An Apollo Landing Near the Great Ray Crater Tycho (1969)

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.

Of the seven automated Surveyor spacecraft NASA launched to the Moon between May 1966 and January 1968, only the last, Surveyor 7, aimed for a target selected specifically for its scientific value. Surveyors 2 and 4 failed, while Surveyors 1, 3, 5, and 6 soft-landed at flat mare (basalt plain) sites in the "Apollo Zone," the near-equatorial band readily accessible to piloted Apollo Lunar Module (LM) spacecraft. The successful Apollo Zone Surveyors performed valuable scientific investigations, but their main purpose was to image their landing sites and test surface bearing strength to help assure mission planners that the lunar terrain was smooth and stable enough to permit Apollo astronauts to land safely.

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 ejecta blasted out when Tycho formed about 110 million years ago. As ejecta fell back onto the Moon, it stirred up more material, generating a ray cascade extending up to 1500 kilometers from Tycho.

Surveyor 3 (above) served as a pinpoint landing target for Apollo 12 astronauts Charles Conrad and Alan Bean in November 1969. During their second moonwalk, they stopped by the derelict lander to collect parts and take pictures for engineering analysis. Surveyor 7 resembled Surveyor 3, but included noticeable differences; most obvious was the addition of the deployable alpha-scattering instrument. Image Credit: NASA.

Hand-laid mosaic of images from Surveyor 7 illustrating the rocky, rolling nature of the terrain north of Tycho. Image credit: NASA/USGS.

Surveyor 7 lifted off from Cape Kennedy atop an Atlas-Centaur rocket on 7 January 1968. It landed on 10 January at 40.9° south latitude, 11.4° west longitude, just 2.5 kilometers from its intended target and 30 kilometers from Tycho's rim, on the ejecta blanket surrounding the crater.

Less than an hour after touchdown, the three-legged, solar-powered lander returned the first of more than 21,000 images it would beam to Earth. Some of these were stereo pairs, enabling scientists to precisely locate the many varied rocks and boulders visible in the field of view of Surveyor 7's scanning camera. Other images were assembled into panoramic mosaics that show lunar landscape features up to 13 kilometers away from the lander.

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.

Tycho, its ejecta blanket, and the Surveyor 7 landing site as imaged by NASA's Lunar Reconnaissance Orbiter (LRO). The spacecraft entered lunar polar orbit in June 2009. The ejecta surrounding the crater partly covers and "blurs" lunar surface features that existed before Tycho was formed. Image credit: NASA.

In keeping with its science-focused mission, Surveyor 7 carried more scientific apparatus than any of its predecessors. Besides its camera, the lander carried an alpha-scattering device for determining the composition of rocks and dirt and an arm-mounted digger. The former had flown previously on Surveyor 5 and Surveyor 6; the latter on Surveyor 3.

At first, the alpha-scattering device failed to deploy, but flight controllers were able to direct the digger to push it down into contact with the lunar surface. They later used the arm/digger to position the alpha-scatterer on a rock and in a trench the digger had excavated. They found that the surface material at Surveyor 7's highlands landing site contained more aluminum than did that at the mare sites the other Surveyors explored.

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.

Lunar Orbiter image of the Surveyor 7 landing area. The two dotted lines originating at the Surveyor 7 ("S.VII") touchdown point indicate the limits of the field of view of the lander's scanning camera. North is toward the top. Prominent in the right half of the image is a dark lake-like feature, the "shore" of which is located about a kilometer away from Surveyor 7. Image credit: NASA.

The lakes and the tantalizing variety of rocks near Surveyor 7 caused some lunar scientists to call for an Apollo mission to the site. It was far outside the Apollo Zone, but could be reached during certain times of year if conservative Apollo mission rules were relaxed.

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.

LRO image of the Surveyor 7 landing area. Please refer to the previous image for a scale bar. The arrow points to the derelict lander, which is just visible because of the shadow it casts on the surface. Technology advancement means that the image is sharper than the previous Lunar Orbiter image: individual boulders about the size of the lander are clearly seen, as are details of the lake-like melt "pond" and small impact craters. Image credit: NASA.

The Bellcomm/USGS team planned that, during Traverse I, one astronaut would deploy an Apollo Lunar Scientific Experiment Package (ALSEP) about 1.1 kilometers east of the LM. The ALSEP would include a passive seismometer. In addition to establishing a "far southern" station in the Apollo seismic network, the instrument would exploit natural moonquakes and asteroid impacts to chart Tycho's subsurface structure. The ALSEP might also include a heat-flow experiment to help scientists determine whether volcanism had occurred recently at the site, a laser retroreflector, a magnetometer, and a gravimeter.

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 the radio antennas on their LM as they walked. The LM would relay signals from their space suit radios, so they might 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. Though doing so would enable a landing in a well-characterized area, it would 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 lunar equator. Image credit: NASA.

During 1970, in the aftermath of the near-disastrous Apollo 13 mission, NASA engineers, mission planners, managers, and astronauts, never enthusiastic about the Tycho site proposal, rejected the region as too rugged for a safe Apollo landing. Some scientists were, however, not easily deterred: they continued to sing the site's praises as late as 1972.

They pointed to the fact that Surveyor 7 had successfully landed without the precise terminal guidance an astronaut would provide. They hoped that Apollo 16 or 17 might be diverted to Tycho. In the end, however, no Apollo mission visited Surveyor 7, leaving to it the honor of the highest-latitude/farthest-south landing site of any spacecraft that has soft-landed on the Moon.

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 close to older craters because the steady rain of micrometeoroids and small asteroids that strikes the Moon splinters them into dust and boulders and gradually renders them indistinct.

Sources

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.

More Information

"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