NASA Mars Curiosity Rover: Rocky Mars Terrain Wheel damage

Engineers are faced with surprising wheel damage on the Curiosity Mars rover mission.

Credit: NASAJPL-Caltech/MSSS

The Curiosity rover's wheels have taken a beating thus far on Mars, and the road ahead may be even rockier.

The 1-ton robot has just crossed out of its landing ellipse; the 12- by 4-mile (19 by 7 kilometers) zone that was targeted for its dramatic August 2012 touchdown, and is now moving toward an increasingly challenging landscape called the Zabriskie Plateau, mission team members said.

"We are heading out into very rough terrain," Curiosity project scientist John Grotzinger, a geologist at the California Institute of Technology in Pasadena, said during a presentation at the 8th International Conference on Mars, which took place at Caltech last week. "These rocks have been a problem for us."

Curiosity embarked last July on a roughly 5-mile (8 km) drive to the base of Mount Sharp, which has long been its ultimate science destination.

The car-size rover has about 2 miles (3.2 km) left to go, researchers said.

Toward the end of 2013, Curiosity encountered a region studded with sharp rocks, which presented the mission with a major technical challenge.

Unlike what had been experienced by other Mars rovers, these rocks were embedded in the surface like spikes in a parking lot exit.

In previous encounters with such obstacles, most rolled over and did not present a risk to the rover wheels.

The sharp rocks, looking like 3- and 4-inch (7.6 and 10.2 centimeters) shark’s teeth, appeared to be wind-sculpted.

Soft formations apparently overlie harder rock, and as the wind scours the region, what is left behind are the jagged remains of the tough subsurface stuff.

"The wind becomes a big problem for our wheels," Grotzinger said. "As the rocks fall apart, they are sculpted by the wind to points that we see as we drive along."

Chris Roumeliotis

Grotzinger and Curiosity rover planner team lead Chris Roumeliotis displayed to the audience at Caltech graphic images of wheel wear captured by Curiosity’s cameras.

"We did an inventory of the wheels," Grotzinger said, "and here’s the image that set us on into a constructed panic."

The mosaic showed wheels that had been dented, punctured and even torn by the rocks below.

"To figure out what to do… you take a picture of a metal wheel," he added, "and when you see the planet on the other side [i.e. through a large hole in the wheel], unless it says 'JPL,' it's a problem."



The JPL phrase refers to the holes that had been engineered into the wheels to mark the rover’s path in the sandy surface; these holes spell out 'JPL' in Morse code but the Martian landscape could be clearly seen through additional rips and tears in the metal.

An extensive testing campaign was immediately initiated both at JPL’s "Mars Yard," a rocky surface set up at the lab, as well as in the field near California's Death Valley.

Roumeliotis showed a video of one such test. It used a roughly 3-inch by 1-inch (7.6 by 2.5 cm) aluminum spike with a dull point to simulate a sharp rock.

"Welcome to 'The Impaler,'" Roumeliotis said as the rover drove over the spike and the wheel’s surface tore like wet paper. There was a visceral gasp from the audience.

Roumeliotis pointed out that such damage only occurred when the rover was driving forward, due to the pivot points of the suspension system.

A similar video of the rover driving backward showed the wheels traversing the spike with no ill effects.

In the months ahead, the rover will therefore be driving backward across some of the worst areas, as it did when crossing the last rocky patch.

This results in less damage, and what does occur tends to affect two wheels and not four when driving in this mode, team members said.

Mars Curiosity Wheel Damage: Adds Reverse driving for wheel protection

This map shows the route driven and route planned for NASA's Curiosity Mars rover from before reaching "Dingo Gap", in upper right, to the mission's next science waypoint, "Kimberley" (formerly referred to as "KMS-9") -- in lower left. 

Credit: NASA/JPL-Caltech/Univ. of Arizona

Terrain that NASA's Curiosity Mars rover is now crossing is as smooth as team members had anticipated based on earlier images from orbit.

On Tuesday, Feb. 18, the rover covered 329 feet (100.3 meters), the mission's first long trek that used reverse driving and its farthest one-day advance of any kind in more than three months.

The reverse drive validated feasibility of a technique developed with testing on Earth to lessen damage to Curiosity's wheels when driving over terrain studded with sharp rocks.

However, Tuesday's drive took the rover over more benign ground.

Jim Erickson

"We wanted to have backwards driving in our validated toolkit because there will be parts of our route that will be more challenging," said Curiosity Project Manager Jim Erickson of NASA's Jet Propulsion Laboratory, Pasadena, Calif.

The rover team used images taken from orbit to reassess possible routes, after detecting in late 2013 that holes in the vehicle's aluminum wheels were accumulating faster than anticipated.

Getting to the chosen route, which appeared to be less hazardous for the wheels, required crossing a 3-foot-tall (1-meter-tall) dune. Curiosity crossed the dune on Feb. 9.

Erickson said, "After we got over the dune, we began driving in terrain that looks like what we expected based on the orbital data."

"There are fewer sharp rocks, many of them are loose, and in most places there's a little bit of sand cushioning the vehicle."

This look back at a dune that NASA's Curiosity Mars rover drove across was taken by the rover's Mast Camera (Mastcam) during the 538th Martian day, or sol, of Curiosity's work on Mars (Feb. 9, 2004). 

Credit: NASA /JPL-Caltech /MSSS

The mission's destinations remain the same: a science waypoint first and then the long-term goal of investigating the lower slopes of Mount Sharp, where water-related minerals have been detected from orbit.

The science waypoint, which may be where Curiosity next uses its sample-collecting drill, is an intersection of different rock layers about two-thirds of a mile (about 1.1 kilometers) ahead on the planned route.

This location, formerly called KMS-9 from when it was one of many waypoint candidates, is now called "Kimberley," for the geological mapping quadrant that contains it.

The mapping quadrant was named for the northwestern Australia region with very old rocks.

While the rover is headed for the Kimberley waypoint and during the time it spends doing science investigations there, the team will use orbital imagery to choose a path for continuing toward the long-term destination.

"We have changed our focus to look at the big picture for getting to the slopes of Mount Sharp, assessing different potential routes and different entry points to the destination area," Erickson said.

"No route will be perfect; we need to figure out the best of the imperfect ones."

Curiosity has driven 937 feet (285.5 meters) since the Feb. 9 dune-crossing, for a total odometry of 3.24 miles (5.21 kilometers) since its August 2012 landing.