Of course I wasn't counting you, Doug.

Sol 472-473 Update On Curiosity From USGS Scientist Ken Herkenhoff: Watching the Wheels:

Besides monitoring, whether there ever could occur wheel damage justifying some concern:
Can wheel "damage" be quantified in a way, such that the wheels can be interpreted as an additional mechanical science instrument to retrieve some statistics about an appropriate physical/geological property of the traversed surface?

Yes it can and just such an experiement was done using the Sojourner Rover. It was called the Wheel Abrasion Experiment (WAE). Thin films of different metals were emplaced on the right centre wheel. A photovoltaic cell was used to periodically measure the reflectivity and record the level of abrasion thus characterizing the surface properties.

Thanks a lot! So there is a good chance, that we get additional science benefit from the wheel monitoring.

A small question: could the recent wheel status worsening be related to the long, high average speed drive on Sol 472?

No - 472 was the last drive, so we have not seen any MAHLI images since then. The 472 images are from before the drive.

Phil

There's no such thing as a 'high speed drive'. When in motion the rover is typically at the same speed. 3cm/sec.

What you're perceiving as 'high speed' is a blind-drive ( no stops ) compared to autonav ( many freqent stops )

The speed of the rover when in motion is the same regardless. Go look at some speed plots at http://curiositylog.com/ to see the typical speed over time of the rover.

I would hesitate to call this science. At best we get a crude idea about rock size statistics, cruder I expect than what you could get just by counting rocks in images.

Thanks Phil/Doug, I already linked Joe's speed plot in my previous post, however I had the (probably wrong) impression that constant "high" speed with no stops can cause more damages!

lt's been said before. The most likely manner of damage to the wheels is the rover climbing up one rock and dropping off onto another. No direct evidence, just logic.

I doubt there is a huge amount of straight scientific data about the surface to be gleaned from examining the wheels. However, there is good engineering data about the interaction of this particular wheel design with a fairly representative Martian surface to be acquired.

There has always been a dissonance between "pure" scientific data gathering during space flights, and the acquisition of engineering data that can be useful in the design phase of the next vehicle to come along. Both are "scientific" goals, but the engineering data is used to both improve future spacecraft and to better understand and improve our engineering models (which usually led to the designs and materials being used and evaluated).

Or, to put it another way, the "pure" scientific data is usually all about studying the environment we've come all this way to look at, while the engineering data is to look at the systems that got us there and keep us running so we can pursue the pure science goals.

-the other Doug

Well said Doug, and both areas of analysis are of equal importance on a mission.

Why you would come to that conclusion I don't know - but yes - it's wrong.

What surprises me about the wheel damage isn't the dents. That would be expected in thin aluminum. But the gaping holes are surprising to me. It's like the alloy is very brittle.

I assume (with no evidence to support it) that the wheels are 7075 alloy, which is somewhere near the upper end of aluminum alloys for fracture toughness -- http://en.wikipedia.org/wiki/Brittle_strength . http://en.wikipedia.org/wiki/Fracture is a useful intro to various types of fracture; see "ductile fracture". I'm not a mechanical engineer or a materials scientist and a hole is a little surprising to me too, but I'd rather see a clean hole than a big propagating rip.

To anticipate a question, aluminum is less subject than say, steel, to low-temperature embrittlement. See http://www.keytometals.com/page.aspx?ID=Ch...e=ktn&NM=23 -- "Below zero, most aluminum alloys show little change in properties; yield and tensile strengths may increase; elongation may decrease slightly; impact strength remains approximately constant. Consequently, aluminum is useful material for many low-temperature applications."

To sum up (note that this is all my opinion derived from public information because I've intentionally not looked at any project-internal sources on this): yes, there are holes in the wheels. Some level of wheel damage was clearly anticipated by the designers. There are a lot of tradeoffs in the MSL wheel design and it may well have not been feasible to make wheels that were impervious to puncture. Imaging the wheels is being done, presumably to assess the amount and evolution of damage. Exactly what the level of concern is, if any, is something for the project to say.