Slashdot Mirror
Wheel Damage Adding Up Quickly For Mars Rover Curiosity
An anonymous reader writes: The folks in charge of the Mars rover Curiosity have been trying to solve an increasingly urgent problem: what to do about unexpected wheel damage. The team knew from the start that wear and tear on the wheels would slowly accumulate, but they've been surprised at how quickly the wheels have degraded over the past year. Emily Lakdawalla at the Planetary Society blog has posted a detailed report on the team's conclusions as to what's causing the damage and how they can mitigate it going forward. Quoting: "The tears result from fatigue. You know how if you bend a metal paper clip back and forth repeatedly, it eventually snaps? Well, when the wheels are driving over a very hard rock surface — one with no sand — the thin skin of the wheels repeatedly bends. The wheels were designed to bend quite a lot, and return to their original shape. But the repeated bending and straightening is fatiguing the skin, causing it to fracture in a brittle way. The bending doesn't happen (or doesn't happen 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 ground is utterly impervious to the rover's weight — hard bedrock. The stresses from metal fatigue are highest near the tips of the chevron features, and indeed a lot of tears seem to initiate close to the chevron features."
The planned mission duration was 2 years not 3 months.
http://en.wikipedia.org/wiki/Curiosity_%28rover%29
You are thinking about Spirit and Opportunity, whom both enormously exceeded their planned mission duration.
In short: Because they aren't idiots and know enough about this field to make informed comment. The rover has reached its planned mission life, everything beyond this is a bonus. The wheels survived and will likely, with proper management, last considerably longer still. It's a great success.
Your comment on the other hand is a great example of how people who are ignorant on a field automatically assume it must be simple and that they have some valuable insight. You know when you hear people who don't have a clue say something stupid about something you know a lot about? That's you when you comment on wheels for vehicles travelling on other planets (unless you'd like to point out what makes you remotely credible in this field).
Plastics don't do very well in a vacuum like atmosphere full of radiation with wide temperature swings in the long term. Plus the low average surface temperature of -82F/-63C makes plastics less malleable and in many cases, brittle.
In the low atmosphere they can become brittle from outgassing and are susceptible to cracking and can simply shatter like glass. Nylon wire ties in a vacuum chamber simply fall apart after a few months. Though the 6 mbar (4.5 Torr) Atmospheric pressure of Mars isn't a hard vacuum, it is still 0.6% That of Earth's average sea level pressure.
Then you have radiation degrading the plastics which again makes them brittle. A friend worked on RHIC out in Brookhaven National Labs and since he was small and skinny he was tasked with changing out a lot of the sensor cables on the ring. The insulation simply disintegrated from radiation. There was nothing they could do about it save for bulky shielding which would have made servicing impossible.
In the end, metals are simply more suited to the task.
The landing site was chosen because it allowed it to fulfil its primary mission AND had a more interesting secondary mission than other landing sites. The mountain was not the primary mission. The primary mission is long since complete. Please don't re-write history.