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Curiosity Rover Report (August 5, 2016): Four Years on Mars

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Winston

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It truly stinks to have a nuclear powered vehicle that unlike the MERs is not hampered by dust on solar arrays or Mars seasons, but whose wheels are being torn apart. A mission done 99.999999% right, but it's that 0.000001%...

[video=youtube;E6t1kxX-EpI]https://www.youtube.com/watch?v=E6t1kxX-EpI[/video]

Detailed article with lots of photos:

Curiosity wheel damage: The problem and solutions

http://www.planetary.org/blogs/emily-lakdawalla/2014/08190630-curiosity-wheel-damage.html

Excerpts:

What is causing the damage to the wheels?

The punctures result from pointy rocks -- sort of. I've heard a lot of snarky comments from people about what a surprise it is that there are pointy rocks on Mars. (Not.) Obviously, the presence of rocks on Mars was no surprise to the Curiosity team; the wheels were tested on rocks and did fine. The presence of a pointy rock alone is not sufficient to puncture a wheel. You will regularly see images of the wheels where a wheel is perched on top of a pretty pointy rock, and the wheel skin has no problem resisting the force of that pointy rock. Another clue that pointy rocks alone are not the problem is that the rear wheels have no punctures, even though the weight of the rover is balanced evenly among the six wheels. Something else has to be going on.

It turns out that there are mechanical aspects of the mobility system that actively shove the wheels into pointy rocks. A wheel can resist the force of one-sixth of the rover's weight pressing down on a pointy rock, but it can't resist the rover's weight plus the force imparted by five other wheels shoving the sixth wheel into a pointy rock. The forces are worse for the middle and front wheels than they are for the rear wheels. If you look at the design of the rocker-bogie system, you can see that the arms that support the middle and front wheels are angled downward. If a front or middle wheel hangs up on a rock and the rest of the rover keeps driving, the arm is exerting a downward force on the wheel. But the rear wheel doesn't experience that same downward force -- it's dragged behind the arm, like a wheeled suitcase.

Again, though, these forces were understood before Curiosity launched to Mars, and are not, on their own, enough to cause the large punctures. If the pointy rock can move, all that pushing force behind it will just shift the pointy rock to one side or another, or it can roll beneath the wheel, and the wheel will get over it without damage. The key to wheel punctures is immobile pointy rocks. If the pointy rock is stuck in place, partially buried, or if it is a pointy bit of intact bedrock, then there's nowhere for it to go. At the landing anniversary event, rover driver Matt Heverly showed a video of a test where they had a sharpened metal spike embedded in the ground, and drove a wheel over it. The spike pierced the wheel like a can opener slices into a can. The entire audience sucked in its teeth.

No place we've ever been on Mars before has these kinds of embedded, pointy rocks. "To the layman, it all looks the same, but it's not," Erickson told me. "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. [The damage rate] got significantly worse towards middle or end of November....unfortunately, we had driven into an area that was full of these rocks."

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.

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.

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.
(For now... - W) 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.
 

NAR29996

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Damage to Curi's wheels has not increased notably since these procedures were implemented. My reading is that the situation is under control. Note that I'm just a guy who reads a message board.

I've also seen a couple of pics of the redesigned wheels for the next rover. They do seem to be thicker, but are narrower, presumably to keep the mass the same as before.
 

dhbarr

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Remind me again how long the original mission design was?
 

Winston

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Remind me again how long the original mission design was?
Well, that's always the absolute minimum target. There certainly wouldn't be any need for a nuclear power source if that was the actual goal.
 

ksaves2

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One Martian year.
Wasn't Opportunity and Spirit supposed to be something like 90 days? Spirit is RIP but Opportunity is still alive.

What was it? Something like 68 pyrotechnic events had to occur on time with Curiosity's landing and we fliers sometimes have trouble pulling off one or two? That was simply amazing. Kurt
 

ThirstyBarbarian

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It's disappointing about the wheels, but they've definitely learned something new about the hazards of Mars. It's to be expected that they can't anticipate everything. If they already knew everything there was to know about the planet, there'd be no need to go.

I'm interested in seeing some missions to other regions, like up in the poles, were they know there is ice. And some rovers with drills that can go deep under ground, looking for water and ice.
 

Winston

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Wasn't Opportunity and Spirit supposed to be something like 90 days? Spirit is RIP but Opportunity is still alive.

What was it? Something like 68 pyrotechnic events had to occur on time with Curiosity's landing and we fliers sometimes have trouble pulling off one or two? That was simply amazing. Kurt
For sure. That's why it's amazing to me that these sort of efforts can ever be pulled off at all.

My point was that it's a real shame that that last tiny percent of required perfection had to be something as relatively mundane but as vitally important as the wheels and something which, if you read the full analysis I linked to, could have been prevented simply by someone saying "Well, just because we haven't seen embedded pointy rocks on Mars before, let's prepare for their 'can opener' effect on our very thin wheels just in case. After all, it is the evidence of huge volumes of flowing water that's sending us to this particular spot and water could easily be expected to produce those, unlike at any spot we've been to before. This is a very heavy rover on a long duration, nuclear powered mission, so durable wheels will be essential." I realize hindsight is always 20/20.
 
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