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THE SCIENCE AND TECHNOLOGY BEHIND APOLLO IN RETROGRADE
Rosemary Claire Smith
Spoiler alert: You may wish to read Apollo in Retrograde before turning to this article.
Orbital mechanics, astrodynamics, and gigantic rocket engines get an out-sized share of the glory when it comes to the scientific and technological underpinnings of a story that re-imagines the Apollo 17 Moon landing. A great deal has been written about the physics and chemistry of space exploration, pitched to readers whose comprehension ranges from rudimentary to highly proficient. These are, however, by no means the only sciences, or even the most critical ones, at work in Apollo in Retrograde. Geology and geography played crucial roles. While I find them downright exciting, I can see your skeptical glances my way. Stay with me.
Geography: The Big Picture Is Not Enough
Most of us live where the geology tends to be covered up and our vistas are limited. Much or all our daily travel is on roadways by motorized means. It is all too easy to take hills, valleys, lakes, rivers, coastlines, and other natural features for granted when we can rely on buildings, road signs, cell phone apps, and automobile navigation systems to find our way around and reach new destinations. To be sure, people have always explored. We tend to forget, however, that our ancestors—the first ones to enter uninhabited regions of the Earth—could usually follow coastlines or travel along navigable rivers and animal trails.
On the Moon, none of these exist. There is only what Buzz Aldrin described as a “magnificent desolation.” It is a world of lifeless, airless, trackless craters, hills, valleys, and boulders, in mostly black and white. The absence of trees, buildings, and familiar landmarks make it tricky to guesstimate distances. The foreshortened horizon does not help matters.
Even before the Apollo years, NASA’s solution was to call in the geographers and cartographers from the U.S. Army TOPOCOM.FN1 In 1964, the cameras on Ranger 7 transmitted thousands of photographs before it crashed into the Moon. Over the years, map-makers worked to transform photo mosaics into usable maps, albeit with a one-meter resolution for the high-resolution camera and an eight-meter resolution for the medium-resolution camera. This may sound more reassuring than it turned out to be. When the Sea of Tranquility was selected for the first crewed Moon landing, the intended target area was believed to be reasonably flat and level. As Apollo 11’s lunar module descended, the astronauts could see that the site lay inside a small crater strewn with large boulders. With the LM running very low on fuel, Neil Armstrong manually guided their craft to a safe site about six kilometers away. That is the reason CAPCOM’s response to Armstrong’s famous words, “the Eagle has landed,” was: “Roger Tranquility. We copy you on the ground. You got a bunch of guys about to turn blue. We’re breathing again.”
What a difference three years made. By the end of 1972, when Apollo 17 became the last of the six Apollo missions to complete a landing on the Moon, NASA could rely on improved maps and was ready to select a more challenging landing site. The Taurus-Littrow valley was chosen. The upland area at the targeted landing point is relatively smooth. From there, the astronauts embarked on traverses to collect samples of the lunar highland crust, ancient lava, boulders that may have rolled down from the mountains, and ejecta from meteorite impacts. These locations, which were a few miles from the landing site, delivered on the promise of geological riches and then some. On their second and longest EVA, Gene Cernan hammered off a hunk of dark breccia containing white clast. It later proved to be 4.6 billion years old, rendering it the oldest rock thus far returned from the Moon. Not to be outdone, Harrison Schmidt found and collected orange soil, a stunning surprise in a monochrome environment. Subsequent analysis showed it to be the product of a volcanic eruption.
In contrast to actual events, the alternate timeline of Apollo in Retrograde hinges upon a crucial moment when NASA agrees to send an astronaut out from the LM to save a cosmonaut in trouble at a permanent Soviet base. This occurs soon after Apollo 17 landed, thereby supplanting all efforts to set up scientific equipment or to collect rock and soil samples. Consequently, a different pair of astronauts have no time to make Cernan and Schmidt’s marvelous discoveries. In fact, the traverse down a rille into Le Monnier Crater doubtless meant passing through or near wondrous geological formations that can reveal so much. However, time constraints do not permit any stops other than those that are strictly necessary. If only it were otherwise. I console myself with the reminder that in the alternate timeline, the 4.6-billion-year-old rock and the orange volcanic soil still await the next geologists to pay a visit to the Moon.
