Fig. 1 - The Apollo 15 lunar module, "Falcon", proudly
displays its flag. The decal is brightly illuminated by
reflected light from the Beta cloth MESA insulation
blankets. (NASA: AS15-88-11866)
The U.S. flag and the
United States nameplate are always clearly visible on the lunar
module, even if the surrounding areas are dark. This could only be
accomplished by using supplementary lighting. [David
Fig. 1 is a passable example of Percy's claim. However it is not
true that the decal is always brightly lit. Percy simply shows only
those that are and asserts that they are all like this. But there are
still questions to be answered. Why do some appear brightly lit while
others are relatively dim? Why is the decal in Fig. 1 bright while
the surrounding panel is very dark?
First let us establish the geometry and lighting angles. The
descent stage of a lunar module is a square with the corners clipped
off, making a sort of irregular octagon. The landing legs are
arranged so that there is one straight in front, one on each side, and
one in the rear. The forward hatch opens above the forward landing
For several reasons lunar modules always land with their backs
generally to the sun. This puts two of the longer octagon sides in
fairly direct sunlight and two of them in relative shadow. They do
not face directly away from the sun, but rather at a forty-five degree
angle from the down-sun direction.
In Fig. 1 we are looking at the right side of the lunar module in
the pilot's reckoning. The forward strut is to our right, with the
ladder and porch rails clearly visible. The light is coming generally
from our left. Look at the bulbous ascent fuel tank housing on the
ascent stage immediately above the strut nearest the photographer.
The pattern of lighting on the panels suggests that the near side of
the lander is rotated to be slightly more sunlit; the sun azimuth does not point directly down
the fore-aft axis of the spacecraft.
The photo of the lunar module in Fig. 2 is taken from an almost
identical angle as Fig. 1. The decal is partially obscured by the S-band antenna of the lunar rover, but
we can see that it appears noticeably darker in this photograph than
in the one above.
Fig. 2 - The Apollo 16 lunar module photographed from
the same angle as in Fig. 1, but with the sun at a slightly
different angle. The decal doesn't seem as bright in this
photo. (NASA: AS16-113-18340)
There is evidence that the sun azimuth angle is different in this
photograph. Our tank housing is lit quite differently. More of the
panels are dark. And we can see a distinct shadow from the right rear
RCS quad falling across the tank; that
shadow is missing in the top photo because it's falling somewhere
We conclude that the visible side of the lunar module is turned
more away from the sun in Fig. 2 than in Fig. 1. This is important
because it determines how much light falls on a key piece of Apollo
hardware -- the MESA.
In Fig. 1 we can see the MESA deployed. With the lander turned
relative to the sun as we suppose, the MESA is subjected to direct
sunlight. The white cover blankets are in full sun. They act like
photographic reflectors to cast indirect light on the decals.
In Fig. 2 the lander is turned a different direction. And
although the MESA is difficult to see, the white blankets visible
below the dishlike LRV S-band antenna
are not as fully lit as those in the above photo. Less indirect
light, and therefore less visibility. The MESA blankets are made from
Beta cloth, the same highly
reflective material that covers the space suits.
And consider the ubiquitous indirect lighting from the lunar
surface. The more a lunar module feature points down-sun, the more
likely it will be in the lunar module's shadow and not benefit as much
from indirect lighting from the lunar surface. In Fig. 1 the decal
directly faces portions of the lunar surface likely to cast indirect
light on it. Not quite as true for Fig. 2.
The decal placement changed on the upgraded lunar modules used for
Apollo 15, 16, and 17. Earlier missions placed the U.S. flag on the
LMP's side and "United States" on the
commander's side. In later missions both decals appeared on the MESA
side. Obviously the MESA can't be the primary light source for decals
on the opposite side of the LM. Indirect light from the lunar surface
accounts for that.
