The InSight mission's HP3 heat flux sensor - dubbed the "mole" by NASA - was just declared dead by the US agency on Thursday after failing to burrow deep into the Red Planet's soil.
The American lander InSight successfully landed in November 2018 on the surface of Mars. Its main objective:to detect tiny seismic waves passing through the lower layers of the planet, in order to learn more about its internal composition.
In its instrument "baggage", InSight also offered its Heat Flow and Physical Properties Package (HP3), provided by the German Space Agency (DLR). Basically, it is a heat flux sensor (a 40 centimeter tube) which, on paper, was supposed to sink about five meters into the Martian subsoil. Hence his nickname "mole".
To do this, the tube sinks under the action of a tungsten mass which slides inside between two springs, set in motion by an electric motor. By studying the thermal properties of the planet's interior, scientists could have learned a lot about its geological history.
That said, HP3 is probably the most complicated and sensitive instrument on the lander. Since the start of its deployment in March 2019, it has encountered quite a few problems.
Initially, the tube successfully sank three-quarters into the ground, but its progress was quickly halted. After several other unsuccessful attempts, NASA finally considered that the tube was not sinking because of the frictional forces exerted by the ground on the tube, considered insufficient to prevent it from bouncing after each impact of the tungsten block.
The surface of Mars where the InSight lander is located is indeed covered with a very compact layer of soil that does not fall back into the hole of the mole when it descends . Hence the insufficient friction forces (and yet necessary to sink). Note that this was the first time a mission had attempted to dig into the Martian surface in this way. Also NASA had no way of anticipating with certainty the behavior of its instrument.
Since then, several approaches have been attempted to solve this problem, to no avail. Eventually, the "Martian Mole" was pronounced dead by the US agency. " We gave him everything we had, but Mars and our heroic mole remain incompatible " , said HP3 principal investigator Tilman Spohn of DLR.
Despite this hard blow, the mission continues all the same until at least 2022. The other instruments of the lander are indeed still working. The SEIS (Seismic Experiment for Internal Structure) is busy measuring earthquakes, while the RISE (Rotation and Interior Structure Experiment) instrument focuses on the size of Mars' iron-rich core. The lander's weather station is still working too.
And while the mole failed, despite the best efforts of the folks at NASA and DLR, that failure will help foster future dig missions . Eventually, astronauts may indeed one day have to dig into the Mars soil, looking for frozen water or to probe for the presence of past life.
