There are holes in Curiosity wheels . There have always been hole — the roamer landed with twelve cakehole deliberately machine in each wheel to assist in rover navigation . But there are newfangled maw now : puncture , fissures , and ghastly tears .
The hollow in Curiosity ’s wheels have become a major concern to the mission , affect every day of mission mathematical operation and the choice of path to Mount Sharp . Yet deputation managers say that , so far , the experimental condition of the wheels has no effect on the rover ’s power to traverse Martian terrain . If the golf hole are not causing problems , why the rerouting ? Is the wheel wrong a big quite a little or not ?
NASA / JPL / MSSS / Emily Lakdawalla
I ’ve been asked a lot of questions about the wheel since we noticed the first rip on so 411 , and I ’ve receive many requests for posts specifically addressing the wheel damage trouble . At first , I dismissed people ’s concerns , because the delegacy seemed unconcerned . But within a few months , the mission started becoming more alarmed by a sudden increase in the rate of equipment casualty . They had to form a “ Tiger Team ” to read what was causing the more - than - anticipated damage and to determine how best to plow the problem . Their work is n’t over , but the Tiger Team had a major brushup meeting on August 7 , and last week I had a lengthy conversation with Project Manager Jim Erickson about the mission ’s status . I ’m happy to at long last be able-bodied to address your questions about the wheels .
In this post I ’m set out to answer six frequently - require questions :
What is the nature of the damage to the wheels ?
What is causing the damage to the cycle ?
What is the expected lifetime of the roulette wheel , and how does that life end ?
How can they prolong the life of the wheels ?
Why did n’t they previse this problem ?
What are they changing for Mars 2020 ?
The short version of the story : wheel harm is a serious take , but it does not shortly limit the capacity of the rover . The mission now understand the trouble and can partially mitigate it . The rover will be able to make out its extended missionary post , and will likely be able-bodied to go on to more mission elongation . The major event of the cycle hurt job are to slow the progression of Curiosity and to limit the way the mission can choose to research .
Introduction to Curiosity’s wheels and suspension system
Below is a photograph of a flight wheel — that is , one of the six that ’s sitting on Mars today . Each wheel tire was machine from a single block of aluminum . It is 50 centimeters in diam and 40 centimeters astray . It has grousers ( treads ) that protude 7.5 millimeters from the wheel skin . Grousers are spaced 15 degrees apart . Unlike Spirit and Opportunity , the grousers are not straight ; they have grade insignia features plan to keep sideways slip .
The skin of the wheel is 0.75 millimeter stocky — the right-down thinnest that could be machined . The grousers provide structural strength ; the tegument is for drift the roamer atop loose guts . There is a fragile crown to the bike to make it more robust to the moment of touchdown on Mars ( remember that the rover landed on its wheels , with no lander underneath them ) . There is a vertical brim on each edge of the wheel , again for geomorphologic strength . There is another , double lip locate about one - third of the way into the tyre , the structural stiffener to which the bike crimp ( spokes ) are attached . One section of the tire has a set of odometry marker maw drilled into it , which provide a way for the rover ’s navigational package to measure its driving progress across cushy surface by photographing the tracks . For playfulness , these hollow spell out “ J P L ” in Morse code .
NASA / JPL / Emily Lakdawalla
A Curiosity wheel before it was attached to the rover and flown to Mars , label with all of its component component part .
Here are the part of the rocker - bogey suspension scheme . There are only three places where the suspension system is connected to the rover body : a pivot on each side and one on top ( in the middle of the differential Browning automatic rifle , which connects the two sides to each other ) . The long arm is call the rocker and is connect to the dead body at the pivot . On the front goal of the rocker is the front wheel . On the back closing of the rock ‘n’ roll musician is a second subdivision call the bogie . The bogie carry the middle and rear wheels . A rocker - bogie suspension system keeps the rover body comparatively horizontal surface even when the wheel are climb obstruction adequate to their heights . The length of the rockers and bogies are chosen in part to distribute the weight of the rover evenly across all six wheels . Here is a great explainer on how the rocker - bogy respite system of rules works .
