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LUNAR   D I G A B I L I T Y   COMPENDIUM


Welcome  to the Lunar Digability Compendium, a source for information on the many times, with varying degrees of difficulty/success, humans and machines have managed to dig, drill, push, or rake through the Moon's surface soil (regolith).

What follows is the result of extensive compilation by yours truly (Paul Warren) of the records of the Apollo missions and other available constraints (e.g., from the Soviet Luna missions). The Apollo Lunar Surface Journal is a great resource for the transcripts of communications among the astronauts and Capcom during the lunar surface operations. The ALSJ also includes many reduced-resolution snippets from the Apollo video recordings. However, I have worked from high-resolution copies of the videos. Some of the many printed documents that helped in this work are cited at the bottom of this page.

To anticipate the upshot from all of this evidence, the lunar regolith grows more compact and strong just a few centimeters (cm) below the surface. It seems the Moon's lower gravity is to a considerable extent offset by ubiquitous "seismic shaking" caused by the same process of impact bombardment that supplies the tiny rock particles that make up the regolith. This shaking enhances compaction, and the
effectiveness of compaction for strengthening the lunar regolith is enhanced by the typically irregular (even "hackly") grain shapes of the impact-shattered rock particles. David Carrier makes an analogy between the deeper lunar regolith and velco!

In the deeper lunar regolith, the role of pressure becomes significant. The overburden creates pressures that increase in a near-linear fashion with depth. At about 30 meters (only very rarely,
in highlands, is the regolith quite so deep), this pressure reaches 1 bar, the same as sea-level atmospheric pressure on the Earth. The dramatic effect that 1 bar of pressure can have on the cohesiveness of a granular, in-vacuo substance like the lunar regolith is clear from a homely analogy. A vacuum-sealed bag of coffee grounds will show rock-like inflexibility as long as the vacuum is preserved, because the pressure of the atmosphere squeezes the bag such that the grains' rough, craggy surfaces "lock up" into one another. But as soon as the vacuum seal is broken, the same 1 bar of pressure takes effect in all the pore spaces between the grains of the coffee grounds, and suddenly, ssssss voila!, the grounds pour easily, almost like a fluid. Even at the 10 times lower depth of the deepest lunar drill cores, overburden pressure significantly enhances the regolith's cohesivity.

The evidence reviewed here shows that the upper half-meter, at least, of the lunar regolith can be dug or penetrated about as easily as a freshly dumped pile of topsoil or beach sand, self-compacted in air by terrestrial gravity. And that conclusion is supported by results for a wide variety of lunar terrains
. What is more, since the cratering process that forms regolith is similar everywhere on the Moon, this conclusion can be extended to the vast majority of the Moon's surface.

But I hope you will not just take my word for it. I hope you will use this site as a starting point for checking out those Apollo videos, which are fascinating in all sorts of ways. Then judge for yourself about the "digability" of the lunar regolith.

Links to a few examples of the video are included below. The files are in mpg format. However, mpg is a non-specific format that requires diverse "codecs" that your computer may or may not have. In case the videos do not play, or play without sound, I recommend that you download and install the excellent Windows Essentials Media Codec Pack (free, from MediaCodec.org). T
he WEMCP includes a wide array of codecs, as well as a program, Media Player Classic, that has only a vanilla set of video player controls, but enables a Windows PC to play almost any mpg file. Another free player app that copes with nearly all flavors of mpg is the cross-platform VLC.

As a reminder in case it's been a while since your last science class, 1 centimeter (cm) is about 4/10 of an inch (1 inch = 2.54 cm), and 1 meter is about 3.3 feet.

 
 
 
 
 
 
SUMMARY of Apollo operations involving penetration of the lunar regolith.
 

 

raking procedures 

trench digs

SWC* experiment implants 

flagpole implants

core tube drive-downs

penetrometer tests

powered drilling♥

typical depth (cm)^

8

20

20

30

50

50

250

Apollo 11

-

-

3

1

2

-

-

Apollo 12

-

2

3

1

3

-

-

Apollo 14

-

2

1

1

4

3

-

Apollo 15

3

2

1

1

3

6

3

Apollo 16

16

2

1

1

5

11

2

Apollo 17

5

5

-

1

5

-

3

Apollo totals

24

13

9

6

22

20

8

* SWC: Solar Wind Composition.

Powered drilling includes holes drilled for heat flow probes as well as deep sample cores.

^ Typical maximum depth of penetration involved in the listed operation.

