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Lunar Excursions

In the mid-twentieth century, much science fiction focussed on the challenge of getting to the Moon. However it was commonly assumed that once there, settlement and utilisation of the Moon would follow naturally.

So another challenge had to be faced: how was one to cross the airless surface of Earth’s satellite?

Image of full moon. Source: wikimedia, Gregory H. Revera

Rovers and Buggies

Arguably the most common solution, particularly in the era of the Apollo programme, was the use of traditional ground vehicles in the form of wheeled or tracked rovers.

This was, for example, the approach of Herge’s Explorers on the Moon in which Tintin, Haddock, Calculus and others anticipated the advent of balloon-wheeled buggies to travel on the surface. Another example can be found in Murray Leinster’s City on the Moon (novel, 1957) - the third of Leinster’s novels featuring young engineer-adventurer Joe Kendrick. Here the settlers in the Moon’s first civilian city use custom-built “jeeps” with large wire-mesh wheels to traverse the rough surface:

 The vehicle moved among mountains, crawling on twenty-foot, spidery wheels over the fantastic terrain […]

Moon-jeeps were extremely exotic vehicles, developed from the straddle trucks of Earth for use in airless frigidity. Each of their four wheels revolved at the bottom of a stalk; each could be separately steered, and separately lifted over over obstacles. The tubelike cabin was raised some thirty feet off the surface; it contained an insulated cargo compartment and a vast assortment of apparatus. Crawling among senselessly unflung masses of stone, among craters and debris and the craters-wthin-craters of the moon, this jeep looked rather like a silvery stick-insect on wheels.

These robust vehicles carry larger cargo slung beneath their cabins, but can reach significant speeds and travel large distances over the lunar surface.

Ground vehicles were also the method of choice for Arthur C Clarke’s early lunar settlers in most of his short stories. In “Lest I forget thee, oh Earth” (short story, Clarke, 1951), a boy is taken to view the ruined home planet in a “balloon-wheeled” buggy. By contrast, the short stories Earthlight and Holiday on the Moon both feature caterpillar-tracked vehicles and both these stories are based in and around a lunar observatory, sited to make optimum use of the clear and dark skies and minimal atmosphere of the lunar surface.

In the first, “Earthlight” (short story, Clarke, 1951) astronomers from the observatory (which hosts a thousand inch diameter telescope) are asked to make a perilous drive across the surface in a lunar night illuminated only by Earth-light, in support of the opening skirmish in a conflict between Earth and its colonies in the Federation of Outer Planets. Their caterpillar-tracked vehicle is a small affair, carrying a usual complement of two, but able to accommodate a couple more for a matter of a few days.

By contrast “Holiday on the Moon” (short story, 1951, Heiress magazine, as by Charles Willis) is a coming of age story written by Clarke for a girls’ magazine under a pseudonym. In its narrative, an eighteen year old visits her father who is director of the lunar observatory, and is taken on a tour to the Moon’s second permanent base, which is on the dark side. The vehicle for this excursion is similar in design but rather larger:

“It still seemed strange to Daphne that the only means of transport on the Moon was something as old-fashioned as a motor-bus, but like many of the peculiar things she had met here it was reasonable enough when explained. Rockets were much too expensive for journeys of only a few hundred miles and as there was no atmosphere air transport was, of course, impossible.The big vehicle was really a sort of mobile hotel in which a couple of dozen people could live comfortably for a week or more. It was about forty feet long and mounted on two sets of caterpillar tractors, operated by electric motors. The driver had a little raised cabin at the front and the passenger compartment was fitted with comfortable seats that became bunks at night. At the back was a kitchen, storeroom and even a shower-bath.”

 The story concludes with Daphne realising that women could be interested in science and even become astronomers (!), and returning to Earth with broader horizons.

