Made from a pair of two-wheeled vehicles, NASADuAxel is designed to descend crater sides and nearly vertical cliffs on the moon. Mars, and beyond.
A rover rolls over rocky terrain, its four metal wheels clattering along until they encounter a seemingly insurmountable danger: a steep slope. Below is a potential treasure trove of scientific goals. With a typical rover, operators would have to find another target, but this is DuAxel, a robot built for just such situations.
The rover actually consists of a pair of two-wheeled rovers, each called an axel. To divide and conquer, the rover stops, lowers its chassis, and anchors it to the ground before essentially splitting into two. When the rear half of the DuAxel (short for “Dual-Axel”) is firmly in place, the front half is undocked and rolls away on a single axle. All that now connects the two halves is a line that loosens as the front axle approaches the danger and rappels down the slope. Instruments stored in the wheel hub are used to investigate a scientifically attractive place that is normally inaccessible.
A flexible rover capable of traveling long distances as well as abseiling hard-to-reach areas of scientific interest has been field tested in California’s Mojave Desert to demonstrate its versatility. DuAxel consists of two Axel robots and is designed to explore crater walls, pits, drop-offs, openings and other extreme terrain on the moon, Mars and beyond. Photo credit: NASA /JPL-Caltech
That scenario played out during a field test in the Mojave Desert last fall when a small team of engineers at NASA’s Jet Propulsion Laboratory in Southern California challenged the modular rover to test the versatility of its design.
“DuAxel has performed excellently in this area, successfully demonstrating its ability to approach challenging terrain, anchor, and then undock its attached Axel rover,” said Issa Nesnas, robotics technologist at JPL. “Axel then autonomously maneuvered down steep and rocky slopes and deployed his instruments without the need for a robotic arm.”
The idea behind the development of two single-axle rovers that can be combined into one with a central payload is to maximize versatility: the four-wheel configuration is suitable for long distances in harsh landscapes. The two-wheeled version offers maneuverability that larger rovers cannot.
“DuAxel opens up access to more extreme terrain on planetary bodies like the moon, Mars, Mercury and possibly some icy worlds like Jupiter‘s moon Europe, ”added Nesnas.
The flexibility was built with a view of crater walls, pits, drop offs, vents and other extreme terrain in these different worlds. That’s because some of the best places on earth to study geology are in ledges and on rock faces, where many layers of the past are neatly exposed. They are hard enough to get here, let alone other celestial bodies.
The rover’s mobility and ability to access extreme locations is a compelling combination for Laura Kerber, a planetary geologist at JPL. “That’s why I find the Axel Rover very attractive,” she said. “Instead of always trying to protect yourself from hazards like falling or tipping over, it is designed to withstand them.”
A two-wheeled story
The radical concept of two robotic vehicles functioning as one has its roots in the late 1990s when NASA began exploring ideas for modular, reconfigurable, self-repairing rovers. This inspired Nesnas and his team at JPL to develop the rugged, flexible two-wheeled robot that would later become known as Axel.
They envisioned a modular system: for example, two axes could dock on both sides of a payload, or three axes could dock on two payloads, and so on, Creation of a “train” from Axels that can transport many payloads. This concept also fulfilled the “self-repair” requirement of the NASA challenge: If one Axel should fail, another could take its place.
Axel development focused on modular transportation until 2006 when satellite imaging of the Martian surface revealed canyons in crater walls. Later, the discovery of what appeared to be seasonal runoffs of liquid water – dark features known as recurring slope lines – increased interest in using robots to take samples. Scientists wanted to know if ravines and recurring slope lines were caused by water flows or something else.
However, the slopes are too steep for a conventional rover – even for Curiosity or the soon landing Perseverance rover, both of which are designed for slopes of up to 30 degrees. To explore these features directly, a different type of vehicle would be required.
So Nesnas and his team started developing a version of Axel that would be connected to a lander. Not only did they descend a crater side or a steep canyon wall, but electricity was also supplied and communicated with the lander. The wheels could be fitted with extra tall grousers or treads for added traction, while the wheel hubs could accommodate microscopes, drills, sample collection shovels, and other instruments for surveying the site. To turn, the two-wheeled axle would only turn one of its wheels faster than the other.
Interest in the flexibility of the concept has resulted in a growing family of two-wheeled designs, including A-BUFFER and BRUIE from NASA JPL, which expand the possibilities of exploration to new destinations and uses, including underwater on icy worlds.
Despite the versatility of the tethered Axels, there was one notable limitation associated with a stationary lander: the lander had to be within abseiling distance from the crater side – which requires a level of landing precision that may not be possible for a planetary mission.
To remove this requirement and increase mobility, the team reverted to the original modular design, adapted it to the new connected Axel, and named it DuAxel.
“The main benefit of using DuAxel becomes apparent when you have uncertainties about the landing site like we do on Mars, or when you want to move to a new location to abseil and explore with Axel,” said Patrick Mcgarey, robotics technologist at JPL and a DuAxel team member. “It allows for unrestricted driving from the landing pad and allows for temporary anchoring in the terrain, as it is essentially a transformation robot designed to explore the planet.”
While DuAxel remains a technology demonstration, waiting to be given a target, its team will keep improving its technology. That way, in due course, the robot would be ready to roll where other rovers are afraid to kick.