In the early 1990s, NASA experienced a sea change in the way it approached space exploration. Gone were the days when all their programs would be massive projects with audacious goals. The bulk of NASA’s projects would fall under the Discovery Project and hew to the mantra “faster, better, cheaper,” with narrowly focused goals and smaller budgets, with as much reuse of equipment as possible.
The idea for what would become the Mars InSight mission first appeared in 2010 and was designed to explore Mars in ways no prior mission had. Where Viking had scratched the surface in the 1970s looking for chemical signs of life and the rovers of the Explorer program had wandered about exploring surface geology, InSight was tasked with looking much, much deeper into the Red Planet.
Sadly, InSight’s primary means of looking at what lies beneath the regolith of Mars is currently stuck a few centimeters below the surface. NASA and JPL engineers are working on a fix, and while it’s far from certain that that they’ll succeed, things have started to look up for InSight lately. Here’s a quick look at what the problem is, and a potential solution that might get the mission back on track.
After a six-month journey, InSight arrived on Mars in spectacular fashion in November of 2018. NASA’s coverage of the landing was outstanding: so good that it was nominated for two Emmy awards. Unlike so many times before, Mars had failed to claim InSight as a victim, and with the lander safely on station on Elysium Planitia, not far north of the Martian equator, the science began in earnest.
Most landers are self-contained platforms, with all the instruments needed to perform the science mission built into the spaceframe of the vehicle. This reduces complexity and keeps costs lower – but only when the science allows it. Since InSight was designed to explore Mars both seismically and thermally, eliminating vibrations and thermal effects from the equipment aboard the lander made it necessary to provide two tethered instrument packages which could be deployed to the surface using a robotic arm.
The first of the two instruments to be deployed was the SEIS, or Seismic Experiment for Interior Structure. Once that was safely in place and commissioned, the robotic arm placed the HP³, or Heat Flow and Physical Properties Package, a couple of meters away. Equipped with a penetrator designed to burrow itself up to five meters into the Martian regolith while trailing a string of thermal sensors and heating elements, HP³ was designed to study the thermal profile beneath the surface, leading to a better understanding of heat flows within the Martian core.
The HP³ penetrator, dubbed “the mole”, began pounding itself into the regolith on February 28, 2019. Mission planners had allowed the penetrator two months to dig down to its full depth, but within a week, the mole called it quits. Having paid out a mere 30 cm of its instrument-studded tether, a quarter of the length of the mole was still inside the HP³ protective enclosure. The probe was far away from its minimal useful depth of three meters, and NASA and JPL engineers needed to figure out why.
Early speculation centered around that which causes consternation to almost every hole-digger on Earth: that the mole had hit a rock. Mission planners had used data from other landers to select an area where that would be as free from rocks as possible, and early estimates were that the mole would have about a 4% chance of hitting a rock that it couldn’t go around. Either NASA was spectacularly unlucky in placing the instrument, or something else was going on.
Another possibility is friction, or more specifically, the lack thereof. The innards of the mole are not very different from a common electric impact driver. In the mole, an electric motor drives a pin against a hammer with a ramped cam around its outer circumference. As the pin rotates, the came lifts the hammer up against spring pressure until it reaches the end of the ramp. The hammer is then forced down against the penetrator’s tip by the spring, directing a powerful force downward. The force of friction from the regolith against the hull of the penetrator, along with the force of a second damping spring, keep the mole from simply bouncing back up out of the hole.
And therein lies the problem, at least according to the current thinking. It appears that the individual grains of regolith the mole is trying to burrow into are not compacted well, thanks to the lower gravitation of Mars. It could be that the mole has made no progress because there’s nothing to hold it in place, and that the device is just bouncing around and not digging deeper.
To assess this, mission controllers decided they needed a look at the mole. Covered by the HP³ tower, there is no way to tell exactly what’s happening, so a plan was developed for moving it. This was no easy feat; aside from the fact that it would necessarily unspool some of the science tether, possibly resulting in a tangle, there was a chance that the mole would snag on something and be pulled completely out of the hole, ending its mission entirely. Add to that the complexity of operating a robotic arm over a high-latency connection, and the operation would require exquisite planning.
The HP³ removal was done painstakingly, in three very slow, very deliberate stages. On July 26, NASA revealed pictures of the uncovered mole which seem to corroborate the low-friction regolith theory: the end of the mole is sitting askew inside a wide conical hole, exactly what you expect from something that has been bouncing around in one place rather than hitting a rock and getting stuck.
Mission planners are now using all the data to develop a plan to save HP³. Tests on Earth using a duplicate penetrator and simulated Martian regolith seem to confirm that the friction issue is to blame, in which case there may be options. The mole might just need a little extra friction to get started; if enough of the hull is in contact with the regolith, it may just be enough to begin moving downward. To that end, NASA is reportedly concocting a plan to use the sample scoop on the robotic arm to fill in the hole and compact the regolith around the mole a bit. That might just get things started again.
Any way you slice it, the InSight team has done a tremendous job working this problem under difficult conditions. If they’re lucky, we may soon see the mole’s long Kapton tail disappearing down the deepest extraterrestrial hole we’ve ever dug, and we’ll start learning more about the Red Planet’s interior.