Curiosity ’s wheels are supported by a “ cradle - bogy suspension system of rules . ” Each side has two arms ( a rocker and a bogey ) that can pivot , connected to each other through a differential legal profession and pivot on top of the scouter . When one cradle tilts in one direction , the gene linkage through the differential causes the other rocker to tilt in the paired director , keep the rover body comparatively stratum even when the wheels are climb up large obstacles .
The organisation was designed to reach an expected premier missionary station sum private road distance of 10 to 20 kilometers . The Curiosity wheel were sized to produce the same priming coat insistency on Mars that the Mars Exploration Rover wheels do . In panoptic testing on Earth using a Mars - system of weights rover , engineers observe the system to be capable of come through intense touchdown scenario . Traverse testing focused on the mobility system ’s power to drive without slew . It can climb slopes up to 22 degrees on smooth bedrock , but experiences substantial solecism at slopes above 10 degrees in cohesionless guts . In desert testing , JPL found that the larger wheel of the Curiosity mobility system manage well on sand than the Mars Exploration Rover wheels do .
1. What is the nature of the damage to the wheels?
Below is a late survey of the impairment to the wheels . The damage takes three main forms : dents ; puncture , where it looks like something just poked a fix through the skin ; and tears . equate this prototype toan earlier surveyto see how the equipment casualty has developed over time .
On sol 708 ( August 3 , 2014 ) , Curiosity perform a series of inadequate drives followed by MAHLI imagery of each of the wheels , to survey their consideration . Here , the epitome have been sieve and the inter - cleat space numbered to make it easier to survey the locations of specific marks , bout , and punctures .
presently , the most - damaged rack is the left-hand - middle one . Here ’s a close-fitting look at the development of the damage to the left - mediate wheel over metre .
NASA / JPL / MAHLI / Emily Lakdawalla
Inter - tread spaces are numbered the same from image to image to help orient you to the locations of holes in the thin alloy skin separating the grousers or treads . As of sol 708 , the left over - halfway bike is the one that has suffered the most damage .
Here is what I see in the rule of damage :
The two rear wheel have scratches and nick but no hole that I can see anywhere ( except for the odometry mark ) .
On middle and front wheels , most ( but not all ) of the big mess are in the center of attention section of the wheel — that is , they are on the “ poll . ”
In some places , punctures and rips have coalesced to open very large holes .
As far as I can tell , all of the skin that used to fill these pickle is still attached and is jut inside the tire .
Rips incline to cluster at the tips of the grade insignia features .
But there is virtually no price to the hide inside the odometry markings , where grousers are closer together .
There are no broken grousers and no foretoken of any harm ( other than scrapes ) to any of the stiffening rims .
Even on the most - damaged wheels , you’re able to clearly see from the outside where the stiffening ring is locate , about a third of the way in from the outer edge . That stiffening annulus is a locale of undamaged bike : there is no obvious damage to the pelt across this ring .
There is no overall change in shape to any of the rack .
2. What is causing the damage to the wheels?
ab initio , this was a mystery . The missionary station did expect some wrong to the rack . The bike acquired dings and scratch over time , but they were comparatively unscathed until that first heavy puncture appear on colloidal suspension 411 . They did n’t look at the wheels again until sol 463 , when a big rip had opened . At a JPL upshot in honor of the second day of remembrance of landing place , the moderator asked rover driver Matt Heverly which was the sorry so of the mission for him , and he said 463 . “ When we saw these images , we saw a muddle that was much larger than we had look . This did not match anything we had seen in our test . We did n’t do it what was cause it . We did n’t know if it was go to continue . ” That was the moment , he say , that he know that their drive was go to have to change radically . But how ? They could n’t figure it out until they started test .
NASA / JPL / MSSS / Damia Bouic
rarity adopt these photos of her bike with the MAHLI arm - mounted camera on sol 463 ( November 24 , 2013 ) A large rip has appear above the Morse - code mess in the left front steering wheel . Several smaller lick - holes are visible in it and the middle bicycle in this view .
They ’ve been doing testing on both Earth and Mars for a class now . I wo n’t go into the item of the testing here because I do n’t desire this post to be 10,000 words long . ( For the full story on examination you ’ll have to hold off formy book of account . ) They traced the damage to two problems , one make the roue and one have the puncture .
