Dust

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The Dust Hazard to a Lunar Settlement

The threats caused by lunar dust are probably the most difficult problem lunar settlers will face.

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The Problem:

Sometimes the biggest problems are caused by the smallest things.

General description

Regolith blankets nearly the entire surface of the Moon. It is everywhere and even simple rock outcroppings are rare. The depth of the regolith dependents on the location, slope, and the geologic history and usually is at least several meters. The regolith is made of mineral grains, largely of the local rock, with some glass and some metal particles.

The particles tend to be elliptical in shape but often have sharp points and edges. The grains have never been weathered by water or rolled by wind which would remove sharp edges.

Particle size ranges from rocks, through gravel, sand, and down to dust with an average particle size of about 70 um (10^-6 meter). This is smaller than you can see with the human eye. The dust grains are defined to be any particles less than 100 um in diameter. Fine dust with particles less than 20 um can make up 10% to 20% of the mass of the regolith. </-- Note: um may need to be replaced with it's verbal designation due to wikimedia's inability to display the proper character for u. Jarogers2001 -->

For more details see:

[1] Timothy J. Stubbs, Richard R. Vondrak, and William M. Farrell, “Impact of Dust on Lunar Exploration”

Layers in the Regolith

The regolith surface is covered with nearly pure dust 6 cm to 10 cm thick. This is a very fine powder but is nothing like the talc you are familiar with despite its appearance. The particles are hard and abrasive even if small. This powder has been generated by billions of years of bombardment by micrometeorites.

The bulk of regolith is coarser grains mixed with rock to a depth of several meters. Below that is crushed rock in increasingly larger sizes.

There are no Brazil nuts on the Moon. If you shake a can of mixed nuts, the larger nuts will raise to the top. This is because it is statistically easier for a small nut to work its way under a big nut than for a big nut to work its way under a lot of small nuts. On the Moon the big rocks tend to be found in the lower levels of regolith. There are also a few rocks of all sizes scattered throughout all levels of the regolith and across the surface.

Particle Mobility

As long as regolith particles do not move they are not a problem. We want nothing more that to let sleeping dogs lie. The problem is that the dust can be moved by natural causes or by the actions of man. It can then get into the wrong place and do a lot of damage.

Dust can be moved significant distances naturally by:

• Micrometeorite hits
• Electrostatics (see below)

Total mass movement by natural causes is very small. If the mass was significant then crater rays would vanish in a few thousand years. They do vanish, but it takes millions of years.

Or, dust can unintentionally be moved by people:

• Kicked by astronaut shoes
• Adherence to outer coverings
• Thrown up by robot wheels
• Thrown up by rover wheels
• Thrown out by rocket exhaust on landing or takeoff
• In industrial activity, Sandworms
• During construction

The mass of dust moved by human activity can be controlled and we must make what ever effort is required to do so.

Electrostatic Movement

Electrostatic movement of lunar dust is a very special case that needs specific study. A combination of strong sun light and the solar wind produces strong electrostatic charges in the regolith that change over the night/day cycle.

Course particles, between 100 um and 40 um, can be levitated to at least 6 cm and move comparable distances. They can form a dust cover over metal and painted surfaces. If they can reach optical surfaces the dust will significantly reduce light throughput. If the dust makes it way into mechanical systems it will greatly increase wear leading to breakdowns.

The fine dust particles, under 40 um, are subject to major electrostatic movement. They can be thrown kilometers above the lunar surface in dust fountains. Such fountains most often occur at dawn or night fall. The dust can be thick enough to disperse the sunlight and be seen with the unaided eye.

For more information see:

[2] Timothy J. Stubbs, Richard R. Vondrak, William M. Farrell, “A dynamic Fountain Model for lunar dust”

[3] Timothy J. Stubbs, Jasper S. Halekas, William M. Farrell, and Richard R. Vondrak, “Lunar Surface Charging: A Global Perspective using Lunar Prospector Data”

Dust Inside

If the fine dust gets inside living spaces, it can float in the air and slowly settle on surfaces. It smells like spent gun power and can cause hay fever like symptoms. It has a tendency to react with wet surfaces such as the lining of your breathing passages.

The long term health effects of exposure to lunar dust are not known.

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The Response:

A major effort to control dust will be needed.

Protecting Living Space

Dust control will be an important design requirement for the architecture of any lunar structure. Dust must be controlled at entry points and filtered from the air. All areas must be easy to clean by simple robots. For more information see Architecture as Mole Hills.

The entry ways will complex and expensive to build, man, and maintain. It will take people a significant length of time to enter a building. They will probably need help from a robot.

There will probably need to be outside robots and inside robots. It will simply be too much trouble to clean an outside robot to bring it inside.

Protecting people

American spacesuits are being totally redesigned for the new lunar missions. The outer Beta cloth on the Apollo suits could not have been better designed if we had intended them to collect lunar dust. The new suits will have smooth, dust shedding outer layers. They will also have few cracks for dust to hide in and will be much easier to clean.

The long term heath effects of breathing dust will be studied in detail. The design of permenant living facilities will be strongly affected by the results of these studies.

Protecting machinery

Lunar industrial activity will require handling lunar regolith at the rate of tens of tons an hour (see Sandworms). This activity could throw dust great distances if not controlled. Once a dust particle is moving, there is no air to slow it down.

Critical systems of the machines themselves will also be degraded by layers of dust. These include:

• Solar collectors
• Thermal radiator panels
• Photovoltaic panels
• Cameras

It may be necessary to design some devices, such as solar collectors sections, so that they can be turned periodically perpendicular to the ground and shaken to induce dust to fall off.

Dust seals on moving equipment will be very important. It is much easier to design seals for rotary movement, like a motor, than linear movement, like a hydraulic piston. Rotary movement tends to throw the dust away from the seal. For linear movement the seal must repeatedly push the dust out of the way. Seals will be a major maintanence problem.

Sandbags full of regolith will be very useful devices for providing radiation and thermal protection. They will have to be made in such a way so that they will not leak dust and can be filled by robots.

Outside robots will probably need to wear coveralls to help keep their insides free of dust to provide a stable temperature.

Protecting Electronics

All electronics will require electrostatic shielding and grounding. The thermal blankets for all equipment will have to have metalized outer layers and ground wires.

Electrical Connectors are particularly susceptible to dust damage. The dust can act as an abrasive and wear at the outer conductive coatings on the pins, and it can get between electrical contacts and cause an intermittent connection that can be very difficult to find. Lunar dust may also react chemically with some cable insulations. Dust can also partially block or misalign fiber-optic connections.

Protecting optics

Optical devices will be used extensively on the Moon and require an exposed surface to function. They will be critical for:

• Science instruments
• Robot vision
• Industrial control
• General health and safety

Keeping entrance optics clean will require a serious design effort. It will help if the apertures can be at least one meter above the regolith, but this is difficult for small instruments.

All optical apertures may require fully actuated covers that can be opened and closed remotely. However these can fail, especially in a dusty environment.

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Conclusion

We can beat the lunar dust problem but only by a major effort on many fronts.

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