BERLIN (DLR PR) — In his logbook, Instrument Lead Tilman Spohn who is back in Berlin since April and communicating with JPL via the web, gives us the latest updates regarding the InSight mission and our HP3 instrument – the ‘Mole’ – which will hammer into the Martian surface.
Logbook entry 28 October 2019
More surprises on Mars! Unfortunately, we saw that the Mole had backed-out of the Martian soil instead of going deeper as we had expected. How could that happen? After all, this Mole does not have a reverse gear as the Mole that DLR built for the ill-fated European Beagle II lander had. (That probe was designed as a sampling device that would go down and then come up again with a sample.)
But we have seen the phenomenon of backing-up before: in the lab at small atmospheric pressure such as on Mars. Remember, the Martian atmospheric pressure is only 0.6 percent of the Earth’s! At Earth’s atmospheric pressure, if the Mole starts to rebound and if the rebound is not compensated by friction on the wall, the Mole rapidly opens a cavity below the tip of the Mole. The difference in pressure in the expanding cavity and in the atmosphere creates a suction effect that helps damping the rebound. The atmospheric pressure on Mars, however, is so small that the suction effect does not play a role and cannot really help the Mole. This is another reason why the friction on the Mole hull is so important! It is the major force to balance the recoil! In addition to the recoil you need some collapsing of the borehole at the tip in order to let the Mole move up.
When we analyzed the images from the previous hammering, we had an indication that the Mole´s forward motion had slowed down towards the end of that session. Therefore, we played it safe by commanding a smaller than originally planned number of hammer strokes and then a readjust of the pushing of the arm and scoop on the soil.
I, for one, would have never thought that the Mole could back out as much during a few tens of hammer strokes. Maybe, if we were out of luck, it would not penetrate or possibly come up a bit. Well, operating on Mars is not only time consuming. It is full of surprises! The interactions of low atmospheric pressure, low gravity, unknown regolith mechanical properties and Mole dynamics are a challenge.
What to do next? First, we want to be sure that the Mole will not tip over. Then, we want to inspect the hole it is sitting in, the view of which is blocked by the scoop. We may then do another pinning trying to bring the Mole back to where it was before the recent hammering. And start again to get it to dig below the surface. But give us some time to think!
Stay tuned, it is not at all over, but the Mole is not making our lives easier these days! It is good that we have such a great team from JPL and DLR working on the problem. And it is great that NASA and DLR continue to support us. After all, our goal is to eventually go much deeper!