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Headshot of Emily Lakdawalla

Yes, there seems to be a hole in Curiosity's left front wheel, and no, that's not a problem

Posted by Emily Lakdawalla

02-10-2013 23:22 CDT

Topics: pretty pictures, pics of spacecraft in space, mission status, spacecraft, Mars, Curiosity (Mars Science Laboratory)

Some brand-new images just arrived from Curiosity on Mars, and two of the most recent are Mars Hand Lens Imager (MAHLI) images of the wheels. Curiosity uses her arm-mounted camera to check on the status of her undercarriage from time to time. I absolutely love these photos of wheels sitting on the surface of Mars -- there's something both comforting and thrilling about seeing those pieces of human-wrought technology sitting firmly on the dusty ground of another planet.

However, today's images contained two little surprises. In one photo, you can see that the left rear wheel appears to be lifted slightly off the ground -- it's perched on a little rock. More worrisome, though, is this view of the left front wheel, which clearly has a hole in it. You can see Mars right through it. But let me spoil the tension right now: holes in Curiosity's wheels are expected and are absolutely not a problem at all. Don't freak out.

Curiosity's left front wheel, sol 411


Curiosity's left front wheel, sol 411
Curiosity took this photo of its left front wheel on sol 411 (October 2, 2013). A hole in the wheel is visible. Holes in the wheels are not a concern for the Curiosity mission; such wear and tear is expected, especially in the thinnest areas of the wheels between the treads.

Here's a zoom in on the hole that's been punched into the wheel by some random rock or another:

Detail view of Curiosity's left front wheel, sol 411 (October 2, 2013)


Detail view of Curiosity's left front wheel, sol 411 (October 2, 2013)

Now that you've seen the big hole, look closer at the whole wheel and at the sol 411 photo of the other two left wheels and you will discover various other spots where you can see bright Mars in holes punched right through the wheels, which are clearly similar in thickness to soda cans. I can understand why this would cause some consternation among viewers. We spent two billion dollars to put this thing on Mars! Couldn't we have sent it with better wheels? Is this going to endanger the mission?

Let me answer the second question first: no. It's not going to be any kind of problem at all. All the wheels are required to do is to rotate and to grip the ground. The motors in Curiosity's wheels (each one has its own motor) have insanely high torque. I have it on good authority that if Curiosity had square wheels, she would still roll across Mars just fine. They won't ever be square, but they may become oval, and that will be okay, according to official documents from Curiosity engineers. If you took the wheels away and just had those six-spoke titanium spindles she would operate just fine. Round wheels are only really a requirement if you want a vehicle to be able to coast frictionlessly. We never want Curiosity to coast, because then we wouldn't know precisely where she was. Curiosity's top speed is the same as Spirit and Opportunity's -- barely 5 centimeters per second. Most of the time, she grinds along at 1 or 2 centimeters per second. Get up from your computer now and try to walk at 2 centimeters per second. That's not walking. It's shuffling. It's almost standing still. And you don't need round wheels to be able to shuffle; you need to have your feet firmly connected to the ground. Curiosity's wheels could be just about any shape and still be firmly grounded. The wheels are going to be thoroughly battered by the time the mission is over, but they'll still be essentially round and have a lot of raised tread and a lot of grip; she'll be fine.

Okay, now for the first question. Couldn't we have sent Curiosity with better wheels? Sure, we could've. Curiosity was built from scratch; the wheels could've been made any thickness, from any material. But making them heavier and thicker would've added weight without improving her ability to accomplish her mission. And adding weight adds cost -- or even worse, it requires you to remove weight somewhere else on the spacecraft. Spacecraft engineers don't add weight without good reason; they shave excess weight off anywhere they can, so they can use that precious mass to make some other part better that needs to be better.

It seems really wrong for there to be holes in Curiosity's wheels. But it was actually the right thing to do, to allow that to happen. So don't freak out. Enjoy those holes as badges of honor, marks of the miles traversed already.

