For more than half a century, planetary mobility has suffered from a terrible lack of imagination. We have placed machines on other worlds and, with very few exceptions, asked them to do one of two things: sit still or roll. Sometimes they sat still heroically. Sometimes they rolled heroically. Occasionally, after landing, they bounced around inside airbags like expensive scientific popcorn, which is not really locomotion so much as a controlled embarrassment. Spirit and Opportunity used airbags for landing; Ingenuity later demonstrated powered flight on Mars; but the workhorse of planetary exploration remains the wheel.
This is not because engineers lack imagination. It is because Mars is an accountant with a murder weapon.
Every gram must justify itself. Every motor is a liability. Every joint is a future failure report. Every clever mechanism has to survive launch vibration, cruise, radiation, dust, thermal cycling, landing, months of idleness, and then perform perfectly while being debugged from another planet at the speed of regret. Under those conditions the wheel starts to look less primitive and more like a monk: simple, patient, austere, and annoyingly correct.
Still, the wheel is not having an easy life. Curiosity’s aluminium wheels have been punctured and torn by sharp Martian rocks, forcing engineers to adapt routes and driving strategies. The rover survived, because it was built with large safety margins and because Martian engineers have the emotional temperament of medieval watchmakers. But the lesson is clear: even the humble wheel is not a solved problem on Mars. NASA’s own lessons-learned material notes that Curiosity’s wheel design proved susceptible to puncture on some terrain.
Which brings us to the sandfish lizard.
A recent New Atlas piece describes an experimental Mars rover wheel inspired by Scincus scincus, a small desert lizard that “swims” through sand by undulating its body. Researchers at the University of Würzburg and the University of Bremen built wheels that still roll but also wiggle, imitating that sideways swimming motion. Earlier versions sank and slipped, but after making the wheels lighter and wider, the prototype reportedly outperformed similar vehicles with conventional wheels in loose sand. The project belongs to the German Space Agency’s VaMEx programme, which explores swarms of driving, walking, and flying robots for Valles Marineris.
This is exactly the sort of idea that looks ridiculous until one remembers that Mars is mostly ridiculous terrain.
On Earth we know what to do with soft ground: use tracks. Tanks, bulldozers, excavators, snow vehicles, agricultural machines. The track spreads weight over a larger area, reduces ground pressure, and gives excellent traction. So why not send a small Martian Panzer?
Because tracks are wheels that have attended too many meetings.
A tracked vehicle needs multiple rollers, idlers, tensioning systems, drive sprockets, flexible belts or linked plates, and a tolerance stack that remains friendly after years of dust exposure. Tracks are superb when somebody can hose them down, tighten them, replace a pin, swear at them, and call a mechanic. On Mars, a thrown track is not a maintenance issue. It is a geological monument.
Tracks may make sense for a heavy lunar construction vehicle, a bulldozer near a crewed base, or some future mining machine with human maintenance nearby. For a lone scientific rover expected to crawl for years across unknown terrain, tracks carry a frightening number of failure modes. Wheels can be individually driven, individually steered, partly damaged, and sometimes sacrificed. A six-wheeled rocker-bogie rover can limp. A tracked rover with one jammed belt may become a very expensive coffee table.
Legs are more tempting, especially now that Earth is full of quadruped robots trotting over stairs, rubble, mud, and conference stages. A four-legged Mars robot could step over rocks, choose footholds, crouch for inspection, climb slopes, and look wonderfully alive. It could also fall over.
This is the central problem with legs: they convert terrain into computation. A wheel asks the ground a simple question: “May I roll here?” A leg asks a questionnaire. Where is the next foothold? Is it stable? Is that dust a crust over a void? Is the slope too high? Is the leg actuator warm enough? What is my body attitude? How do I recover if one foot slips? How do I stand up if I fall?
Legged robots are mechanically and computationally impressive, but their advantage is not universal. They shine in discontinuous terrain: boulder fields, steps, collapsed structures, caves, cliffs, lava tubes. They are less attractive on long traverses over mostly continuous ground, where wheels are usually more energy-efficient and mechanically simpler. A rover crossing kilometres of plains does not need the dignity of a goat. It needs the metabolism of a bicycle.
Then there is hopping.
Hoppers are seductive for low-gravity bodies. On the Moon, asteroids, comets, or small icy moons, a hop can cover terrain that wheels cannot handle. But hopping is navigation by apology. You launch, wait, land, hope you did not hit something unfortunate, then reorient. On Mars the gravity is high enough and the atmosphere annoying enough that hopping loses some charm. It may still work for small scouts, but it is a poor default for a laboratory that weighs hundreds of kilograms and contains instruments that dislike being treated like luggage.
Flying is now real. Ingenuity proved that powered, controlled flight on Mars is possible, flying 72 times before its final flight in January 2024. That is no longer science fiction. But flight on Mars is energetically expensive because the atmosphere is thin. Rotorcraft are excellent scouts: fast, high vantage point, able to cross terrain that would trap a rover. They are not yet replacements for large rovers carrying heavy instruments, drills, caches, arms, and power systems.
So what is the most efficient locomotion system?
Annoyingly, the answer is: wheels, until they are not.
On hard, relatively flat terrain, wheels win on energy, simplicity, reliability, and control. On sand, wide compliant wheels with grousers, active geometry, or even lizard-inspired wiggle may be best. On deep dunes, tracks or swimming wheels look attractive, but tracks pay in complexity. On rubble, cliffs, caves, and lava tubes, legs become serious. On small bodies, hopping may be the elegant answer. In atmospheres, flying scouts are invaluable. In mixed terrain, the best solution may not be one heroic machine but a small ecosystem: a rover as mothership, drones as scouts, tiny hoppers as disposable scouts, and perhaps a legged robot for the places where wheels become decorative.
And what about sending a folded Optimus robot to Mars?
As a publicity stunt: magnificent.
As an exploration system: dubious.
A humanoid robot is shaped for human environments: stairs, doors, handles, factories, kitchens, tools designed by primates with elbows. Mars has no door handles. Mars does not care about bipedal elegance. A humanoid brings too many joints, too high a centre of gravity, too many exposed mechanisms, and a body plan optimized for a planet full of furniture. Unless we first build a Martian IKEA, the humanoid is the wrong animal.
A folded Optimus might be useful in a future crewed base, where tools, habitats, connectors, and maintenance tasks are human-shaped. But for first exploration, the best robot is not the one that looks like us. It is the one that looks like the problem.
On Mars, the problem is sand, rock, dust, slopes, cold, distance, latency, mass, power, and boredom.
The perfect Martian explorer may therefore look less like a person and more like an argument between a bicycle, a lizard, a spider, and a helicopter. Which, now that one says it aloud, is probably why engineers are so fond of wheels. They are not glamorous. They do not wave. They do not do backflips on YouTube.
They just keep moving.
And on Mars, that is already close to magic.
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