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Steep Grade Ahead: Can the Rover Make It?

June 10, 2004

A steep grade beckons as the Mars rover Opportunity retraces its tracks to be sure it can get back out of Endurance Crater.
Opportunity eyes the abyss
A steep grade beckons as the Mars rover Opportunity retraces its tracks to be sure it can get back out of Endurance Crater. Image Credit: NASA/JPL
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At the edge of a crater on Mars, a solitary robot waited. Surrounded by miles of salty, sandy, wind-strewn terrain and carrying a large toolbox of scientific instruments, it waited for instructions from a planet 200 million miles away.

Back on Earth, those who gave life to the rover named Opportunity were about to give it the biggest challenge of its short existence by sending it over the edge of the crater, nicknamed Endurance Crater. Though the crater was not terribly steep, the rover had never descended such a steep incline before.

Its keepers were fairly certain that the rover would be OK. The reason for their optimism was that they had tested the Opportunity's abilities with its look-alikes on Earth. The only thing they weren't so sure of was whether the rover could climb back out again. If not, this lonely, dune-filled crater would become its final resting place.


Small-scale ripple patterns, seen in a rock at the southeast side of Endurance Crater's rim, are suggestive of past water processes on Mars.
Up-close View of Water Record "Written in the Rocks"
Small-scale ripple patterns, seen in a rock at the southeast side of Endurance Crater's rim, are suggestive of past water processes on Mars. Image Credit: NASA/JPL
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Why take a risk in venturing down the walls of a crater?

By sending the rover into the crater, scientists hope to learn about environmental change and the fate of ancient water on Mars. As Firouz Naderi, Manager of the Mars Exploration Program Office at NASA's Jet Propulsion Laboratory, explained, the layers of rock in the crater are like chapters in the geologic history of Mars.

Opening up those "pages" through close-up investigation by the rover will enable scientists "to read the record in the rocks," providing unprecedented knowledge about the martian climate history and the red planet's potential as a habitat for past or present microbial life. These are central scientific goals for NASA's Mars exploration missions. [More on the rovers' science goals.]


Picture of: Randy Lindemann, the lead mechanical engineer for the Mars Exploration Rovers.
Randy Lindemann
Randy Lindemann is the lead mechanical engineer for the Mars Exploration Rovers. Image Credit: NASA/JPL
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Weighing the risks: experience from the people who built the rovers

But, even with possible answers tantalizingly within reach, the mission team also didn't want to lose the rover.

Randy Lindemann, lead mechanical engineer in charge of the team that built the Mars Exploration Rovers, described the situation in this way: "We have no concern that the rover is going to tumble. We have no concern that the rover is going to perish in any sort of way. So it's not an end-of-mission sort of risk. However, it is a risk that this very large crater has very steep slopes that were far beyond what we had originally planned on ever driving the rovers on."

Lindemann, a tall, dark-haired, outgoing guy who is deeply dedicated to his job, has a lot staked on the rover's performance. His team of JPL engineers built almost everything on the robot except for the science instruments, electronics, and software. Independent contractors provided some components, while JPL machinists and technicians created designer, one-of-a-kind parts from scratch.

"We built the chassis, the wheels, the instrument arm, the mast, everything that moves or is a structural piece but not the things that go at the end," he said. "We built the gimbal but not the antenna, the arm but not the instruments. We built the body and the heart but not the brain or the eyes."

With such structural knowledge, if anyone knows what the rover is capable of doing, it's Lindemann.


Engineering model of the rover during test outside of the Mars Yard. at NASA's Jet Propulsion Laboratory, May 27, 2004.
Testing Rover Performance at JPL's Mars Yard
Engineering model of the rover during test outside of the Mars Yard. at NASA's Jet Propulsion Laboratory, May 27, 2004. Image Credit: NASA/JPL
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Preparing for crater entry through extensive testing

But even Lindemann was taking no chances. In the outdoor "Mars Yard" at JPL, he and his team of mechanical engineers had previously built a rotating platform on a hill overlooking Pasadena, California, and covered it with rocks and sand to look like those on Mars. To determine the boundaries of its capabilities, they tested the rover on it many times prior to its launch to Mars on July 7, 2003.