Go Curiosity!

See other posts from October 2013


Or read more blog entries about: pretty pictures, pics of spacecraft in space, mission status, spacecraft, Mars, Curiosity (Mars Science Laboratory)


Patrick Wiggins: 10/03/2013 01:20 CDT

Hi Emily, In addition to saving a bit of weight the series of squarish holes that were intentionally put in the wheels serve two other purposes. One is to help controllers visually confirm how far the rover has moved. But the other (my favorite) is that the holes leave Morse code impressions in the regolith spelling "J P L". More on this at the following URL once the ludicrous situation in DC is resolved and NASA's websites start working again.

Kenny Chaffin: 10/03/2013 07:01 CDT

so much dust there....

Donald Morrow: 10/03/2013 09:33 CDT

I just posted this in your other blog post stating that Curiosity is still operating, even with the government shutdown. I was hoping to get an update that she is still moving along towards Mount Sharp. How far did she drive on Mon, Tue and Wed? Or has she stopped for any other reason?

Philip Metschan: 10/03/2013 11:52 CDT

So then the next obvious question should be...why does she have wheels at all?

Casey Dreier: 10/03/2013 11:54 CDT

Donald: the website will answer your drive question, though it doesn't provide the reasoning for each drive record.

Gene Van Buren: 10/03/2013 12:01 CDT

My brain must be weak. I cannot figure out where there's an unintended hole in the "zoomed" image. Can someone describe in words where they see a hole there? I do see a small slit hole in the larger image just above the axle on the far side of the wheel, at normalized image coordinates of (~0.35,~0.60) with the origin at lower left. But that hole isn't visible in the zoom.

MarsFKA: 10/03/2013 01:48 CDT

Kenny, the dust will always be a problem for Mars exploration. On Earth, the dust ends up in the bottom of the oceans and lakes, but on Mars, it has nowhere to go and so it just blows around the planet, year after century after millennium. The dust grains are worn down until they are so fine they make talcum powder look like gravel. This is why the winds in even an atmosphere as thin as Mars' are able to blow up dust storms that can blank out the entire planet, as Mariner 9 found when it arrived in orbit in November 1971.

hugo : 10/03/2013 04:02 CDT

wish that the wheels are not compounded over that ...

Emily Lakdawalla: 10/03/2013 05:49 CDT

Patrick: I'm not talking about the intentional holes, I'm talking about the puncture located above the intentional holes. Gene: Look to the left of the "dash" hole, at dead center of the photo, and you will see a large tear, with a stripof the wheel actually ripped, in the direction of the viewer. There is a shiny glint off of the strip where it is reflecting sunlight into the camera. Where the strip has lifted, you can see Mars right through the hole.

aswy: 10/03/2013 08:40 CDT

Isn't it likely that most of the damage was done during initial landing? The wheels had the opportunity to hit unfortunately placed sharp rocks on the ground at more than a few cm/sec. Maybe there is shock data from onboard sensors?

Emily: 10/03/2013 09:48 CDT

No, it was done by driving a rover on paper-thin wheels across several kilometers of Martian surface. Compare to earlier images of the wheels before they drove so much to see the changes. They looked quite clean right after landing (sol 34) and not too bad even on sol 274.

Gene: 10/03/2013 10:38 CDT

Emily, thanks for pointing me to the raised strip, which even casts a shadow that helps to understand it spatially (definitely a crack and some bent metal). It is indeed rather thin, much thinner than the thickness visible at the intentional holes, obviated here: So this wheel's got at least two punctures.

Dan Griscom: 10/03/2013 11:15 CDT

I think you're overstating how little Curiosity needs these wheels. Remember what happened to Spirit? If Curiosity's wheels get completely trashed, it will become more vulnerable to getting captured by a sand trap.