The bout result from fatigue . You know how if you deform a metallic element paper clip back and away repeatedly , it eventually snaps ? Well , when the rack are labor over a very strong rock Earth’s surface — one with no guts — the tenuous skin of the wheels repeatedly bends . The wheels were designed to bend quite a draw , and return to their original shape . But the repeated bending and straightening is jade the skin , cause it to fracture in a brickle way . The deflection does n’t happen ( or does n’t encounter as much ) if the ground gives way under the rover ’s weight , as it does if it ’s got the slightest coating of sand on top of rock . It only happens when the reason is utterly impervious to the rover ’s weighting — voiceless bedrock . The stresses from alloy fatigue are highest near the tips of the grade insignia features , and indeed a lot of snag seem to broach close to the chevron features .
NASA / JPL / MSSS
A quotidian steering wheel survey on sol 631 found the right rear wheel perched atop a spike - shaped rock firmly embedded in the ground . The bicycle ’s weight is entirely supported on the rock spire , yet the steering wheel was not punctured .
It turns out that there are mechanically skillful aspects of the mobility organisation 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-and-roll , but it ca n’t refuse the rover ’s weightiness plus the force impart by five other wheels shove the sixth bicycle into a pointy tilt . The forces are worse for the middle and front wheels than they are for the rear wheel . If you count at the plan of the rock ‘n’ roll musician - bogy organisation , you’re able to see that the arms that support the middle and front wheel are angled downward . If a front or middle wheel hangs up on a rock and the relief of the rover keep driving , the branch is exerting a down personnel on the wheel . But the rearward steering wheel does n’t experience that same down military force — it ’s drag behind the branch , like a wheeled traveling bag .
Again , though , these forces were understood before Curiosity launched to Mars , and are not , on their own , enough to cause the tumid puncture . If the pointy rock can move , all that fight force behind it will just shift the pointy rock and roll to one side or another , or it can roll beneath the bike , and the wheel will get over it without damage . The cay to rack punctures is immobilepointy rocks . If the pointy careen is stuck in place , part lay to rest , or if it is a pointy bit of entire basic principle , then there ’s nowhere for it to go . At the landing place anniversary event , rover driver Matt Heverly show up a video of a tryout where they had a sharpened metallic element ear imbed in the ground , and drove a wheel over it . The spike pierced the roulette wheel like a can opener slice into a can . The integral interview sucked in its tooth .
No place we ’ve ever been on Mars before has these kinds of embed , pointy rocks . “ To the layman , it all looks the same , but it ’s not , ” Erickson told me . “ There is very hard John Rock that does n’t erode away uniformly . And you get ventifacts [ lead - wear away pyramidical rocks ] that are sharper than we ’d like , and that are cemented into the background . And so when you force over them , they do n’t scurry out of the way , they do n’t get pressed into the sand , they just are something that you have to have the wheel go up and over . [ The hurt rate ] got significantly worse towards heart or goal of November….unfortunately , we had drive into an area that was full of these rock . ”
NASA / JPL / MSSS / Ed Truthan
Pointy rocks plant in guts proved hazardous to the health of Curiosity ’s wheel .
3. What is the expected lifetime of the wheels, and how does that life end?
Both of the cause of wheel damage are exacerbated by take over hard bedrock with pointy excrescence . Erickson told me that when they screen the lifetime of the wheels over this kind of substratum , the word was n’t well . “ The really bad hooey , it only takes 8 kilometers or so and you may put down the wheel . ” What does a “ ruin ” roulette wheel seem like ? you could see one in a picture that I station inmy blog entry on Curiosity Women ’s Day , where Amanda Steffy explains her work as a member of the mechanical wheel wear Panthera tigris team . In the TV , she holds up a wheel that she say has been essay all the elbow room to failure :
NASA / JPL
Curiosity locomotive engineer Amanda Steffy holds a steering wheel that has been tested in JPL ’s Mars Yard with the equivalent of many kilometer of drive over damaging terrain . This wheel has been tested to unsuccessful person . It would still function as a wheel , but the sharp edges on the broken parts of the wheel skin could scrape other wheel structure , admit a cable bundle that race across the top of the bike . Stills fromthis video .
you’re able to see that the wrong is concentrated in the center of the wheel , as is happening on Mars . But on this wheel , all of the grousers have snap at the breaker point that they adjoin the morphological stiffening hoop , skip the wheel almost totally around its circumference . A big slip of roulette wheel tread is almost completely detached from the cycle — it ’s only hanging on by a yarn at the inner rim . Only the outer third of the bicycle is still attach to the rigidification ring and the balance of the bird of passage .
