Exploring Lunar Lava Tubes: An Idea
As the rate of advancements in the field of space begins to increase once again, we’re nearing another age of lunar exploration. Most notably, NASA is aiming to establish a human presence on the Moon within the next ten years through the Artemis Missions. Both unmanned and manned spacecraft will be sent to the Moon, with humans on lunar soil by 2024.
These missions will bring about a new age of space exploration, allowing us to fully study and possibly colonize the Moon. One of the many regions of the Moon that we will explore will be lunar lava tubes, which are geological structures that formed on the Moon millions and billions of years ago.
Lunar Lava Tubes
For decades, NASA scientists have identified multiple pits along the Moon’s surface, and after careful analysis, have determined that they are likely lava tubes.
These tubes formed when the Moon was still geologically active and lava flowed. As lava was flowing, basaltic lava around the sides, bottom, and the top of lava channels would chill and crystallize, producing a rock-encased structure known as a lava tube. The surrounding crust then remained relatively hot and insulated the interior lava, preventing it from cooling. Once the interior lava stopped flowing, the channels drained, leaving behind an empty tube.
The largest lava tubes on Earth are about 40 meters in diameter, which is about the size of a large motorway tunnel. However, due to the lack of gravity on the Moon, lunar lava tubes are estimated to be about 500–900 meters in diameter. To put that into perspective, the Empire State Building (381 meters) and the Burj Khalifa (828 meters) could easily fit into one of these tubes.
Tubes of this immense size are extremely appealing to scientists looking to colonize the Moon. Settlements or storage areas could be established in these tubes, which would prevent the need to create settlements on the surface. This could save a lot of time and money (due to the high costs of launching materials into space).
Lunar lava tubes could also protect astronauts from external dangers such as meteors, extreme temperatures, and solar radiation. Unlike the Earth, the Moon lacks an atmosphere. This means that incoming debris won’t burn up, heat cannot be trapped, and there is no shielding from the Sun’s radiation. Compared to existing technologies used to protect astronauts from these dangers, living in lunar lava tubes would be far more effective.
These lava tubes would also be very useful for in-situ resource utilization efforts. In-situ resource utilization is using resources on the Moon to sustain a colony, rather than sending resources from the Earth. In lava tubes near the poles of the Moon, there are likely massive ice formations. These could be mined and turned into water much easier than excavating water from large amounts of icy regolith.
Current Exploration Efforts
Lunar lava tubes may be extremely advantageous as humans begin to establish a presence on the Moon. However, scientists have only gathered satellite imagery of skylights into these tubes. No technology has directly examined the inside of these tubes.
Many missions aiming to explore the lunar lava tubes are through the use of small autonomous rovers. These rovers would be deployed by a lander and connected to a probe that can collect data from the rovers as they are exploring the tubes.
You can read more about a mission like this from ESA here.
The primary disadvantage to using wheeled autonomous rovers is that they are generally limited to a smooth terrain. Yet, it is highly likely that the entryways to the lava tubes (skylights) are filled with collapsed rubble. It’s also unlikely that the surface and environment of the lava tubes are smooth, due to how lava cools. Both of these factors could prevent the wheeled rovers from properly examining the tubes and collecting data.
Wheeled rovers could also be particularly difficult to deploy since they are entering the tubes through a natural opening rather than rolling into them. These opening may not be structurally sound either, since they have remained untouched and near regions of meteor bombardment for millions or billions of years.
Many of these disadvantages and difficulties could be avoided through the use of soft robots.
Soft robotics is a subfield of robotics that, as its name implies, creates robots out of soft and flexible materials, rather than rigid ones. The use of highly flexible materials allows for the robots to perform movements that traditional robots cannot replicate.
Generally, rigid robots have difficulty working in unpredictable and complex environments (the environment of a lava tube). They can also be dangerous to humans (if the astronauts interact directly with the rovers), and damage to rigid robots can be fatal to their operation.
Previous attempts to achieve both adaptability and robustness in rigid robots have often been inspired by biped and quadruped animals, like Honda’s Asimo or Boston Dynamic’s Big Dog. However, they require a high power input and highly sophisticated controls. They can also be dangerous since they operate with powerful actuators and stiff materials.
Soft robots, on the other hand, are extremely compliant and produce self-adapting structures that decrease the demands on environmental sensing. For instance, soft-bodied organisms can conform to a complex 3D substrate without knowing its exact geometry (think of an octopus). Additionally, soft robots are much safer to handle, since they can dissipate forces and minimize risk or damage through sudden impacts.
Soft robots are gravity independent since they have a much lower mass density than traditional robots. They can be constructed to operate in any orientation or in low gravity situations, such as on the Moon. Think of how many soft-bodied animals can use the same strategy to crawl upside down, vertically, or even underwater.
