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

Posted by Emily Lakdawalla

2013/10/02 11:22 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.

NASA / JPL / MSSS

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:

NASA / JPL / MSSS

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!

Comments:

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. https://www.nasa.gov/mission_pages/msl/news/msl20120829f.html

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 http://curiositylog.com/ 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: http://www.americaspace.com/wp-content/uploads/2013/01/Curiosity-wheels.jpg 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.