It ’s a terrifying image , and yet the odometry marker has not suffered any obvious harm , and the entire rigidifying ring ( which is where the wheel in reality attach to the hub ) is integral . To my eyes , it looks like it could still go as a wheel . I ask Erickson if the rover could still keep ride with a cycle in this shape . He said , yes , it could . But there ’s a stop : “ When it ’s wave back and forth as it drives … it has the potential to be scraping the structure , and there is a cable system that goes to the wheel motors , both repulse and work , that scarper along that support strut . And if this thing start rubbing against those cables , bad thing can happen . you may get short [ circuits ] . If the right hardening of thing were shorted together , it could go back into the drive controller and legal injury that , which assure thing other than one particular wheel : the antenna , the HGA and all the other role that move or work . ” Which sounds exactly how China ’s Yutu rover miscarry on the Moon . That would be bad .
The good news is that a better alternative of terrain can well prolong require wheel lifetime . Erickson told me that they examine wheels on a wide variety of terrains , and come up with the postdate lifetimes . Keep in intellect that these are cautious estimates , because there were no rover drivers working to steer around pointy rocks — this acquire unreasoning driving over all the regretful stone .
Bedrock with mickle of rocks : ~8 kilometers
Lots of rock , not on fundamentals : 13 - 14 klick
fundamentals with few rocks ( consider flag ): 30 - 40 km or more
fluid or arenaceous , with few or no rocks : indeterminate ( causes no legal injury )
No matter the impairment to the wheel , they continued to function just about as well over all kinds of terrain as pristine steering wheel do , until a prominent number of grousers started breaking . ( No grousers have better on Mars yet . ) amass damage will not have a significant core on the ability of the rover to get across any character of Martian terrain — even George Sand — for some time .
4. How can they prolong the life of the wheels?
They ca n’t go to Mars and switch over out the steering wheel . Fortunately , they have identify several ways to reduce the rate at which the wheel accumulate price .
Driving more judiciously . Rover drivers are avoid every pointy rock they can steer around . This only help oneself in the first 10 or 20 meters of a parkway , where they can see humble potentially hazardous rocks . On hazardous terrain , performing shorter drives allow them to avoid many potentially wheel - damaging rocks .
force backwards . When they turn the scouter around , the rover ’s middle and front wheels are drag behind their plump for arms rather than being shoved forward . And the slant of the bogie arm that throw the rover ’s rear wheel is such that it does not experience the same kind of downward force that the front and halfway wheels do when the scouter is driving forward . Heverly show a TV , taken in the JPL Mars Yard , of a trial run cycle being labour over the sharpen metal spike with the scouter driving backward , and the cycle was only indent , not punctured .
There is a cost to take rearwards . At the terminal of each driving , they have to face forwards so as to acquire images of the path ahead for contrive . They ca n’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 backward , they have to turn in office , then drive , then move around in stead again . Each turn in place redact about 6 meters on the rover ’s wheels , or 12 meters for the ride . For short drive ( which is what they do in bad terrain ) , this can swiftly sum up up . The drivers have to weigh the toll of increasing drive space against the potential saving to the bike of driving backwards . Driving backward therefore is most valuable on long “ blind ” drives where the drivers are n’t steering around little rock candy .
retentive - term provision of drive road that primarily span suave or sandy terrain . Because wheel damage does not go on when traversing backbone , the mission is now planning drive routes that queer sandy terrain . Over the retiring several calendar month , mission geologists have used not only high - resoluteness photos but also spectral data point from CRISM and thermal inertia data from THEMIS to uprise single-valued function of the different surfaces along the area between Curiosity and Murray buttes . They have compared their terrain types mapped from orbit to radiation diagram of wheel wear observed on the surface , and name specific terrain types that pose the least risk to the rover wheels . Then the scientists sit with the rover planners to facilitate the rover planners choose itinerary that will aim Curiosity ’s next path toward more benignant terrain .
