|Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions|
MISSION AND SCIENCE PLANNING
Operational Constraints on Landing Sites
Picking a spot where a lunar module could land was a complex exercise requiring tradeoffs among dozens of factors. Predominant among these were the topography and texture of the lunar surface and the requirements of the lunar module's guidance and navigation system. Other restrictions included the elevation of the sun at the landing site; the temperature of the lunar surface; the radiation environment in space and on the moon; and the earth lighting conditions desired for launch and recovery.3 The most restrictive mission rule, so far as landing sites for the earliest lunar landing missions were concerned, was the requirement to place the spacecraft on a "free-return" trajectory - a flight path that allowed for failure of the service module's main engine. If the service module engine should fail to put the spacecraft into lunar orbit, the joined CSM and LM would loop around the moon under the influence of lunar gravity alone and head back to earth. This free-return trajectory required the spacecraft to leave earth orbit on a path that would bring it to the moon within five degrees of latitude of the lunar equator. As early as mid-1963 MSC's Space Environment Division selected four sites from a list compiled by various lunar scientists, balancing scientific interest against this mission rule. A few months later five more sites were picked for preliminary trajectory studies.4
The choice of lunar longitude for a landing site depended mainly on two considerations. To land at a predetermined site it was essential to determine the position and the flight path of the lunar landing craft as accurately as possible before beginning the final descent to the surface. Navigational sightings taken from the spacecraft on stars or on lunar surface landmarks provided the data from which ground-based computers determined the spacecraft's orbit and calculated the necessary course corrections. Accurate calculation of the orbit required the astronauts to take sightings on five prominent lunar landmarks some distance east of the landing site, and since the spacecraft went behind the moon on the west and was out of radio and radar contact with earth until it emerged around the eastern edge, those sightings could only be taken after earth contact was reestablished. The position of the navigational landmarks had to be known with an error of no more than 1,500 feet (450 meters). In mid-1963 this was not possible; at the eastern and western edges of the visible face, surface features might actually be as much as 6,000 feet (1,800 meters) from where the best lunar maps showed them. In light of these navigation requirements, early planning assumed that a landing could be plotted no farther east than 40 degrees east longitude. The "Apollo landing zone" thus defined extended to 40 degrees west longitude; other operational considerations made a more westerly landing undesirable.5
The second limitation on the longitude of the landing site was the elevation of the sun at the time of landing, which was the major factor considered in choosing the time of launch. To the pilot looking for a safe spot to land within the time the lunar module could hover, it was vital that the sun be high enough in the lunar sky to highlight the surface topography without casting long, confusing shadows, but not so high that all surface detail was washed out. After landing, the lunar explorers would also be hindered by a low or high sun. The moon has no atmosphere to scatter light and therefore shadows are completely black; at either low or high sun angles, visual observations can be difficult. Furthermore, solar heating of the lunar surface varies with sun angle, complicating the problem of protecting the astronauts and the spacecraft against extreme temperatures. Conditions would be best when the sun was 15 to 45 degrees above the horizon. Mission planners could choose a launch time so that the lunar module would land at a time when solar illumination was near optimum. Launch time was subject to the further constraint, however, that lunar missions had to leave the launch pad well before last light in case an aborted launch required emergency recovery operations.6
Two potential hazards to the lunar mission were more difficult to take into account: meteoroids and radiation. In 1963 no one knew how dangerous meteoroids were. It seemed prudent to avoid the predictable (and dense) swarms that recur annually, but the earth-moon system constantly encounters a smaller number of randomly distributed meteoroids. The last three test flights of the Saturn I launch vehicle carried meteoroid-detecting satellites into earth orbit to determine how serious this hazard might be. Radiation (subatomic particles, x-rays, and gamma rays) was more worrisome. Of special concern were the high-energy protons shot out from the sun during major solar flares, which could subject astronauts on the lunar surface to lethal doses of radiation.7 Solar flares were more troublesome because they are completely unpredictable. Protection was extremely difficult and warning all but impossible: by the time a flare could be detected on earth and its magnitude assessed, the most energetic (and dangerous) particles would already have reached the moon.
