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January 3, 2006: The
moon is utterly familiar. We see it all the time, in the blue
sky during the day, among the stars and planets at night.
Every child knows the outlines of the moon's lava seas: they
trace the Man in the Moon or, sometimes, a Rabbit.
This
familiarity goes beyond appearances. The moon is actually
made of Earth. According to modern theories, the moon was
born some 4.5 billion years ago when an oversized asteroid
struck our planet. Material from Earth itself spun out into
space and coalesced into our giant satellite.
Yet when
Apollo astronauts stepped out onto this familiar piece of
home, they discovered that it only seems familiar. From the
electrically-charged dust at their feet to the inky-black
skies above, the moon they explored was utterly alien.
Thirty
years ago their strange experiences were as well-known to
the public as the Man in the Moon. Not anymore. Many of the
best tales of Apollo have faded with the passage of time.
Even NASA personnel have forgotten some of them.
Now,
with NASA going back to the moon in search of new tales and
treasures, we revisit some of the old ones, with a series
of Science@NASA stories called "Apollo Chronicles."
This one, the first, explores the simple matter of shadows.
Dark
Shadows
On
the next sunny day, step outdoors and look inside your shadow.
It's not very dark, is it? Grass, sidewalk, toes--whatever's
in there, you can see quite well.
Your
shadow's inner light comes from the sky. Molecules in Earth's
atmosphere scatter sunlight (blue more than red) in all directions,
and some of that light lands in your shadow. Look at your
shadowed footprints on fresh sunlit snow: they are blue!
Without
the blue sky, your shadow would be eerily dark, like a piece
of night following you around. Weird. Yet that's exactly how
it is on the Moon.
To
visualize the experience of Apollo astronauts, imagine the
sky turning completely and utterly black while the sun continues
to glare. Your silhouette darkens, telling you "you're
not on Earth anymore."
Shadows
were one of the first things Apollo 11 astronaut Neil Armstrong
mentioned when he stepped onto the surface of the moon. "It's
quite dark here in the shadow [of the lunar module] and a
little hard for me to see that I have good footing,"
he radioed to Earth.
The
Eagle had touched down on the Sea of Tranquility with its
external equipment locker, a stowage compartment called "MESA,"
in the shadow of the spacecraft. Although the sun was blazing
down around them, Armstrong and Buzz Aldrin had to work in
the dark to deploy their TV camera and various geology tools.
Above:
Blinding sunshine, dark shadows and the lunar lander Antares.
From the book FULL MOON by Michael Light, Alfred A. Knopf
©1999.
"It
is very easy to see in the shadows after you adapt for a while,"
noted Armstrong. But, added Aldrin, "continually moving
back and forth from sunlight to shadow should be avoided because
it's going to cost you some time in perception ability."
Truly,
moon shadows aren't absolutely black. Sunlight reflected from
the moon's gently rounded terrain provides some feeble illumination,
as does the Earth itself, which is a secondary source of light
in lunar skies. Given plenty of time to adapt, an astronaut
could see almost anywhere.
Almost.
Consider the experience of Apollo 14 astronauts Al Shepard
and Ed Mitchell:
They
had just landed at Fra Mauro and were busily unloading the
lunar module. Out came the ALSEP, a group of experiments bolted
to a pallet. Items on the pallet were held down by "Boyd
bolts," each bolt recessed in a sleeve used to guide
the Universal Handling Tool, a sort of astronaut's wrench.
Shepard would insert the tool and give it a twist to release
the bolt--simple, except that the sleeves quickly filled with
moondust. The tool wouldn't go all the way in.
The
sleeve made its own little shadow, so "Al was looking
at it, trying to see inside. And he couldn't get the tool
in and couldn't get it released--and he couldn't see it,"
recalls Mitchell.
"Remember,"
adds Mitchell, "on the lunar surface there's no air to
refract light--so unless you've got direct sunlight, there's
no way in hell you can see anything. It was just pitch black.
That's an amazing phenomenon on an airless planet."
(Eventually
they solved the problem by turning the entire pallet upside
down and shaking loose the moondust. Some of the Boyd bolts,
loosened better than they thought, rained down as well.)
Tiny
little shadows in unexpected places would vex astronauts throughout
the Apollo program--a bolt here, a recessed oxygen gauge there.
These were minor workaday nuisances, mostly, but astronauts
were jealous of the minutes lost from their explorations.
Right:
Apollo 14's ALSEP, deployed. The shadow belongs to Al Shepard.
From the book FULL MOON by Michael Light, Alfred A. Knopf
©1999.
