing with the chalk will work GREAT! The astronauts and spaceship that has to deal with the liberated chalk dust will NOT do so well. [This, this, and this comment on the question "Can we write with chalk on blackboard in space?"]

The stated reasons have to do with the risk of floating crumbs or particles getting into machinery, lungs, or eyes due to their not being pulled to the floor by gravity.

But:

  • Proper air circulation (more important in microgravity than in macrogravity to begin with, due to the need to prevent pockets of carbon dioxide from building up and asphyxiating astronauts) and filtration should effectively clear floating particles from the air;
  • Anything floating into an astronaut's eye would trigger the blink reflex and thereby be cleared;1
  • Electrical and electronic machinery tends to be quite insensitive to electrically-nonconductive dust until so much of it has accumulated for it to impede equipment cooling by its sheer airflow-blocking bulk (something we know because electronics are dust magnets even in macrogravity, as anyone who's ever opened up a well-used desktop knows), and equipment with conformally-coated circuit boards will ignore even electrically-conductive dust (e.g., graphite from pencil shavings), and will shrug off liquids to boot.

So why is dust so feared in microgravity?

(Thanx to DrSheldon for prompting me to ask this question.)


1: In fact, eye-attacking particles would probably be less dangerous in microgravity, where they'd just be floating around, than in macrogravity, where particles that get into eyes tend to have been thrown with considerable force from whatever generated them.

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    $\begingroup$ The "why can't you just filter it out" aspect particularly needs to be answered here. The "why is it a problem" part has been answered in a piecemeal fashion by the answers you cite (though an answer which unifies the issues would be nice). $\endgroup$
    – DrSheldon
    Sep 9 at 5:59
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    $\begingroup$ "Anything floating into an astronaut's eye would trigger the blink reflex and thereby be cleared." What if the particle is water/tear soluble? $\endgroup$ Sep 9 at 12:06
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    $\begingroup$ Just look underneath the desk on which you wrote this question. Imagine if the dust and crumbs that are there were floating around your eyes... $\endgroup$
    – Jeffrey
    Sep 9 at 13:20
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When you have gravity heavy particulates like shavings, crumbs, large dust granules, etc. will fall to the ground where they can be cleaned or form part of the environment. In microgravity everything is suspended no matter the size, so there is a lot more stuff to deal with, including dust. Heavier particles take more energy to move, so they will take longer to filter out in microgravity

Now, imagine a room in microgravity with a ventilation system which pulls in one side of the room, filters it and then blows it out a vent on the other side, which is a simple model but good enough for our purposes. There are 2 *nauts in the room, one of them bashes 2 chalkboard erasers together, the other eats toast while sharpening pencils. Now picture what's going to happen to the dust and shavings: first it's going to spread, then it's going to start to get pulled towards the intake vent. However, it's not going to be filtered instantly, in general the lighter particles will start to move to the vent, the heavier ones have more mass so move slower, although density and shape are factors in how quickly they will move.

Anyone in the path of the airflow is going to get a face-full of the mass of particulates. Cranking up the fans will move the air faster and speed up filtering, at the same time it will make all the particulates move more energetically into people's faces.

So filtering is very important, but it's better to prevent it from the outset.

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    $\begingroup$ Good point, it's not like the airflow is a hurricane howling through the cabin; there's plenty of time for the particles to spread out and get into/onto stuff. $\endgroup$ Sep 9 at 12:10
  • $\begingroup$ "Heavier particles take more energy to move, so they will take longer to filter out in microgravity" Doesn’t it depend on the aerodynamic properties vs. mass? Surface area vs. mass etc. $\endgroup$
    – Michael
    Sep 9 at 12:19
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    $\begingroup$ Sure @Michael, shape and density will be factors in how quickly particles move, in general heavier particles will move slower than lighter ones. $\endgroup$
    – GdD
    Sep 9 at 12:45
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    $\begingroup$ @Michael As a very rough first order model, you have a square/cubed issue. As radius goes up, aerodynamic forces are proportional to area while mass is proportional to volume. $\endgroup$
    – Cort Ammon
    Sep 10 at 5:30
  • $\begingroup$ As an aside, such a windy room would also constantly move the nauts themselves, no? I'd say that's an additional unwanted side effect. $\endgroup$
    – Flater
    Sep 10 at 9:43
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Getting dust in one's eyes is a mild inconvenience at best.

However, what happens if the dust particle lodges itself in a person's eye just when that person needs to do something critical?

What happens if a dust particle cannot be easily or safely dislodged and the person has to endure having something in their eye for a prolonged period? The stress caused by the discomfort will affect the person's ability to perform tasks and will be a constant distraction.

What happens if something abrades the cornea resulting in blurred vision and potentially very uncomfortable spikes of light which can result in headaches, discomfort and stress for the person involved?

All this is bad enough on Earth were medical attention would be available quickly, but for someone confined to a space craft it becomes a more serious matter.

