By Steve Bartlett
As any L.A. driver knows, getting around can be a hassle. Between the traffic, the freeways, the potholes on the roads, and the endless construction work, you never know just what you’ll be facing. Now imagine what it’s like trying to get around on the surface of the Moon or Mars or an asteroid, where not only are you dealing with rough terrain and a lack of roads, but you’re in a harsh environment with low gravity that puts your life at risk every time you go outside. That’s the challenge faced by NASA rover designers such as Matthew Frost, Chief Engineer and Task Manager at the Extreme Environments Robotics Group at the Jet Propulsion Laboratory. Frost gave a presentation to OASIS on “Moonbase Mobility: NASA’s All-Terrain Hex-Limbed Extra-Terrestrial Explorer (ATHLETE) Vehicle” on June 22 at the Long Beach Public Library.
Frost and his team were tasked with coming up with an innovative way to get crews and equipment to the surface of the Moon and to move them around once they were down. He explained that for most lunar and planetary human mission scenarios, the landing has to be within line-of-sight from any valuable assets, including habitats, exploration equipment, power plants, etc. The problem with this approach is that the landing thrusters tend to kick up a considerable amount of dust, dirt, and debris, which can travel for a great distance on low-gravity worlds. Oftentimes, this dirt and debris land on the assets. When the Apollo 12 lunar module touched down on the Moon in 1969, it came to rest 1,180 feet from the landing site of the Surveyor 3 probe. In the process, the lunar module’s thrusters threw large amounts of debris on the robotic craft.
But if the assets are mobile, Frost argued, the landing site can be in a safe zone several kilometers away and the assets and rover can then move to the landing site.
Additionally, NASA rules require that astronauts stay within walking distance of habitats, even when driving a lunar roving vehicle. (This is to avoid stranding astronauts in the event that their vehicle breaks down and they’re unable to repair it.) This restriction severely limits the amount of exploration and useful work that astronauts can do from a fixed site. “After a few weeks, they’ve done all they can do at that location and they’re left sitting around playing cards,” Frost said. But if the habitats are mobile, the entire surface of the world is opened for exploration. Astronauts can explore an interesting site for a few weeks and then pull up stakes and move to the next site.
So his team developed ATHLETE -a multi-purpose robotic and human/robotic exploration vehicle that can be tailored for lunar, Martian, asteroid, and Near-Earth Object missions.
The ATHLETE vehicle consists of six articulated legs attached to a central platform. At the end of each leg is a small, motorized wheel. When traveling over level, hard-packed ground, the ATHLETE rolls around on its wheels. The compliant wheels, developed by JPL and the Michelin Tire Company, allow them to roll over rocks of moderate size with ease. Active sensing and adaptive compliance in the legs allow the vehicle to travel over rough terrain, dirt, sand, and grades with a smooth ride that even the best SUVs cannot match. But when larger rocks, hills, or gullies are encountered, the ATHLETE can lift its legs and walk around the obstacle.
Frost provided a brief tutorial on why the combination of small, high-torque, low-speed wheels and compliant legs on the ATHLETE save considerable weight compared to the conventional wheel and suspension design on rovers currently in use (e.g., the Mars Exploration Rovers and the Curiosity Rover.) He showed the OASIS audience a video of a JPL crew testing a half-scale lander/ATHLETE/habitat in the Arizona desert near Flagstaff. In the video, the ATHLETE vehicle picked up and moved a habitat from one location to another while maneuvering around rocks, uneven ground, and small hills. The half-scale model was configured to fit inside the payload shroud of the proposed Ares V vehicle that NASA was developing for the Constellation lunar/Mars program.
The basic ATHLETE vehicle configuration has the six legs firmly attached to the central platform. But the JPL engineers realized that this could be a problem when trying to separate the vehicle from its payload. So they came up with an improved version that allowed the robot to split into two, independent, 3-legged robots to leave their payload where needed and come back together for other work. The upgraded version, called Tri-ATHLETE, has been built and is undergoing testing. Its articulated legs also have accommodations for handling tools, such as drills, shovels, scrapers, and other equipment needed on a planetary surface.
On the Earth’s surface, a typical vehicle can carry a payload weighing about as much as the vehicle. Frost pointed out that on the surface of Mars, with the lower gravity, a vehicle can carry more than four times its own weight in payload. But on the Moon, with only one sixth of Earth’s gravity, a rover can carry eleven times its weight in payload. These conditions also work in ATHLETE’s favor.
When President Obama cancelled the Constellation program and shifted NASA’s focus to asteroids and Near Earth Objects, the ATHLETE team looked at ways to adapt to the modified mission. One of the major issues with landing on an asteroid is how to land and work in the very low gravity environment there. A spacecraft landing on an asteroid could kick up a great deal of dirt and debris and spend considerable time and propellant bouncing around on the surface before finally settling down. The ATHLETE team thought that their combination of wheels and compliant legs could absorb the landing energy without bouncing around and stirring up as much debris. So they built a low-gravity test bed to simulate the landing conditions and a modified ATHLETE to demonstrate how the hardware handled the situation. Their tests showed that the ATHLETE-based lander could settle down on the surface of a typical asteroid with minimal bouncing and debris being raised.
Frost concluded his talk by discussing some of the robotics team’s other projects including improved robotic grippers, climbers, an under-ice rover that travels upside-down on the bottom surface of the ice of a frozen lake, and a “RoboSimian” to work in hazardous environments following a natural or man-made disaster. The RoboSimian is under development for a Defense Advanced Research Projects Agency program in response to the Fukushima nuclear power plant accident following the earthquake and tsunami that struck Japan in 2011.