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James Fincannon of NASA took the two images of the Marius Hills lunar pit taken at different times by Lunar Reconnaissance Orbiter (which I posted here) and did an overlay so that the shadow produced by pit’s rim could be easily compared with the rim itself (see below). He then did some calculations based on the sun’s angle of light shining into the cave and came up with the following calculations:
I estimate it is 60 meters from rim to bottom. The floor is flat below the surface. The rocks on the flat surface below ground are in stark relief (hard shadows) compared to above ground due to the sun coming only at one angle while above ground the albedo/reflections makes for soft shadows at this high sun angle (65 deg elevation). I cannot tell if the black portion of the combo image is a slope or more flat floor. Need a different high sun angle or azimuth to fill that in. Still I like the general pattern of the rim matching the shadow on the floor, although the image I found originally has that edge of the cave rim in shadow for a large extent.
A 60 meter drop is about 200 feet deep. This result is reasonably close to the depth estimated by Japanese scientists, 88 meters or 288 feet, based on images of the same lunar pit taken by their Kaguya probe.
Knowing the approximate depth of the entrance pit raises the much more important question: How will future lunar explorers to get to the bottom of this pit? It is ironic that, after flying almost a quarter of a million miles from Earth, the task of traveling this measly additional 200 to 300 vertical feet is actually a significant and hardly trivial engineering challenge.
This isn’t the movies, so flying a spacecraft down through the opening is probably not practical and is certainly too risky. Nor can the astronauts simply jump in, since even in the Moon’s weak 1/6 gravitational field the fall would probably be fatal at 200 feet.
So, how will those first astronauts do it?
On Earth, traveling up and down a 300 foot pit has become somewhat routine for cave explorers. You rig a sufficently long rope to a solid anchor, put on a harness, attach your rappel device to both the harness and rope, and rappel in. The rappel device, usually a rack, uses friction on the rope to control the rate of descent.
To exit the pit, you attach ascenders both to the rope and to your harness. The ascenders are mechanical devices with a cam that can slide up the rope, but will lock into place when you put any weight on it. Thus you literally walk up the rope, alternating ascenders as you climb.
The Moon, however, presents technical difficulties for these rope-climbing techniques. For one, the astronaut will be wearing a big spacesuit, with thick unwieldy gloves. Using both a rack and ascenders requires some delicate fingertip control. A heavily-clad astronaut might find such tasks difficult. For another, how will the astronaut put on his harness?
The simpliest and most practical solution here would be to modify these devices to fit the situation. The harness attachments could be incorporated directly into the spacesuit, thereby eliminating the need for a harness. The rappel device as well as the ascenders could be modified so that they will be easier to use by a thickly gloved astronaut. (This is exactly what engineers at the Goddard Space Flight Center did in preparing the tools for the Hubble Space Telescope shuttle mission: the powered tools were all carefully modified or redesigned from ordinary Earth-based tools to make it easy for a spacesuited astronaut to use.)
Then there is the question of the lower lunar gravity: Will it require a redesign of the rack and ascenders to make them work properly? For example, in climbing it helps to have some weight pulling down on the rope so that the ascenders will slide up easily, then lock instantly when you stand on them. The Moon’s low gravity might make it more difficult for an Earth-designed ascender to slide up the rope. Similarly, the low gravity might make it difficult for the rappel rack to provide the proper amount of friction during descent.
I am not an engineer, so I am very curious to hear what other vertical experts (cavers, rock-climbers, etc) as well as the engineers out there think of this, and if they have any suggestions of their own. The key is to keep the solution as simple as possible, and as small as possible so as to not add any unnecessary weight.