You can see that the damage is concentrated in the center of the wheel, as is happening on Mars. But on this wheel, all of the grousers have snapped at the point that they meet the structural stiffening ring, cutting the wheel almost completely around its circumference. A large strip of wheel tread is almost completely detached from the wheel -- it's only hanging on by a thread at the inner rim. Only the outer third of the wheel is still attached to the stiffening ring and the rest of the rover.

It's a terrifying image, and yet the odometry marker has not suffered any obvious damage, and the entire stiffening ring (which is where the wheel actually attaches to the hub) is intact. To my eyes, it looks like it could still function as a wheel. I asked Erickson if the rover could still keep driving with a wheel in this condition. He said, yes, it could. But there's a catch: "When it's flapping back and forth as it drives...it has the potential to be scraping the structure, and there is a cable that goes to the wheel motors, both drive and turn, that runs along that support strut. And if this thing starts rubbing against those cables, bad things can happen. You can get short [circuits]. If the right set of things were shorted together, it could go back into the drive controller and damage that, which controls things other than one particular wheel: the antenna, the HGA and all the other parts that move or turn." Which sounds exactly how China's Yutu rover failed on the Moon. That would be bad.

The good news is that a better choice of terrain can substantially prolong expected wheel lifetime. Erickson told me that they tested wheels on a wide variety of terrains, and came up with the following lifetimes. Keep in mind that these are conservative estimates, because there were no rover drivers working to steer around pointy rocks -- this assumes blind driving over all the worst rocks.

Bedrock with lots of rocks: ~8 kilometers

Lots of rocks, not on bedrock: 13-14 kilometers

Bedrock with few rocks (think flagstones): 30-40 kilometers or more

Smooth or sandy, with few or no rocks: indeterminate (causes no damage)

No matter the damage to the wheels, they continued to function just about as well over all kinds of terrain as pristine wheels do, until a large number of grousers started breaking. (No grousers have broken on Mars yet.) Accumulating damage will not have a significant effect on the ability of the rover to traverse any type of Martian terrain -- even sand -- for some time.

4. How can they prolong the life of the wheels?

They can't go to Mars and switch out the wheels. Fortunately, they have identified several ways to reduce the rate at which the wheels accumulate damage.

Driving more judiciously. Rover drivers are avoiding every pointy rock they can steer around. This only helps in the first 10 or 20 meters of a drive, where they can see smaller potentially hazardous rocks. On hazardous terrain, performing shorter drives allows them to avoid many potentially wheel-damaging rocks.

Driving backwards. When they turn the rover around, the rover's middle and front wheels are dragged behind their supporting arms rather than being shoved forward. And the angle of the bogie arm that holds the rover's rear wheel is such that it does not experience the same kind of downward forces that the front and middle wheels do when the rover is driving forwards. Heverly showed a video, taken in the JPL Mars Yard, of a test wheel being driven over the sharpened metal spike with the rover driving backwards, and the wheel was only dented, not punctured.

There is a cost to driving backwards. At the end of each drive, they have to face forwards in order to acquire images of the path ahead for planning. They can't take those images while facing backwards, because the RTG and antennas on the rover's rear deck obscure the view from the cameras on the mast. So to drive backwards, they have to turn in place, then drive, then turn in place again. Each turn in place puts about 6 meters on the rover's wheels, or 12 meters for the drive. For short drives (which is what they do in bad terrain), this can swiftly add up. The drivers have to weigh the cost of increasing drive distance against the potential savings to the wheels of driving backwards. Driving backwards therefore is most valuable on long "blind" drives where the drivers aren't steering around smaller rocks.

Long-term planning of drive routes that primarily traverse smooth or sandy terrain. Because wheel damage does not occur when traversing sand, the mission is now planning drive routes that cross sandy terrain. Over the past several months, mission geologists have used not only high-resolution photos but also spectral data from CRISM and thermal inertia data from THEMIS to develop maps of the different surfaces along the region between Curiosity and Murray buttes. They have compared their terrain types mapped from orbit to patterns of wheel wear observed on the surface, and identified specific terrain types that pose the least risk to the rover wheels. Then the scientists sit with the rover planners to help the rover planners choose routes that will aim Curiosity's future path toward more benign terrain.

They applied these methods to the path between the Kimberley and Hidden Valley, and have demonstrated that scientists can successfully use orbital data to predict the hazard that observed terrain types pose to the wheels. They have also demonstrated that they consistently overpredict the hazard -- that is, their hazard estimates are conservative. For instance, Erickson told me that some "red" terrain that they have traversed (the worst kind, bedrock plus lots of rocks) has turned out to have rocks spaced far enough apart that rover drivers can steer around them, mitigating the hazard. It's a spectacular collaboration between scientists and engineers. With judicious terrain choice, Erickson suggested they could go 30 to 50 kilometers before experiencing wheel failure. And the higher proportion of the time spent on sand, the better. You can see why it's worth attempting to cross sandy terrain like that in Hidden Valley, even if they experience some wheel slip -- that kind of terrain is "free" in terms of wheel wear. Erickson was confident, based upon the work done to date, that Curiosity could complete its first mission extension without wheel failure even if they drove on the worst possible terrain. By being choosy about terrain, they can survive significantly longer than that.

Changing driving software to reduce the forces experienced by wheels hanging up on pointy rocks. This one has not been implemented yet, but Erickson told me they are trying to develop and test some software fixes in time for the next rover software update, scheduled for December or January. The rover can sense wheel currents, so it can sense when a wheel is sticking. Also, the damage may be exacerbated by the fact that the software requires all six wheels to rotate at a constant rate, even though a wheel climbing an obstacle has a longer path to travel than one traversing flat ground. By implementing a "smart controller" on the wheel current and allowing wheel rotation rates to vary intelligently in response to sensed conditions, they might be able to mitigate the damage.

