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Updated information on the Mars InSight Lander's Mole probe.
NASA has now announced that the plan to investigate the cause of the mole's depthless digging was implemented successfully and they have the results
As was previously reported the Mars InSight Lander's heat probe was only able to reach a depth of 13cm in its attempts to drill to a final depth of five meters. Earlier this month NASA announced their plan to move the lander's support structure out of the way so it could view the hole and determine the problem.
Initially, the InSight team thought that the mole had hit a rock and was blocked. But after analysis and experimentation with a mock-up lander at test-bed facilities, they came up with another explanation: a cavity in the soil.
They couldn't be sure without seeing into the hole, which lead to the June effort to move aside the lander support structure.
Now that the mole's support structure has been moved aside, camera's[sic] on the lander's instrument arm are able to see into the hole. And they've confirmed what the InSight team suspected. A small pit has formed around the mole, depriving it of the necessary friction to penetrate deeper.
"The images coming back from Mars confirm what we've seen in our testing here on Earth," said HP3 Project Scientist Mattias Grott of DLR. "Our calculations were correct: This cohesive soil is compacting into walls as the mole hammers."
This is important because the mole works differently from your garden variety post hole digger and
relies on friction with the [soil] surrounding it to hammer its way into the ground [...]. Without that friction, the mole will just recoil from the hammering action, and bounce around in the hole, rather than penetrate.
This was actually the hoped for result as a large blocking rock would have effectively been game over. NASA will next put into action its plan to attempt to remedy the situation
The robotic instrument arm has a small scoop on the end, and they intend to use that scoop to pat on the hole and compress the soil, hopefully eliminating the cavity.
There is a NASA Q&A page on the mole's situation available as well.
(Score: 4, Informative) by Anonymous Coward on Saturday July 06 2019, @10:27AM
The 'probe' is a cylindrical tube with a spring-loaded weight inside. When that spring is released, the weight is accelerated downwards by the spring until it hits a stop at which point it transfers its energy to the casing and drives it downwards. A small electric motor is used to repeatedly wind up the spring mechanism.
When the weight is accelerated downwards? The force is applied to the casing which then wants to move upwards. This 'acceleration phase' requires enough friction on the outside of the casing to resist moving the probe up.