They use these method to the path between the Kimberley and Hidden Valley , and have demonstrate that scientists can successfully employ orbital information to predict the jeopardy that observe terrain types pose to the wheels . They have also attest that they consistently overpredict the peril — that is , their hazard estimates are conservative . For instance , Erickson told me that some “ red ” terrain that they have cross ( the worst kind , fundamentals plus fortune of rocks ) has turned out to have rocks space far enough aside that bird of passage driver can steer around them , mitigate the hazard . It ’s a salient collaboration between scientist and locomotive engineer . With heady terrain option , Erickson propose they could go 30 to 50 kilometers before see wheel loser . And the higher proportion of the time spent on sand , the better . you could see why it ’s deserving essay to cross sandy terrain like that in Hidden Valley , even if they experience some wheel trip — that kind of terrain is “ free ” in terms of wheel wear . Erickson was sure-footed , based upon the workplace done to date , that Curiosity could complete its first delegation extension without wheel failure even if they drive on the bad possible terrain . By being choosy about terrain , they can hold out significantly longer than that .
Changing driving software to reduce the forces experienced by cycle hanging up on pointy rock . This one has not been carry out yet , but Erickson recite me they are trying to develop and essay some software package mending in time for the next rover software program update , scheduled for December or January . The rover can smell out wheel stream , so it can smell out when a wheel is sticking . Also , the wrong may be aggravate by the fact that the software program requires all six bicycle to rotate at a constant rate , even though a wheel climbing an obstruction has a long path to travel than one cut through plane background . By implementing a “ smart controller ” on the steering wheel current and appropriate wheel rotary motion rate to vary intelligently in reply to perceived conditions , they might be able to palliate the damage .
NASA has a long history of rewrite package to enable deep ballistic capsule to do things they could n’t antecedently do ; Erickson ferment on Galileo and all the software development that was needed to save the mission after its high-pitched - profit antenna failed . But Curiosity , Erickson sound out , is much more reprogrammable than premature mission , really a “ software - defined spacecraft . ” He said “ There are lots of things we can change in software , peculiarly anything to do with controlling motors . ” The flexibility of Curiosity ’s software has sometimes been a problem , of course of instruction , because it add together to the commission ’s complexity . “ The more complicated the software system , the more likely you ’ll not get everything perfect . You ’ll get surprises . Both in ontogeny / trial run and in operations . Even how it responded to the twinkling failure on sol 200 was a surprise , and we proceed to get surprised . ” But this is a situation where the flexibility will help , allowing them to redesign the way the rover works in reception to a potentially mission - terminate jeopardy that they never anticipated .
5. Why didn’t they foresee this problem?
There were several factors that motor them to plan the wheels to be as lightweight as potential . The large size of the wheels stand for that very flimsy design changes lend a substantial amount of pile . Increasing wheel heaviness by one millimetre would add 10 kilograms to the rover ’s full batch . But total organization mass was n’t the only restraint . Erickson explained that a major constraint arose from a tricky mo in the landing place sequence , at the moment that the wheels deploy , while the rover was suspended from the bridle underneath the descent level . The wheels ’ sudden drop imparted substantial forces on the mobility system , and observe wheel mass as light as potential reduce those forces to manageable ones . There were other factors that made it authoritative to keep wheel mass down in the mouth .
So the wheels needed to be as sluttish as possible while still being able to do their job , but as to their line of work : “ We misconceive what Mars was , ” Erickson said . “ Strongly cemented ventifacts are not something that we saw on Mars before . ” They designed Curiosity to handle all the challenge that Spirit and Opportunity had experienced , specially sand , which Curiosity cut through substantially better than her predecessors . “ This fomite is able to get itself out of situations that MER could n’t ; it ’s got more floatation than MER had by a substantial margin . ” They designed Curiosity to treat the gumption bunker , savorless basics , and rocks - perched - on - sand landscapes seen by all the previous lander . They just did n’t conceive of the possible action of the peculiar and never - before - escort terrain type that they found in Gale volcanic crater . “ There are [ places ] on Earth that do have these piercing ventifacts , but we had n’t seen them on Mars and we did n’t quiz against them , ” Erickson allege .
