Dust

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Sometimes the largest problems are caused by the smallest things. The threats caused by lunar dust are probably the most difficult problem lunar settlers will face.

Please note:

This article uses the µm abbreviation for microns, micrometers, or 10-6 meters. This is equivalent to one millionth of a meter (1 m/1,000,000), a thousand nanometers, or approximately one tenth of the diameter of a droplet of mist or fog.

General Description

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

A nice illustration of lunar dust.

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

Particle size ranges from large rocks to gravel, sand, and down to dust with an average particle size of about 70 µm. This is too small to be visible with the human eye. The dust grains are defined to be any particles less than 100 µm in diameter. Fine dust with particles less than 20 µm can make up 10% to 20% of the mass of the regolith.

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. This powder has been generated by billions of years of bombardment by micrometeorites, resulting in hard, abrasive particles with often jagged edges.

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 rise to the top because it is statistically easier for a small nut to work its way under a large nut than for a large nut to work its way under a lot of small nuts. Unlike a can of nuts, on the Moon the large rocks tend to be found in the lower levels of regolith. There are also a few rocks of all sizes scattered across the surface and throughout all levels of the regolith. See Regolith.

Particle Mobility

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


Dust can be moved significant distances naturally

due to:

  • 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.

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 and other regolith processors
  • During construction

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

Electrostatic Movement

A combination of strong sunlight and bombardment by the solar wind produces strong electrostatic charges in the regolith that change over the night/day cycle. This movement of lunar dust is a very special case that needs further study.

Coarse particles, between 100 µm and 40 µm, can be levitated to at least 6 cm above the surface and can move comparable distances. They can form a dust cover over metal, painted surfaces, and optical surfaces (significantly reducing light throughput). If this dust makes its way into mechanical systems its highly abrasive attributes will greatly increase internal wear and tear and lead to breakdowns.

The fine dust particles, under 40 µm, are subject to major electrostatic movement and can be thrown kilometers above the lunar surface in "dust fountains". Such fountains most often occur at dawn or night fall, and the dust can be thick enough to disperse the sunlight, making the fountain visible to 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 reportedly smells like spent gun power and has caused hay fever like symptoms in Apollo crew members. The dust has a tendency to react with wet surfaces such as the lining of human breathing passages.

The long term health effects of exposure to lunar dust are not yet known. Some have suggested that inhaling lunar dust could cause a form of "Silicosis", a serious chronic lung condition which has no cure and can be fatal.


The Response:

A major effort to control dust will be needed[1].


Protecting Living Space

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

The entry ways will likely be complex and expensive to build, man, and maintain. It will take people a significant length of time to enter a building, and they will probably need robotic assistance.

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.

To support industrail activity, one or two shop areas will be needed. These will represent particarlly difficult problems for controlling dust as complex equipment will need to be brought in and out, often on an emergancy basis.

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 collector 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 or full space suits to help keep their insides free of dust and to provide a stable temperature. Hydraulic pistons could be completely inside space suits covering robotic machines or the pistons could be covered by stretchable accordion pleated sleeves with gas tight connections to the machine above and below the pistons.


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.

Conclusion

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

Related Articles


External Links

References

  1. ↑ Solving Settlement Problems: Dealing with Moon Dust Ad Astra By Edward D. Flinn, posted: 23 February 2006 07:11 am ET


Hazards
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