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WASHINGTON — An instrument on NASA’s InSight Mars lander that has struggled for more than a year to make its way into the Martian surface is now making steady, but slow progress with the help of the lander’s robotic arm.
The Heat Flow and Physical Properties Package instrument on the InSight lander was to deploy a probe, or “mole,” into the surface of the planet, using a hammering mechanism to burrow as deep as five meters below the surface to measure the heat flow from the planet’s interior. The probe, though, got stuck shortly after it started burrowing in February 2019, getting no deeper than about 30 centimeters.
The project has tried several ways to get the mole moving into the surface again. Most recently, spacecraft controllers positioned the scoop on the end of the lander’s robotic arm on top of the mole, pushing down on it to help it move into the surface and to prevent it from moving back out, which has happened in the past.
That approach is working so far. “The mole is going down by its hammering mechanism, but it is aided by the push of the scoop that balances the force of the recoil,” said Tilman Spohn, principal investigator for the instrument at the German space agency DLR, during a May 4 webinar about results from the mission that was part of the European Geosciences Union General Assembly, a conference that moved online because of the coronavirus pandemic.
However, the progress is slow because of the need to reposition the arm as the mole gets deeper. “That is a very tedious operation,” he said. “We can only go like 1.5 centimeters at a time before we have to readjust.”
Another issue is the angle at which the mole is penetrating into the surface. The mole was originally designed to go down vertically, but is now at an angle of nearly 30 degrees from the vertical. “It’s not something we like to see,” he said. If the mole is able to get completely below the surface, he expects that it will “rectify itself to some extent.”
The problems have given scientists some insight into the properties of the surface at InSight’s landing site. There is a “duricrust” about 20 centimeters thick, which he described as sand that has been cemented into place by salt. That duricrust didn’t provide enough friction to keep the mole from recoiling as it tried to hammer into the surface initially.
Another issue, he said, is that there is now a region of compacted sand created by the mole as it hammered in place without moving deeper. That will make it more difficult for the mole to penetrate into the surface, even with the assistance of the robotic arm.
While Spohn didn’t state how long the current effort to get the mole into the Martian surface would last, other project officials have suggested it may take a couple months. The latest effort had just started when Bruce Banerdt, principal investigator for the overall mission, gave a briefing at a meeting of NASA’s Mars Exploration Program Analysis Group April 17, noting that the lander’s other instruments, including its seismometer, were working well.
“We anticipate that we’ll have the mole down flush with the ground within another month or two months,” he said. By then, the arm will no longer be able to help push the mole further into the ground. “At that point, it’s either going to be able to go on its own or not.”