With the Apollo landings, lunar buggies were cemented in the public consciousness, and their extrapolation to much larger mobile laboratories or caravans seemed obvious - and indeed was in development by NASA itself. Many of these concepts had been around for years and were widely reported in articles aimed at the public and children. In the TV Century 21 Annual 1971, for example, a lengthy article describes the Apollo 17 lunar jeep (with wire-mesh wheels reminiscent of Leinster’s forecasting of the 1950s) and also concepts for a mobile laboratory, or Molab, that future - never undertaken - moon missions might deploy.

By this point, the use of crawling or rolling ground vehicles was sufficiently common to be no more than background world-building in many science fiction narratives through the 1970s, 80s, 90s and later. Stephen Baxter's Time (1999), for example, has lunar buses with large wire mesh tyres mentioned in passing and incidnetal to the story. However a notable more recent example in which the vehicle takes centre stage the 2009 film Moon (dir. Jones) in which the protagonist oversees lunar surface mining for Helium 3, using a giant caterpillar-tracked mobile factory, and smaller balloon-wheeled rovers to travel between this and his permanent base.

Still from Moon (2009) showing ground vehicles

 Certainly ground vehicles have the advantage of familiarity and are more efficient than rocket travel for short distance point-to-point travel. The recent improvements in battery technology and performance make electric vehicles more feasible (rather than the oxygen-hungry internal combustion technologies that still dominate on Earth). Given the surface roughness of the moon (where fractured rock is not eroded by weather) and the lack of pre-made roads in any hypothesised early colonisation era, Earth-normal rubber tyres are unlikely to be well suited to conditions. Hence logical extrapolations to the use of balloon-tyres (which provide more flexibility, extra suspension and allow larger obstacles to be overcome) or caterpillar tracks (which are already used to improve traction for vehicles crossing challenging landscapes on Earth). However these suggestions are not without their problems. A constant concern in the perilous midnight drive of Earthlight was the fear that the drivers would tumble into a deep crevasse in the lunar surface, and the impenetrably deep shadows cast by lunar hills even with the vivid reflected light of Earth by which to see. Given the rough and faulted volcanic surface of much of the lunar plains this is a serious peril for lunar drivers.

Lunar Railways 

While lunar rovers now have the advantage of practical demonstration, a number of authors have taken the analogies to Earth surface travel further and imagined the transport infrastructure at a slightly more advanced stage of lunar settlement. Here railways come into their own as a form of transport. Running along set rails they avoid (or, rather, must already have bridged) most of the perils of canyons, crevasses or unexpected rock outcrops in the shadows. In more advanced forms, they can even be sited underground, or in surface tubes, allowing their environs to be pressurised and providing some additional protection from radiation. On the other hand, they require a significant investment of time and resources to construct the required infrastructure, and only become logical if several sizeable settlements are developed at some distance from one another.

An example of lunar railway networks can be found in John M Ford’s Growing Up Weightless (novel, 1993), a coming-of-age story focussed on a boy born several generations after lunar independence. Together with a group of friends, he plans to take a lengthy rail trip of more than 3000 kilometres from a city at Copernicus crater to Tsiolkovsky observatory on the lunar far side and back, in order to go “cold” - off-grid and away from (largely imagined) parental surveillance. Much of the book is taken up first with planning and then with the trip itself, and we’re treated to a fairly detailed description of the extensive railway network and its technical construction:

Book cover of Growing Up Weightless. Source: isfdb

“Where the ground was flat, the rails were laid on blocks of waterless concrete, tied together with thin glass rods. Obstacles had mostly been burned through with electron guns. Where there was a gap, or a ridge to climb, the track was supported on X-shaped trusses of the same structural glass as the rails. Occasionally there was a rainbow bridge, suspended from glass towers. Sometimes the track split into two parallel lines, so the trains could pass”
(SF Masterworks Kindle edition, pg 147)

It’s also notable that Ford (and hence his lunar transport authorities) has considered some of the practicalities and dangers of crossing the atmosphere-free surface. The train carriages are described as effectively self-contained space craft, complete with suit lockers and pressure curtains for emergencies. There are also structural measures taken against more extreme dangers, for instance:

“Fifty kilometers out there was a sort of tunnel: a tube of cretebonded regolith, almost a meter thick, surrounding the tracks. They were only a couple of hundred meters long, a sudden blackout for a quarter of a minute. They were flare shelters, long enough to park a train inside and ride out a sunstorm.”

