Rescue plan for InSight lander’s stuck heat probe gets underway

Ever since its successful landing at Elysium Planitia on Mars back on November 26th of last year, NASA’s InSight spacecraft has been feverishly studying the planet’s internal structure to get a sense of how it formed and subsequently evolved over billions of years.

These studies are currently being conducted by use of a plethora of scientific instruments and experiments, including an advanced self-hammering probe (affectionately nicknamed the “mole”) that can monitor heat flow through the Martian surface. However, this probe has suffered from some issues that require a resolution plan to be implemented.

This instrument, known officially as the Heat and Physical Properties Probe – or HP3 for short – was developed by the German Aerospace Center (DLR) and provided to NASA for use on the InSight lander.

The small, 35 centimeter-long probe was designed to burrow as deep as 5 meters (16 feet) below the surface of Mars while connected to a tether, which is embedded with temperature sensors that can determine how efficiently heat travels from the planet’s core.

Data taken from could then be used to reveal new information about Mars’ core and its state of matter – whether it is currently a liquid or solid core.

The heat-flow probe is supported by a larger, rectangular structure, which brings the total mass of the HP3 package to 3 kilograms (6.6 pounds). After being placed on the surface by InSight’s robotic Instrument Deployment Arm (IDA), the support structure would ensure that the probe would be kept in place during the initial hammering sequence.

If everything to this point went well, the probe would keep on burrowing to 5 meters in depth, provided that it did not encounter any obstacles.

DLR scientists and engineers built the heat-flow probe with the purpose of using friction to its advantage – that is, the Martian soil would absorb the recoil from the probe’s hammering action, thereby providing enough friction for movement.

A small motor and gearbox is located inside, which periodically loads a spring connected to a rod. After release, this “hammer” accelerates downwards and into the outer casing of the probe, which allows it to self-burrow into the ground.

The HP3 package was successfully deployed onto the Martian surface on February 12, 2019, and the heat-flow probe was released from its support structure on February 22. NASA and DLR initially gave the go for hammering to begin on the 26th, but were forced to hold off until the 28th.

On that day, the probe began hammering into the regolith as expected, and made it about a foot (12 inches) deep into the surface. However, after about a half-hour, both the NASA and DLR teams discovered a problem: the probe could not hammer any further than that.

At first, both agencies presumed that the probe had made contact with a rock just below the surface. However, they had prepared for this scenario – the probe had previously destroyed small rocks and even maneuvered around others during testing back on Earth. As such, they commanded the probe to continue hammering, in case the force would shatter the obstacle or push it aside. Unfortunately, nothing of the sort occurred during that day’s operations.

NASA and DLR also began to suspect that the Martian soil itself was to blame for the probe’s sudden stoppage. The usually loose regolith was supposed to flow evenly around the device during hammering, as observed during simulated testing in Earth-based laboratories.

JPL engineers conducting testing on models of the HP3 package – credit: NASA/JPL-Caltech

However, it has been posited that the probe may have instead found an empty space in the subsurface soil, which is unable to provide the friction necessary for movement – the device would instead just bounce in place.

After about 4 months of investigation, engineers from the NASA and DLR teams came across another potential issue: in its attempts to hammer through the obstacle, the probe had gotten itself inclined by about 15 degrees with respect to vertical – this was observed when the support structure moved by about 2 centimeters from its last position.

This, combined with its shallow depth into the Martian surface, meant that the probe may have wedged itself inside the HP³ package’s support structure, thereby preventing it from moving regardless of the obstacle it encountered. However, NASA nor DLR could tell for sure, since the support structure was blocking InSight’s view of the probe itself.

Illustration of heat probe wedged inside support structure – credit: NASA/JPL-Caltech

Therefore, both agencies spent the early days of June concocting a plan to “rescue” the probe and resume hammering operations: they would send commands to InSight’s Deployment Arm to have it slowly lift the support structure upwards, thereby allowing the teams to get a better look at the device.

These observations would confirm the issue at hand, and would also allow both NASA and DLR to move forward with a solution.

However, this lifting procedure was not without risk. If the heat probe was mistakenly lifted out of the Martian soil, it could not be reinserted, meaning that the experiment’s mission would have come to an end.

The Instrument Deployment Arm successfully grappled the HP³ support structure in the second week of June, and lifting operations began on Saturday, June 22. The structure was successfully lifted about 12 centimeters (4.7 inches) off of the ground that day.

A 13-centimeter lift on Tuesday, June 25, along with a 27-centimeter lift and 20-centimeter translational movement towards the InSight lander on Friday, June 28, allowed NASA engineers to successfully lift the structure off of the Martian surface and obtain a clear view of the half-submerged heat probe.

Now, the InSight team is taking a short break to analyze the situation and determine the next best course of action. This would involve further use of InSight’s Instrument Deployment Arm, which has a small scoop attached to it for soil-based observations.

Current plans have NASA engineers using this scoop to compress the soil surrounding the probe with a force of about 40 newtons (9 pounds) to possibly create enough friction within the regolith, thereby allowing the probe to effectively begin hammering once again.

However, this is dependent on the fact that the device has found a gap within the soil, which allows the InSight team to devise solutions. Unfortunately, there is not much the team can do in the event that the probe has encountered a large rock that is unmovable.

Replica of IDA pressing down on simulated Martian soil using scoop during lab testing – credit: NASA/JPL-Caltech

According to a blog post created by DLR, the team hopes to set the next step of this plan into motion later this month, if not early next month.

Regardless of the heat flow probe issues, InSight’s mission on Mars has been going according to plan. On December 7, 2018, the Seismic Experiment for Interior Structure (SEIS) instrument recorded sounds of Martian winds, which were estimated to have traveled at around 16-24 kilometers (10-15 miles) per hour. These winds were detected by recording vibrations coming from the lander’s large solar arrays.

On April 6, the same instrument detected a small, but long-duration tremor coming from the planet’s interior. This event, along with many other “marsquakes” detected by this instrument, lends credibility to the assumption that Mars may have an active, liquid center.

InSight’s mission on Mars is projected to last about 2 Earth years, or 709 sols (Martian days). So far, the spacecraft has survived for 219 sols, with many more scientific discoveries to come in the near future.

About the author