Janet : 10/04/2013 11:42 CDT

Those wheels are tougher than they look! One of the videos I remember from the Curiosity landing event at the Museum of Flight (Seattle) was a stress test on one of these wheels. They weren't just built to be wheels, but also landing gear, and they didn't assume that the sky crane was perfectly gentle, either.

Dan: 10/06/2013 09:25 CDT

From the photo link that Gene provides, it's clearer that the wheels have a solid frame structure that ought to hold up to a lot to stress. It looks like they could lose a lot of the thinner area and still be wheels. They might look like skeletons of wheels but they would still work. The thin areas though look ridiculously thin.

Andyj: 10/07/2013 05:12 CDT

Emily, don't protect anyone on this or eyes will be on you too. The actual skin of the wheel is structural. If not, why is it there in the first place and not fully machined away? . NASA continuously tested an MSL on what pitifully resembled a kindergarten play pit with a few soft pebbles thrown in. Sure it ripped due to its weight, on sand (not spiked pebble punctures). Which on lessons learned with spirit, they avoid sand like the plague. Duh! . Wherever I look on the MSL box it is built lovely and strong. Nothing is less than 3mm thick right down to the stout, sturdy legs before these jokes called wheels. The 3mm thick rule impinges on much of the non structural items too! . With the inner wheel skins ripped up and an off-centre load we find a 60Kg+ load (per wheel on Mars) supported by the remaining ~24 bits of ~3 mil square Aluminium tread. Each say 20cm long from the drive rim to each outer rim with tears and cracks all around them. If that's not asking for a disaster (x6), nothing is. . A proper redesign would of added very little mass indeed which could easily of been shaved off elsewhere with very, very little trouble. . NASA has stated this is not a recipe for disaster to this $Bn+ rover? Every person who works on aluminium aircraft production knows *any* stressed rip point is the part that will eventually bring the aircraft down. Once Al rips, it tears more and more. No tear cannot be engineered in as a known quantity. You will find the rate of wheel damage will speed up now. . Sure I'll hazard a guess it will reach it's destination but more? . While I'm on a rant. Remember the Kaptan tape? They lauded it must of come off the sky crane. Rubbish! That's beyond astronomical odds. Blame MSL. It is covered with external wiring strapped over many moving parts. My first guess would be a motor cable after they rotated to face the sand. Then the mast cabling. Then then gauze issue. No cold work hardening math gone into MSL at all?

Gene Van Buren: 10/07/2013 12:34 CDT

Just an addendum that the photo link I provided earlier seems to also show a small slit of light coming through a hole in the middle wheel too. The MSL team likely has a more accurate count, but we can at least spot three holes in two wheels.

Rob Dorsey: 10/16/2013 01:04 CDT

Andy said "The actual skin of the wheel is structural. If not, why is it there in the first place and not fully machined away?" I must disagree with this as he is not taking into account the engineers no doubt kept the thin webbing between the much stiffer treads to "float" the wheels in soft material. An excellent notion. Also Andy's assertion that aircraft designers know and allow for "stressed rip points" is pure fiction. No engineer allows for known rip points in material unless the vehicle is intended to fail for testing purposes. Curiosity is meant to last beyond mission and hopefully forever. No such rip points would be design tolerated. I know that this sounds like "beat up Andy" but he said a lot, some of it observational, some poppycock. I am concerned by the rip, not the hole, and a crack can propagate in Al at virtually any heat treat but generally stop when the level of stress is lowered, either by stop-drilling the end of the crack - something I imagine is beyond Curiosity's capabilities - or that it propagates to a point where the stresses are not sufficient to continue the process. If sand was the cause of the travel of the crack then keeping sand time to a minimum should keep the crack from becoming a problem, which at this state it is not. Apologies Andy, just trying to keep it factual, an often unpopular pursuit. Best To All (first post) Rob

Donald Barbee: 10/16/2013 02:35 CDT

Actually there appears to be another tear that you can actually see through just to left of the hub of the axle, in the shadow of the top portion of the wheel!