THE OTHER HALF OF THE
We've determined that the apparent brightness of the decal roughly
correlates to the degree of indirect light from the MESA blankets, and
potentially from the lunar surface. This is considerably more
parsimonious than postulating the existence of supplementary
lighting. Since David Percy categorically rejects the lunar surface
as a source of indirect light and tends to disbelieve any explanation
which involves straightforward sources of indirect lighting, he is
likely to reject this explanation as well. But the correlation does
not go away, and it explains the apparent lighting anomaly in terms of
what is visible in the photos without resorting to speculations of
hidden equipment for which there is no proof.
But we've only answered the first of Percy's questions. We've
determined that the source of the light is likely to be the indirect
light from the MESA blankets and possibly from the lunar surface. But
that doesn't explain why the decal is so brightly lit compared to its
To answer this question we have to look at the two broad modes of
"Specular" reflection is the kind we see from shiny objects like
mirrors (Latin "speculum") or polished metal or glass. Like a
billiard ball bouncing from a rail, the light rays bounce off a
surface at the same angle at which they struck it. When we see the
blinding glare of the sun in the chrome or windows of the car in front
of us, that's an example of specular reflection.
"Diffuse" reflection on the other hand is what we see from
textured objects. Light striking such an object is scattered in all
directions. Slightly weathered rubber is a good diffuse reflector, as
Most surfaces are both diffuse and specular reflectors. That is,
the total phenomenon of reflection from any single surface is a
combination of specular and diffuse reflections in different
proportions, depending on the microscopic texture of the surface.
Whether an object appears bright or dark to our eye depends on
whether light reflected from the object arrives at it. And that in
turn depends on whether an object is primarily a diffuse reflector or
primarily a specular reflector. Since specular reflectors reflect
light only in one direction, you have to be in the right place to
intercept the reflected light and thus perceive the object as bright.
But diffuse reflectors reflect light in all directions. As long as
light is falling on it, you can be almost anywhere and still see light
from a diffuse object.
The lower half of the lunar module is almost completely covered
with thermal insulation made of Mylar coated with a very thin coat of
aluminum. It is affixed with the Mylar side facing outward. This
material -- similar to the anti-static bags in which computer
components are packaged -- mimics the reflective capabilities of shiny
metal but without the associated weight. By reflecting away about
half of the sunlight that strikes the spacecraft the insulation keeps
the spacecraft from becoming too warm.
The decals, on the other hand, are made from the same lightweight Beta
fabric as the space suits and most of the other fabric coverings used
in the Apollo program. The difference should now be apparent; the
Mylar is specular while the fiber is diffuse (like the space suits).
But both are good reflectors.
Fig. 3 - The footpad of the Apollo 11 lunar module.
The individual creases are either wholly dark or brightly
lit. This indicates a specular surface. (NASA:
Fig. 3 shows the LM footpad in full sun. Some of the golden
insulation appear black. This is because the crinkled insulation
forms facets, each of which points in a different direction. And each
therefore reflects light only in a direction controlled by the
orientation of the facet and the angle of light incidence. Another
way to think about it is to treat each insulation facet as a mirror.
Some facets are oriented to reflect an image of the pitch black sky
(e.g., those areas near the top of the photo). Some are oriented to
reflect the brightly lit lunar surface -- even from far away (e.g.,
the areas near the "ankle"). And some are oriented to reflect the
full light directly from the sun (e.g., the hot spot near the toe).
This seems complicated, but the bottom line is that when this
insulation looks "dark" it's not necessarily because there's no light
falling on it (from whatever source). Many of the facets in this case
are reflecting that light away from the photographer where the camera
can't capture it. And so while light may be streaming onto the
surface, none of it is getting to the camera. On other hand, the
decal reflects a portion of the incident light in all directions and
some of it will find the camera no matter where the camera is.
I don't buy your shadow
analysis. The Apollo 16 photo looks like it was taken with darker
exposure and this might account for the differences in shadow
Then it might also account for why one decal is lighter than the