Slopes

The Moon is not Flat

The Moon is not flat. Almost every surface slopes this way and that. The harsh sunlight accentuates this problem my making the shadows dark and un-Earth like. This is very different situation from what we experience on Earth and our lack of experience can create real problems in our planning of a lunar settlement.

Why the Earth is Flat and the Moon is Not

The great flat spaces on Earth, the prairies, ice fields, plateaus, the deserts, and sea bottoms are made by the action of water, wind, and plate tectonics. None of these agents are at work on the Moon and never have been.

The primary force at work on the moon is impact cratering. This simply does not build flat land. Instead it digs deep holes, throws up mountain ranges, and sends out ejecta rays.

Small, persistent cratering has rounded off all shaped rock edges that appeared in illustrations of thousands of pre-Apollo SF stories. For our purposes, rounded off does not qualify as flat.

The Maria are the flattest areas on the Moon. Even these are crossed by collapsed lava tubes, wrinkled by pressure ridges, pitted by small craters, and scattered with rocks from ejecta.

A Small Globe

Also the Moon is simply a smaller sphere than the Earth. This makes the horizon much closer than a person is used to and leads to difficulties in distance estimation. This effect also directly affects the usefulness of specific sites for solar power and Earth communication.

What this Means to Lunar Settlement

Various lunar activities can only tolerate a certain amount of slope to the ground. This was a real problem in the Apollo missions as the Lunar Lander could only successfully launch from a nearly flat surface. One mission nearly failed and another nearly ran out of fuel looking for a flat spot.

Landing Sites

The new Lunar Lander design carries much larger fuel tanks than Apollo. This gives it both the capability to return from more distant landing sites and tolerate more sloped ones. Still the landing sites will probably have a maximum slope limit of less than 5 degrees along with a restriction on rock size.

Living Arrangements:

Living areas will also have serious slope restrictions. First it is very inconvenient to live in a house that slopes. Even the plumbing becomes a real problem. Extensive site preparation that requires the movement of tons of regolith and rocks will be a very expensive proposition. Constriction machines are by nature massive and ones designed for operation in 1/6 g with poor traction will be quite a challenge.

Covering the buildings with two meters of regolith for radiation protection is much less of a problem if the ground is level to start with. Then not nearly so much bulk material has to be moved from high spots to fill low ones.

Mining:

The huge mining rovers, the proposed sandworms, are unlikely to tolerate much slope at all. Designing them for operation on a slope would significantly increase their already gross size. Slope operation would also make access to solar power and access for maintenance more difficult. They will probably be limited to only about 2 degree slopes.

Rovers:

Here is the biggest problem. Apollo rovers were limited to a 30 degree slope and that was pushing the limit. The 1/6 g and the loose powdery surface really limit the traction of any possible wheel. There is no obvious way to get around this problem. If this 30 degree slope limit is not surmounted, then steep slopes, both up and down, will be major barriers to transportation.

For example, we want to site the solar power stations and Earth communication arrays on high ground away from other operations to avoid the shadows of our own equipment. This high ground must have a path back to the settlement and mining areas that can be transversed by wheeled vehicles. Without a path with a slope below 30 degrees all the way, the high ground may as well be half a world away.

Local land slopes are therefore critical selection criteria for any good settlement site.

A New Map is Needed

We need a refinement of standard contour maps for all sites being considered for lunar operations, manned and unmanned. We need a very detailed map of not just the altitude but specifically the slope of the land down to 2 degree steps. The current data, largely from Clementine, is not adequate for this task. We will have to wait for the Lunar Reconnaissance Orbiter (LRO) data to be collected and analyzed. This data set should be available in late 2009 to mid 2010.