An update on that troubled Heat Flow and Physical Properties Package (HP3) experiment deployed by NASA’s InSight lander on Mars.

The “mole” is part of the HP3 and has only managed to partially bury itself since it started hammering in February 2019.

“Mars (and the mole) continue to make our lives…how should I say…interesting,” explains Tilman Spohn of the German Aerospace Center’s (DLR) Institute of Planetary Research in Berlin via a February 17, 2020 newsletter.

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

“You may recall that by Christmas we were almost back in after the mishap of the mole backing-out in October. There were a few (2-3) centimeters still to go before we would have called “pinning” off. At that depth of burial with only a couple of centimeters of the mole sticking out of the ground, the mole would have not provided enough surface area for the scoop to safely press against the mole hull,” Spohn explains.

“We had planned and commanded a final pinning and hammering for the first week of operation in the New Year to bring us to that point. But because we had seen a decrease in the rate of downward motion before Christmas, we decided that we would readjust the pinning before we started the hammering. To save time, we took a different approach to readjusting the pinning than we had used before: Rather than removing the scoop from the mole entirely and then re-pin, we loosened the push and then retightened it,” Spohn observes.

Backed out

When team members studied the images returned from Mars on Sunday, January 12th it was seen that the mole had backed out again.

The detailed images showed that it had penetrated through the first 20 strokes by about 1.5 centimeters and then had reversed motion and had backed-out by 3.5 centimeters for the remaining 110 strokes.

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

“This is only half of the length it had backed-out during sol 325, late October, but still unwelcome and puzzling,” Spohn says. “It had to be concluded that our retightening of the pinning had not been successful! But why did the mole first move forward before it then reversed motion?”

Spohn senses that a possible explanation starts with observing that the rebound force (the force that the pinning is supposed to balance) depends on the resistance of the soil that the mole is penetrating. The more resistant the soil, the larger the rebound force. 

“In any case, after considering the options, we decided not to use the pinning technique again but rather press the scoop against the backcap of the mole,” Spohn notes.

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: Spohn/NASA/JPL-Caltech

Collapsed duricrust

“This week the scoop has been positioned above the backcap and pushing will start soon…however, only after carefully checking the placement of the scoop,” Spohn adds.

Before the positioning, InSight’s scoop performed two experiments in preparation of a possible filling of the pit.

“First, it was successfully demonstrated that the wall of the pit could be collapsed by pressing against it with the tip of the scoop. The collapsed duricrust fell into the pit and partly filled it,” Spohn notes.

“Second, it was shown that the scoop could be used to scrape loose sand on the surface together and move it towards the pit,” Spohn says.

Both techniques may eventually be used to fill the pit and then…to press on the surface of the filled hole to provide friction to the mole underneath.

“Stay tuned,” Spohn concludes.

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