Rocks and Regolith Are Not Our Friends
The dangers of landing in a boulder field, as nearly happened to Apollo 11, are readily apparent. The hazards presented by the regolith beneath an astronaut’s boots are less obvious. Indeed, for several reasons, familiarity with Earth soil and sand does little to prepare one for dealing with the challenges presented by lunar regolith. First, this material consists of a very fine mixture of basalt fragments, breccias, agglutinates, and tiny glass particles. Its most common elements are oxygen, silicon, iron, calcium, aluminum, and magnesium. Unlike the sand and soil on Earth, lunar regolith has not been subject to much abrasion after the meteorite impacts that created it. Astronauts have reported that it tends to have the consistency of powdery snow if snow were dark and needle-sharp. Regolith not only sticks to spacesuits, visors, and equipment, but it absorbs heat. Thus, it can play havoc with machinery such as radiators, as well as a spacesuit’s neck ring and glove joints. Look closely at photos of astronauts on the surface of the Moon or inside a LM. Do you see dark smudges on what was once white equipment and clothing? That means regolith found its way inside the landers and into the astronauts’ eyes and lungs.
Getting Around: Moon Buggies to the Rescue
A total of six astronauts set foot on the Moon over the course of Apollo 11, 12, and 14. Their explorations of the lunar surface were circumscribed by the length of time it took them to trudge to nearby locations while wearing cumbersome spacesuits, boots, and a Portable Life Support System, which took their toll on the astronauts’ physical stamina. Their ability to collect geological samples was constrained by the need to haul the rocks and soil back to the LMs in wheeled hand-carts. In contrast, the commanders of Apollo 15, 16, and 17 drove Lunar Roving Vehicles (LRVs) to preselected collection stations. Not only did these Moon buggies reignite public interest in the later Moon missions, but they were also tremendous boons to human understanding of the formation of the Moon, and the Earth as well. This chart illustrates the total distances traveled on the Moon’s surface and the quantities of material obtained for each of the six Apollo missions. Note that LRVs were used for the last three missions, but not the previous ones. Nearly 75% of the materials retrieved and 95% of the total distance traveled across the lunar surface was made possible using motorized transport.
| Mission | Combined Surface Distance Traveled for All EVAs | Materials Collected
(Earth Weight) |
| Apollo 11 | 300 feet approx. | 47-48 lbs. |
| Apollo 12 | Just under 1 mile | 70 lbs. |
| Apollo 14 | 1.7 miles | 94 lbs. |
| Apollo 15/LRV-1 | 17.5 miles | 170+ lbs. |
| Apollo 16/LRV-2 | 16.6 miles | 209 lbs. |
| Apollo 17/LRV-3 | 19 miles | 243 lbs. |
Although, the LRVs had about a 57-mile range, it is hardly surprising that NASA refused to permit the crews to drive the rovers any farther away from the LMs than they could walk back to the base, should the need arise. After all, these trips presented the ultimate challenge in off-road driving across uncharted and unforgiving territory. Hence, none of the actual EVAs took the landing crews more than five miles from the LM. Even so, driving had several obvious advantages over walking. Longer distances were covered in a lot less time, thereby increasing the time available for locating and hammering rocks, taking core samples, and picking up surface materials. Importantly, the astronauts could reach the collection stations while they were still relatively fresh. Comparing the total distances traveled and materials obtained by each of the teams for Apollo 11, 12 and 14 with the comparable figures for the last three missions, which employed LRVs, illustrates how much of a difference the rovers made.FN 2
Powering the Moon Buggies
Every explorer knows that the success of the expedition depends upon the sturdiness of the vessel or vehicle used and the reliability of its power source. For obvious reasons, it was not feasible for the Moon buggies to rely on internal combustion engines and gasoline. Instead, a pair of silver oxide-zinc batteries, each containing 23 cells, produced the 36 volts needed to power most of the equipment on the LRVs. This technology had the advantage of storing a good deal of power in a relatively small volume. It did, however, necessitate protecting the batteries from temperature fluctuations. In the Taurus-Littrow area, the daytime high could reach 110 degrees C (230 F) and the nighttime low -171 degrees C (-275 F). However, the rovers’ batteries would not work above 57 degrees C (135 F) or below 4.5 degrees C (40 F).