 Indeed, many of the settlements described in the story are located underground in deep trenches, or in crater walls - presumably to protect against the radiation threat associated with space weather. Ultimately, Growing up Weightless is about the journey of the young protagonist through life, and the misunderstandings and assumptions which shape his world, but the narrative plays out in a journey across the landscape, and against a well-realised lunar colony infrastructure.

 

Other lunar railway networks appear in the short story “Counterpoint” (appearing in Nebula SF, Ian Wright, 1955) - in which a Soviet Federation infiltrator masquerading as an American spaceman reaches the Moon with a view to enabling a future invasion - and in the novel-length expansion of Earthlight by Arthur C Clarke (novel, 1955). In the latter, the caterpillar tracked vehicles of the short story are complemented by a more limited but established railway network between major settlements. The novel opens with an evocative journey across the lunar landscape on this monorail system, with the protagonist noting the huge construction task needed to wind the track around lunar landmarks, and considering the speed the monorail can reach so close to the ground in the airless and weather-free environment of the Moon (much higher than permitted by the air resistance of Earth’s atmosphere). More recently, in The Expanse (e.g. TV series, season 5, 2022; novel, Babylon’s Ashes by James S A Corey, 2016), an underground mass transit tube system links the major settlements (also sited underground) on an extensively-settled Luna. Of course, an earlier extensive underground tube capsule network was also described by Robert Heinlein in his classic 1966 novel The Moon is a Harsh Mistress. In this story of a fight for lunar independence, control of the tube network becomes an important factor in the insurrection of its residents against Earth authority.


Although the disparate railway examples mentioned here appear similar to those on Earth from the passenger point of view, the difference in environment must necessarily result in different technologies. As Clarke noted, lunar trains might travel faster than those on Earth. However a key aspect of ground-based railways which would need to be reconsidered is traction. Ground-based railway locomotives transfer the energy of their motors into forward motion through rotating wheels held against the ground by gravity and friction. With the much lower gravity of the lunar surface, friction (which exerts a force proportional to the weight of an object) is less effective. Hence a common suggestion is that alternate technologies, such as magnetic levitation monorails, which are hindered by friction on Earth’s surface might prove ideal in the airless, low-friction, low magnetic field environment of the Moon. Such ideas are taken seriously by space technologists and futurists. Indeed engineering firm Northrup Grumman have recently won a NASA and DARPA contract to develop concepts for a lunar railway between hypothetical future surface settlements. Of course other problems, including the erosive potential of the angular dust grains which coat the lunar surface, would have to be overcome.

 A more unusual suggestion, made in some science fiction, is that the Moon may prove to be an effective location for a so-called gravity train. This technology would involve tunnels bored deep through the interior of the satellite, such that a carriage could effectively fall toward the centre of the Moon, gaining speed, and then be slowed down again by the resistance of gravity as it rises again towards the surface. In theory, and in the absence of friction losses, this is a highly efficient form of transport. However that argument must be balanced against the colossal challenge of boring through the diameter (i.e. close to the centre) of a body such as the Moon. The Moon may have a solid core, rather than Earth’s molten mantle and core, but it would still be extremely hard, and require a tunnel thousands of kilometres long, with the debris from this tunnel hauled outwards against gravity as it grew.

