Why don't we see huge
clouds of dust in the low gravity?
Because dust clouds depend on air. If an atmosphere is dense
enough, it can hold small particles suspended against gravity for a
short time. Particles suspended in the air like that are called
aerosols. Smoke and dust are both solid aerosols. Smoke can be
carried for a long distance in earth's atmosphere. Dust particles are
heavier than smoke particles and so they settle out faster. Both show
us the movement of the air that's carrying them; we see billows and
curls in the clouds of smoke and dust.
None of that would happen on the moon because there's nothing to
keep the dust particles suspended. Galileo showed us that gravity
accelerates all objects the same regardless of any difference in
mass. On earth the air prevents some objects from falling as fast as
others, especially very, very small objects like dust particles. But
on the moon only gravity affects them, so dust will fall immediately
to the lunar surface.
The behavior of the dust in the video and film taken on the lunar
surface is one of the most compelling reasons we have for believing it
was shot in a vacuum. The dust is clearly dry, but it falls
immediately to the surface and does not form clouds.
But natural earth
particulates like sand don't raise much dust. If you filled a
soundstage with dry sand you could duplicate the observed behavior of
This scenario doesn't produce all the effects of lunar soil that
can be simultaneously observed in the videos: dryness, dustlessness,
and imprintability. Sand will not hold a footprint unless it is wet,
and if it is wet it will land in clumps after being disturbed. If it
is dry, but the particles are small enough to hold a print, it will
form dust clouds in air when disturbed.
A Grumman engineer who
worked on the lunar module says that it was absolutely necessary to be
able to see the surface to land the LM. With swirls of dust blown by
the descent engine there would be no way to see the ground during the
latter phase of the critical landing procedure. [David
Percy fails to name his informant, so we have know way of knowing
whether this witness really is who he claims to be and has the
authoritative knowledge relevant to his question. We find that many
of Percy's sources are anonymous, relieving his critics of the
possibility of verifying their claims.
However, no less an authority than Tom Kelly, Grumman's chief
design and manufacturing engineer on the lunar module, has a very
"He [Apollo 12 commander Pete Conrad] performed the final
one hundred feet of descent primarily on instruments, since his view
of the surface was largely obscured by dust kicked up by the descent
engine's exhaust." [Kelly01, p. 223]
The note referenced here reads, "This confirmed the design
requirement that we and NASA had included from the outset that the
lunar module must be capable of landing from one hundred feet under
instrument flight rules in anticipation of the lunar-dust visibility
requirements." [Ibid., p. 272]
This is amply confirmed by Pete Conrad's statements in the audio and
transcripts: "It's a good thing we had a simulator, because
that was an IFR landing." [ALSJ, Apollo
12, GET 110:33:56] IFR stands for "instrument flight rules" and
refers to a landing made entirely using instruments -- no visual
Dave Scott also reports that his Apollo 15 lunar module made an
IFR landing. This was a routine maneuver practiced in simulation. [Chaikin94] In fact, the astronauts
seemed to believe an instrument landing was more probable than a
Again we find that David Percy consistently places himself at the
mercy of "experts" who tend not to know as much as expected.
NASA's estimates of dust turned out to be quite conservative.
None of the lunar missions encountered as much dust as was expected.
And so it seems incongruous that an agency so obsessed with dealing
with dust would require precise surface visibility in order to effect
the landing. It makes more sense to suppose that NASA would hedge its
bet and require the lunar module to be able to deal with a dusty
landing. And so they did. David Percy is simply wrong.
The landing radar (Fig. 1) is used to to determine altitude and rate
of descent. Normal aircraft altimeters use differences in air
pressure to measure altitude, but obviously that won't work in a
vacuum. So the only way to measure the distance and descent rate is
by radar, which was accurate to about 15 feet (5 meters). But the
descent rate was already fixed before switching to instruments, so it
was just a matter of riding it down until the contact light
Fig. 1 - The lunar module's landing radar antenna,
normally mounted on the underside of the LM descent stage.
The angled panels send out beams for differential range
altitude and attitude sensing.
The "eight ball" instrument, familiar to pilots as a gyroscopic
horizon reference, provides the pilot with tilt information. Since it
is necessary to set the lunar module down as vertically as possible
the pilot's attention is usually fixed on this instrument during the
final descent. The lunar module pilot is responsible for reporting
the radar data.
The pilot must still select his landing site visually. There is
no instrument which informs the pilot of obstacles like craters and
boulders or slopes which may exceed the lander's tolerance. But this
selection takes place at a much higher altitude where dust is not a
factor. One cannot land the lunar module from orbit on instruments
alone. This may be what Percy's engineer refers to. But it is
absolutely false to argue that displaced dust would have prohibitively
impaired the pilot's ability to complete the landing.
Grumman's reference manual for the lunar module describes the
"At approximately 200 feet above the lunar surface the LM
pitches to orient its X-axis perpendicular to the surface; all
velocity vectors are near zero. Final visual selection of the landing
site is followed by touchdown under automatic or manual
control. [ANR-LM, p. GN-13] (emphasis
Why would the descent
engine produce clouds of dust at all? Above you argue that dust
clouds require a sufficiently dense atmosphere.
The descent engine's exhaust is a gas. Although it will
eventually disperse into the vacuum, it is momentarily dense enough to
hold some dust particles suspended. The gas impacts the lunar surface
and displace some dust with it. Even when the gas disperses to the
point where it can no longer suspend the dust, the particles will
follow ballistic trajectories dictated by the initial impulse from the
exhaust gas. You can see in the film how the dust immediately settles
as soon as the engine is turned off.