Those batteries provided a good deal of pep. The astronauts needed it, especially to propel the LRV-3 up some steep terrain in the Taurus-Littrow region. In the one-sixth gravity, there were places where Gene Cernan and Harrison Schmitt felt more comfortable switch-backing upward instead of charging straight up. Descents presented their own challenges, particularly where the terrain undulated and the regolith got abruptly deeper or shallower or was apt to crumble. Even on relatively flat ground, it felt like driving over snow or sailing across a choppy sea. At times, it was not possible to keep all four wheels on the ground, even at low speeds. The rovers were not designed to exceed eight miles per hour. Cernan is thought to have set the speed record on a steep downhill, which is estimated to be somewhere around a whopping eleven miles per hour.FN 3
Where the Rubber Meets the Regolith
Hard rubber wheels were used on the hand carts dragged by the Apollo 14 astronauts. The LRVs, however, sported wide zinc-coated steel-mesh wheels mounted on solid aluminum rims. Photo 1. Titanium treads in a chevron pattern provided traction, not to mention the distinctive imagery seen in numerous photographs. This technology worked well on the Moon. When it came time to manufacture the Curiosity Rover to explore Mars, wide aluminum wheels were fabricated with chevron-patterned treads. After traveling roughly sixteen kilometers across Gale Crater, Curiosity’s wheels began to show dents, punctures, and tears. Photo 2 It is thought that this is due to metal fatigue and the presence of pointy rocks.
Here is another difference between the Mars rovers and the earlier LRVs: The Moon rovers have fenders but Curiosity and Perseverance do not. We take these humble wheel coverings for granted until something goes wrong. The fender extensions were thin fiberglass affairs whose function is to prevent fine lunar material beneath the tires from being kicked up onto the vehicle, the equipment, and the astronauts. The fenders do not obviate the need for periodic pit stops so the astronauts can brush the sticky, sharp regolith from themselves and their gear.
At one point in Apollo in Retrograde, the LRV’s wheels slip on loose debris, causing it to slew sideways and brush up against a boulder. The vehicle is not badly damaged, but part of the right rear fender gets torn off. The idea for this scene stemmed from a real mishap to LRV-3’s right rear fender, although the actual event came about somewhat differently than the one in the story. Apollo 17’s Gene Cernan walked too close to the fiberglass extension to the back fender and accidentally caught it with the handle of a hammer protruding from a pocket on his spacesuit. This was enough to tear it off. Driving with the broken fender quickly caused a spray of needle-sharp dust and fine debris to fly up like a rooster tail and coat the rover, both astronauts, and the delicate equipment on board. Weight restrictions meant that the LM carried no replacement part. NASA scrambled to come up with a way to fix the fender using nothing but the items the two astronauts had on hand. The solution was duct-taping several survey maps together, being careful not to get regolith on them. The maps were then bent into a curve and clamped to the intact portion of the fender. You can see the result in Photo 1.
Houston, Do You Read?
Twenty-first-century technology has improved the ability to stay in touch over vast distances on land, sea, and air to so great an extent that it is easy to forget how difficult communication was on the Moon fifty years ago. For one thing, there was no network of communications satellites racing overhead. Astronauts standing on the surface relied on VHF radio links to each other, as well as to the Lunar Communications Relay Unit (LCRU) attached to the front of the Moon buggy. The LCRU was connected to a small satellite dish, known as the parasol, which was mounted on the rover and pointed in the appropriate direction to send voice and data communications to Earth. From Mission Control, communications were relayed to the command module in lunar orbit. Drawing 1 shows how these transmissions worked.
What is missing from this picture? Somewhere beyond the Moon’s foreshortened horizon sits the lunar lander. Thus, the Moon’s rocky surface prevented communications the instant the LRV ventured too far for line-of-sight signals to the LM. Running transmissions through Mission Control introduced a barely-noticeable time lag of 1.2 seconds each way. Communications with the CM were necessarily intermittent given the forty-eight minutes it took to swing around the far side of the Moon. Sidenote: The time lag for the Earth to receive signals from the Perseverance and Curiosity rovers on Mars varies from five to twenty minutes depending on the proximity of the planets to each other. The Ingenuity helicopter on Mars experienced a planned two-month communications hiatus (April 26-June 28, 2023) with the Perseverance rover while exploring rugged terrain in Jezero Crater.
