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The New York Times' The Moon Is a Hazardous Place to Live says:

Throw in the intense contrast between light and dark — unlike anything we see here on Earth — and those changes in the terrain can play tricks on your eyes.

When Pete Conrad and Alan Bean landed on the moon during the Apollo 12 mission, one of their tasks was to enter the 650-foot-wide Surveyor Crater. But as they skirted its rim, searching for the best path down, they informed Houston that the crater was far too steep.

Topographic maps, however, revealed an easy, 21-degree slope. The sharp shadows had fooled the astronauts.

That means the act of simply walking around on the moon might be perilous.

Did Conrad and Bean "inform" NASA that the crater was too steep to enter, or did they simply express concern? I'm guessing that they would have been intimately familiar with whatever topographical data was available at the time and would have studied every last bit of information.

Question(s):

  1. Did Conrad and Bean balk at entering the crater or just express concern?
  2. What data was used to generate the ">Topographic maps"? Was it stereographic images from satellites and/or from Apollo 11, or inference from analysis of the motion of shadows as the direction of the Sun changed, or something else?
  3. Is it likely that the astronauts would have had access to this data and been confident of the slope before the mission?
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  • $\begingroup$ Wow, what an incredibly uninformed article (at least that sentiment). I am shocked a reputable newspaper would publish something like this. Obviously the author has never considered the slope of anything yet they write about a 21-degree angle with great confidence... $\endgroup$ – user2705196 Aug 24 '20 at 13:04
  • $\begingroup$ @user2705196 it looks like I left out the actual link to the article, I've fixed that, thanks for the heads-up! $\endgroup$ – uhoh Aug 24 '20 at 13:11
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Overall it seems that the NYT article has garbled the details: although the initial low-angle sunlight made Surveyor Crater look too steep, the estimated angle of about 11 degrees proved manageable by sidling down the rim. From the Apollo 12 Lunar Surface Journal transcripts and commentary by the astronauts (https://www.hq.nasa.gov/alsj/a12/a12.html):

Commentary on Conrad's first view of Surveyor 3:

[Bean - "But there's one thing we don't say here but we do later. I can remember the first time I looked at it and I thought it was on a slope of about 40 degrees (instead of the actual slope of about 10 degrees). And I remember us talking about it in the cabin, about having to use ropes. How are we going to get down there? How come they screwed up so badly (on the slope estimate)? And I think I was fooled because, on Earth, if something is sunny on one side and very dark on the other, it has to be a tremendous slope. We weren't getting (scattered) light in there like you do on Earth. So when light finally did strike, it was real..."]

[Conrad - "It turned out it was real flat."]

[Bean - "Yeah. But I can remember us talking about ropes and how were we going to get there and what can we do. And there it was, sitting there at 11 degrees like it should be."]

On the second EVA, 20 hours later:

131:51:50 Conrad: Okay. (Long Pause) Hey, look at that Surveyor, Al. That's not anywheres near as bad a slope (as it had seemed during EVA-1).

131:52:09 LM Crew: (Garbled) shade.

131:52:11 Bean: Hey, Houston, that Surveyor looks a lot better today.

131:52:13 Conrad: Yeah, now that the Sun's up on it.

When discussing the neighbouring Bench Crater angles of more than 20 degrees are described as perilously steep, with a further comment that the rope was never used:

[A contour map in the mission report indicates that Bench Crater is about the same depth as Surveyor Crater and, with half the diameter, would have an average slope of 25-30 degrees.]

[Conrad - (Chuckling) "I wasn't going to go down. It was really steep."]

[Bean - "Yeah, that wasn't a good crater to be inside. I mean, that would be real bad. Plus, your chances of going down in there and not falling over, head first, is zilch."]

[Conrad - "I don't think anybody went down anything that steep, ever."]

[Bean - "Uh-uh. They'd be crazy to do it."]

...

[Conrad - "Did anybody ever use the rope?"]

[Jones - "Nope. Nobody ever did."]

In the actual descent into Surveyor Crater Conrad confirms that the rope plan had proved unnecessary:

133:53:18 Gibson: Pete and Al, could you give us a comment on how far you're sinking in?

[By now, Houston has realized that Pete has already started into the crater. Plans for this part of the traverse, while not spelled out in the checklists, were cautious. Al is carrying a tether (30-foot safety line) in his saddlebag. Because no one has yet worked on a slope as steep as the inner wall of Surveyor Crater, the plan is for one of them to stay on the rim, paying out the tether while the other makes his way part way down the slope. If the surface proves to be too soft for a safe descent, they can use the tether to help him back up to the rim. An Ernie Reyes cartoon on page 15 of Al's cuff checklist playfully shows the tether being used. Clearly, Pete has decided that the surface on the upper wall looks good enough that the tether won't be necessary.]

133:53:24 Conrad: Not sinking in very far at all. This is fairly firm stuff. And I'm down in the crater about the same distance down that Surveyor is. I'm just going around it radially (means "circumferentially"). Wouldn't you say so, Al?

133:53:41 Bean: Yeah, I would say that...I think Houston is just as concerned about us getting down in this crater. We been thinking about it, too, Houston. (Garbled under Pete)

133:53:47 Conrad: Okay. Yeah, don't worry about it, Houston, because, really, it's no strain; I'm 200 feet away from it; I'm at the same level; the ground is firm; and I can go right back up the way I came down with no strain at all.

So:

  1. they didn't balk, though the initial impressions were that it was far steeper than the estimated angle
  2. (guessing) as well as shadows on Lunar Orbiter photos (e.g. https://www.nasa.gov/content/lunar-reconnaissance-orbiter-looks-at-apollo-12-surveyor-3-landing-sites) the photographs from Surveyor 3 were surely useful (e.g. https://www.planetary.org/space-images/surveyor-3-panorama)
  3. it seems they were confident about the expected angle, but less confident about how problematic it would prove.
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It helps to do the math. Tangent 21 degrees = .384 = 38.4 % grade. This is around 5 times steeper than most anything you'll see driving around the Rocky mountains of Western USA. Very difficult to climb out of, even when not wearing a bulky space suit.

Important to remember, even though moon gravity is 1/6th earth (1.62m/s$^2$), there is no "terminal velocity" due to lack of air drag. A long lunar fall would be lethal.

A 21 degree slope is very steep, and the astronauts exercised proper caution. One held a rope tether while the other began to descend into the crater. The plan was to walk down the crater wall in a spiral fashion, and early on they found the footing firm enough to proceed, making it to Surveyor 3, which had landed there in April 1967.

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  • $\begingroup$ Please see edits. $\endgroup$ – Robert DiGiovanni Aug 24 '20 at 11:03
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    $\begingroup$ Do you have any sources for the information in your 3rd paragraph? Confirming the plan, the use of the rope, etc? Unsourced statements like this are worth very little. $\endgroup$ – Organic Marble Aug 24 '20 at 14:34
  • $\begingroup$ In the Lunar Surface Journal transcript (hq.nasa.gov/alsj/a12/a12.surveyor.html) I don't see any belaying on the walk. Several times they do support each other when bending to collect a rock, e.g. "A few minutes ago, Pete wanted to pick up a rock, so I held onto that strap of the Surveyor bag and he leaned right over and picked it up and I helped him get back up. It's not that you're heavy or anything, it's the fact that you have such poor balance." $\endgroup$ – Tom Goodfellow Aug 24 '20 at 20:49
  • $\begingroup$ The tether is mentioned further on in that journal. They did not need it. $\endgroup$ – Robert DiGiovanni Aug 24 '20 at 20:52

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