Shadows
could also be mischievous:
Apollo
12 astronauts Pete Conrad and Al Bean landed in the Ocean
of Storms only about 600 yards
from Surveyor 3, a robotic spacecraft sent by NASA to the
moon three years earlier. A key goal of the Apollo 12 mission
was to visit Surveyor 3, to retrieve its TV camera, and to
see how well the craft had endured the harsh lunar environment.
Surveyor 3 sat in a shallow crater where Conrad and Bean could
easily get at it--or so mission planners thought.
The
astronauts could see Surveyor 3 from their lunar module Intrepid.
"I remember the first time I looked at it," recalls
Bean. "I thought it was on a slope of 40 degrees. How
are we going to get down there? I remember us talking about
it in the cabin, about having to use ropes."
But
"it turned out [the ground] was real flat," rejoined
Conrad.
What
happened? When Conrad and Bean landed, the sun was low in
the sky. The top of Surveyor 3 was sunlit, while the bottom
was in deep darkness. "I was fooled," says Bean,
"because, on Earth, if something is sunny on one side
and very dark on the other, it has to be on a tremendous slope."
In the end, they walked down a gentle 10 degree incline to
Surveyor 3--no ropes required.
A
final twist: When astronauts looked at the shadows of their
own heads, they saw a strange glow. Buzz Aldrin was the first
to report "…[there's] a halo around the shadow of my
helmet." Armstrong had one, too.
Right:
A silvery glow surrounds the shadow of an Apollo astronaut's
helmut. From the book FULL MOON by Michael Light, Alfred A.
Knopf ©1999.
This
is the "opposition effect." Atmospheric optics expert
Les Cowley explains: "Grains of moondust stick together
to make fluffy tower-like structures, called 'fairy castles,'
which cast deep shadows." Some researchers believe that
the lunar surface is studded with these microscopic towers.
"Directly opposite the sun," he continues,"
each dust tower hides its own shadow and so that area looks
brighter by contrast with the surroundings."
Sounds
simple? It's not. Other factors add to the glare. The lunar
surface is sprinkled with glassy spherules (think of them
as lunar dew drops) and crystalline minerals, which can reflect
sunlight backwards. And then there's "coherent backscatter"--specks
of moondust smaller than the wavelength of light diffract
sunlight, scattering rays back toward the sun. "No one
knows which factor is most important," says Cowley.
We
can experience the opposition effect here on Earth, for example,
looking away from the sun into a field of tall dewy grass.
The halo is there, but our bright blue sky tends to diminish
the contrast. For full effect, you've got to go to the Moon.
Luminous
halos; mind-bending shadows; fairy-castles made of moondust.
Apollo astronauts discovered a strange world indeed.
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Author: Dr. Tony
Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
More
Information |
The
author would like to thank
- photographer
Michael
Light for allowing his prints of Apollo
photos to be reproduced in this story. Displayed full
size in his book "Full
Moon," Light's prints are breath-taking.
- Eric
Jones who penned the
Apollo
Lunar Surface Journal. The Journal is a must-read
for historians of the Apollo program, and it is the
source of the astronaut recollections reported here.
- Les
Cowley for sharing his insights about
lunar shadows, and for several unattributed contributions
to the narrative. Les's web site Atmospheric
Optics is a wonderful source of information about
all things light and shadowy.
More
crazy shadow tricks
After
the incident with Surveyor 3, Bean and Conrad were tricked
again: "A real interesting thing has happened to
the solar wind collector," Bean radioed to Earth
from inside the Intrepid. "When I left it yesterday
it was just a flat sheet of foil, but as I look out
there now, it has folded back around the pole that's
holding it. Looks almost like a sail in the wind. It's
sort of bulging in front and bent back on the sides.
It's real crazy."
Right:
The Apollo 12 solar wind collector. Bright reflections
from the foil combined with dark shadows around it tricked
astronauts into thinking that the foil was in motion.
From the book FULL MOON by Michael Light, Alfred A.
Knopf ©1999.
"You've got a fairly strong solar wind, I suspect,"
joked Ed Gibson by radio from mission control in Houston.
Gibson knew, as did Bean, that the solar wind is too
diaphanous to flutter a sail.
Indeed,
when Bean went out to the collector to take pictures,
he discovered that the foil looked absolutely normal.
"I guess it was sort of an optical illusion from
inside the spacecraft," he reported. "The
thing that fools you," he explained later, "is
the relative lightness and darkness of shadows on the
object. [We] looked at the solar wind, and we think
'Man, that thing is really bent around the pole.' But
we go out there and see that it's not."
The Opposition Effect
In the main text, the author says that you can experience
the opposition effect on Earth "by looking into
a field of tall dewy grass." This is an example
of the opposition effect that combines the backscattering
of light by water droplets and the shadow-hiding
of the grass. The lunar opposition effect combines similar
phenomena: backscattering by "lunar
dew drops" and shadow-hiding by moondust.
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