Common Questions about InSight’s ‘Mole’

Signs of the Heat Probe Shifting on Mars: The support structure of the Heat Flow and Physical Properties Package (HP3) instrument moved slightly during hammering, as indicated by the circular “footprints” around the instrument’s footpads. (Credit: NASA/JPL-Caltech)

PASADENA, Calif. (NASA PR) — There’s a new plan to get InSight’s “mole” moving again. The following Q&As with two members of the team answer some of the most common questions about the burrowing device, part of a science instrument called the Heat Flow and Physical Properties Package (HP3).

HP3 Principal Investigator Tilman Spohn of the German Aerospace Center (DLR), which provided the instrument.

Q: What do you suspect is preventing the mole from digging further?

A: We don’t know for sure, because we can’t see underground. That’s why we want to lift the support structure. But we’re fairly confident the problem is a lack of friction in the soil.

The mole is designed for loose soil to flow around it, providing friction that keeps the mole from bouncing backwards with recoil. The soil around InSight provides much less friction than what we’ve seen before on Mars. We have several ideas for why this is. One is that the soil compacts and holds together so well that the mole may have created a small cavity around itself while hammering, preventing it from getting the friction it needs to dig. Tests conducted in Germany have confirmed this could happen.

There’s also the possibility that we’ve hit a rock. The mole was designed to go around smaller rocks — no bigger than a few inches [or centimeters] in size — but it may be pinned between the rock and the support structure. If that’s the case, moving the support structure may allow it to keep digging. If it’s hit a larger rock, there’s not much we can do.

Q: Why couldn’t the team anticipate the soil issue or hitting a rock?

A: We always knew there were environmental risks that could stop the mole. What we did is prepare as best we could so that they were calculated risks.

The low-friction soil was unexpected: Everywhere else on Mars, soil has been different, which was what the mole was built for. We also knew there would be rocks underground, so we deliberately chose a landing site that was flat and had as few rocks as possible on the surface. Based on what we can see, the chances of hitting a rock this close to the surface that the mole can’t go around is only a few percent.

Q: What will happen once you lift the support structure?

A: It depends on what we see. First, we’ll try a short hammering sequence with the mole. Then, regardless of whether there’s a cavity around the mole, we will likely press on the soil with a small scoop on the robotic arm to provide a load on the surface. Our calculations suggest that this should increase the friction and could help it dig.

Q: When would that occur?

A: We hope to lift the support structure by the end of June and to have our next steps decided by mid-July. After that, we will need time to run more tests.

Engineer-scientist Troy Hudson of NASA’s Jet Propulsion Laboratory in Pasadena, California, which leads the InSight mission.

Q: Why are you lifting the support structure in three steps?

A: There are springs inside of the support structure that may still be in contact with the top of the mole. If that’s the case, we want to be careful lifting the structure so that we don’t accidentally pull the mole out. If that happened, we won’t be able to insert it back into its hole in the soil. So we’ll lift the support structure a little bit at a time, checking to make sure the mole isn’t coming with it.

Q: Why has it taken so much time to plan these next steps?

A: We don’t want to take an action that makes the situation worse, so we’ve been moving very carefully. InSight is also a Discovery class mission, meaning it was meant to be more affordable — and to accept more risk — than a flagship NASA mission. So the team is very small; we don’t command the lander every day, and commands that are sent up need to be rigorously tested to make sure they’ll work the way we want them to. We want every action to be safe for HP3, the robotic arm and InSight’s seismometer, which is very close by.

Q: Why can’t you just pick up the mole and move it to another spot?

A: The robotic arm, which we inherited from another mission, wouldn’t be able to pick up the mole. We designed HP3‘s support structure to be grasped by the arm’s grapple, but the mole was never designed to be handled that way.

We think the issue is a lack of friction in the soil under InSight. So even if we could pick up the mole, it wouldn’t matter where we put it — we would still have the same friction problem. If the issue is a rock, there’s no way to guarantee you wouldn’t just hit another rock.

About InSight

JPL manages InSight for NASA’s Science Mission Directorate. InSight is part of NASA’s Discovery Program, managed by the agency’s Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space in Denver built the InSight spacecraft, including its cruise stage and lander, and supports spacecraft operations for the mission.

A number of European partners, including France’s Centre National d’Études Spatiales (CNES) and the German Aerospace Center (DLR), are supporting the InSight mission. CNES provided the Seismic Experiment for Interior Structure (SEIS) instrument to NASA, with the principal investigator at IPGP (Institut de Physique du Globe de Paris). Significant contributions for SEIS came from IPGP; the Max Planck Institute for Solar System Research (MPS) in Germany; the Swiss Federal Institute of Technology (ETH Zurich) in Switzerland; Imperial College London and Oxford University in the United Kingdom; and JPL. DLR provided the Heat Flow and Physical Properties Package (HP3) instrument, with significant contributions from the Space Research Center (CBK) of the Polish Academy of Sciences and Astronika in Poland. Spain’s Centro de Astrobiología (CAB) supplied the temperature and wind sensors.