Within the zone defined by all these constraints - roughly 185 miles (300 kilometers) wide, stretching l,500 miles (2,400 kilometers) along the moon's equator - geologic factors would determine the choice of a landing site. Surface topography could not be known in any detail until Ranger and Surveyor provided information; but from lunar maps available in 1963, several landing areas* about 400 square miles (900 square kilometers) in size could be picked out where slopes apparently were not too great and craters not too numerous. Balancing the need to pick landing areas near the center of the moon (where lunar maps were most accurate) against the requirement to spread the areas as far as possible along the equator (which allowed maximum flexibility in launch dates), MSC's Space Environment Division found 10 landing areas that seemed promising enough to warrant reconnoitering by unmanned spacecraft and close scrutiny by mission planners. The areas were spotted in a zone extending from the southeastern edge of Mare Tranquillitatis (not far from where Apollo 11 would eventually land) to a point northeast of Flamsteed crater in Oceanus Procellarum (some 375 miles [600 kilometers] west of Apollo 12's touchdown point). Even among these, however, none was completely satisfactory with respect to all the known constraints.8
The effect of all these restrictions on the landing site was to reduce drastically the number of consecutive days per month on which a lunar mission could be launched. Considering only the two most important factors - sun angle and surface temperature - a given site could be reached only if the spacecraft were launched during a 2.3-day period each month. Experience to 1963 indicated that launch operations stood a good chance of being interrupted and launches postponed because of systems problems, and no one was willing to count on launching an Apollo mission within 2.3 days. But if flight planners could be prepared to land at more than one site for each launch - changing to a more westerly target if launch delays prevented reaching the first site - the number of consecutive days on which a launch was possible could be substantially increased. A Bellcomm study in early 1964 pointed out that choosing multiple sites for each launch would make the program considerably more flexible, though it would require certifying more sites through the Surveyor and Lunar Orbiter programs. That, however, might cost no more than postponing a few launches by a month each.9
The greatest uncertainty in the program at that time, pointed up by all these early studies, was the physical nature of the moon's surface. Astronomers held widely different views of what a lunar module would encounter when it touched down. Gerald Kuiper of the University of Arizona, one of the principal investigators in the Ranger project, was convinced that the surface was firm, though it might be unconsolidated and might be covered by a thin layer of dust. Cornell University astronomer Thomas Gold asserted, however, that the apparently smooth areas on the moon were likely to be covered with a layer of fine dust several meters thick, raising the prospect that the lunar module might sink out of sight with only a short-lived dust cloud to mark its disappearance.10 There was the further possibility that the surface might be so cluttered with boulders and pitted with small craters that the lander would find no level spot large enough to land - or if it tried to land, would turn over or come to rest tilted at an angle that made return to orbit difficult.
NASA had been hoping that Ranger's television photographs would shed light on these questions, but by the end of 1963 Ranger had experienced its fifth failure in as many attempts and was undergoing a critical reappraisal. [see Chapter 2] 11 Spacecraft engineers at Houston's Manned Spacecraft Center, meanwhile, in spite of their real need for this information in designing the lunar landing module, had to go ahead without it.12 Lunar Orbiter, still in the early stages, would have to provide the information that mission planners needed for site selection. The spacecraft builders could only hope that data from Surveyor, when they got it, would not force them to revise their design too drastically.
* A landing area was a fairly large segment of the lunar surface which appeared sufficiently level and smooth to permit landing; a landing site was an ellipse a few hundred meters in size within which the lunar module would actually touch down. A site would be picked for exact targeting after the hazards of the landing area had been assessed.
3. MSC, "Environmental Factors Involved in the Choice of Lunar Operational Dates and the Choice of Lunar Landing Sites," NASA Project Apollo Working Paper (AWP) No. 1100, Nov. 22, 1963. In 1972 an entire number of The Bell System Technical Journal (vol. 51, no. 5, pp. 955-1127) was devoted to a single article, "Where on the Moon? An Apollo Systems Engineering Problem," J. O. Cappellari, Jr., ed. This is a narrative description of the many factors that entered into the choice of an Apollo landing site, how the factors were weighted, and how the interaction of those factors changed as experience with Apollo systems accumulated. It is not so technical as to be incomprehensible to most readers. Not surprisingly, it lays considerable emphasis on the role of Bellcomm, Inc., in the site selection process.
4. AWP 1100, p. 23.
5. Ibid., p. 32.
6. Ibid., pp. 7-13.
7. Ibid., pp. 6-7.
8. Ibid., p. 33.
9. W. E. Thompson (Bellcomm, Inc.), "Lunar Landing Site Constraints: The Arguments for and Against One Preselected Site Versus Several Sites," Jan. 31, 1964.
10. Benjamine J. Garland, memo for Apollo proj. off., "Characteristics of the Lunar Surface," Aug. 14, 1961; Thomas Gold, "Structure of the Moon's Surface," in J. W. Salisbury and P. E. Glaser, eds., The Lunar Surface Layer (London: Academic Press, 1964), pp. 345-53. Gold interpreted radar, optical, and thermal properties of the lunar surface as indicating a layer of fine particles up to several meters thick whose mechanical properties would be difficult to predict. He suggested that at the very least a lunar landing would be compromised by blinding clouds of dust raised by the exhaust from the descent engine. In the popular press, Gold's hypothesis was taken to mean that a lunar module could sink out of sight. The pictures returned by Ranger 7 (July 1964) gave Gold no reason to change his thinking; see R. Cargill Hall, Lunar Impact: A History of Project Ranger, NASA SP-4210 (Washington, 1977), p.285; also Gold, "Ranger Moon Pictures; Implications," Science 145 (1964): 1046-48. Most Ranger scientists agreed that the TV pictures from Ranger 7 gave no basis for drawing conclusions about the load-bearing characteristics of the surface. They did give some useful information about the distribution and size of craters and the slopes of lunar terrain on a smaller scale than had been possible. Gold remarked after the conclusion of Ranger that its pictures were like mirrors: "everyone sees his own theories reflected in them." Hall, Lunar Impact, p. 309. For a bibliography of studies on the lunar surface up to the first successful Ranger mission, see J. W. Salisbury, ed., Bibliography of Lunar and Planetary Research - 1960-1964, AFCRL-66-52, U.S. Air Force Office of Aerospace Research, Cambridge Research Laboratories, Jan. 1966.
11. Hall, Lunar Impact, pp. 156-82.
12. Courtney G. Brooks, James M. Grimwood, and Loyd S. Swenson, Jr., Chariots for Apollo: A History of Manned Lunar Spacecraft, NASA SP-4205 (Washington, 1979), pp. 150-54.