Phil: 10/16/2013 04:16 CDT

Great photos of the wheel and apparent "hole". What can you tell me about the camera taking these pictures (make, aperture setting, etc.) I am a retired engineer who was on the original Mars mission Viking with it's ITEK manufactured facsimile camera. (one per lander) I worked for ITEK, and then Martin Mariatta during the Mission Ops phase of the project at JPL in Pasadena, CA. Thanks for your help.

James: 10/17/2013 09:05 CDT

All in all, I believe that there is still many years of life in Curiosity's wheels. Hind sight is always 20-20. Just remember at the o-ring seal redesign that was taking place before the Space Shuttle incident. Years of testing wheel designs resulted in the ones that were installed on Curiosity. I'm sure there are better and more durable ones now available. We just need a human up there to change them out.

Emily Lakdawalla: 10/17/2013 03:35 CDT

@Phil: You can read all about it here: The paper describing MAHLI (as are many of the official papers describing Curiosity instruments) is open-access. To those of you who claim the engineers didn't foresee the punctures and that the wheels are badly designed: evidence that you are wrong is in the link I posted in the original article. But I've been receiving emails for years from people telling me *Opportunity's* wheels are badly designed and the fact that she's driven 38 kilometers on those "badly-designed" wheels hasn't deterred them, so I'm resigned to people arguing about Curiosity's wheels forever too.

Mike H: 10/22/2013 05:06 CDT

I have high confidence in our NASA engineers. Truly. As long as they keep their metric and US measurements straight. ;)

ToSeek: 10/22/2013 07:31 CDT

Curiosity's top speed is actually less than that of the MERs - 4 cm/second vs. 5.

Andrew Jones: 12/01/2013 01:06 CST

For Rob Dorsey: I'm not sure you read Andyj's post succinctly. You replied, "Curiosity is meant to last beyond mission and hopefully forever. No such rip points would be design tolerated." May I inform, the wheels ARE tearing up. As Andyj stated (disjointly) the MSL team do and will deliberately avoid driving on any sand if possible. Worse still if the drums are completely torn up, sand is not an option. So what is the the 0.03" wheel skin for if it's meant to tear itself to pieces in use before crossing the sandy stretches before the mountain? My take on the design is the lack of thought behind machining and structural strengths - over all the vehicle! The treads ought to of crossed over (diamond) each other to multiply the load spreading and bearing. Not half aimlessly mill out the same very easy to machine tread most way around the wheels. That was lazy! Andyj is basically right but I'd dispute his use of "allow" in that sentence. However, tearing in aircraft grade alloys are not structural at all. Your point about drilling a hole to stop further tearing is not a viable fix - Especially on Mars! not unless the holes were pre-drilled but they were not. The treads are what, 3x5mm? The existing wheel drum accommodates the wheel torque to protect what he said will effectively become long pieces of wire if all the drum is busted through. The vehicle is now one year old and travelled a mile. If it survives 14 years multiplied by the 6 month damage rate, it's going to be one interesting hobble as the media make lots of fun about it. I'll agree with him. These wheels are a bad compromise between weight, politics and pussy footed handling on easy tracks of soft sand and round pebbles. A huge amount of weight could of been skimmed off elsewhere with no structural risks at all. If they doubled the wheel drum skin thickness it would be :- 6 wheels 0.0075cm thick 2.7g/cm^2 density of Al 50cm dia x pi circumference 40cm wide wheels 6*0.0075*0.0027*60*3.142*40 = 0.916

SpaceKadet: 06/22/2014 01:17 CDT

Regarding the condition of Curiosity's wheels. I believe that this damage has not been caused by sharp rocks, rather the damage has been caused by the intricate complex steel 'mesh' (for want of a better word) the rover is travelling over. Also, your explanation of the speed of the rover does not align with the speed of Curiosity published in Wikipedia

Max: 08/27/2014 06:20 CDT

Can the next mars rover project deliver new wheels and replace the worn out wheels

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