NASA's Mars Rover

NASA’s Jet Propulsion Laboratory (JPL) in California tested navigational software created by Carnegie Mellon University on the Mars rover Opportunity about two weeks ago. This new software enables the rover to maneuver around obstacles in search of past water on Mars.

In the summer of 2003, scientists at JPL sent two rovers, Opportunity and Spirit, to Mars as part of the Mars Exploration Rover Mission (MERM). MERM is a project that aims to find evidence of past water by studying bits of rock and soil on Mars.

Opportunity and Spirit are semi-autonomous rovers that have been operating on Mars since arriving there in January 2004. Last month, scientists tested the new navigational software, called Field D-Star, on Opportunity.

Field D-Star is an upgraded version of the robot’s original navigational software. It allows the robot to create maps of its terrain and thereby avoid obstacles in its path.

Professor of robotics Anthony Stentz helped develop Field D- Star. He said that the rover’s original software only allowed it to function in two modes: directed driving or auto-drive.

In directed driving mode, the rover moves blindly in a certain direction up to 50 meters based on pictures taken by the rover.
“That is a relatively quick way to get around, but it requires that the path be safe,” said Stentz.

In the auto-drive mode, the rover moves short distances based on images taken by on-board cameras. If there were obstacles in the vehicle’s path, it was told to maneuver around them.

“It’s good for avoiding isolated obstacles … a single rock or hole … but it’s not the best strategy for more complicated obstructions like dead ends,” Stentz said.

If the vehicle drove into a dead end, Stentz said, it turned the other way. But, it then turned back toward its destination, thereby running into the dead end again.

Carnegie Mellon alumnus and JPL mobility engineer Mark Maimone helped create the vehicles’ original software. He said that the vehicles’ software sometimes caused them to become stuck in a back-and-forth motion between rocks.

Maimone said that although Field D-Star uses the same output as his program, it improves the rovers’ ability to navigate around obstacles. During its first test run, Field D-Star successfully maneuvered Opportunity around a keep-out zone on Mars.

“It takes the output of my system and is smarter about navigating through it,” said Maimone.

Field D-Star, which was upgraded to both Opportunity and Spirit, enables the rovers to create virtual maps of their environment that determine their course.

“It always plans the best path to the object point … from the rover’s current location taking into account everything that the rover’s seen so far,” said Stentz.

Stentz said that scientists on Earth do not have constant contact with the robot, and Field D- Star thus allows the robot to maneuver out of difficult situations on its own.

When the robot reaches a dead end, for instance, it knows that it needs to turn around and drive all the way out of the dead end.

“This software has the potential to steer the robot out of very complicated, maze-like environments,” said Stentz.

Maimone said that Opportunity is currently exploring an 800-meter-wide crater called Victoria Crater. The vehicle analyzes the bedrock surfaces of the crater’s cliffs and alcoves.

Bedrock contains layers of aged rock, providing researchers with a summary of the crater’s past.

Opportunity uses a spectrometer to analyze the different parts of the bedrock. A spectrometer is a device that measures wavelengths of light to determine the bedrock’s mineral and element composition.

According to a December JPL press release, Opportunity has collected over 80,700 images of its territory since the beginning of its mission.

“They can see the true history of what happened in order” across millions of years, Maimone said.

If scientists find salt in the bedrock, they know that water used to be present. Maimone said that the rover has already found evidence of past water in two craters.

Within minutes of beginning the interview, Maimone received a message on his cell phone from the Mars rover Opportunity. Maimone said that when the rover changes positions, it sends a message to his cell phone that contains images of its current location.

Over the course of three years, Opportunity has traveled about 10 kilometers. Stentz said that Opportunity and Spirit move at a rate of one centimeter per second, and, “for long stretches, they stay in one place.”

The rover’s computer is slow due to cold temperatures. In fact, the rover’s computer has one-hundredth the processing speed of a desktop computer.

Maimone said that field-testing was one of the most challenging parts of the project.

“The hard part was … testing it on Earth because the rovers are built for Mars,” said Maimone.

He said that the cameras, for instance, overheat at a temperature of 30°C, ceasing to function in California by mid to late morning.

Stentz said the team also faced the challenge of developing a program fast enough to construct terrain maps during the rover’s operation.

“The biggest challenge was to really come up with a computationally efficient algorithm for solving this problem,” said Stentz.

Maimone said that Opportunity’s crew of engineers faces the present challenge of maintaining the rovers’ three-year-old parts. In particular, Maimone said that the vehicles’ parts are beginning to wear out, and Opportunity’s front right wheel does not steer.

“We’ll have to see how to best take advantage of it,” said Maimone.

Nonetheless, the Mars rover Opportunity is already a record-setting vehicle. According to JPL’s website, Opportunity is the first robot to travel 10 kilometers on Mars.

“It’s been a stunning success for NASA,” said Stentz. “I hope that this is a capability that NASA will decide is beneficial for any rover on Mars.”

Stentz said that he foresees NASA investigating how to best utilize the new programming system for its present purpose, and hopefully, for future missions.

Maimone said, “Long-term, we’re going to keep driving these things until we don’t hear from them anymore.”