As for dust on equipment, are you really sure you want to risk a component failing in space and not being able to repair it quickly?

As to designing and making a "proper air circulation" system there are a few problems.

Firstly air speeds need to be to such to ensure timely circulation. Too slow and gases and dust can collect in pockets and people could become too hot , too fast and dust could more easily lodge itself into someone's eye, low mass objects could more easily fly around uncontrollably and people could become too cold.

The placement of ventilation intakes and exhausts cannot easily be modeled because of the variables involved. Heat generated by equipment. The location of people in the system and how they obstruct airflow. A well built person has a different affect to a slightly built person. Short hair has different affect to long flowing hair. Energetic hand movements, a person in stream compared to nearer a wall. Two people acting as obstructions close to each other.

Minimizing dust and it affects is a way to mitigate risk associated with operations.

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  • $\begingroup$ Good answer on impact being bigger due to mission critical tasks. But misses the gravity part. Gravity keeps the crumbs on the floor on earth. Micro-gravity would make your whole work space as dirty as the floor on earth. $\endgroup$
    – Jeffrey
    Sep 9 at 13:19
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    $\begingroup$ Even if it turns out not to be mission critical, consider that the ISS's yearly budget is $3-4 billion. That's about $350,000/hr, or around $100/sec. Even a small annoyance shows up as a good chunk of one of our terrestrial salaries. $\endgroup$
    – Cort Ammon
    Sep 10 at 5:32
  • $\begingroup$ @Cort you're kidding me, why don't they spend that money on practical stuff that will help regular folk here on earth? Like a 13th aircraft carrier? $\endgroup$ Sep 12 at 2:42
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Dust will damage machinery through more than just conducting electricity or hindering passive cooling. Fans in particular are vulnerable. Dust can slow down fans by adding weight to the blades or caking onto the shaft and increasing friction. Fine particles that get into the fan's casing have a hard time getting out. They grind against the fan's bearings as it spins, adding friction, wearing down the bearings faster, and increasing the heat generated. In some cases, the dust can get hot enough to ignite. These are all problems you can observe on Earth, they'll only get worse when you don't have gravity helping control the particle count.

Many of these particles are also quite abrasive, crystalline substances (like salt) in particular. Scratches on the surface of a solar panel can reduce its efficiency. A single particle isn't likely to do much, but the cumulative effect of a large number of particles over a long period of time can do some serious damage, slowly grinding away at any moving parts they get lodged between. A buildup of dust can even cause equipment to seize up, or prevent a membrane switch from making contact. Equipment failures like that in space are a serious problem, both due to lack of replacement parts and due to some equipment not being easily accessible. Abrasive particles between your eyelid and cornea can do permanent vision damage.

It's true that you have a blink reflex to help protect your eyes, but that doesn't mean that it won't hurt like the dickens. Black pepper irritates mucous membranes, you definitely don't want it in your eyes or nose. Astronauts do a lot of things where concentration is critical and unpredictable attacks from invisible flying irritants can have disastrous consequences. During the emergency abort procedure for Apollo 13, the crew had to steer their corrective maneuvers by visually keeping the moon aligned with a specific point in one of the windows. Such procedures would be next to impossible when the pilot's eyes started watering uncontrollably due to irritants in the air.

Don't forget that dust can affect things chemically as well. Dust can mix with the lubricants used in motors and bearings and change that lubricant into a thick, viscous fluid that impedes motion instead of promoting it. Salt accelerates the corrosion of metals.

In other words, there are lots of possible problems here. You either have to design a system that can effectively filter out all this dust, or just avoid creating the dust in the first place. The latter is significantly easier and less risky.

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  • $\begingroup$ Also rember that without gravity, tears wont flow down, specifically, so its harder to clear them from the eyes. $\endgroup$
    – Stilez
    Sep 10 at 22:04
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AFAIK, the main problem is static electricity, dust tends to stick to everything with a minimum charge, my personal guess is that if not properly handled the ventilation system could make it worse because dust will generate more static electricity by rubbing the vents.

I know my answer is not as complete as others, however I noticed that nobody mentioned static electricity so I wanted to contribute. I read about the subject in an article about the moon rover issues dealing with dust.

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    $\begingroup$ Indeed, in micro-gravity forces from electric charges don't compete with gravity, so dust will tend to stick to all surfaces before it has a chance to be filtered-out. Air flow is not getting in motion with convection either in micro-gravity, so you cannot count on flushing dust. Dusts can be very flammables or even explosive like flour dust. In micro gravity, dust will stay as a cloud for longer and will not settle. $\endgroup$
    – Léa Gris
    Sep 12 at 3:44
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The problem is the route the pinxy-wee particles take.

When you guarantee that this, that or the other contaminant goes through a filter before hitting the astronaut's eye or nose or mouth… or any system that might mind being invaded by dust, everything might be fine and dandy.

How, though, will you make that guarantee?

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