NASA has a long history of rewriting software to enable deep spacecraft to do things they couldn't previously do; Erickson worked on Galileo and all the software development that was needed to salvage the mission after its high-gain antenna failed. But Curiosity, Erickson said, is much more reprogrammable than previous missions, really a "software-defined spacecraft." He said "There are lots of things we can change in software, particularly anything to do with controlling motors." The flexibility of Curiosity's software has sometimes been a problem, of course, because it adds to the mission's complexity. "The more complicated the software, the more likely you'll not get everything perfect. You'll get surprises. Both in development/test and in operations. Even how it responded to the flash failure on sol 200 was a surprise, and we continue to get surprised." But this is a situation where the flexibility will help, allowing them to redesign the way the rover works in response to a potentially mission-ending hazard that they never anticipated.

5. Why didn't they foresee this problem?

There were several factors that drove them to design the wheels to be as lightweight as possible. The large size of the wheels means that very slight design changes add a substantial amount of mass. Increasing wheel thickness by one millimeter would add 10 kilograms to the rover's total mass. But total system mass wasn't the only constraint. Erickson explained that a major constraint arose from a tricky moment in the landing sequence, at the moment that the wheels deployed, while the rover was suspended from the bridle underneath the descent stage. The wheels' sudden drop imparted substantial forces on the mobility system, and keeping wheel mass as light as possible reduced those forces to manageable ones. There were other factors that made it important to keep wheel mass low.

So the wheels needed to be as light as possible while still being able to do their job, but as to their job: "We misunderstood what Mars was," Erickson said. "Strongly cemented ventifacts are not something that we saw on Mars before." They designed Curiosity to handle all the challenges that Spirit and Opportunity had experienced, especially sand, which Curiosity traverses substantially better than her predecessors. "This vehicle is able to get itself out of situations that MER couldn't; it's got more flotation than MER had by a substantial margin." They designed Curiosity to handle the sand traps, flat bedrock, and rocks-perched-on-sand landscapes seen by all the previous landers. They just didn't imagine the possibility of the peculiar and never-before-seen terrain type that they found in Gale crater. "There are [places] on Earth that do have these sharp ventifacts, but we hadn't seen them on Mars and we didn't test against them," Erickson said.

6. What are they changing for Mars 2020?

Erickson did not have specific insight into how the wheel design is being changed for the 2020 mission, because he is not directly involved; but the design is definitely being changed. Erickson said that they had already developed several solutions and are now in the process of trying to identify the best solution.

I personally believe that there's another way that Mars 2020 can prevent this kind of problem, besides redesigning the wheels. That is: select a landing site where they can reach interesting rock targets inside the landing ellipse, rather than having to rove out of the ellipse in order to find good targets. Given that Curiosity only exited its landing ellipse at the end of the prime mission, I think that the scientific community that will participate in the Mars 2020 landing site selection will place a higher value on landing sites with in-ellipse science. There were already strong advocates for non-go-to sites in the Curiosity landing site selection process; they will feel vindicated and emboldened by Curiosity's challenges. It would limit the landing site choices, and I'm sure there will be a lot of debate about the scientific trade-offs. But, thanks to the work of the four modern Mars orbiters, we know much more about Mars than we did when Curiosity's landing sites were being selected, and I believe that the community can locate a compelling landing site for Mars 2020 that will not require quite so much driving to reach the rocks of interest.

At the end, I asked Erickson to put the wheel problem into context with his experience on many other missions. He said that the problem of damage to Curiosity's wheels has definitely had a significant impact on the mission, and mentioned for comparison the wheel failure on Spirit, when they had to start dragging the right front wheel behind them and drive exclusively backwards. But the Curiosity problem is not as bad as Spirit's because Curiosity is no less mobile than it was before. They can choose to accept wheel damage if they determine the scientific value to be worth it. So while Spirit's mobility problems limited the scope of what the rover could do, Curiosity's mobility problems do not -- at least, not directly. The biggest effect of the wheel damage problem is to slow the mission down. And that's what will limit how much Curiosity accomplishes. By not traveling as fast, and by having to limit their path choices, the amount of exploration that they can do is necessarily less than if they could go gallivanting across the bedrock outcrops at will.

The slowed pace of the mission is frustrating, but that's the way it is. The good news is that the mission went from being surprised and dismayed by unexplained damage, to a full understanding of what's causing the damage, and of what they have to do to prevent the wheel damage from prematurely bringing the mission to an end. "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. The slowed pace has delayed their arrival at Mount Sharp, but they will get there, and the science will be good, Erickson said. "Our whole goal in life is to bring a set of instruments to the good stuff. Right now we're driving from restaurant to restaurant. But we're about to get to a smorgasbord. We've got a lot of things to pick from there. Instead of driving from place to place, we're going to hunker down and start pigging out." We can look forward to much more drilling -- and much more in-situ science -- once Curiosity's tender wheels finally bring her to the rocks of Mount Sharp.

We may already be there. Curiosity is drilling this week, into a rock that the mission's geologists have mapped as one that may be part of the basal units of Mount Sharp. If so, it would be the first such rock that Curiosity has seen -- and the beginning of the science that specifically brought Curiosity to the landing site in Gale crater.

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Reference: Here is a conference paper (PDF) by engineers Sean Haggart and Jaime Waydo describing the Curiosity wheel design in detail.