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On Mars: Exploration of the Red Planet. 1958-1978

 
 
PREPARING FOR SITE SELECTION
 
 
 
[284] Besides considering the imaging system and discussing desired landing site characteristics at its October 1970 meeting, the landing site working group also considered what it could gain from Mariner 71. Dan Schneiderman introduced the group to the Mariner project, and Edwin Pounder reviewed mission operations plans for both the prime 90-day mission and the extended mission (for the remainder of the first year in Mars orbit). Pounder went on to outline problems and promises of the project, one of the promises being data that would assist the Viking team in landing site selection. Patrick J. Rygh and Robert H. Steinbacher briefed the working group on mission operations and participation by scientists.
 
In turn, Hal Masursky and Carl Sagan told the Mariner specialists what the Viking team hoped to learn from Mariner 71. What they wanted was not in the written mission plans but was rather, How do we learn as we go along and then modify our plans accordingly? In NASA shorthand, this tactic was called the adaptive mode-acquiring data from a spacecraft and quickly using it to modify the mission. The Viking team was certain it would need this skill, and it would require discipline, planning, and timely responsiveness to succeed. In the plans for Mariner 71, data processing was not scheduled to catch up with acquisition for a year, and Masursky feared that unless adequately supported, the complete process could take 5 to 10 years, which was obviously too slow to be of value to Viking. Years of work had to be compressed into weeks. On occasion, time for data processing would have to be whittled down to days and even hours.12
 
At its next meeting, 2-3 December 1970, the landing site working group made its initial recommendation for landing sites, so that Howard Robins' mission planning staff could proceed with its work. These proposed [285] sites had been chosen after only four months. Carl Sagan, who had been urging that the site selection process be completely documented, prepared a convenient summary of the thinking-as he saw it-that went into the choices. "The following is a preliminary attempt to integrate coherently a range of ideas which have been suggested on the Viking landing site question, to point out inadequacies in the existing data and to serve as guide for future discussion." He noted that the "present cycle of discussion on landing site selection is to aid development of the Viking Project Reference Mission #1," a theoretical model that would be used in planning mission operations and designing the spacecraft. Since in some respects this was a training exercise. there was no commitment to the specific landing sites they had selected.
 
In considering landing sites for the two Vikings, some factors would be certain to change. But those that would likely remain unaltered fell into two categories, engineering and scientific. Under the engineering heading, the 30 south to 30 north latitude range for landing sites was dictated by the angle at which the spacecraft would have to enter the Martian atmosphere to obtain optimum aerodynamic deceleration and proper thermal conditions. Second, nearly all of the working group members agreed that the lander should sit down where atmospheric pressures were the highest. As on Earth, high pressure corresponds with lower elevation, but whereas sea level pressure on Earth averages about 1013 millibars, surface pressures on Mars are 100 times lower. Pressure at the lowest elevation was believed to be close to 10 millibars and at the top of mountains less than l millibar but the uncertainty in these values was 20 or 30 percent at the time. The Viking scientists hoped that Mariner photographs and ground-based radar studies would give them more exact information on atmospheric pressure relative to topographical features. A third engineering concern was the effect that Martian surface winds would have on the spacecraft. The Mars engineering model with which the team was working predicted winds of less than 90 meters per second, but Sagan noted that newer calculations indicated the possibility of winds up to 140 to 200 meters per second.
 
If such winds are encountered during landing maneuvers, the survivability of the spacecraft is very much in question; and such winds, even after a safe landing, might provide various engineering embarrassments. It will shortly be possible to predict which times and places are to be avoided.Such considerations obviously require further theoretical study and (with Mariner Mars '71) observational study. But they do indicate how new parameters, not previously considered, can severely impact landing site choices. Such considerations imply that any landing site selected at the present time should not be too firmly imbedded in the Project's thinking. 13
 
Other technical factors affecting the choice of a landing spot included the time of day on Mars at touchdown, the size of the landing target, and a pair strategy calling for one very safe (but perhaps less interesting) site and [286] one of greater scientific potential. Depending in part on progress made in developing the lander tape recorder. Sagan thought that it might be desirable to land in the late afternoon to ensure that some lander images of the planet would be transmitted to the orbiter before it passed out of view of the lander, giving the team at the Jet Propulsion Lab maximum assurance of obtaining at least some initial pictures of the surface. They had to face the possibility that the lander could die while the orbiter continued on its way around Mars; it would be 24.6 hours before the orbiter passed over the lander a second time. Should a late afternoon touchdown be called for, those areas with dense cloud development at the time of day would have to be excluded. Turning to the target, or landing ellipse, Sagan indicated that it was currently 400 by 840 kilometers, which would eliminate areas appreciably smaller than this zone. The pair strategy had been devised for reasons of "survivability." One landing site would be selected with "safety considerations weighed very highly"; One landing site would be selected with "safety considerations weighed very highly''; if the first mission failed on entry, the team would want to have a preselected, extremely safe site for the second lander. "It is therefore necessary to consider some sites almost exclusively on engineering grounds." Sagan hoped planners could "back off from this requirement a little bit and seek out safe contingency sites with at least acceptable science.'' Alan Binder had made this same point earlier but somewhat more bluntly: "The engineering criteria must reign since it hardly need be mentioned that a crashed lander is not very useful even if it did crash in the most interesting part of the planet." 14 Sagan wrote, ''Before any Viking lander is committed to a given site, there must be reasonably extensive Mariner Mars '71 type data, including but not restricted to imagery." He thought that selection of alternative candidate sites should be based on Mariner 71 data, and certification of the various candidates should be based on Viking data, which would be of higher resolution.
 
