April 24, 2009
Lunar dust sticks to everything! It’s electrically charged! It causes silicosis – astronauts on the Moon will get “black lung” disease, just like coal miners on Earth! It’s so abrasive that under its obnoxious influence, moving parts slowly grind to a halt! We can’t possibly cope with it! So much for our plans to live and work on the Moon. Guess we better stick to low Earth orbit.
To see the headlines and read some of the articles written on this topic over the last few years, one might get these impressions. Is any of it true? We actually have some hard data on this subject from measurements made on the Moon, experience dealing with dust during the Apollo missions, and from studies of returned lunar samples. Being a unique product of its environment and the processes that made it, the dust of the Moon is fascinating material. But is it a “show-stopper” for lunar return?
The surface of the Moon is covered by a fine powder, the ground-up reside of surface rocks produced over eons by micrometeorite bombardment. The lunar soil or “regolith” covers everything and as a surface layer, its thickness increases with time. Because lunar soil is formed by impact disaggregation, the edges of the dust grains are angular and sharp; no water or wind erosion is present to round off the grain edges as on the Earth. The highly angular nature of lunar dust makes it self-compacting. The upper few centimeters of soil are loose and fluffy, but the soil becomes considerably denser and well packed below this depth. The dust is very fine grained; the mean grain size of lunar regolith is about 40 microns, roughly half the width of a human hair and a little coarser than talcum powder (about 10 microns), but hard and abrasive instead of soft.
The experience of the Apollo astronauts with lunar dust was that of the unprepared meeting the unknown. We didn’t really understand the nature of the dust when the Apollo EVA suits and equipment were designed. The crews went to the Moon with brushes to clean off dusty surfaces, but the adhesive nature of the dust (caused by the high degree of angularity of each grain) was not appreciated. We also didn’t (and still don’t) understand the electrostatic charging properties that makes the dust “cling” to things, just as your clothes do when you first take them out of the dryer.
Dust got into everything during the Apollo missions. Plastic bags refused to seal properly. Fenders fell off the lunar rover, spraying dust all over the astronauts and their equipment. Metal seal rings on space suits because clogged with dust and refused to seat properly. As if all this weren’t bad enough, the astronauts themselves seemed to revel in getting down and wallowing in the stuff, covering their lower bodies with black, charcoal-like smudges. After a hard day exploring the Moon, the crews noted the acrid smell of dust in the LM cabin (Buzz Aldrin said that it smelled like gunpowder.) They breathed it into their lungs during the rest periods.
So, will lunar dust prohibit long-term habitation of the Moon? Hardly. Lunar dust has some unique properties that require careful consideration during the design of surface systems, but many of its alleged hazards can be avoided or mitigated. One of the simplest things to do is to avoid contaminating things with dust. The Apollo astronauts got covered with dust largely because they could not bend at the waist in their suits. They would fall forward on their faces, stop the fall with one hand, grab a rock or tool with the other and then do a one-handed push-up to stand up. This technique worked great – except for covering their suits with dust. New suits will allow crews much greater flexibility, including the ability to bend at the waist.
Another idea is to keep the suits outside, leaving the interior of the habitat completely dust free. But no matter how carefully we avoid it, some dust will get into places we want to keep clean. Brushing only seems to grind it into porous surfaces. But amazingly enough, we have found that much of the dust is magnetic. Vapor-deposited metallic iron coats the surfaces of many mineral and glass dust grains. This so-called “nanophase iron” (from its extremely small size) makes the dust easily attracted to a simple magnet. A brush made with magnetic bristles will clean surfaces of most of the dust. Incidentally, this same property permits the lunar soil to be fused into glass using low-energy microwaves, allowing us to “pave” roads and landing pads near and around the lunar outpost and to make bricks for construction and radiation shielding.
Although we do not fully understand the electrical charging properties of lunar dust, several experiments on current and future robotic missions will characterize these properties thoroughly. The idea that charged dust can levitate and coat exposed surfaces is widely believed, but there is no solid evidence that this process occurs to any significant degree and there is considerable evidence that it does not. The Surveyor 3 spacecraft, exposed on the lunar surface for over 30 months before examination by the Apollo 12 crew, was not covered by any significant amount of dust, other than that thrown up by the nearby landing of the Lunar Module.
Dust is both an asset and an issue. We will process the dust of the Moon to extract useable products, like hydrogen and oxygen. At the same time, we will learn how to live with it. We’d better – dust occurs throughout the Solar System. On Mars, it may have toxic chemical properties. It’s not a deadly hazard –just another property of the new worlds to which we journey.
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