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Saturday, September 27, 2014

Reading List: Destination Moon

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.

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