- Byers, Bruce K.
Destination Moon.
Washington: National Aeronautics and Space Administration, 1977.
NASA TM X-3487.
-
In the mid 1960s, the U.S. Apollo lunar landing program was at the
peak of its budget commitment and technical development. The mission
mode had already been chosen and development of the flight hardware was
well underway, along with the ground infrastructure required to test and
launch it and the global network required to track missions in flight.
One nettlesome problem remained. The design of the lunar module made
assumptions about the properties of the lunar surface upon which it would
alight. If the landing zone had boulders which were too large, craters
sufficiently deep and common that the landing legs could not avoid, or
slopes too steep to avoid an upset on landing or tipping over afterward,
lunar landing missions would all be aborted by the crew when they
reached decision height, judging there was no place they could set down
safely. Even if all the crews returned safely without having landed,
this would be an ignominious end to the ambitions of Project Apollo.
What was needed in order to identify safe landing zones was high-resolution
imagery of the Moon. The most capable Earth-based telescopes, operating
through Earth's turbulent and often murky atmosphere, produced images which
resolved objects at best a hundred times larger that those which could
upset a lunar landing mission. What was required was a large area, high
resolution mapping of the Moon and survey of potential landing zones, which
could only be done, given the technology of the 1960s, by going there,
taking pictures, and returning them to Earth. So was born the
Lunar Orbiter
program, which in 1966 and 1967 sent lightweight photographic reconnaissance
satellites into lunar orbit, providing both the close-up imagery needed
to select landing sites for the Apollo missions, but also mapping imagery
which covered 99% of the near side of the Moon and 85% of the far side,
In fact, Lunar Orbiter provided global imagery of the Moon far more
complete than that which would be available for the Earth many years
thereafter.
Accomplishing this goal with the technology of the 1960s was no small
feat. Electronic imaging amounted to analogue television, which, at the
altitude of a lunar orbit, wouldn't produce images any better than
telescopes on Earth. The first spy satellites were struggling to return
film from Earth orbit, and returning film from the Moon was completely
impossible given the mass budget of the launchers available. After a
fierce competition, NASA contracted with Boeing to build the Lunar
Orbiter, designed to fit on NASA's workhorse
Atlas-Agena
launcher, which seriously constrained its mass. Boeing subcontracted
with Kodak to build the imaging system and RCA for the communications
hardware which would relay the images back to Earth and allow the
spacecraft to be controlled from the ground.
The images were acquired by a process which may seem absurd to those
accustomed to present-day digital technologies but which seemed
miraculous in its day. In lunar orbit, the spacecraft would aim its
cameras (it had two: a mapping camera which produced overlapping
wide-angle views and a high-resolution camera that photographed
clips of each frame with a resolution of about one metre) at the
Moon and take a series of photos. Because the film used had a very
low light sensitivity (ASA [now ISO] 1.6), on low-altitude imaging
passes the film would have to be moved to compensate for the motion
of the spacecraft to avoid blurring. (The low light sensitivity of
the film was due to its very high spatial resolution, but also reduced
its likelihood of being fogged by exposure to cosmic rays or
energetic particles from solar flares.)
After being exposed, the film would subsequently be processed on-board
by putting it in contact with a band containing developer and fixer, and
then the resulting negative would be read back for transmission to
Earth by scanning it with a moving point of light, measuring the transmission
through the negative, and sending the measured intensity back as an analogue
signal. At the receiving station, that signal would be used to modulate
the intensity of a spot of light scanned across film which,
when developed and assembled into images from strips, revealed the details
of the Moon. The incoming analogue signal was recorded on tape to
provide a backup for the film recording process, but nothing was done
with the tapes at the time. More about this later….
Five Lunar Orbiter missions were launched, and although some experienced
problems, all achieved their primary mission objectives. The first three
missions provided all of the data required by Apollo, so the final two
could be dedicated to mapping the Moon from near-polar orbits. After
the completion of their primary imaging missions, Lunar Orbiters continued
to measure the radiation and micrometeoroid environment near the Moon,
and contributed to understanding the Moon's gravitational field, which
would be important in planning later Apollo missions that would fly in
very low orbits around the Moon. On August 23rd, 1966, the first Lunar
Orbiter took one of the most iconic pictures of the 20th century:
Earthrise
from the Moon. The problems experienced by Lunar Orbiter missions and
the improvisation by ground controllers to work around them set the
pattern for subsequent NASA robotic missions, with their versatile,
reconfigurable flight hardware and fine-grained control from the ground.
You might think the story of Lunar Orbiter a footnote to space
exploration history which has scrolled off the screen with subsequent
Apollo lunar landings and high-resolution lunar mapping by missions
such as
Clementine
and the
Lunar Reconnaissance Orbiter,
but that fails to take into account the exploits of 21st century space
data archaeologists. Recall that I said that all of the image data from Lunar
Orbiter missions was recorded on analogue tapes. These tapes contained about
10 bits of dynamic range, as opposed to the 8 bits which were preserved by
the optical recording process used in receiving the images during the missions.
This, combined with contemporary image processing techniques, makes for breathtaking
images recorded almost half a century ago, but never seen before. Here are a document and video
which record the exploits of the
Lunar
Orbiter Image Recovery Project (LOIRP). Please visit the
LOIRP Web site for more restored
images and details of the process of restoration.
September 2014 