Dust Cruising

A more unusual form of long-distance transport proposed for the Moon is one that treats a key property of the Moon - its regolith - as equivalent to a key property of the Earth - its precipitation. The regolith is a thick layer of dust, made up of stone pulverised by billions of years of asteroid impacts, thermal shock between the extremes of lunar day and night, and erosion by the solar wind. While the regolith is unlikely to be as thick as some early estimates (which feared the Apollo landers might sink entirely out of sight), the thick dust of the Moon, in which the Apollo astronauts left their footprints, does certainly exist over much of the surface, and may well drift to considerable depths in places. This, perhaps in combination with the ancient naming of the lunar “seas”, has led to speculation that dusty regions of the lunar surface might be better treated as fluid surfaces (i.e. analogous to oceans or snow-fields) rather than solid land.

Cover image for A Fall of Moondust. Source: isfdb

The best known example here is almost certainly another novel by Arthur C Clarke. A Fall of Moondust (novel, Clarke, 1961) describes the journey of a “dust cruiser” known as Selene, which sails the Sea of Thirst and is the pearl of the nascent lunar tourist industry.

“Nothing could move upon its treacherous surface, except the small two-man dust-skis - and Selene herself, an improbable combination of sledge and bus, not unlike the Sno-cats that had opened up the Antarctic a lifetime ago.” [...] “Selene was the very first of her line, created in the brains of a few engineers who had sat down at a table and asked themselves: ‘How do we build a vehicle that will skim over a sea of dust?’ Some of them, harking back to Ole Man River, had anted to make her a stern-wheeler, but the more efficient submerged fans had carried the day. As they drilled through the dust, driving her before them, they produced a wake like that of a high-speed mole, but it vanished within seconds, leaving the Sea unmarked by any sign of the boat’s passage.”

Unfortunately a tremor on the surface of the Moon causes Selene to sink under the dust surface, and - after initially being assumed lost - its passengers and crew must be rescued in a feat of scientific and technical innovation.

While Selene is the best developed ship of the lunar seas, the idea that dust might provide a low friction fluid surface has been used by others. The use of human-powered skiing on the lunar regolith was shown on the Apollo 17 mission (1972) by astronaut Harrison “Jack” Schmitt. He confirmed that he was able to move relatively easily by adopting a skiing motion to slide his feet (albeit in the absence of skis), despite the bulky spacesuit that restricted his movements. Admittedly he did so on the relatively flat and deep dust of a lunar sea, rather than in more mountainous or cratered regions. He has recently advocated training in cross-country skiing for astronauts returning to the moon, and suggested its adoption as a lunar sport. Such lunar dust-skiing (either human-powered or hauled by vehicles like water-skis) was proposed as a future recreational development for the moon at the end of A Fall of Moondust. It was also proposed as a practical method of transporting themselves and their belongings for individualist lunar mineral prospectors in Heinlein’s poignant short story Requiem. Less seriously, lunar skiing was memorably demonstrated by stop-motion animation characters Wallace and Gromit, on a Moon literally made of cheese, in their A Grand Day Out (animated short, Nick Park, 1989). Unfortunately, lunar regolith is far more abrasive than water-ice snow, and its practicality is yet to be fully demonstrated.

The Weird and the Wonderful

Of course, the imagination of writers hasn’t been limited to these more or less conventional forms of transportation. Explorers and prospectors in Robert Heinlein’s juvenile novel Space Family Stone (aka The Rolling Stones, 1952) use a balloon-wheeled vehicle but a human-powered one - a bicycle - for their excursions, and the Stone twins even attempt to export these devices from the Moon to Mars:

The solitary prospector, deprived of his traditional burro, found the bicycle an acceptable and reliable, if somewhat less congenial, substitute. A miner’s bike would have looked odd in the streets of Stockholm; over-sized wheels, doughnut sand tires, towing yoke and trailer, battery trickle-charger, two-way radio, saddle bags, and Geiger-counter mount made it not the vehicle for a spin in the park - but on Mars or the Moon it fitted its purpose the way a canoe fits a Canadian stream. (NEL UK edition, 1973, pg 46)

Despite this, the efforts of Castor and Pollux Stone to market their refurbished stock on Mars are not entirely successful.

