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Extricating Spirit, Digging out from Troy
djellison
post Apr 11 2010, 03:05 PM
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It's a see-how-we-go situation.

If the thing gets cleaned to 800Whr+ come spring, can you really imagine them sitting still and not trying to get out?
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elakdawalla
post Apr 11 2010, 03:59 PM
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Scott told me that they can actually move some centimeters without affecting the radio science campaign, so they can continue working on extrication efforts up to a point even while they are considered "stationary." In the event that they find they are successful and could move so much that they would ruin the radio science experiment, my impression is that the radio science experiment would be completed first, but obviously that's a decision that would be made if and when they actually face that situation.


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centsworth_II
post Apr 11 2010, 06:52 PM
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In view of all the signage going up on Meridiani, I thought Spirit could use one.

Attached Image

Using Stu's "Stuck" image.
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Guest_Bobby_*
post Apr 11 2010, 11:14 PM
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Guests






Does anyone have an image of the area taken by Spirit where she is stuck now? I would like
to see a ground level shot of where she is now and what direction she is going from that spot.
There is probably an image somewhere in an earlier thread. I just want to know if she has
to go up hill from where she is now to escape the sand trap???

Thanks
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djellison
post Apr 11 2010, 11:23 PM
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QUOTE (Bobby @ Apr 12 2010, 12:14 AM) *
Does anyone have an image of the area taken by Spirit where she is stuck now?


The post previous to yours - with added rover for scale and perspective - is EXACTLY that.
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Guest_Bobby_*
post Apr 12 2010, 01:28 AM
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Thanks Doug.
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tanjent
post Apr 13 2010, 03:39 AM
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QUOTE (elakdawalla @ Apr 11 2010, 11:59 PM) *
Scott told me that they can actually move some centimeters without affecting the radio science campaign, so they can continue working on extrication efforts up to a point even while they are considered "stationary."


Sols 1406 to 1782 (=386 sols) saw only small movements down the slope at Winter Haven 3. Tesh's maps are not quite accurate enough to estimate the minimum number of centimeters moved over a six month span, but whatever it may be, isn't that an upper bound for the amount of permissible movement during the core composition experiment? IIRC Spirit was able to call home during the previous winter, and yet the stationary observations that could have been done then were apparently not accurate enough. Anyway, it seems to me that if we are going to perform such a sensitive and difficult-to-repeat experiment, the observation error ought to be reduced as small as we can possibly make it by just keeping the rover stationary. I don't understand how some error can be permissible and more error can be impermissible.
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stevesliva
post Apr 13 2010, 05:07 AM
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QUOTE (tanjent @ Apr 12 2010, 11:39 PM) *
I don't understand how some error can be permissible and more error can be impermissible.


I can speculate. The wavelengths are 2-4 cm and the frequencies are 12-8 GHz. A few cm movement of the rover isn't a big deal... it's still in the same place as far as the experiment's accuracy is concerned.
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ElkGroveDan
post Apr 13 2010, 05:10 AM
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There was an entire press conference on this topic and they went round and round on the issue of small movement. Ultimately, what Scott told Emily was described as within the tolerances of the experiment. I don't see why that is difficult to understand. Have you ever used a measuring tape to find the length of something? Did you stop to worry that the molecules at the end of the tape are vibrating back and forth the entire time? It doesn't affect your measurement though because that movement is within the tolerance you have accepted for that datum.


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Stu
post Apr 13 2010, 05:48 AM
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QUOTE (ElkGroveDan @ Apr 13 2010, 06:10 AM) *
Have you ever used a measuring tape to find the length of something? Did you stop to worry that the molecules at the end of the tape are vibrating back and forth the entire time?


Doug and I measured out the whole solar system with a tape measure last year, and I can honestly say that by the time we got down that long hill to Pluto, the state of the molecules at the end of the ******** tape were not foremost in our thoughts...! laugh.gif


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tanjent
post Apr 13 2010, 03:26 PM
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Is there a transcript of the above mentioned press conference available somewhere? I am interested in this experiment and I want to understand it better.

Meanwhile, limping along on what I have read and heard, I am reasoning as follows:
To build on the tape measure analogy, measurement error can usually be discounted when the question at issue is binary.
Like: "Will this bookcase fit through the door of my den?" Even with just a centimeter to spare, it's a done deal, unless I have a very badly frayed tape measure or I confuse centimeters with inches or something like that.
If I thought the center of Mars was either wholly liquid or wholly solid, (whatever that means when applied to metals) it would still be a binary call and measurement errors less than a certain size would probably not affect whether the hypothesis is to be rejected or sustained.
But in this case there are a number of continuous variables to be estimated - coefficients of plasticity, or the diameter of whatever "liquid" core may exist.
If I take your photo, your face may be recognizable even if my hand trembles or you blink your eyes. But photographers usually try to minimize that noise to get the best picture possible.
BTW, if we are going to sacrifice some measurement accuracy to perform an extrication maneuver essential to Spirit's survival, I am OK with that, but it is not the same argument as saying that movements of less than n centimeters do not have consequences and movements of more than n centimeters do have consequences.
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Phil Stooke
post Apr 13 2010, 03:54 PM
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I think you'll find that it is not centimeters that are being measured, but doppler shifts and times.

Phil


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fredk
post Apr 13 2010, 04:13 PM
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What matters is that the movement of the rover is small compared to the relevant scale. No doubt the rover shifts a bit each time the wind changes direction, but probably by only a very small fraction of a millimetre. Surely such movements would be unimportant for the core studies, but those same movements could be critical for the seismology experiments.

In the case of the core studies, there is some relevant scale, but I don't know what it is. As mentioned above the signal wavelength sets an absolute lower limit to movement that you'd care about, in the cm range. There could be many factors that make that lower limit larger. If it was several metres, then movements of tens of cm would be unimportant. The errors induced by the rover movement would be swamped by other errors.

Another point is that it might be that the movements could be corrected for in the core studies. Remember that the driven rover movements aren't random noise - the distances and directions and orientations are known with high precision.

Edit - definitely you're right, Phil, that cm are not measured directly. But still the relevant timing errors correspond to some position errors, since the signals travel at a fixed speed, 300 000 km/s.
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centsworth_II
post Apr 13 2010, 04:46 PM
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So as I see it, they measure the effect of Mars' wobble on Spirit's radio signal in real time. Spirit can move between measurements without the overall study being effected. It seems to me that a much bigger problem would be motions due to thermal changes or a stiff breeze during the measurement.

NASA - Spirit's Journey to the Center of Mars
"Mars wobbles, or precesses, as it spins," says Bruce Banerdt of NASA's Jet Propulsion Laboratory. "We'll measure that wobble by looking at the Doppler shift of Spirit's radio signal."
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stevesliva
post Apr 13 2010, 05:24 PM
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Well, since it is the doppler shift they're interested in, the distance of the Spirit from the center of Mars affects its radial velocity, so that movement upslope...

...is going to be a probably immeasurable difference. ph34r.gif

Actually, compared to the frequency variability implicit in the temperature of the oscillator on the rover... yeah, it would be very lost in the wash.
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