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.

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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