This Transforming Rover Can Explore the Toughest Terrain
Made of a pair of two-wheeled vehicles, NASA's DuAxel is designed to descend crater sides and near-vertical cliffs on the Moon, Mars, and beyond.
A rover trundles over rocky terrain, its four metal wheels
clattering along until they encounter a seemingly insurmountable hazard: a
steep slope. Down below is a potential trove of science targets. With a typical
rover, the operators would need to find another target, but this is DuAxel, a
robot built for situations exactly like this.
The rover is actually made of a pair of two-wheeled rovers,
each called
Axel.
To divide and conquer, the rover stops, lowers its chassis and anchors it to
the ground before essentially splitting in two. With the rear half of DuAxel
(short for "dual-Axel") firmly in place, the forward half undocks and
rolls away on a single axle. All that connects the two halves now is a tether
that unspools as the lead axle approaches the hazard and rappels down the slope,
using instruments stowed in its wheel hub to study a scientifically attractive location
that would normally be out of reach.
This scenario played out last fall during a field test in the
Mojave Desert, when a small team of engineers from NASA's Jet Propulsion
Laboratory in Southern California put the modular rover through a series of
challenges to test the versatility of its design.
"DuAxel performed extremely well in the field, successfully
demonstrating its ability to approach a challenging terrain, anchor, and then
undock its tethered Axel rover," said Issa Nesnas, a robotics technologist
at JPL. "Axel then autonomously maneuvered down steep and rocky slopes,
deploying its instruments without the necessity of a robotic arm."
The idea behind creating two single-axle rovers that can
combine into one with a central payload is to maximize versatility: The
four-wheeled configuration lends itself to driving great distances across rugged
landscapes; the two-wheeled version offers a nimbleness that larger rovers
cannot.
"DuAxel opens up access to more extreme terrain on
planetary bodies such as the Moon, Mars, Mercury, and possibly some icy worlds,
like Jupiter's moon Europa," added Nesnas.
The flexibility was built with crater walls, pits, scarps,
vents, and other extreme terrain on these diverse worlds in mind. That's
because on Earth, some of the best locations to study geology can be found in
rocky outcrops and on cliff faces, where many layers of the past are neatly
exposed. They're hard enough to reach here, let alone on other celestial bodies.
The rover's mobility and ability to access extreme locations
is an enticing combination to Laura Kerber, a planetary geologist at JPL.
"This is why I find the Axel rover to be quite delightful," she said.
"Instead of always trying to safeguard itself against dangers such as
falling or flipping over, it is designed to withstand them."
A
Two-Wheeled History
The radical concept of two robotic vehicles functioning as
one has 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 robust, flexible two-wheeled robot that would come to be known
as Axel.
They envisioned a modular system: Two Axels could dock to
either side of a payload, for example, or three Axels could dock to two
payloads, and so on, creating a "train"
of Axels capable of transporting many payloads. This concept also fulfilled the
"self-repairing" requirement of NASA's challenge: Should one Axel
fail, another could take its place.
Axel development remained focused on modular transportation
until 2006, when satellite imaging of the Martian surface revealed gullies in crater
walls. Later, the discovery of what appeared to be seasonal outflows of liquid
water - dark features known as recurring
slope lineae - heightened interest in using robots to take
samples. Scientists wanted to know whether gullies and recurring slope lineaewere
caused by water flows or something
else.
During warm seasons on Mars, dark streaks called "recurring slope lineae" often appear on crater slopes, as seen in this series of observations captured by the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter. The DuAxel rover is designed to rappel to such inaccessible areas to study them. Credit: NASA/JPL-Caltech/University of Arizona
› Larger view
But the slopes are too steep for a conventional rover - even
for Curiosity or
the soon-to-land Perseverance
rover, both of which are designed to traverse slopes of up to 30 degrees. To
explore these features directly would require a different kind of vehicle.
So Nesnas and his team began developing a version of Axel
that would be tethered to a lander, using the tether not just to descend a
crater side or steep canyon wall, but also to supply power and communicate with
the lander. Its wheels could be equipped with extra-high grousers, or treads,
for added traction, while the wheel hubs could house microscopes, drills,
sample-collection scoops, and other instrumentation to study the terrain. To turn,
the two-wheeled axle would just rotate one of its wheels faster than the other.
Interest in the concept's flexibility has led to a
burgeoning family of two-wheeled designs, including NASA JPL's A-PUFFER and BRUIE, which
extend the possibility of exploration to new destinations and applications,
including under water on icy worlds.
Despite the tethered Axel's versatility, there was a
notable limitation when used in conjunction with a stationary lander: The
lander would need to be within rappelling distance of the crater side - demanding
a degree of landing precision that may not be possible for a planetary mission.
To remove this requirement and boost mobility, the team
reverted to the original modular design, adapted it to the new tethered Axel,
and named it DuAxel.
"The key advantage of using DuAxel is made clear when
you have landing site uncertainty, such as we do on Mars, or you want to move
to a new location to rappel and explore with Axel," said Patrick Mcgarey,
a robotic technologist at JPL and DuAxel team member. "It enables
untethered driving from the landing site and allows for temporary anchoring to
the terrain because it is essentially a transforming robot made for planetary
exploration."
While DuAxel remains a technology demonstration and is waiting
to be assigned a destination, its team will continue honing its technology;
that way, when the time comes, the robot would be ready to roll where other
rovers fear to tread.
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