In one such test, they tilted the platform 20 degrees to make it look like a hillside and put the rover on it to see how it fared. In this test, the test robot kept roving. Proving performance on a twenty-degree tilt seemed sufficient.

Suddenly, in May and June of 2004, as the rover waited at the edge of Endurance Crater, the engineers needed to know if it could handle an even steeper hill. So, they created a new surface that looked like the one they saw in new images from Mars.

They covered the platform with concrete flagstones purchased at a large-chain hardware store. They mixed sand and quick-drying cement to create a granular texture. To make the flat surface irregular, they pressed pieces of cotton rope into the concrete before it dried to leave indentations in the wet cement. They also dotted the surface with ball bearings to simulate the iron-rich spherules found on Mars.

Then they tilted the platform to 25 degrees and put the rover on it. The rover kept going up the hill. Next, they decided to make the test a little tougher. As the rover's wheels turned, they pulled backward on the rover with a cord to make the rover lose its footing and slip. They had also attached a fish scale to the cord to measure the force exerted on the robot. A fish scale is what fishermen use to weigh their catch. This calculation would be sort of like joining a tug of war and measuring how strong your team can pull.


Navigation camera mosaics like this one, taken May 1, 2004 from the southeast side of the crater, help the team assess possible entry points into Endurance Crater.
Navigation camera mosaics like this one, taken May 1, 2004 from the southeast side of the crater, help the team assess possible entry points into Endurance Crater. Image Credit: NASA/JPL
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A well-performing rover helps analyze safe paths into Endurance Crater

Even with 40 pounds of backward pull, the test rover kept going like the Energizer bunny. These results gave Lindemann and his engineers confidence that, as long as the simulation was accurate, Opportunity would make it both in and out of the crater.

But only as long as the simulation was accurate. That required another set of investigations.

"For those of us who have the most information, it's still a little scary. Even with all of the testing that tells us what we can anticipate, there are 'curve balls' that Mars can throw at us," said Lindemann. "The rover won't change, but the nature of the ground and the soil can change."

Meanwhile, back on Mars, Opportunity was helping the team assess the best entry points into the crater too.


Steve Squyres, Principal Investigator, assesses a potential path into Endurance Crater.
Mission Team Considers Safe Pathway into Endurance Crater
Steve Squyres, Principal Investigator, assesses a potential path into Endurance Crater. Image Credit: NASA/JPL
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The team assesses safe pathways into Endurance Crater

In meetings that went on for several weeks, scientists and engineers on the Mars Exploration Rover team studied hundreds of images from Endurance Crater that they instructed the rover to acquire. The rover followed the instructions that the mission team transmitted by radio, traveling around the edge of the crater, taking snapshots, collecting scientific data, and stopping every now and then to rest and recharge its batteries with light from the sun.

Along the way, the rover measured the distances to various rocks and sand patches within the crater. It sent a steady stream of data to Earth. Computer scientists took the data and translated it into three-dimensional topographic maps with special software they had developed at JPL.


Opportunity looks back at the edge of the crater from well below the rim.
Far below the edge
Opportunity looks back at the edge of the crater from well below the rim. Image Credit: NASA/JPL
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Rockier slopes provide safer passages into the crater

Ultimately, scientists and engineers agreed that as long as they could identify a path into the crater that was mostly covered with rocks, they wouldn't have to worry about how deep the sand might be inside the crater. As long as four of the rover's wheels were on a reasonably solid surface with only a few millimeters of sand on top, they agreed the rover could make it.

Up on that crater rim, Opportunity received the command to roll on into the crater.

Stay tuned for Opportunity's continuing story!


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