Finally, soft robots are light and morphable, which is very valuable when it comes to space travel. The cost of space travel puts weight and volume limitations on the amount of payload that can be launched. Since soft robots are lighter and morphable, both space and money are saved.
To explore lunar lava tubes, we want robots that can adapt easily to the environment, don’t require a lot of energy input, are light, and can explore the caves quickly.
- Robots need to be able to adapt easily to their environment because it is unlikely that we will know the exact characteristics of the landscape being navigated, and rubble could collapse.
- The more energy our robot requires, the more complex its system will be. Thus, its ideal to create a robot that requires less energy input. This also allows the robots to continuously explore the lava tubes for a longer period of time, since constantly moving back to a base or to the surface to replenish energy is inefficient.
- Launching payloads into space is extremely expensive. The cheapest launch prices at present are still about $3000/kg. Therefore, it’s ideal that the robots are light to save mission costs.
- Depending on what operating systems the soft robots use, it may be crucial that they complete their exploration within 1 lunar day (14 Earth days). This is because after the lunar day ends, the temperatures become extremely cold (the Moon has no atmosphere to trap heat), which could prevent any technology from operating and astronauts from retrieving the technology.
As such, I propose a concept to explore lunar lava tubes — myriapod-inspired soft robots.
The Idea — Myriapod-Inspired Soft Robots
I propose that we use small, myriapod-inspired soft robots to explore lunar lava tubes. Myriapods are a subphylum of arthropods that includes centipedes, millipedes, and related organisms. They most notably share elongated bodies with multiple leg-bearing segments. Having multiple segments allows myriapods to move efficiently in many directions.
Some myriapods can move very quickly, such as centipedes. Centipedes have flattened bodies that allow them to travel quickly and in between crevices, as well as across very complex terrains. In fact, a study from the City University of Hong Kong determined that myriapod robot designs have yielded advanced adaptivity to harsh environments with incredibly fast locomotion speed (>40 limb length/s), ultra-strong carrying abilities(>100 times its own weight), and excellent obstacle-crossing abilities (standing 90° and across obstacles >10 times the body height).
Watch the following videos to see how myriapods move.
Many myriapods can also climb up and even upsidedown. If you’ve ever encountered a house centipede, you would know.
These basic characteristics make myriapod-inspired robots ideal for exploration. The robots would be flexible enough to traverse the complex terrain of lunar lava tubes and could even explore it in different orientations. They could also explore it within a lunar day, since they can travel fairly quickly.
One of the most difficult parts of creating a soft robot is creating a soft actuator. An actuator is just something that allows a robot to move. Ideally, a soft actuator should be similar to a muscle, deformable, and powered locally.
Currently, common synthetic soft actuators have major limitations. The most deformable actuators are chemically reactive gels and electroactive polymers. However, it is difficult to interface these polymers with other materials or conduct to surfaces necessary for their activation. Some polymers must also be kept wet, while others require extremely high electrical fields.
However, there are many possible alternative actuators that can be used in these myriapod robots. Compliant actuators, such as pneumatic pistons, hydraulics, or inflating elastic compartments are popular. They require the ability to pressurize and store gas or liquids using rigid materials and off-board motors. Obviously, the major drawback to this type of actuator is the off-board motor, although Harvard has developed a pneumatic soft robot with an onboard motor.
Another alternative is to use the crystalline transition properties of shape memory alloys. For example, SMA wires are normally rigid, but when drawn to a diameter of fewer than 200 microns and tightly coiled, they are as soft as fabrics and able to exert strains of 100%. The main drawback to this type of actuator is that they are temperature-dependent (usually they rely on heat), which can be difficult to work within the cold environment of lunar lava tubes.
Currently, most soft robots are made of silicone. However, as scientists discover more about the properties of lunar lava tubes, it may not be possible to use silicone as the primary body material of our robots.
As with many lava tubes on Earth, the lava in lunar lava tubes may have cooled into basalt, an extremely sharp volcanic rock. You may know basalt as the tool you can buy from the gardening store to use as edging. If you know, then you’d also understand that you can cut yourself if you mishandle the tool.
That rock has already been tumbled around and dulled. If lunar lava tubes have basalt, then it will not have been weathered at all, since the Moon doesn’t have water or wind. This extremely sharp rock could easily cut through the silicone bodies of our robots.
One possible solution could be to implement a self-healing fabric coat over the robot. The field of self-healing material has already had major developments and is being applied to commercial products.
If you’d like to learn more about self-healing fabrics, navigate here.
Examining and studying lunar lava tubes will be difficult, but gaining that data could be very valuable if we want to build bases in them. Myriapod-inspired soft robots could quickly explore the tubes with their speed and adaptability. They’ll be light too, which means more can be deployed to explore the tubes. Compared to wheeled autonomous rovers, they could study these tubes far more efficiently.
It may just be an idea, but it’s not out of the realm.
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