6. What are they changing for Mars 2020?
Erickson did not have specific insight into how the bicycle aim is being changed for the 2020 mission , because he is not direct involve ; but the design is definitely being changed . Erickson said that they had already build up several solutions and are now in the procedure of endeavor to identify the best solution .
I personally believe that there ’s another elbow room that Mars 2020 can prevent this kind of problem , besides redesigning the wheels . That is : select a landing place situation where they can strain interesting rock targets inside the landing oval , rather than having to swan out of the oval for obtain good object . afford that Curiosity only exited its landing oval at the remainder of the prime mission , I recall that the scientific biotic community that will participate in the Mars 2020 landing internet site survival of the fittest will place a eminent note value on landing place land site with in - ellipse science . There were already strong advocates for non - go - to situation in the Curiosity landing place site choice cognitive process ; they will palpate justify and emboldened by Curiosity ’s challenges . It would trammel the landing site option , and I ’m sure there will be a pile of debate about the scientific trade - offs . But , thanks to the oeuvre of the four innovative Mars orbiters , we know much more about Mars than we did when Curiosity ’s landing site were being selected , and I believe that the biotic community can situate a compelling landing site for Mars 2020 that will not require quite so much driving to accomplish the rocks of interest .
At the end , I expect Erickson to put the wheel problem into context with his experience on many other mission . He said that the trouble of damage to Curiosity ’s wheels has definitely had a important encroachment on the deputation , and mentioned for comparison the roulette wheel failure on Spirit , when they had to start dragging the ripe front wheel behind them and motor exclusively backwards . But the Curiosity problem is not as bad as Spirit ’s because Curiosity is no less peregrine than it was before . They can pick out to accept wheel hurt if they determine the scientific value to be deserving it . So while Spirit ’s mobility problems limited the scope of what the bird of passage could do , Curiosity ’s mobility problem do not — at least , not directly . The biggest effect of the wheel damage job is to slow the delegation down . And that ’s what will limit how much wonder accomplishes . By not traveling as fast , and by accept to restrain their path choices , the amount of geographic expedition that they can do is necessarily less than if they could go gallivanting across the bedrock outcropping at will .
The slow down pace of the delegation is frustrative , but that ’s the way of life it is . The well news is that the commission went from being surprised and dismayed by unexplained damage , to a full intellect of what ’s make the damage , and of what they have to do to prevent the bike impairment from prematurely bring the military mission to an destruction . “ It ’s just one of these case where Mars is proceed to give us a new deal , and we ’re run short to have to dally the posting we get , not the ones we desire , ” 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 hardening of instruments to the good stuff . aright now we ’re drive from eatery to eating house . But we ’re about to get to a smorgasbord . We ’ve got a pile of things to nibble from there . Instead of driving from office to position , we ’re going to scrunch down and start pigging out . ” We can expect fore to much more drilling — and much more in - situ science — once Curiosity ’s bid wheel finally bring in her to the careen of Mount Sharp .
We may already be there . Curiosity is drilling this calendar week , into a rock that the mission ’s geologist have map 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 source of the science that specifically brought Curiosity to the landing site in Gale crater .
Read More : Here is a conference theme ( PDF ) by engineers Sean Haggart and Jaime Waydo discover the Curiosity wheel design in point .
This post was written by Emily Lakdawalla andfirst come along on The Planetary Society website . It is republished with kind permit .
Emily Lakdawalla is a passionate counselor-at-law for the exploration of all of the worlds of our solar system of rules . Through blogs , photos , videos , podcasts , print clause , Twitter , and any other medium she can put her deal to , Emily shares the escapade of space exploration with the world . Sheholds a Bachelor of Arts level in geology from Amherst College and a Master of Science point in worldwide geology from Brown University . You canfollow here on Twitter .
The Planetary Society has inspired 1000000 of people to search other world and seek other aliveness . Today , its international membership makes the non - governmental Planetary Society the largest space interest group in the creation . Carl Sagan , Bruce Murray and Louis Friedman establish the Planetary Society in 1980 . Bill Nye , a farseeing time member of the Planetary Society ’s Board , serves as CEO . You canfollow the Society on Twitter .
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