It’s not easy off-roading on an alien planet. Harsh conditions abound, and you can’t call AAA if you get a flat.
NASA’s car-sized Curiosity rover continues to experience exactly these challenges. A routine inspection on March 19 uncovered two breaks in the raised ribs that reinforce the left-middle wheel’s delicate skin, further evidence of the wear and tear that has been a constant challenge for the mission. Scientists say the unwelcome discovery doesn’t jeopardize scientific goals, but it underscores the need for continued careful driving and perhaps some creative thinking.
"All six wheels have more than enough working lifespan remaining to get the vehicle to all destinations planned for the mission," said Curiosity project manager Jim Erickson in a press release. "While not unexpected, this damage is the first sign that the left middle wheel is nearing a wheel-wear milestone.”
Each of Curiosity’s six knee-high wheels features a skin just half as thick as a US dime. Much-needed reinforcements come in the form of zigzag ribs about as thin as an iPhone. A suspension system evenly distributes the buggy’s 800-pound Martian weight over the finely machined cylinders.
Designing the wheels required a balancing act weighing the need for strength with the intricate landing system. Upping the thickness by just four hundredths of an inch would have added more than 20 pounds, which would have complicated the physics of the “seven minutes of terror” sky crane scheme used to land the rover on the surface of Mars, according to science journalist and planetary geologist Emily Lakdawalla.
Driving nearly 10 miles on the Red Planet has come at a cost. Various punctures and holes in the wafer-thin wheel skin have been worrying scientists for years, but they were largely cosmetic blemishes. Horrible to look at, but not structurally significant.
Last week’s check caused concern because it revealed the first and second instances of rib breakage, a sign that the middle-right wheel has been significantly weakened.
The good news is that scientists suspect they’ll be able to complete their current objectives as planned. "This is an expected part of the life cycle of the wheels and at this point does not change our current science plans or diminish our chances of studying key transitions in mineralogy higher on Mount Sharp," said Curiosity project scientist Ashwin Vasavada.
The farthest potential target sits 3.7 miles up the mountain from Curiosity’s current location, and to get there, mission planners will have to rely on a bag of driving tricks they’ve developed ever since the damage first started cropping up four years ago.
For rover driver Matt Heverly, the worst day of the mission came a little more than year in. “When we saw these images, we saw a hole that was much larger than we had expected. This did not match anything we had seen in our tests. We didn't know what was causing it. We didn't know if it was going to continue," he said at a JPL event celebrating the second anniversary of the landing.
The problem was a never-before-seen feature of the Martian surface: pointy rocks embedded in the ground that didn’t give or roll out of the way.
"We misunderstood what Mars was," Mr. Erickson told Mrs. Lakdawalla. "There is very hard rock that doesn't erode away uniformly. And you get ventifacts [wind-eroded pyramidal rocks] that are sharper than we'd like, and that are cemented into the ground. And so when you drive over them, they don't skitter out of the way, they don't get pressed into the sand, they just are something that you have to have the wheel go up and over.
After the discovery, NASA sought out places on Earth with similar conditions and ran extensive tests with a Curiosity clone, finding that mileage until wheel failure depended on what the driving surface was like: from five miles on rocky bedrock to essentially indefinite driving on sandy plains, according to Lakdawalla’s blog post on the topic.
Equipped with a better understanding of the problem, NASA scientists have learned to be better drivers and route planners. The name of the game is avoiding the rocks. Early on in the mission there were more long drives, where the rover would navigate “blind” from A to B without worrying too much about what’s in the middle.
Now NASA does more short drives, especially in bad terrain. Using Curiosity’s cameras to peer dozens to a hundred feet ahead, drivers can micromanage the path to avoid any particularly spiky-looking rocks. This approach has slowed down the mission, but can significantly extend the wheels’ lifespan.
Another possibility for the future is to put the rover into reverse. While the weight falls evenly on all six points of contact, the front and middle wheels bear the brunt of the burden because they’re constantly being driven into oncoming rocks, while the rear wheels drag behind.
"The rear wheels are still almost pristine," Erickson told Space.com in 2015.
Driving backwards for a while could even things out, but it too comes at a cost. The rover would be riding blind with antennas blocking the view, and turning it back around at the start and end of each drive would add over 30 extra feet to each drive, spending an increasingly valuable currency.
But NASA has a history of long-lived rovers and using creative engineering to restore capacity after a system failure. No one expected exploring Mars to be easy, and every broken rib offers new insight into the challenges Curiosity’s successor will face.
"It's just one of these cases where Mars is going to give us a new deal, and we're going to have to play the cards we get, not the ones we want," Erickson said.