ASI W9800013r1.0

Moon Miners' Manifesto

#102 February 1997

Section the Artemis Data Book

Technical Comments on the MMM # 101 Lava tube Articles

Bryce Walden, Oregon Moonbase

Technical Comments on the MMM # 101 Lava tube Articles

Congratulations to Moon Miners' Manifesto on its 100th issue! We have enjoyed every issue, chock full of interesting and provocative ideas on the development of the space frontier.

The Oregon Moonbase team especially appreciates the extensive coverage given to the topic of lava tube caves in Issue 100. Peter Kokh did a lot of work developing the arguments and provided some very nice illustrations of various concepts.

In the interest of completeness, I would like to add a few technical comments as they occur in my reading of the articles.

Lava tube Stability vs. Human Activity

In "Twelve Questions About Lunar Lava tubes" Kokh correctly states that caverns that have survived for over 3 billion years are probably very stable. Yet I feel obliged to add the caveat that human activities could at times threaten that safety. A lava tube that has survived may still have suffered trauma that makes certain parts of it weaker, such as a meteorite strike over part of the roof. Sections that survived the relatively minor moonquakes over the millenia may fail if we blast during construction. Also, lava that is strengthened by incorporating super-strong anhydrous glass may weaken over time if exposed to water vapor from atmosphere or other gas-releasing activities. Sealing and pressurizing a lavatube will also introduce new stresses, as he himself mentions in a subsequent article. To mitigate these effects, there are engineering precautions that can be taken on a case-by-case basis.

Lava tube Temperatures

The "steady temperature" of -4 xF is based on Apollo temperature measurements that reached equilibrium within several centimeters from the surface and stayed fairly constant from there to as far down as the astronauts could measure, roughly 2-3 meters. Deep mines on Earth get quite hot from heat bleeding away from the mantle; this could happen on the Moon, too, but probably to a much reduced extent due to the relative coldness of the small lunar core. As a rule lava tubes don't have much vertical development but run parallel to the surface. There may be older lava tubes in deeper layers of lava, as his article points out. Once again, the real problem is likely to be human activity. Lava tubes are good insulators. On Earth, cold air can fall into a lava tube in winter and remain below freezing through summer heat. Our case will be just the opposite. Human activity generates a great deal of heat, and the lava tube is a relatively closed environment. For awhile this could be an advantage, and raise lava tube temperatures to comfortable levels, but we are likely sooner rather than later to have to engineer some heat-sink solutions. Changing temperature can also be a source of stress to the cave vault.

Gross Available Lunar Lava tube Volumes

In terms of ready volume available now, we did a poster session at the 22nd Lunar and Planetary Sciences Conference that partially addressed this question. Cassandra Coombs, for her doctoral dissertation under Dr. B. Ray Hawke, identified a number of probable lava tube sites from high-resolution Apollo photographs and Lunar Orbiter pictures. Only the largest possible candidates were resolved by these sources. Cheryl Lynn York and I selected the largest 20 of these sites. Making a working assumption of circular caves of width and length identified by Coombs, then half-filled with congealed lava or breakdown, we computed over 3 billion cubic meters of volume, nearly 14 million square meters of "floor" area, or about 0.0531 of Peter's "O'Neill Units" of 100 square miles. The average of these twenty large lava tubes was 470m diameter, length 1,370m, roof thickness 66m, floor area 687,685 m2, and volume 157,908,640m m3. Incidentally, these "Top 20" lava tube caves were located in only four rille formations, with rille "collapse trenches" separating the various caves.

Lava tube SVolumes vs. O'Neill Habitats

I checked Kokh's 100 square mile "O'Neill Unit" with O'Neill's figures in The High Frontier. He claims an Island Three habitat, 20 miles long and 4 miles in diameter, would have 500 square miles of land area. Each of the three "valleys" in the interior would be 20 miles long by 2 miles wide, or 40 square miles. Three of these totals 120 square miles. Total cylinder interior surface area (including windows) is 251 square miles, while endcaps area equals a sphere of radius 2 miles, or 50 square miles. The remaining 199 square miles must be made up by numerous small "agricultural modules" outside of the main habitat, in O'Neill's total design. But for convenience in figuring, 100 square miles is very roughly correct for the popular conception of the "valley" areas in an Island Three habitat.

Lava tube Remote Mapping

On "Remote Mapping of Lunar Lava tubes," Tom Billings' paper "Radar Remote Sensing of Lunar Lava tubes from Earth" was published in the Journal of the British Interplanetary Society, Vol. 44 pp. 255-256, 1991. A more inclusive treatment of "Lava tube Remote Sensing" was given to a seminar sponsored by the Lunar and Planetary Institute in 1992. In regard to side-looking infrared, the detection of a lava tube temperature signature would, we think, be easier during lunar night, when the exposed surface temperature reaches -240 xF. The comparatively "warm" -4 xF lava tube interior would then be virtually the only "warm spots" on the volcanically inactive Moon. During lunar day, it would probably be harder to differentiate cave interior temperatures from normally shadowed areas on the surface. Such an investigation would have the serendipitous (or even primary) effect of finding any volcanic "hot spots" that may be expressed at the surface (there are indications of a few areas of recent lunar volcanism). Such areas would be mineralo-gically (= resources) interesting.

Kokh's articles about lunar lava tube habitats and environmental manipulation were right on the money. Beside our own work on these topics, including a study performed for Lockheed, another researcher who has given some thought to lunar lava tube habitats is Andrew Daga,

In all some very inclusive articles, "in depth" coverage of lava tubes, as it were, most welcome and well done. Thank you, Peter!

 Bryce Walden,
 Oregon Moonbase
 P.O. Box 86
 Oregon City, OR 97045-0007
"The Moon - It's just like Motel 6
- a bucket of ice and no atmosphere!"
Louise Rachel Quigley

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