|Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions|
LUNAR EXPLORATION BEGINS
Target: Surveyor III
Some of that preparation time would be needed, for after Apollo 11 the objectives for Apollo 12 began to change. Shortly after the site selection board had settled on its fist of 10 landing sites for lunar exploration, Sam Phillips selected the Surveyor III site as the target for the next mission.36 Even though the scientists had unanimously rejected its location as unsuitable, the inert Surveyor offered some opportunities that could not be passed up. A demonstration of point landing (i.e., within 1 kilometer [0.62 mile] of a preselected spot) needed to be made as soon as feasible. For that purpose, something like a Surveyor was more appealing to mission planners than picking some specific crater. Scientists might have good reasons for landing next to one crater rather than another, but to nonscientists (including the public) all craters were pretty much alike.37 Besides, valuable engineering and scientific information could come out of examination of a spacecraft that had spent more than two years in the space environment,* as Milwitzky had pointed out to the site selection board (see above). Houston had already determined that certain components could be removed from a Surveyor with no special difficulty and was working with Hughes Aircraft Company, builder of the Surveyors, on procedures the astronauts could use.38
As soon as that decision was made, changes in mission techniques became a major effort for Apollo 12 planners. Howard W. Tindall, Jr., MSC's chief of Apollo data priority coordination, wrote as the planning began, "It is clear that lunar point landing capability is absolutely necessary if we are to support the exploration program the scientists want [emphasis added]. That is, mission success intrinsically depends upon it."39 A preliminary look at the problems of accomplishing it, however, showed that it could not be done as early as Apollo 12. Too many procedural changes were involved, and there was not enough time to incorporate them and prove their suitability. Accordingly, mission planning teams at MSC set out to refine existing techniques, in the hope that they could reduce the unavoidable errors to the point of landing within a mile (1,600 meters) of a preselected spot.40
Trajectory planners first needed to know the precise location of Surveyor III, which, they discovered, had been determined not long after the spacecraft had touched down.** Geologists had located the spacecraft inside and just below the eastern rim of a subdued crater about 200 meters (650 feet) across and 15 meters (50 feet) deep, some 480 miles (775 kilometers) west-southwest of the moon's center, at lunar latitude 2 degrees 57 minutes south, longitude 23 degrees 20minutes west.41
Given the precise location of their target, trajectory planners next went to work improving mission techniques. The fundamental difficulty in making a precision landing was determining the lunar module's orbital position and velocity in lunar orbit (its "state vector") with sufficient accuracy. Earth-based radar acquired this information and the communications network transmitted it to the lunar module's primary guidance computer just before the landing craft began its powered descent. No further change was made until the spacecraft reached low altitude and the landing point was in sight. Then the mission commander could assume manual control and adjust the landing point to avoid hazards, as Neil Armstrong had done on Apollo 11, or land at a desired spot.
After the state vector was determined and before descent began, the spacecraft's actual path could deviate from its predicted path for several reasons. Perturbations caused by the irregular shape and gravity field of the moon, unwanted changes in velocity during required spacecraft maneuvers, and the guidance system's tendency to drift slightly from its inertial alignment, all contributed to inaccuracy in the state vector.*** 42
A month of considering the sources of these errors and ways of minimizing them boosted MSC's confidence considerably. Still, after a "three-day mission techniques free-for-all" at the end of July, Tindall concluded that "if we land within walking distance, it is my feeling we have to give most of the credit to 'lady luck.'"43 Four weeks later, numerous productive suggestions had been explored. Among the more significant changes were separating the lunar module from the command module earlier in the mission and eliminating several attitude changes normally made in the following two orbits. These changes gave more accurate tracking of the LM, could significantly reduce unwanted velocity changes, and might enable Mission Control's computers to measure drift in the inertial navigation system and compensate for it. Another proposed refinement was to give the spacecraft computer a revised landing point, based on radar tracking just before the lunar module started down, while the lander was making its descent, thus taking out all the error accumulated to that point in one step.44 All these changes got careful study in the next two months, with encouraging results. Two weeks before launch Tindall was more sanguine. "For whatever it's worth," he reported, "my feeling now is that as long as the systems work as well as they have in the past, we have a pretty good chance of landing near the Surveyor." Beyond that he would not make any predictions about the accuracy of the landing.45
* Surveyor III landed on April 20, 1967, and transmitted 6,326 television pictures and large amounts of scientific data for 14 days.
** Prominent features appearing in Surveyor III's television photographs had also been located in high- resolution pictures of the same area taken by Lunar Orbiter III.
*** These random (indeterminate) errors led engineers to use statistical methods in predicting landing points. The most commonly used designation was the "3-sigma ellipse," centered on the desired landing point, within which there was 99 percent confidence that the' spacecraft would land. For Apollo 11 the 3- sigma ellipse was roughly 11.7 miles long by 3 miles wide (18.8 by 4.8 kilometers); for Apollo 12, planners were aiming for 2.6 by l.6 miles (4.2 by 2.6 kilometers).
36. MSC News Release, "Apollo 12 Landing Sites," July 25, 1969; John G. Zarcaro to multiple addressees, "Apollo 12 Mission H-1 planning," July 28, 1969.
37. Sevier interview.
38. B. E. Oldfield (Hughes Aircraft Co.) to John Hodge, July 10, 1969; Hodge to mgr., Apollo spacecraft program, "Science and Technological Items Available Upon Landing at a Surveyor Site," July 31, 1969.
39. Howard W. Tindall, Jr., "A Lengthy Status Report on Lunar Point Landing Including Some Remarks about CSM DOI [Descent Orbit Insertion]," Aug. 29, 1969. Tindall's clear and breezy style in writing memos ("Tindallgrams," as they came to be called), usually prefaced with an attention - grabbing title, gained him a well-deserved local reputation. He had the gift, rare among engineers and still rarer among managers, of compacting large amounts of information into a few clear and understandable paragraphs, leavened with considerable humor.
40. Tindall, "How to Land Next to a Surveyor - a Short Novel for Do-lt-Yourselfers," Aug. 1, 1969.
41. Surveyor III: A Preliminary Report, NASA SP-146 (Washington, 1967), pp. 12-16; Walter Sullivan, "Surveyor Tracked Down By a Photographic Sleuth," New York Times, Nov. 19, 1969.
42. John P. Mayer, "The Search for Surveyor III," MSC Internal Note 69-FM-329, Jan. 16, 1970. Its language shows that the report was written before the launch of Apollo 12, although it is dated two months later.
43. Tindall, "How to Land next to a Surveyor," Aug. 1, 1969.
44. Tindall, "A Lengthy Status Report," Aug. 29, 1969.
45. Tindall to multiple addressees, "Apollo 12 Descent - Final Comments," Nov. 4, 1969.