Sagan's report then turned to the working group consensus on science criteria for the landing sites. Many members believed it would be useful to pair the first two landing sites in such a manner that each one would be a control for the measurements made at its companion location. A reason for varying from this plan would be positive results from the biology experiment on the first lander; then the Viking team might wish to land the second craft as near the first one as possible to determine if the results could be duplicated. The best guess at the time was that Martian life, "or at least that subset of Martian life which the Viking biology package is likely to detect,'' would be found where there was water near the surface. But there was still considerable debate about the nature and amount of water that might be found. Low atmospheric pressures and temperatures always below 0°C did not augur well for the presence of liquid water. Still, Sagan and others believed that it was possible to have life-sustaining water present in other forms.
 
The uncratered terrain observed in the Mariner 4 photographs was of possible interest. Sagan hypothesized that such terrain must have been [287] recently (in geological terms) reworked. "Whatever the cause of the reworking, but particularly if it is due to tectonic activity, such locales are much more likely a priori to have had recent outgassing events and therefore to be of both geological and biological interest." Taking into consideration all these factors, Sagan listed his six favorite landing spots, but several of his colleagues came up with other suggestions of their own. 15
 
After considerable freewheeling debate of the kind that characterized many of the working group's meetings, the group recommended three sites for each lander. It wanted to find water and it wanted to land one craft in the north and one in the south. The mission planners indicated that it would be best to land the first Viking in the northern latitude, or during the Martian summer. Immediately following the working group sessions, the mission analysis and design team subjected the six candidate sites to a preliminary examination, and its first quick look revealed no apparent difficulties. On 7 December, Jim Martin directed Martin Marietta to proceed with the design of the two Viking missions using Toth-Nepenthes (15°N, 275°) * for the touchdown area of the first lander and Hellas (30°S, 300°) for the second craft. 16
 
Early in February, Dan Schneiderman and Jim Martin signed a "Memorandum of Agreement for Viking Participation in Mariner '71 Operations." Two areas were identified for direct Viking participation-mission operations and scientific-data analysis. Viking personnel would work as part of the Mariner team. The Viking data analysis group would be housed in the Science Team Analysis Facility at JPL, and a Viking representative would act as an observer at the Mariner science recommendation team meetings, watching the interplay between the science advisers and the mission operations personnel. 17
 
The Viking landing site working group did not meet again until April 1971. Meanwhile, the mission planners and the Martin Marietta Corporation evolved the "Mission Design Requirements Objectives and Constraints Document," which outlined for the first time in detail how the two missions would be conducted from launch through operation of the science experiments on Mars. Members of the landing site team and the Science Steering Group met in joint session on the afternoon of 21 April to discuss that document and mission planning in general, but earlier that day the landing site team had considered at length its participation in the Mariner 71 operations.
 
Tom Young opened the morning session, noting that Robert A. Schmitz would serve as manager of the Viking-Mariner Mars 1971 participation group. His duties included overseeing the Viking data analysis team, which would examine areas related to proposed Viking landing areas. This team would be drawn from two groups of scientists, those who would be working as part of the Mariner 71 operations team-Geoffrey [288] Briggs. Michael Carr, Hugh Kieffer, Conway Leovy, Hal Masursky, and Carl Sagan-and part-time participants from the Viking team. ** Schmitz also was to act as the Viking observer on the Mariner 71 science recommendation team, which would give him a much broader understanding of the entire Mariner project.
 
Hal Masurksy raised two problem issues in data management for Mariner 71, computer data processing and preparing Mars maps. The flow of data from the Mariner spacecraft would be so rapid that only one-fourth to one-third of the information could be processed in real time or near real time by the Mariner 71 system. At that rate, Masursky predicted it would take 18 months to get a complete set of reduced data records. a serious lag for Viking planners who wanted to use this information to land their spacecraft. And to prepare maps from Mariner 71 photography, stereo -plotters and computers for analytical cartography, as well as more experienced cartographers, must be brought in. The photogeologist noted that these problems would be discussed with the JPL Mariner people later in the month. But at Carl Sagan's request, these issues were raised that afternoon at a joint session with the Science Steering Group. The advisory body agreed that modest expenditures of Viking funds would be justified if supporting Mariner 71 data processing would contribute to the success of Viking. Masursky would prepare a letter to Jim Martin that clearly defined items that needed support and justifications for using Viking funds. 18
 

* Longitude on Mars is always determined in a westerly direction, 0-360°. For more on Martin place names, see T.L. Macdonald, "The Origins of Martian Nomenclature," Icarus 15 (1971); 233-40.
 
** C. Snyder, T. Mutch, D. Anderson, W. Baum, A. Binder, B. Farmer, R. Hutton, J. Lederberg, H. Moore, T. Owen, R. Scott, J. Shaw, and R. Shorthill.