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Visibility During the Lunar Landings

Copyright © 2007 by the Editors of Working on the Moon.
Last revised 24 August 2007.

Summary

During the final moments of each of the Apollo landings, the descent engine exhaust swept fine particles away from the area immediately beneath the spacecraft, much of it (most of it?) launched into flat, radially-outward trajectories.  During a 19 September 2001 interview with historians Stephen Ambrose and Douglas Brinkley at NASA Johnson, Neil Armstrong commented: "I was absolutely dumbfounded when I shut the rocket engine off and the particles that were going out radially from the bottom of the engine fell all the way out over the horizon.  [Rephrasing the thought] When I shut the engine off, they just raced out over the horizon and instantaneously disappeared, you know, just like it had been shut off for a week. That was remarkable. I'd never seen that. I'd never seen anything like that. And logic says, yes, that's the way it ought to be there [because of the lack of an atmosphere], but I hadn't thought about it and I was surprised."

Armstrong and several of the other CDRs commented that they could see rocks and craters thru this veil of racing dust and were able to judge spacecraft motions - forward and back, left and right.  Pete Conrad on Apollo 12 and Dave Scott on Apollo 15 had more trouble with dust and both described their landings as being IFR (Instrument Flight Rules).  Scott said in the Apollo 15 Technical Debrief that below "about 50 to 60 feet, the total view outside was obscured by dust."  In the Apollo 15 film shot out Jim Irwin's window, one small rock can be seen throughout that period; but, generally speaking, there were very few rocks in the immediate area.  In the Apollo 12 film shot out Al Bean's window, rocks and small craters can be seen throughout the landing and Conrad's comment may refer to the craters large enough to give the landed LM significant tilt.

Lessons

(1) The most signficant problems due to dust that were encountered during the landings were those of determining lateral (left/right) and horizontal (fore/aft) motions of the spacecraft during the final descent and of determining spacecraft attitude.  Rocks that were visible through the dust sheet provided sufficient information for a safe landing.  Raised markers at a prepared landing site would provide a low-tech, visual backup to onboard displays.

(2) Although the total amount of dust swept from beneath the LM during the landing was not great (see the article on scouring), the dust particles flew away from the LM at high speed and low departure angles, suggesting the possibility that lens, windows, solar cells and other sensitive surfaces on other spacecraft or lunar base structures could be subject to energetic impacts by fine regolith particles.

Dust Observations

We have two sources of information about  dust raised during  a landing: (1) verbal reports from the crew and (2) the 16-mm DAC film shot out the LMP's window.  In the following table, audio recordings are used to estimate the elapsed time between the start of the first verbal report of dust and the subsequent report of illumination of the blue Contact Light ("Contact"); and the DAC film has been used to estimate elapsed time between the first appearance of dust and the time the spacecraft hits the ground and becomes stationary.  Times when a LM was at 100 ft, 50 ft, or 20 ft are estimates from the LMP's verbal reports of what he was seeing in the output of the LM Guidance Computer.


Mission

First Verbal Report:
Time to "Contact";
Altitude (ft);
ALSJ Link
Crew Comments on Visibility;
ALSJ Links
First Dust in Film:
Time to Landing;
Altitude
Visibility in DAC Film at 100 ft
Visibility in DAC Film at 50 ft Visibility in DAC Film at 20 ft
Apollo 11
> 17 sec
> 40 ft
102:45:17

(there is considerable dust visible by the time Buzz mentions it)
Began as transparent sheet  that thickened;  some difficulty sensing fore/aft/left/right rates because of the dust;  rocks helped.  The dust sheet was gone immediately after shutdown because the individual particles raced to the horizon on ballistic trajectories.
102:45:44
102:46:38
55 sec
100 ft
55 sec; first dust, many small craters and some rocks
28 sec; intermittent general obscuration but small craters and rocks visible
about 12 sec; rocks always visible thru more general obscuration
Apollo 12
no dust call

110:31:31
The sheet looked thicker to Pete than what he expected from the A11 film; no trouble with fore/aft/left/right rates but couldn't see craters (large enough to be a concern?)  Had difficulty determining spacecraft attitude - primarily pitch - using the horizon.  Pete called it an IFR (Instrument Flight Rules) landing.  Crew descriptions give the impression of a much dustier landing than the impression given by the A12 DAC film.
110:32:06 to 110:33:56
110:38:19

45 sec
80 ft
50 sec; no dust yet; may small crater; some rocks
34 sec; some dust streaks; little obscuration
12 sec; white-out
Apollo 14
43 sec
110 ft
108:14:28
"We had less problem with dust than they've had before ... The dust was obvious, but you could also see the rocks through the dust." Had modified training and final approach procedures to accomodate the Apollo 12 experience. Used cross-pointers to confirm he'd nulled left/right velocities. Mitchell comments that the dust in the film looks worse than it did in reality. "The dust is no great problem at all."
108:15:18
43 sec
110 ft
35 sec; some small craters and only a few rocks; some light obscuration from the dust.
15 sec; light obscuration; small craters and rocks easily seen
6 sec; heavier obscuration but shadowing in small craters still visible; one rock faintly visible
Apollo 15
52 sec
120 ft
104:41:39
"At about 50 to 60 feet, the total view outside was obscured by dust.  It was completely IFR (Instrument Flight Rules)."  (Fewer rocks in the immediate area than most sites.)
104:41:39
104:42:48
104:58:40
48 sec
120 ft
51 sec; very few rocks and few sharp-rimmed craters
36 sec; some streaks of heavy obscuration; rocks and craters visible
19 sec; general obscuration; one rock still visible; no distinct craters visible; at lower heights the scene briefly clears twice
Apollo 16
27 sec
80 ft
104:29:08
Charlie commented "the dust started a lot later than I had expected." John had no trouble seeing "rocks all the way to the ground."
104:22:19
104:28:39
104:29:08
33 sec
80 ft
about 30 sec; more rocks than A15; similar numbers of small craters?
24 sec; some general obscuration; rocks and small craters visible
about 12 sec; several rocks always visible, intermittent good crater visibility
Apollo 17
29 sec
80 ft
113:01:15
Jack commented at 60ft, "Very little dust." During extraction of the deep core, Gene speculated that the relative  lack of dust they saw during the landing was related to the compactness of the soil they encountered a the ALSEP site.
113:00:55
113:05:26
121:04:14
>22 sec
80 ft

(The available film is quite dark)
47 sec; dark film copy
28 sec; dark film copy
11 sec; dark film copy


LM Attitude and Velocites at Engine Shutdown


Summary of dust info that can be derived from observations of liftoff, including ALSEP and effects on the TV camera.



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