What Happens if the InSight Mole Hits a Rock?

This is the second most common question I get asked about the InSight mission. (The most common is: how can you take a sensitive instrument like a seismometer, launch it on a rocket, land it on Mars, and expect it to work? I’m happy that question is now obsolete.)

Don’t worry, HP3’s designers have thought about this. The following are the reasons why they’re not overly concerned:

The mole can push aside small rocks. We have dug in Mars soil many, many times before, both with scoops (Viking and Phoenix) and wheels (Sojourner, Spirit, Opportunity, and Curiosity), and scientists have written quite a lot about the properties of various types of Mars soil. The mole has been designed to deal with a range of potential Mars soil properties, and can hammer itself forcefully enough to be able to push aside pebbles and rocks up to 5 or 10 centimeters across that it encounters as it penetrates.

The mole can skitter around larger rocks. If the pointy tip of the mole encounters a rock that is too large to push aside, it will most likely encounter a surface that is not horizontal. In that case, the hammering action will cause the tip of the mole to be deflected, sliding it down the sloping face and around the rock; once it’s detoured around the rock, the mole will right itself and continue driving vertically. The mole contains a tiltmeter that will be monitoring any such deflections so the team will know pretty precisely what the mole’s course into the surface was. The tiltmeter can measure deflections to an accuracy of 1 degree.

Big rocks are not likely. One of the factors that led them to the Elysium landing site was the relative absence of large rocks. The surface of the landing site is less rocky than most previous landing sites, comparable to Phoenix. How about buried rocks? A tricky bit of geomorphology research told them there shouldn’t be many. Here’s how that worked: they mapped the sizes of impact craters and noted whether the craters had blocky ejecta (that is, whether they were surrounded by large rocks that had been dug up during the impact). Larger craters do have blocky ejecta, but smaller craters do not. The transition between small, not-blocky craters and larger, blocky ejecta craters happens at a crater size that would have dug its ejecta from more than 5 meters’ depth. The lack of blocks in these smaller craters suggests that there aren’t big rocks in the top 5 meters or so of soil at Elysium.