|In-situ Exploration and Sample Return: Autonomous Planetary Mobility
Technologies for autonomous planetary mobility enable the rovers to make decisions and avoid
hazards on their own.
Autonomous Planetary Mobility for the Mars Exploration Rover Mission: Overcoming Obstacles
New path-planning software has helped the rovers avoid mission barriers. When rocks are unavoidable,
the inherited suspension system allows for easier maneuverability.
Improved Mobility System
The twin rovers were designed to traverse with a fair amount of ease over the rocky martian terrain.
Sojourner's "rocker-bogie" mobility system was modified for the Mars Exploration Rover Mission.
To account for the extreme difference in weight and center of gravity from Sojourner, the mobility
system on the Mars Exploration Rovers is in the back of the vehicle. The wheels are, naturally, larger
and have evolved in design. Each wheel is approximately 26 centimeters (about 10 inches) in diameter
and has a unique spiral flecture pattern that connects the external part of the wheel with the spoke
to absorb shock and prevent it from transferring to other parts of the rover. The rocker-bogie design
allows the rover to go over obstacles (such as rocks) or through holes that are more than a wheel
diameter in size. Each wheel also has cleats, providing grip for climbing in soft sand and scrambling
The wheels on Spirit and Opportunity are 26 centimeters in diameter (a little over ten inches) and made of aluminum.
The "orange filling" between the spaces in the spiral flecture is an open-cell foam called Solimide.
It was cut into crescent shapes and bonded to the wheel. Mobility engineers decided to fill in the
open geometry design of the wheels to prevent rocks and debris from interfering with drive and steering
actuators. Solimide maintains its flexibility even at very low temperatures so it is ideal for
conditions on Mars.
The Path of Least Resistance
In order to drive themselves the rovers take stereo images, from which 3-D terrain maps are generated automatically by the rover software.
Having more physical capability than 1997's Sojourner rover, Spirit and Opportunity also needed more
autonomy. Engineers improved the auto-navigational driving software to give the golf cart-sized
explorers more freedom.
When the rovers are navigating themselves, they get a command telling them where to end up, and
then evaluate the terrain with stereo imaging to choose the best way to get there. They must avoid
any obstacles they identify. This capability has enabled longer daily drives than would have been
possible by simply depending on step-by-step navigation commands from Earth. As of mid-August, 2004,
Opportunity has used auto-navigation to drive for 230 meters (about 755 feet, or one-third the distance
between Eagle and Endurance craters), and Spirit for over 1250 meters (about 8 tenths of a mile), mostly
during the nearly 3000-meter (nearly 2 miles) drive to the Columbia Hills.
Watch the Rover Navigation 101: Autonomous Rover Navigation animation.
QuickTime 18 MB
The auto-navigation system takes pictures of the nearby terrain using one of the Mars Exploration Rover
stereo camera pairs (body-mounted hazard-avoidance cameras on Spirit, mast-mounted navigation cameras on
Opportunity). After stereo images are taken, 3-D terrain maps are generated automatically by the rover
software. Traversability and safety is then determined from the height and density of rocks or steps,
excessive tilts and roughness of the terrain. Dozens of possible paths are considered before the rover
chooses the shortest, safest path toward the programmed geographical goal. The rover then drives between
0.5 and 2 meters (1.6 and 6.6 feet) closer to its goal, depending on how many obstacles are nearby.
The whole process repeats until it either reaches its goal, or is commanded to stop.
The Mars Exploration Rover autonomous driving software is more advanced than Sojourner's in several ways.
Sojourner's onboard safety system also looked for obstacles, but could only measure 20 points at each step;
Spirit and Opportunity typically measure more than 16,000 points from each pair of images. The average
Mars Exploration Rover obstacle-avoidance driving speed of nearly 34 meters (about 112 feet) per hour is
ten times faster than Sojourner's. During its entire three-month mission, Sojourner drove just a little
more than 100 meters (328 feet) total. Spirit and Opportunity each broke that record in a single day;
Spirit drove 124 meters (407 feet) during sol 125, and Opportunity 141 meters (about 463 feet) during sol 82.
Another improvement over Sojourner is the Mars Exploration Rover Visual Odometry software system. As
the rovers drive over sandy and rocky terrains, they can slip by unpredictable amounts - even backwards
when driving up very steep slopes. But the Visual Odometry system helps by giving the rover a much better
notion of how far it has actually traveled. It works by comparing pictures taken before and after a short
drive, automatically finding dozens of features in the terrain (for example: rocks, rover tracks and sand
dunes), and tracking their motion between images. Combining that with the 3-D terrain shape is more than
enough information to let the rover figure how it really moved, much more precisely than simply counting
how much its wheels have turned. For more on visual odometry see the July 3-9 Week in Review.