People rightly marvel at modern surgical techniques that let surgeons leverage the power of robotics to repair the smallest structures in the human body through wounds that can be closed with a couple of stitches. Such techniques can even be applied remotely, linking surgeon and robot through a telesurgery link. It can be risky, but it’s often a patient’s only option.
NASA has arrived at a similar inflection point, except that their patient is the Mars InSight lander, and the surgical suite is currently about 58 million kilometers away. The lander’s self-digging “mole” probe needs a little help getting started, so they’re planning a high-stakes rescue attempt that would make the most seasoned telesurgeon blanch: they want to use the lander’s robotic arm to press down on the mole to help it get back on track.
All About Friction
We’ve been covering InSight for a while now, starting with a discussion of how the mole, part of the Heat Flow and Physical Properties Package (HP³) experiment, is supposed to work. As a quick recap, the mole is basically an electric impact driver, with a rotating cam that loads a spring to release a burst of mechanical energy into a heavy hammer. The impact is supposed to drive the mole into the Martian regolith a few millimeters at a time, slowly burrowing up to five meters into the regolith while dragging a sensor-filled tail behind it, to measure subsurface heat flow.
Unfortunately, HP³ has been unable to dig itself into the soil. The failure has been attributed to everything from hitting a rock just below the surface to a previously unknown layer of duricrust, a hard layer made of soil particles that have been cemented together by chemical precipitates in the water that once flowed freely on Mars. Add to these possibilities the fact that the Martian soil has proven to be far less cohesive than originally thought, giving the mole little friction to work with, and it’s no wonder it’s stuck.
Back in October of 2019, it looked like the mole was on the move again. JPL engineers had decided to use the lander’s robotic arm to press against the hull of the mole while it tried to dig. The thought was that increasing the friction would give it the boost it needed to penetrate the duricrust and get on the move again.
Initial reports were that the mole was making progress after this pinning maneuver, but that proved to be optimistic. The mole did make progress, but it popped back out of the hole on two occasions. Those failures cleared the way for the current and riskiest idea: using the robotic arm’s scoop to push directly on the back cap of the mole.
On the face of it, this seems to be the approach that makes the most sense. After all, if the mole is having trouble moving downward, applying force downward should help it penetrate the duricrust. But it’s not as simple as giving the mole a little shove.
First of all, consider the physical aspects of the problem. The mole is about 3.5 cm in diameter, which is a pretty small target to hit with the robot arm’s scoop. The back cap is also not the most friendly surface in terms of manipulation, either. It’s bisected by the mole’s tail, which looks like a flexible Kapton PCB. There’s also a protrusion on the back cap to one side of the tail, which would appear to limit the scoop’s ability to bear down on the mole.
The tail itself is a problem as well. Extreme care will need to be taken to ensure that the scoop doesn’t touch the flexible PCB, which would likely be damaged if it got pinched. The tail not only powers the mole but provides data connections; lose any of those and you’ve likely lost the mission. Positioning the scoop will have to be done very gingerly, and is already being practiced on Earth-side mockups and using the actual hardware on Mars.
Latency is certainly going to be a huge problem as well. The current round-trip time for radio signals from Earth to Mars is in excess of six minutes, meaning that realtime control of the operation will be out of the question. The moves will have to be carefully planned in advance and made in very small increments, so as to minimize the chance of damage.
Snatching victory from the jaws of defeat through engineering ingenuity has been the story of space exploration since its very earliest days, so there’s a good chance that engineers will be able to push on the mole without damaging it. Whether that game of interplanetary Whack-A-Mole will yield results is anyone’s guess, but even if it HP³ never gets a chance to dig down into Mars, it won’t be for a lack of trying.