   

   

Raking (to cull out cm-sized rocks)


Jack Schmitt (Apollo 17)

 Geologist/astronaut/senator Jack Schmitt collecting a rake sample (shaking out residual fine material) at Apollo 17 Station 1. Photo AS17-134-20425.
 

Petrologists and geochemists (like yours truly) crave large numbers of rocks greater than a few millimeters in size (minimum dimension). For many purposes, rocks >>1 cm in size are neeedlessly large. To efficiently collect small rocks, on the last three missions the astronauts used special rakes. The design, which was inspired by clam-digging rakes, featured 1 cm spacing between the tines.

 

The following very large table is available for download, with greater detail and in its original format, here.


Apollo raking operations

A video (mpg) of John Young obtaining the Apollo 16 Station 10 rake sample can be downloaded here. NB: This is a LARGE file (58 MBytes). You may want to right-click and invoke "save link as" (or "save target as") rather than simply opening the file within your browser.


   

   

Trenching

 
James Irwin digging trench
 
James Irwin used a between-the-legs technique while digging the Apollo 15 Station 8 trench. Note flying moondirt. Photo AS15-92-12425.
 

In the Apollo context, a "trench" was a small hole no more than about 46 cm deep, and typically about 50 cm in maximum horizontal dimension. Lunar regolith, because it consists of diverse-sized ("well graded") and angular, often even hackly grains, is rather cohesive. According to the Lunar Sourcebook (p. 521), a precisely vertical cut would be stable to a depth of about 3 meters. Other trenching experiments included the scooping by the Surveyor III and VII landers, which went to depths of "15 to 17.5" and "20 to 24" cm, respectively.

 

The following table is available for download, with greater detail and in its original format, here.

Trenching table

A video (mpg) of James Irwin digging the Apollo 15 Station 6 trench can be downloaded here. NB: This is a LARGE file (32 MBytes). You may want to right-click and invoke "save link as" (or "save target as") rather than simply opening the file within your browser.

 

 

  

 

 

 

Pole Implants


Gene Cernan (Apollo 17)

 Gene Cernan with the newly deployed flag on Apollo 17. Photo AS17-134-20386.
 

Flagpoles were implanted on each of the six successful Apollo landing missions. The flagpoles were 2.22 cm diameter tubes, with tube thickness of 0.09 cm.

A video (mpg) of James Irwin implanting the Apollo 15 flagpole base can be downloaded here; and for completeness another video showing the upper part of the assembly (i.e., the part with the flag itself) going into the base can be downloaded here. This is by far the best documented of the flagpole implants, but the procedure was accomplished with similar ease on the other five occasions.

Poles were also implanted a total of 9 times in connection with Solar Wind Composition (SWC) experiments. These implants were not well documented. The SWC poles were 4-cm diameter tubes. In most if not all cases, they were simply shoved down, with no hammering. (Why 9 such implants? On Apollo 11, the pole was pushed down an extra two times, just to dispose of it after detachment of the SWC experiment. On Apollo 12, two extra push-downs were motivated by concerns that the experiment was situated too close to the Lunar Module. Apollo 17 had no SWC experiment.)

 

 

     

 

 

Coring (as done with tubes and a hammer; power drilling not tabulated here)

Charlie Duke practice-coring in Florida

The irrepressible Charlie Duke, practicing core hammering for the Apollo 16 mission. (Yes, he really DID walk on the Moon, just not on this day!) Photo S72-33989.

   

Multiple shallow cores were collected on each of the six successful Apollo landing missions. In addition, the three successful Soviet Moon-sampler missions all acquired their samples with coring mechanisms. Luna 16, 20 and 24 reportedly drilled down 35, 25 and 160 cm, respectively.

   

The following very large table is available for download, with greater detail and in its original format, here.


Coring table


A video (mpg) of James Irwin obtaining the Apollo 15 Station 2 core can be downloaded here. NB: This is a LARGE file (35 MBytes). You may want to right-click and invoke "save link as" (or "save target as") rather than simply opening the file within your browser.

A video (mpg) of Charlie Duke hammering down the Apollo 16 Station 10' double-tube core can be downloaded here. NB: This is another LARGE (57 MByte) file. You may want to right-click and invoke "save link as" (or "save target as") rather than simply opening the file within your browser. Already before the camera pan reached him, and before using his hammer, Duke had simply shoved the core-tube down about 30 cm. Characteristically, Duke endeavors to provide amusing-explanatory commentary as he works, quipping about how a pressure suit makes difficult (even painful, for the hands) an otherwise simple physical task such as this hammering process.

