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June
20, 2008: Imagine landing on the Moon, climbing down
the ladder of your spacecraft, and looking around the harsh
lunar landscape—to see another, older spacecraft standing
only 200 yards away.
That's
exactly what happened in November 1969, when astronauts Pete
Conrad and Alan Bean stepped out of the Apollo 12 lunar module.
There, within walking distance on the edge of a small crater,
stood Surveyor 3, an unmanned U.S. spacecraft that had landed
in April 1967.
Above:
Apollo 12 astronaut Pete Conrad inspects Surveyor 3. Conrad's
own spaceship, the Intrepid, can be seen 200 yards away in
the background. [More]
[Stereo
View]
Apollo
12's landing site had been chosen deliberately near Surveyor
3. The little lander had spent two and a half years exposed
to the worst the Moon had to offer: harsh vacuum, intense
cosmic radiation, meteoritic bombardment, extreme temperature
swings. Back on Earth, NASA engineers wanted to know how metals,
glass and other spacecraft building materials held up to that
kind of punishment. Inspecting Surveyor 3 first hand seemed
a good way to find out.
On
their second four-hour EVA, Bean and Conrad walked over to
Surveyor 3, took dozens of photographs and measurements, and
began snipping off parts of metal tubing and electrical cables.
They retrieved a camera. The very last thing they removed
was a small scoop at the end of Surveyor's extendable arm,
which had dug into the dry moon dust and gravel to make mechanical
measurements of lunar soil.
The
little scoop, the camera, and other artifacts returned to
Earth were analyzed and then put in storage. At some point
in the intervening four decades, the scoop, owned by Johnson
Space Center, was transferred on permanent loan to a space
museum in Kansas. And there matters quietly lay ... until
recently when researchers at NASA's Glenn Research Center
(GRC) realized that that little scoop could hold big secrets.
Namely,
the secrets of digging on the Moon.
NASA
is returning to the Moon with plans to establish an outpost--and
this will inevitably require some digging. The rocky, dusty
lunar soil or "regolith" contains many of the natural
resources humans need to live. For instance, there is plentiful
oxygen bound up in ordinary moon rocks and, in polar regions,
deposits of frozen water may lie hidden in the soil of shadowed
craters. All that's required is a little excavation.
 But
how? Lunar regolith is not like terrestrial soil. Here on
Earth, the sand beneath our feet is shaped by a combination
of biological and meteorological forces. Terrestrial soil
is moist, rounded by weather, and utterly familiar. Lunar
regolith, on the other hand, is a dry, glassy substance pounded
into dusty smithereens by eons of meteoritic bombardment.
It's not going to respond to a shovel--or a scoop--like terra
firma.
Right:
A micro-photo of lunar regolith. The sample is a mixture of
volcanic glass beads, sharp-edged fragments of "impact
glass", rock fragments and more.
Photo courtesy of Larry Taylor, University of Tennessee.
"To
design lunar digging equipment, we need to predict the forces
required to move a scoop or other implement through lunar
regolith," says Allen Wilkinson, team leader of the ISRU
(In-Situ Resource Utilization) Regolith Characterization team
at the Glenn Research Center.
Surveyor
3 and a sister ship Surveyor 7 actually dug into the Moon
and measured how hard their drive motors had to work to scoop,
press, and scrape the soil. To interpret those measurements
more than 40 years later, however, Wilkinson's team needs
to know the dimensions of the Surveyor scoops. Unfortunately,
they learned, the blueprints had been lost! Only a scoop itself
could provide the answer.
That
sent Wilkinson to Hutchinson, Kansas, in April 2007 to borrow
the Surveyor 3 scoop from the Kansas State Cosmosphere in
order to make detailed measurements.
Above:
Surveyor 3 scoop being examined by four of the members of
the Regolith Characterization team; from left to right they
are Xiangwu (David) Zeng, Enrique Rame, Allen Wilkinson, and
Juan Agui. Copyright 2007 Trudy E. Bell.
Measuring
the scoop, however, would prove to be no simple matter. You
can't just lay a ruler along the scoop and read off the dimensions.
Indeed, you can't touch it at all. The Surveyor 3 scoop is
in an airtight triangular container, and NASA curators do
not wish the scoop to be removed because handling in air will
degrade the historical fidelity of the unique artifact.
So
the Glenn team borrowed photogrammetry apparatus from the
Kennedy Space Center. Photogrammetry is a technique of measuring
objects strictly from photographs. They have a photographic
studio setup with a white background. GRC team member Juan
Agui, an expert in digging force experiments, photographed
the scoop in its container next to a standard photogrammetry
cube, which has a precise checkerboard pattern on it. Then,
using software, Robert Mueller of the Kennedy Space Center
extracted dimensions using mathematical triangulation, measuring
from points on the scoop to points where corners of dark checks
meet on the cube. The software was developed for the Columbia
Accident Investigation Board activity.
