[X] My Assistant

[John Moore]

Super animations.

Is there a measurement mark on the tether to (visually) see how far it has sunk?

John

[mcaplinger]

Is there a measurement mark on the tether to (visually) see how far it has sunk?

There's a gray-coded series of marks on the science tether that's read by an optical sensor inside the support structure, and there's also a regularly-spaced series of marks on the tether (See figure 2 in https://www.hou.usra.edu/meetings/lpsc2019/pdf/1344.pdf ) but I'm not sure any of this can be seen in the available imagery.

[Gerald]

In some of the images, the marks can be seen pretty well:

(Sol 633)


(Sol 646)

[John Moore]

Many thanks to you both - mcaplinger and Gerald.

John

[Gerald]

Welcome!
For convenience, here a blink between roughly registered images of Sols 633 and 646:

[PaulH51]

One of the contributors to the InSight Reddit page just compiled this YouTube video documenting the trials and tribulations the mole
LINK

[PaulH51]

There is some apparent interaction between the scoop / mole since the last hammer session on Sol 645?

On Sol 645 there was a long hammer session (I already shared the animation here) however there appears to have been some small downward movement of the scoop in the daily IDC images returned by the lander.

I've assembled a simple cropped GIF using frames from Sols 645 (last in the session), 646, 647, 649 & 651.

This apparent downward movement could be due to mid week hammer sessions (but that would be out of the ordinary) or possibly natural harmonics in the lander caused by the seasonal winds inducing vibrations by interaction with the solar cells?



Spotted by a contributor on the Sub-Reddit for InSight who shared this zooned IDC clip Looking at the zoomed clip it's hard to be sure, but there appears to be some minor sand grain movements on the scoop, I'm not sure if they are caused by shadow / lighting, or are hammer sessions.

[Gerald]

It's looking to me like they'd analyse each single hit of the hammer. Just a more or less educated speculation.

[JRehling]

There is definitely progress between Sol 645 and Sol 651 in terms of the objects we can see. FWIW, the frames are not aligned precisely and appear, possibly, to be aligned on the tether rather than the ground, but I'm blinking 645 and 651 in Photoshop now with precise alignment and we can see the tether moving the right way as the scoop lowers. The movement of the tether appears to be about equal to the width of one of the dark squares on it – something like 4mm? The pebbles in front of the scoop move upwards as the scoop moves downwards.

Reading team blog posts from the past, I note that one of the mysteries for them was the way that quite a bit of the soil they scooped in simply seems to disappear, presumably filling in small voids in the duricrust/soil below the scoop as they compact it. So it is possible that downward motion could occur now without any hammering – if the pressure of the scoop alone is sufficient to compact the soil. Since we're seeing a movement of less than 1mm/day, whatever happened is quite small, but at least it's in the right direction.

[Hungry4info]

I apologise if this is a silly question, but it looks like there's more pressure on the cable than was imagined when the whole thing was designed. With the hole being (partially?) filled plus the pressure from the arm, is there any risk of damage to the mole/cable interface? I suppose I imagine a sort of issue where the cable is pulled on too much during mole drilling and eventually tears.

[Keatah]

I suppose I imagine a sort of issue where the cable is pulled on too much during mole drilling and eventually tears.

..or will that drag stop the mole from progressing?

[Gerald]

They are obviously aware of the risks. But it seems to be the risk of damaging the teather with the scoop:

...I have already mentioned that the placement of the scoop is risky and must be done with millimetre accuracy...

In a slightly different context, they say that the force should be about 7 N:

The force should be enough to offset the hammer mechanism's recoil of about seven newtons.

I presume that this is considerable more than the friction between a layer of a few centimeters of sand and a smooth teather as long as there are no perpendicular forces in the same order of magnitude. The friction force induced between sand and teather by pushing the scoop will push the teather downward, too.
So, the upward motion of the mole might be the result of the elastic reactio of the phase boundary between loose sand and the suggested "duricrust" on hammering, not necessarily just the recoil of the hammering mechanism.

[fredk]

I'm not sure what you mean by elastic reaction, but the thinking seems to be that the recoil happens because of lack of friction due to too stiff duricrust material. Hence adding sand to try increase the friction with the mole. See eg the July 7th 2020 log entry here.

About the cable, I guess that was designed to handle drilling metres into the soil so should tolerate the current configuration.

[Gerald]

In the simplest case, even with friction, if you are hammering on a solid elastic surface, your hammer will bounce back from the surface by Newton's 3rd law, the "reactio" of your hammering, or stated differently by elastic collision of the mole against the duricrust or whatever elastic material around.
The assumption of an inelastic collision doesn't require to be a given. But my impression is that an inelastic collision of the mole with its environment is assumed at least approximately, and only the recoil due to the acceleration of the hammer within the mole is considered appropriately. An elastic collision can cause much larger forces than the seven Newton the scoop balances. The force is depending on the hardness of the (elastic) surface you hammer on.

[fredk]

Well, according to the diagram in that July 7th post, they believe the tip of the hammer is well beneath the duricrust, so it doesn't seem like a collision against the duricrust is likely.