 

 

   

   


Penetromer Tests

 

Apollo penetrometer

 

Unglamorous but not unimportant: The Apollo penetrometers could be configured with a variety of penetration ends, from small cones to a flat plate.
  

The Apollo penetrometers were simple devices for measuring the relationship between applied force and depth of penetration. In addition, the two Soviet Lunokhod rovers (deployed by the Luna 17 and 21 missions) each pushed short (10 cm) penetrometers into the regolith at about 500 different locations. The Luna 13 lander included a very short (5 cm) penetrometer. Other de facto penetration experiments included the footpad penetrations observed on a total of 17 (6 Luna, 5 Surveyor and 6 Apollo) missions.

  

The following table is available for download in its original format, here.

penetrometer tests

A video (mpg) of Charlie Duke performing penetrometer tests at Apollo 16 Station 4 can be downloaded here. NB: This is a LARGE file (113 MBytes). You may want to right-click and invoke "save link as" (or "save target as") rather than simply opening the file within your browser. Caveat: Don't be misled by the abundance of boulders you will see at this location. It's not a typical lunar surface locale. The astronauts deliberately sought out this circumstance, by driving their LRV vehicle halfway up the side of a very large mountain (Stone Mountain) and close to the rim of a fresh 25-meter crater. The camera eventually pans to a different direction, and the abundance of boulders becomes more typical. Do watch for Charlie's "Rats!" escapade starting at about 3:00.

   

 
 

Powered Drilling


Charlie Duke practice drilling

 Charlie Duke training with the Apollo power drill. Photo 71-H-1848.
 

The final three Apollo lunar missions were more ambitious in many ways. The astronauts used powered drills to obtain deep core samples (one per mission), and to drill deep holes for insertion of heat flow probes (two per mission, except an unfortunate mishap terminated the Apollo 16 heat flow experiment after only one hole had been drilled). The documented heat flow holes went to depths ranging from about 1.6 meters (Apollo 15) to 2.5-2.6 meters (Apollo 17). The three drilled sample-core holes went 2.2-2.9 m deep.

A video (mpg) of Charlie Duke drilling the Apollo 16 heat flow hole can be downloaded here. In this LARGE (78 Mbyte) video, the three drilling sequences involved in the drilling of the hole have been spliced together. During the clipped-out intervals (of 2-3 minutes), Duke placed additional tubular "stems" onto the drill. Duke's commentary is interesting. He probably felt some anxiety as he started, because on the sole prior mission that employed this type of drill, Apollo 15, the drilling was at first infamously slow and arduous. It seems that the astronauts, poorly instructed, used improper technique, by weighing down on the drill. The drill functioned better by letting its spinning motion do the work and "not leaning on it ... I guarantee you", in Duke's words (the Apollo 15 astronauts eventually figured this out, but only after a major expenditure of effort and precious time; their difficulty may have been caused in part by a flaw in the initial design of the drill stems, according to W. David Carrier, personal communication).


 

 

 

 

Some Recommended Reading/Viewing

 

The COMPLETE Apollo Video Collection (commercially packaged)

    Spacecraft Films

Apollo Astronaut Lunar-Surface Activities

    Exploring the Moon: The Apollo Expeditions, by David M. Harland (1999)

    On the Moon: The Apollo Journals, by Grant Heiken and Eric Jones (2007)

    and of course, the Apollo Lunar Surface Journal (web site edited by Eric Jones and Ken Glover)

Apollo Astronaut Training Activities (and more)

    To a Rocky Moon: A Geologist's History of Lunar Exploration, by Don E. Wilhelms (1993)


 

More Technical Reading

 

Apollo Astronaut Tools

    Catalog of Apollo Lunar Surface Geological Sampling Tools and Containers, by Judith Allton (1989)

Lunar Soil Geotechnical Properties

    Lunar Sourcebook, Chapter 9 (W. David Carrier, III, et al., 1991)

    Apollo 16 Preliminary Science Report, Chapter 8 (James K. Mitchell et al., 1972)

Lunar Surface Processes

Lunar Sourcebook, Chapters 2 and 4 (1991)

Soviet Automated Lunar-Surface Activities

    Handbook of Soviet Lunar and Planetary Exploration, by David M. Harland (1979)

  

 

Web page designed and implemented by Paul Warren. Please let me know if you have suggestions for improvement.