 Right:
Surveyor 3 scoop inside its glass container. A fountain pen
in the foreground gives the image scale. Another
photo shows the interior of the scoop. Copyright 2007
Trudy E. Bell.
"Photogrammetry
is pretty good," Agui remarks. "We got measurements
of the scoop accurate to 0.030 or 0.040 inch (~1 mm)."
They've
since constructed a replica of the scoop and now they are
using it to dig into simulated lunar regolith.
"Measurements
of digging forces are underway," he says. The replicated
scoop plunges into a rectangular "soil bed" filled
with JSC-1a, a man-made moondust substitute that closely matches
the known properties of lunar regolith, while a computer monitors
bearing forces. "Our team is quite pleased to find that
the measurements appear to be close to reproducing [the best]
Surveyor 7 data from the Moon."
With
this test bed in place, the team can, e.g., move
forward to test alternate scoop designs and refine theories
of lunar soil mechanics. "Obtaining the Surveyor replica
really made the difference," says Agui.
The
secrets of digging on the Moon are being revealed.
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Author: Trudy E. Bell
| Editor:
Dr. Tony Phillips | Credit: Science@NASA
| more
information |
|
More
about the Surveyor program: from
NASA and from
Wikipedia.
The
Surveyor 3 scoop is described in a 1971 paper "Examination
of the Surveyor 3 Surface Sampler Scoop Returned by
the Apollo 12 Mission," by R. F. Scott and
K. A. Zuckerman. Caution: PDF is a large 35MB file.
The
Apollo Lunar Surface Journal for Apollo 12 -- The
transcript for the second EVA, when Conrad and Bean
cut the scoop off Surveyor 3, begins at 134:28:20; the
scoop was removed from Surveyor at the very end, almost
as an afterthought as it was not on the original list
of items to be returned.
Visit
the Kansas State Cosmosphere and Space Center online
at http://www.cosmo.org/.
A
brief history of lunar soil mechanics measurements:
Several
Apollo astronauts during their landings between 1969
and 1972 used a standard soil mechanics tool called
a cone penetrometer to measure some of the regolith's
mechanical properties in the top two feet (60 cm). So
did the Soviet Luna and Lunakhod missions that landed
elsewhere on the Moon. From the U.S. and Soviet data,
soil mechanicians have indirectly calculated digging
forces.
Only
two U.S. lunar spacecraft directly measured actual digging
and trenching forces—direct measurements are the most
accurate and useful for the design of lunar excavators:
Surveyor 3 (which landed on Oceanus Procellarum on April
20, 1967) and Surveyor 7 (which landed near the crater
Tycho on January 7, 1968). Those two Surveyor spacecraft
included an extendable arm ending in a scoop (officially
called the Soil Mechanics Surface Sampler). Among other
tests, each scoop pressed vertically down to measure
the regolith's weight-bearing strength, and pulled the
scoop toward the spacecraft while bearing down to dig
trenches. Only the data from Surveyor 7, though, were
considered reliable enough to provide the needed digging
forces.
Over
a year ago, the Glenn team calculated the expected digging
force for the one published Surveyor 7 digging force
measurement. But something was clearly wrong: their
calculated predictions, based on the size of the scoop
they had estimated from photographs, were 5 to 10 times
smaller than what was actually measured on the Moon.
That discovery motivated a year of effort to find all
still-existing Surveyor digging data and artifacts,
so the Glenn team could calibrate their experiments
and benchmark the strength of their test soils.
This
detective hunt led the Glenn team to find video images
of the Surveyor 7 scoop while digging, the actual returned
Surveyor 3 scoop, possible data files with motor currents
during other Surveyor 7 digging runs, and the paper
calibration curves actually used in the 1960's to convert
motor currents in into digging forces. The Glenn team
learned that the Surveyor digger drawings were lost.
They found two engineering prototypes of the scoop:
one on display at the Smithsonian Air and Space Museum
and one stored in the Garber Restoration facility. Glenn
team member Juan Agui, an expert in digging force experiments,
visited both sites to measure the scoops. To his consternation,
he found the two engineering prototypes differed in
crucial design details. But which one matched the actual
flight units?
The
answer was: neither. This the Glenn team learned from
74-year-old Floyd Roberson, a former JPL scientist who
had co-developed the Surveyor scoops and supervised
their operations with the late Ron Scott of the Californian
Institute of Technology, Surveyor principal investigator
for soil mechanics. From his personal archives, however,
Roberson verified that the Surveyor 7 scoop was identical
to the Surveyor 3 scoop that the Apollo 12 astronauts
had returned to Earth. The key to the puzzle, therefore,
was the Surveyor 3 scoop available for hands-off study
in a museum in Kansas....
NASA's
Future: US
Space Exploration Policy
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