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.
(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.
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.
|First Verbal Report:
Time to "Contact";
|Crew Comments on Visibility;
|First Dust in Film:
Time to Landing;
in DAC Film at 100 ft
||Visibility in DAC Film at 50 ft||Visibility in DAC Film at 20 ft|
||> 17 sec
> 40 ft
(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.
|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
||no dust call
|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
|50 sec; no dust yet; may small
crater; some rocks
||34 sec; some dust streaks;
||12 sec; white-out
|"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."
|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
|"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.)
|51 sec; very few rocks and few
||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
|Charlie commented "the dust
started a lot later than I had expected." John had no trouble seeing
"rocks all the way to the ground."
|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
|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.
(The available film is quite dark)
|47 sec; dark film copy
||28 sec; dark film copy
||11 sec; dark film copy