Components of the HP3 heat flow probe. Top left: the radiometer (RAD), which is used to measure the radiation temperature (roughly equivalent to the ground temperature) of the surface. Right: the casing with the mole penetrometer, the temperature measuring cable (TEM-P) and the data cable (ET) connected to the lander. In addition, the casing contains an optical length meter for determining the length of the temperature measuring cable that has been pulled from the casing. The mole contains the TEM-A active thermal conductivity sensor and the STATIL tiltmeter. Bottom left: the electronic control unit, known as the back end electronics (BEE), which remains on the lander and is connected to the probe via the ET.
Credit: DLR


That troubled heat probe on NASA’s InSight Mars lander continues to be an issue!

The instruments locomotion system, a self impelling nail nicknamed “the mole” was designed to hammer itself down into the surface of Mars.

Called the Heat and Physical Properties Package (HP3), the German-provided mole hasn’t been able to dig deeper than about 12 inches (30 centimeters) below the Martian surface since Feb. 28, 2019.

The self-hammering mole, part of the Heat Flow and Physical Properties Package (HP3) on NASA’s InSight lander, was only partially buried in the soil of Mars as of early June 2019, as shown in this illustration.
Credit: NASA/JPL-Caltech/DLR

Arm work

Spacecraft engineers have interacted with device, working the mole’s immediate surroundings utilizing InSight’s robotic arm.

Credit: NASA/JPL-Caltech

Credit: NASA/JPL-Caltech

Engineers in a Mars-like test area at NASA’s Jet Propulsion Laboratory try possible strategies to aid the Heat Flow and Physical Properties Package (HP3) on NASA’s InSight lander, using engineering models of the lander, robotic arm and instrument.
Credit: Tilman/NASA/JPL-Caltech

Credit: NASA/JPL-Caltech

Credit: NASA/JPL-Caltech

Credit: NASA/JPL-Caltech


“What we saw was somewhat surprising,” reports Tilman Spohn of the German Aerospace Center’s (DLR) Institute of Planetary Research in Berlin. “We had had indications before that the mole might have dug a hole or a pit but we had not expected it to be that large.”

Tilman adds that the diameter of the “mole hole” is a good two times the mole diameter or about 6 centimeters. “Thus, the mole must have precessed (like a spinning top) while it was digging. Moreover, the twist in the tether shows that the mole must have rotated clockwise about its long axis by about 135 degrees.”

Multiple footmarks

InSight imagery also showed that the feet of the Support Structure Assembly (the SSA) had left clear footmarks that had remained stable, indicative of at least some regolith cohesion, Tilman says. “The multiple footmarks are proof of the SSA having been lifted and bouncing with the mole during hammering.”

Imagery also suggested that there was a layer of cohesion with clumps and concretions and maybe caverns, possibly overlying cohesionsless sand.

The interpretation is that about 5-10 centimeters of a thick layer of duricrust was exposed. “On Mars, the term duricrust is used to indicate a mechanically strong layer of regolith, somewhat differently than in terrestrial geology. It is thought by geologists to consist of cemented sand,” Tilman reports.

Loading the surface

In July it was believed that the duricrust around the pit might be easily crushable.

Thus, it was decided to go ahead with the plan of loading the surface using InSight’s scoop to increase pressure and thus friction on the mole hull, but, the pit would have to be collapsed first.

Multiple rounds of pushing on the surface with the scoop have been done. However, Tilman explains, none of these could fully collapse the pit – although a partial collapse can be observed on the right-hand side of the pit.

Six weeks of work

The mission is pausing now until September 10th, Tilman adds, because Mars is entering solar conjunction and communication with spacecraft on Mars becomes impossible.

It has taken the almost 6 weeks “to get to where we are right now,” Tilman points out. “While we have learned a lot about the properties and the layering of the regolith we still have a long way until we can fill the pit.”

Science friction

Lastly, Tilman says that what may be in order is returning to an earlier suggestion of “pinning” the mole with the scoop such that the pinning and the pressing of the mole against the wall of the pit would increase friction.

“We would then hammer and see if we penetrate further. This will be more risky than the previous strategy but with the unexpectedly stiff duricrust, it is thought to be a more promising strategy,” Tilman adds.

Credit: NASA/JPL-Caltech.

The downside could be that the mole may be helped to penetrate but once the backcap gets level with the surface, there is little chance of pinning further,” Tilman reports. “The friction from the regolith would have to have increased enough (by penetrating deeper than now) so that it would finally suffice. If not, one could continue with trying to fill the pit, of course.”

Tilman concludes: “That is it for now. Stay tuned until we come back from conjunction with a report on what the project finally decided to do.”

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