Communications constraints became more significant for the alternate Apollo in Retrograde crew. For much of the time, the two astronauts situated on the Moon were not able to hold a real-time conversation with the command pilot in the orbiter. These limitations were exacerbated when Commander Natalya Orlova made the decision to leave the LRV at the Soviet base camp. Although she could have taken the portable LCRU with her, it did not work with Soviet communications equipment. Thus, she could not communicate with her fellow astronaut at the American base until they came within sight of each other.
Moon Buggies, Soviet Style
Both the United States and the U.S.S.R. overcame many obstacles inherent in operating vehicles on the Moon, but in different ways. NASA opted for lighter LRVs, which the astronauts drove. These first extra-terrestrial “automobiles” informed subsequent design decisions over the years, culminating in the rovers traversing the surface of Mars today. The Soviets successfully deployed two heavier fully-automated Lunokhod rovers to the Moon. In November, 1970, Luna 17 soft-landed in Mare Imbrium, where it deposited the Lunokhod 1 rover. Over the course of the following ten months, that rover covered about 6.5 miles, taking 20,000 photographs. In 1973, the Soviet Union successfully landed Luna 21 in Le Monnier Crater, bearing Lunokhod 2. Over the next three months, Lunokhod 2 transmitted over 80,000 photos to Earth.FN 4.
Comparing the real-life Lunokhods with NASA’s Moon buggies shows several similarities, such as the basic design of the wire-mesh wheels and tread patterns. Not surprisingly, there are a lot of differences due to the fact that the Soviet Union never did land cosmonauts on the Moon, much less send them off to traverse its surface in a rover. Hence, the Lunokhods lacked seats to accommodate cosmonauts as well as life-support systems and a mechanism for them to drive. In contrast to American Moon buggies, which are sometimes compared to golf carts, the Soviet Lunokhods rather resemble metal wash tubs perched on eight wheels. Photo 3. Nevertheless, it seems to me that in an alternate timeline, Lunokhods 1 and 2 could have served as prototypes to develop subsequent cosmonaut-driven vehicles. This would have been more practical than relying on automated instructions from a Soyuz capsule in orbit around the Moon or an Earth-based control center in Russia.
On Earth, Lunokhods 1 and 2 weighed 1667 pounds and 4145 pounds, respectively. By way of comparison, Apollo 17’s LRV-3 came in at 462 pounds, thereby exceeding the 400 pounds in NASA’s original specifications. Because it was bulkier and up to ten times as heavy than the American Moon buggies, a drivable Lunokhod may well have been sturdier. The additional weight would have made it less prone to bouncing off the surface, which could have made the Lunokhod easier to operate. However, this advantage might have been offset if the increase in weight and momentum meant the steering were to feel less responsive. For someone accustomed to driving a golf cart, it may have felt like operating a minivan for the first time.
For those of you contemplating what it may be like to navigate a Moon buggy or Lunokhod across a lunar crater, there is an intriguing place to stare at Twentieth Century space technology, one that’s hundreds of thousands of miles closer to home. The Cosmosphere in Hutchinson Kansas displays an impressive collection of artifacts from the U.S.S.R. that were either launched into space or could have been. Where else can you scrutinize a flight-ready backup of Sputnik 1 or prototypes for the Vostok and Voskhod space capsules dating to the mid-1960s? The Cosmosphere also has a slew of materials from NASA’s earliest days and before. The show-stopper is the full-sized mock-up of Apollo-Soyuz joined together as they were back in 1975 when they orbited the Earth.
Back to the Moon
For those bemoaning the long hiatus since the Apollo Moon landings and not-so-patiently awaiting a possible polar expedition, here are a couple of things to keep in mind: the last Soviet Union mission to the Moon was Luna 24. That successful sample return took place back in 1976. Remember, too, all the vehicles that traversed those inhospitable lunar craters are still there. I cannot help but wonder if a future astronaut might one day slip into the driver’s seat of a Moon buggy. Assuming the vehicle will not have been impacted—I mean this literally—by that point, could someone metaphorically turn the key in the ignition and drive off to explore more of that barely-charted landscape? Perhaps the biggest impediment will be dead batteries. Then again, communication and navigational issues can easily crop up. Putting such considerations aside, how delightful is it to envision that an astronaut who was not born when people last left boot prints in the regolith might one day go bouncing across the craters in the lunar equivalent of a Model T? Let us hope they mind the fenders.
What geological wonders will the next generation of explorers discover on the Moon? I cannot say, but I do know this much: it will be astounding.
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