Air currents do a very poor job of dispersing dust by themselves; you have to really whomp the hell out of the dust to get it to granulate and disperse that well. I would assume that any zero-g space habitat would have constant forced air circulation which would tend to keep the dust collecting in a few places.

But I could be completely wrong. Let's test it out!

I haven't run across a specific set of safety requirements associated with dust explosions in zero-G, but by and large, NASA is *extremely* careful about this sort of thing, ever since the Apollo pad fire, and I'm sure there are such requirements. They probably severely limit the kinds of material you can even *have* in the form of dust or powder on the Shuttle or station, since whether or not there's a risk of explosion, there would be problems of contamination of sensitive equipment, risks of inhalation, risks of blocking filters, etc. Powdered materials needed for experiments probably have to be in sealed containers; flammable powders (if they've had occasion to carry any) probably have to have double containment.

The station and shuttle also have quite sophisticated ventilation and air filtration systems -- after all, *any* pocket of still air could turn into a dangerous CO2 trap if an astronaut happened to put her head into it, and every skin flake or droplet of perspiration is a potential contaminant.

And (strike three) ignition sources (except the ones inside the engines :-) are seriously verboten in spacecraft, in part precisely so that weird accidents (spilled powders, or, say, accidentally pressurizing the cabin with pure oxygen) *won't* result in fires.

So the Station is probably safe. If we get large-scale space tourism or space industry, though, sooner or later someone will manage to ignore all the safety rules and bypass all the safety mechanisms and blow something (probably themselves) up with a powder explosion.

1) Jordin -- How about an ion generator, pushing out negative ions, and a postive plate?

2) Teresa -- I don't think it's an issue. Dust explosions only happen when you have a very large amount of airborne combustible dust, that you can't feasibly control. You can't filter dust in a silo -- too much of it, and you can't wet it down, you'll ruin the grain. In coal mining, where water won't ruin the coal, they mist as they mine, and dust explosions no longer happen.

I don't think it's an issue. Jordin mention the very active air moving and filtration systems. Also, you are going to generate dust, but in grain silo concentrations? No. You'll generate dust on the order of your house, not a silo -- and you don't see houses exploding from dust. (Natural Gas, yes.) Plus, the ISS is probably a much cleaner enviroment.

I'd think if dust was an issue, Mir would have blown to bits during the 1998(?) fire. Mir was also markedly less well kept than the ISS is now -- and the only reason it's not still in orbit is that they splashed it.

Dust explosions? Oh, boy. I'm still trying to figure out what dust solutions are (as in equations for black holes, etc.)

I think the answer to the original question is that, yes, it could happen if a space station's atmosphere were contaminated with a significant amount of an inflammable substance in the form of a fine powder dispersed in the breathing mixture. In fact, given that the breathing mixture in such a structure is likely to have a higher proportion of oxygen than the Earth's atmosphere, such an event would probably be more powerful on a space station than in the Earth's atmosphere, and could probably be initiated more easily.

However, as Jordin noted, letting dust get loose in a space station would be something to be avoided for other reasons. Additionally, I'm not sure there would be any application for grain flour in free fall or microgravity environments, since baking as we now know it seems to require that the oven be undergoing acceleration during the baking process.

Making a hot cocoa drink from scratch would probably be possible, though, and could generate the kind of hazard you're describing. Not in a NASA facility any time soon, though: more likely in a Tsiolkovsky-esque large-scale orbital structure filled with Just Plain Folks.

My chemistry is rusty enough that I can only question Bob Webber's suggestion that a dust explosion would be more powerful and easier to initiate in the ISS than on Earth, but my recollection is that what matters is not the proportion but the partial pressure. (Crude numbers: Earth pressure is 15psi, oxygen is 20% of the atmosphere, partial pressure of oxygen is 3psi -- measures the concentration in units/volume (e.g., how many oxygen atoms are next to this dust particle) rather than percentage.) I know the ISS is running at

Many years ago I saw the woman who designs meals for the astronauts on a science show. She discussed what their concerns were when they created the meals. She mentioned that pretzles would only be allowed if they could be ingested in a single bite, and even then would probably only be included as a special request. I can't see any powdered substance allowed unless it was for an experiment, and even then it would probably have safeguards.

I recently saw the IMAX Space Station 3D movie which was extremely cool. On a couple of occasions in that film they showed people on the ISS spewing food crumbs in all directions without any obvious concern. Though now that I think about it, these demonstrations may have been done right by a ventilation filter so the detritus would be quickly collected and contained... The movie didn't show any flour fights, so they're probably safe for now.

But like Mr. Maroney said, let's try it! Who's going to design the experiment? I'd be happy to go up to conduct it.

I think we need to find out how they sold the idea that popping water balloons in zero gee was a necessary and useful scientific experiment, and then sell this one the same way.

Dust explosions do -not- require "a very large amount" of dust. I've seen film of a demonstration run by the Bureau of Mines, in which they suspended a few grams of coal dust (it was described as "enough to cover a quarter") and ignited it; the effects were quite dramatic. Close to a stick of dynamite, according to them.

That said, it does seem that reasonably intelligent design would reduce this to, at worst, a very small risk, especially compared to the other risks. And I don't mean that ironically, though the ISS has never struck me as an exceptionally fine piece of industrial or human-factors design.
Into this nettle, danger, we throw this flour . . . oh, never mind.

Eric: NASA may well use electrostatic precipitators inside the air filtration system, but I doubt they want any significant ionization sources in the open air, as it were.

