|Splat! Tycho crater (lower center) is the the most prominent bright surface feature in this NASA image of the full moon. Linear rays originating at the crater can be traced outward for hundreds of kilometers.|
Surveyor 7, by contrast, aimed for the rugged northern flank of Tycho crater, one of the most prominent features on the moon's Earth-facing nearside hemisphere. The 85-kilometer-wide asteroid impact scar, centered at 43° south latitude in heavily cratered highlands terrain, is surrounded by an extensive system of bright rays best viewed when the moon is full. The rays are made up of debris blasted out when Tycho formed about 110 million years ago. Some extend for 1500 kilometers across the moon's face.
|Hand-laid mosaic of images from Surveyor 7 illustrating the rocky, rolling nature of the terrain north of Tycho. Image credit: NASA/USGS|
Among the features most intriguing to lunar scientists were so-called "lakes" of relatively dark material. They lay in depressions and had relatively flat surfaces. Curving, branching trenches etched many of these small dark plains. Some scientists interpreted the lakes as signs of recent volcanic activity, the "holy grail" of 1960s lunar exploration.
Controllers were unable to place the alpha-scatterer in contact with boulders on a low ridge near Surveyor 7, some of which might have been blasted from kilometers below the lunar surface by the Tycho impact. They were far beyond the digger's 1.52-meter maximum reach. Nor were controllers able to move the instrument to the dark material of the lakes, the nearest of which lay about a kilometer from the lander. When the Surveyor 7 mission ended on 21 February 1968, much was known about its complex landing site, but much else remained mysterious.
In August 1969, less than a month after Apollo 11, the first piloted moon landing mission, U.S. Geological Survey (USGS) scientists worked with Bellcomm, NASA's Apollo planning contractor, to rough out the surface portion of an Apollo Tycho mission. It would begin with a pinpoint LM landing a kilometer southeast of Surveyor 7.
The pinpoint landing would be required if the astronauts were to follow the geologic traverse routes the Bellcomm/USGS team planned. The LM descent stage would carry enough propellants to enable the Tycho mission crew to at least partly compensate if their LM missed its designated touchdown point. This was deemed an especially important capability because the Apollo 11 LM Eagle had landed off course at the edge of its landing ellipse.
On the basis of Surveyor 7 and Lunar Orbiter V images, the Bellcomm/USGS team judged that the Tycho site was too rocky for a jeep-like lunar rover to navigate. They suggested that the astronauts explore on foot within an operational radius of about 2.5 kilometers centered on their LM. Proposed new "constant volume" hard suits tougher and more flexible than the mostly fabric Apollo suits would, they anticipated, make possible speedy hikes over rugged terrain. The new suits would also permit the astronauts to operate on the surface for up to seven hours at a stretch. They would spend 54 hours at the Tycho landing site, providing enough time for three seven-hour traverses.
The other astronaut, meanwhile, would walk along the low ridge visible from Surveyor 7 and sample the boulders there. The two moonwalkers would then meet up and return to the vicinity of the LM. Traverse I would total about 3.5 kilometers.
During Traverse II, at about 6.25 kilometers the longest of the Tycho mission moonwalks, the astronauts would strike north together to the "shore" of a prominent kilometer-wide dark lake. They would photograph and sample the branching trenches, then walk to a point 2.6 kilometers from their LM to sample "dark flow dome material." On the way back to the LM, they would visit Surveyor 7 to collect samples of lunar materials it had examined and salvage parts of the robot lander for engineering analysis.
The final traverse of the Apollo Tycho mission would see the astronauts walk south about 1.3 kilometers to sample another dark lake, then travel a further 1.4 kilometers to sample subsurface material exposed by a small fresh impact crater. They would then hike half a kilometer to a raised "flow levee" surrounded by "late smooth flow materials." Traverse III would total 5.25 kilometers. In all, the astronauts would walk 15 kilometers and collect between 100 and 200 pounds of samples during their three moonwalks.
The Bellcomm/USGS team acknowledged that the Tycho site presented challenges beyond its position outside the Apollo Zone. It was rugged and undulating, so the astronauts were likely to lose line-of-sight contact with their LM's radio antennas as they walked. The LM would relay signals from their space suit radios, so they would temporarily lose radio contact with Earth. In addition, the site had not been imaged from orbit at the same high resolution as other candidate Apollo sites.
The team suggested that, if no high-resolution orbital images of the site could be obtained and if this continued to be considered a major drawback, then the Apollo Tycho mission could land closer to Surveyor 7, where the surface had been well characterized. This would, however, create its own problems. The most serious of these would be to place much of the Traverse III loop beyond the planned 2.5-kilometer operational radius of the mission's moonwalks.
|This map of the landing sites of all the successful Surveyors shows how far south Surveyor VII landed. No other spacecraft has soft-landed so far from the moon's equator. Image credit: NASA|
The dark lake-like features observed near Tycho are known today to be patches of melt material that flowed and was thrown outward from Tycho during its explosive formation, not signs of recent volcanic activity. Impact melt flows are found inside and around many large young impact craters. Melt flow features are rare near older craters because the steady rain of micrometeoroids and small asteroids that strikes the moon splinters them into moon dust and boulders and gradually renders them indistinct.
Surveyor VII: A Preliminary Report, NASA SP-173, NASA Surveyor Program Office, May 1968
Surveyor Program Results, NASA SP-184, Surveyor Program, NASA, 1969
"Tycho - north rim," H. Masursky, G. Swann, D. Elston, and J. Slaybaugh, 14 August 1969 (revised 15 August 1969)
Memorandum, J. Slaybaugh to J. Llewellyn, "Tycho Rim Engineering Evaluation - Case 320," Bellcomm, Inc., 28 August 1969
To A Rocky Moon: A Geologists' History of Lunar Exploration, Don E. Wilhelms, The University of Arizona Press, 1993, pp. 242, 287, 312
"Essential Data": A 1963 Pitch to Expand NASA's Robotic Exploration Programs
If an Apollo Lunar Module Crashed on the Moon, Could NASA Investigate the Cause? (1967)
"A Continuing Aspect of Human Endeavor": Bellcomm's January 1968 Lunar Exploration Program