 

Another form of personal vehicle, supported above the lunar dust surface by a form of antigravity and known as a volplane, was proposed by Ray Cummings in his pulp novel Blood of the Moon (which first appeared Thrilling Wonder StoriesLink opens in a new window, 1936) [1], in which a young woman uses it to go in search of her father’s crashed spaceship. Indeed a footnote describes this vehicle in detail:

A Moon vehicle - a toboggan-shaped metal air sled, with low siderails equipped with gravity plates electronized in the fashion of a tiny space flyer. The vehicle is impractical, largely used for sport, the safe handling of it requiring extraordinary skill, the manipulator lying flat, working its controls, and maintaining balance largely by rapid shifting of the body from side to side.

While this footnote stresses the use by individuals, larger examples appear which carry rescue teams. It’s worth noting though that the mention of “electronized gravity plates” here puts this suggestion rather further into the realm of future-technology speculation than some others.

A similarly compact form of personal transport, but one a little more firmly rooted in known physics, has inspired the idea that explorers might use jet packs on the Moon. This was popular in the 1950s and 60s when jet-pack experimentation on Earth seemed like a plausible option for the future (as exemplified in cartoon The Jetsons, for example). The 1971 article in TV Century 21 already mentioned above showed pictures of individuals standing on rocket-lifted platforms, as part of a then current research programme at the NASA Langley facility. The article (and NASA’s PR department in the late 1960s) confidently predicted that moon explorers would make use of these FLEEPs (Flying Lunar Excursion Experimental Platforms) in future missions. As with so many other predictions for lunar life, this concept foundered as the Apollo programme was cancelled.

Moon Hopper from Captain Scarlet

Of course, some lunar vehicles are still more peculiar. One of the strangest ground transport concepts for the Moon appears in another television series from the same team - Captain Scarlet and the Mysterons (1967-1968). In the episodes “Lunarville 7” and “Crater 101”, the two lead characters travel to a lunar base, from which they are transported across the surface in a Moon Hopper - a craft that consists of two linked bubble-capsules, supported on insect-like legs. It propels itself by the somewhat unlikely procedure of storing potential energy in compressed springs in its legs. When this is released, the craft is propelled upwards and slightly forwards. Jet thrusters are used to adjust the path of its long hop before landing and again compressing the leg springs, ready for another jump. While this method is fairly energy efficient, requiring relatively little motive power, it hardly looks comfortable, even under one sixth gravity. It would also eventually need extra energy to compensate for losses, and it’s not clear how long the spring mechanism might last under constant compression and extension in the cold and dusty environment of the lunar surface.

Rocket-powered shuttlecraft are perhaps unlikely to be efficient for point-to-point travel on the surface (not least because their reaction mass would most likely still have to be lifted from Earth). They nonetheless commonly appear in science fictional narratives, such as Space:1999 (TV series, 1975-1977) which is famous for its seemingly-unlimited supply of Eagle shuttlecraft. Given the low gravity, and the large overhead required to construct the infrastructure for rail or road, point-to-point suborbital craft may well continue to be the option of choice for long distance travel on the Moon, while off-road buggies are a proven technology for local exploration. However with the ramping up of the Artemis programme and the not-too-far-off return of humanity to the Moon, lunar railways once again look like a plausible - if rather distant - future. Perhaps, one day, even fleeps, hoppers, volplanes and even lunar dust-skis could be common sights on the lunar surface - or perhaps the imagination of lunar engineers will exceed even that of science fiction writers in devising weirder technologies still.

“Lunar Excursions”, Elizabeth Stanway, Cosmic Stories blog, 30th June 2024.


Notes:

[1] The term volplane more correctly describes a steep unpowered glide in an aeroplane but has been coopted by Cummings for his vehicle. [Return to text]

The views and opinions expressed here are those of the author, and not necessarily those of the University of Warwick. Images are sourced online and used under fair-use provisions for commentary and criticism.