But you can make a respectable dust explosion with modest amounts of dust; figure the equivalent of two or three times the dust mass in TNT. It's just harder to get the right concentration of dust in air if the amount of dust is small; as soon as it spreads below a critical concentration, it won't propagate a flame. Back in the days when chemistry sets included sodium nitrate :-) a staple of "50 Experiments for Boys" books was a flour explosion done with a teaspoon of flour in an empty gallon paint can or similar.

Chip: I recalled that the Station ran 7 psi total pressure, 3 psi oxygen partial pressure -- but I was wrong. A bit of Googling indicates that the Station runs approximately a sea level atmosphere.

If it were that big a risk, you'd simply have to sell the cocoa in semi-pre-mixed sludge format (the marketing types might need to come up with a snazzier, less sludgeful name), in special sealed packets, specifically for consumption in space. Or maybe you could sell it in cubes, like sugar cubes, to be mixed with powdered milk cubes and water in a sealed container.

Space Food Sticks: The Nonexplosive Astronaut Snack.

Leaves Tang out the airlock, though.

"What are you doing, HAL?"
"You may recall that it's Frank's birthday, Dave. Please pass the vanilla extract."

Regarding ease of ignition and rate of combustion, the factors I had in mind were the heat capacity and thermal diffusivity of the breathing mixture in which dust combustion was taking place. The effects might be small, and depending on the actual mixture and total pressure of gases might retard or accelerate combustion -- as Teresa says, we really need to have this experiment funded and make some tests.

Expanding a little bit on my thoughts on gas mixture and pressure, combustion of a solid is initiated at some sites on the surface of some dust particles. Continued combustion results when the sensible heat of the reaction raises adjacent areas of the particle, and of other particles, to the ignition point. Energy can go into heating the gas near the particle, and the temperature achieved will be a function of the amount of energy and the heat capacity of the gas.

Any given volume filled with breathing mixture has a heat capacity which varies proportionately with pressure, as the number of gas molecules in that volume increases with pressure. If a low pressure gas mixture has substantially less heat capacity then sea-level Earth atmosphere, the gas will heat to a higher temperature and more of the heat will end up in the particle itself, so temperatures at the gas-solid interface will probably go up and combustion will probably be accelerated.

Another factor is the thermal diffusivity of the gas mixture, using this term to mean the rate at which thermal energy is transferred away from the combustion site. Higher thermal diffusivity will result in lower temperatures near combustion sites, and could retard the rate of combustion.

What the actual result would be is hard to determine in the absence of (a) experiment and (b) specification of the conditions -- which space station are we talking about, what is the total gas pressure of the breathing mixture, how humid is it, and what are the other component gases. On this last factor, note that Nitrogen and (e.g.) Helium have very different thermal characteristics, so a space station that uses He for a make up gas (okay, they're collecting enough from the solar wind to replace loss through leakage, and they're all rabid right wingers who want to talk like Donald Duck on this station) and a space station that uses Nitrogen would be different cases, as would stations running at close to one atmosphere and those with lower total pressures.

Similarly, Mir or a Tsiolkovsky space colony would probably have different levels of combustion risk than the ISS or an Oneill colony, since the Russian versions are generally much more humid.

By the way, the explosion risk might also vary depending on the time elapsed between grinding of the powder and ignition, since freshly ground particulates carry a surface charge known as (if memory serves) the Zeta potential. This potential results in increased dispersion of powders in air because of the electrostatic repulsion between particle surfaces.

As time goes by, the excess electrons on the surfaces of the new particles migrate into the bodies of the particles, or are captured by molecules in the ambient medium, but right after grinding the mutual repulsion can markedly increase particle dispersion.

Next week: the Marangoni Effect and its consequences for Microgravity Oenophiles.

Overall, I think this kind of problem is a fine argument for centrifugal pseudo-gravity. Jordin points out a need for good air circulation, but I don't think he goes far enough. Perhaps in a large space simulated rainstorms to sweep the junk out of the air might be a worthwhile?

Jordin points out a need for good air circulation, but I don't think he goes far enough.

Not only is it a need, it's there. It has to be -- or you end up passing out because you haven't moved your head in 10 minutes, and you've been breathing your exhalation for all that time.

Mike : Yes, dust explosion require 'a very large amount' of dust -- it's just that a very large amount of dust doesn't take a large amount of space. I've seen the coal-dust demonstration. it's amazingly scary -- until you understand what they went through to make that explosion happen.

Dust explosions happen not because dust is inherently explosive (it's not -- it's just flammable) but because certain common circumstances (coal mining and grain storage leap to mind, though I know of one flour mill that isn't anymore.) lead to that magical level of "just enough suspended flammable matter + ignition source" that lead to a (quite) impressive explosion -- and fuel/air exposions in confined spaces are *quite* impressive.

But, in the ISS, this is much harder. You need to fill a large space with the ideal mixture of fine dust -- too much, and it'll clump, too little, and you don't get the flame front you need to get the explosion. Given that we don't store dusty materials or mine dusty materials on the ISS, I don't see where you get that magical mixture of flammable materials and oxygen you need on the ISS or other space stations.

Silos blow on a regular basis because there's no way to control the dust without ruining the product. Coal mining with "wall chewers" and long-wall miners used to cause problems, until they realized that they had a problem -- misting largely solves that.

As a datum point, I recall hearing that one of the first-generation H2-antagonists (related to ranitidine and cimetidine) was 6 years late reaching the market -- a ferociously expensive mistake in the pharmaceuticals industry in the late 1980's -- because they simply scaled up the laboratory synthesis of the drug to industrial scale. One of the intermediates, stored as a fine powder, was, ahem, explosive ... as they discovered when the spanking new $200M factory spontaneously disassembled itself all over the landscape.