Why NASA’s InSight Mission Can't Scratch Mars’ Surface

Why NASA’s InSight Mission Can't Scratch Mars’ Surface 

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NASA’s InSight spacecraft touched down on Mars on November 26th, 2018, with the goal of studying the interior of our rocky red neighbor. But one year into its two-year mission, one of its major scientific instruments, a heat probe attached to a self-burrowing mechanical mole, isn’t where it’s supposed to be.

The probe is designed to go as deep as five meters down in the martian soil, but it’s barely scratched the surface, getting stuck about 0.3 meters down before recently popping out. So, as I said to my dermatologist, what’s up with this mole? Now the mole isn’t popping out because little green people living just under the surface gave it the old heave-ho. Probably, I mean it’s hard to see into the hole it’s made with the cameras onboard InSight.

Really, it’s more an issue with how the mole was designed and the type of soil it's digging through. You might be picturing a device with a drill, but for a number of reasons a drill just wasn’t possible on the mole. A drill would require a powerful motor to work, which would make the instrument bigger than what the lander could accommodate. A powerful motor would also use more energy than InSight’s solar panels could provide. And the designers would have to take the torque of the motor into account, as it would spin the mole the opposite direction of the drill.

The engineers found a solution as follows

To eliminate that, the drill would need rigging anchoring it in place. So instead, the engineers who designed the instrument at the German Aerospace Center took a different approach to bury their thermometer. They realized that the soil that the Mars rovers Spirit and Opportunity encountered was loose and sandy. So they decided to make the digging action function something like a pile driver.

It works by slowly compressing a spring and quickly releasing it, driving a tungsten hammer into the interior of the mole’s tip. It’s literally Whack-a-mole. The hammer tappa-tappa-taps away at a rate of one tap every 3.7 seconds. As it burrows millimeter by millimeter, the idea was for that loose soil to flow around it, providing enough friction to keep it from bouncing back after each strike. And clearly, things aren’t going according to plan. The good news is it doesn’t seem like the mole is damaged.

InSight’s other scientific instrument is a very sensitive seismometer. Using that, the scientists can look for millisecond level variations in the hammer strokes. So far the data indicate that the hammer is working fine. Is it possible then the mole hit a rock? The team doesn’t think so. The mole is designed to knock small rocks aside and can go around medium-sized rocks once it’s fully buried.

The InSight spacecraft landed on Mars

The landing site they chose, Elysium Planitia, is flat with a few surface rocks as possible, which minimizes the odds they’ll hit an impassable rock at this shallow depth to a few percents. But in a cruelly ironic twist, it seems like the landing site that was chosen for its lack of rocks also has soil that the mole’s designers just weren’t anticipating. They think it’s just not providing the friction needed to keep the digger in place.

In October, they tested the solution of pinning the mole against the side of its burrow with the lander’s robotic arm, and for a while, it seemed to work. But it was short-lived, and recently the mole popped halfway out of its burrow. Now the team is going to try and safely move the robotic arm away and reassess the situation. They can’t pick the mole up and try a different spot because there’s no way to grab onto it directly.

If they have no other options, they’ll try and press down on top of the digger directly using the lander’s robotic arm, but that’s very risky considering they could damage the ribbon that provides power to and collects data from the device. Whatever plan they come up with will have to be rigorously tested to make sure it works as anticipated. So we’re just going to have to be patient and see if we can bury this heat probe underground to see how heat flows inside Mars.

NASA says that overall the InSight Mission is going very well

Even if it doesn’t happen, NASA says that overall the InSight Mission is going very well, and burying the sensor is not critical to mission success. And the scientists remain upbeat, saying that the behavior of the soil is still teaching them something new. So when it comes to the mole as my dermatologist said, it’s nothing to worry about yet, but we’ll keep an eye on it.

My mole is on the move again and back to digging. Using my arm to put pressure on the mole from the side has helped it move down ~1.25 inches (~32 millimeters). My @NASAJPL & @DLR_en team’s efforts to #SaveTheMole continue. pic.twitter.com/ZXo31F5xsi

— NASA InSight (@NASAInSight) November 21, 2019

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