Looks like a little slice of a baby Burns Cliff in places.

Doug

I was just surfing the sol 734 images and noticed one of them shows a dune that's rather dark:

http://marsrovers.jpl.nasa.gov/gallery/all...NTP2446R2M1.JPG

Since it's closer than the cliffs, could this be the target Zane Grey?

The dark-toned dune is Payson. Zane Grey is the light-toned patch at the right midground on the image you referenced.

--Bill

I don't understand why they completely stow the arm as soon as a long drive needs to be done. Why is this so important ? Do they really think something will break while the arm is in "half stowed" position (over the solar panel) ? They drive 5 mm's a second. It's not " bumping", it is like a snail creeping across the terrain ! I've been asking this to myself for a long time. Maybe i oversee something, but the risk of another winding breaking in the faulty joint (thereby completely disabling the possibility to UNstow the IDD for the rest of the mission) looks much higher to me, than the chance of breaking something because of driving in the alternative (half stowed) position. There are two primairy windings in these motors aren't there ? There's only one left. And forcing a higher current trough it at this stage (while materials are getting old) doesn't make sense to me. I'm puzzled. It has been unstowed AFTER a pretty bad situation of stalls and erronous behaviour. LEAVE IT THERE ! Or maybe i'm not informed well enough....

If you hunt back in the movie archive at the JPL site, you'll see some rover driving videos - ditto the NOVA programs - and given that there's essentially a tiny tiny bit of suspension within the wheels but nothing else - it's quite a rough ride even over the fairly flat rocks and terrain we have here.

Given that suprisingly bumpy ride, the mass of the instruments on the IDD, and the leverage they'd have on the joints and motors when in the 'hover' position, there is a real risk of damage to IDD joints, motors, even the front of that array and the nearest instrument.

Hence the high-park for short drives, and the proper drive for longer driving campaigns.

Doug

The simulations that have been run with an unstowed arm involve a single wheel sideslipping off a rock and free-falling some distance. This scenario isn't too unlikely, even when driving slowly. And Doug is right, the "suspension" is very hard so the rover and arm get a good jolt when that happens. The worst thing is that the unstowed arm gets thrown sideways as well.

The arm design was highly optimized to stay within a very tight mass budget (~4 kg for the arm, and the instrument package is ~2 kg). Because of those constraints, the arm was never designed to take these kinds of loads. A single-wheel drop off a 3-centimeter rock edge gets pretty close to the documented capability of the arm when in the hover-stow position.

Something else to remember is that the joint 1 winding failure was almost certainly caused by thermal cycling (remember the stuck heater on Oppy's shoulder). So the expectation is that the joint will fail completely from a thermal cycle, not from operation. So for longer drives, where they can't see the terrain in advance, it is considered fairly safe to stow the arm before driving and then unstow after the drive. (Of course, there's still the occasional stall because it's tricky operating the motor with a broken winding.)

Hope this helps explain the logic behind what they are doing.

Perfect!

I am surprised how tricky it seems to be operating the broken motor, I would have expected it to be very predictable. But then what do i know, I'm a scientist not an engineer!

James

They said that they're running the arm by giving it more current. I'd imagine that feeding it too much current all the time would just burn out something, or at the very least, heat it up.

I suppose one thing the stowing problems might mean is that the rover team may be less willing to unstow the arm and use it on passing terrain unless there is something really interesting to look at, meaning fewer stops on the way to Victoria Crater. That should please some people!

Actually, the present strategy means the arm is never permanently stowed. It only gets stowed during a long drive, then immediately unstowed to the ready position at the end of the drive. This is to ensure that the arm is unstowed if/when the joint 1 motor finally gives out.

The joint 1 motor (with the broken winding) is difficult to operate because its electrical properties fluctuate as the motor goes through a turn, as the broken wire commutates. This only causes trouble when the motor is starting or stopping, as it has to overcome the magnetic detents.

Question: does this mean that when the joint motor does fail and the arm can not be stowed anymore, there will be no more long drives for Opportunity? And can we conclude from this that the team has decided that a rover without IDD has less scientific value than a relatively static platform with IDD?

I've taken the time to properly rework the current power generating capability of both rovers. You can see the charts in this post on the Oppy vs Spirit power consumption thread .

Opportunity really is in a very healthy power state right now and will continue to generate more than 457 whr/sol throughout the SH winter season. The chance of anything changing that before the onset of Spring begins to add dust back into the atmosphere is very low. Tau should begin to rise again around Sol 1000/November 16 2006 but by then insolation will be rising fast. The risk of a killer storm after that remains at around 20% but that's a long way off at the moment.

She has no particular need to rush to a wintering spot and should in general be able to drive for more than 90 minutes per Sol. That means that I'm pretty confident that she'll reach Victoria even if she is limited to 20m/sol provided nothing significant breaks. Given that I think it's fair to say the current caution with the arm is very prudent.

Spirit on the other hand absolutely has to get to a favourable wintering spot very soon or she'll find herself unable to move.

One solution for keeping the unstowed the arm with minimal harm due to the surface vibration caused by the unlevel surface and/or passing over any stones is that Oppy must look another ways that is sand. The sand surface are very, very smooth, it is very very nice to drive over that and it is almost alike to surf among sea waves. That is try to travel close to the ripples of sand if the center have outcrops.

Rodolfo

I'm wondering about this too. I certainly hope not. Even without the IDD arm the rover still has all the cameras except the MI, plus the mini-TES. The RAT is at or near the end of its useful life. The whole point of having a rover is to have mobility, so to be mobile should have priority, and the sooner the better.

And very 'Purgatory' They were hunting for rock to drive on around Erebus ( remember the Erebus highway? ) - better to park the IDD, have a slightly rougher ride, but stick on the rock.

Doug