Is that rock going to move? In order to stabilize the arm and rock for the drilling operation, the Curiosity robotic arm is designed to preload the drill against the rock with a minimum of about 300 Newtons of force. But to ensure this minimum force, we might apply a preload to the rock as high as 400 Newtons or more (this is close to 100 pounds of force: there are 4.45 Newtons per 1 pound force). For vertical or bedrock drilling activities, we don’t need to worry as much about the size of the rock, though the rock still needs to be large enough to accommodate the drill contact prongs with some margin, and if we have a rock sitting on soft regolith it might need to be relatively large to avoid sinking during the drilling activity. However, when the science team wants to drill horizontally or with any significant horizontal component, we have to look at the rock and guess at how well it might be anchored in the terrain or how heavy it might be before we can give a green light for drilling. If we determine that the rock is important but we aren’t sure the rock can support the force required for drilling, we have the option of pushing on the rock with the arm/drill combo to see if it stays put, crumbles, or moves before committing to the actual drilling operation.

Is the rover going to slip while we are at this location? Curiosity is much larger than Spirit and Opportunity, and has an aluminum chassis, compared to the Spirit / Opportunity rovers, which have a carbon-fiber composite chassis. An interesting side effect of these seemingly mundane differences is that during the diurnal 100 degrees Celsius thermal swing, the rover aluminum chassis and titanium mobility system end up growing and shrinking a total of roughly 4 millimeters (about one sixth of an inch). It might not seem like it, but this is a pretty big number! It is big enough to make us worry about vehicle stability from one sol to the next if we are on terrain that might pose a slip risk – e.g. being on a significant tilt, on “slippery” terrain like flat bedrock, or even not-so-slippery terrain if it has lots of little bumps where the wheels might slip off of during the diurnal cycle.

Why do we care if we slip? If the arm has not placed any turret tools into contact with the ground then we generally don’t worry about it. However, if the drill is on or in a rock or other tools are in contact with regolith or rocks we need to think through whether or not we might slip and whether any hardware might be damaged if the rover does slip. It is relatively easy for us to figure out what the damage threat is from a certain fixed magnitude of slip, but it is very difficult to judge whether we think we will actually slip enough to cross a damage threshold. It is important to note that we will always slip a little given the huge forces put out by thermal expansion and contraction of the hardware.

So, what do we do? Early in the mission, while we are characterizing how the vehicle behaves on Mars at temperature (roughly the first 3 months or so) we will restrict ourselves to sampling operations at tilts below about 7 degrees and will stay off anything that we think poses a high slip risk. As we get data from rover operations and how the vehicle actually behaves on the Martian surface, we will open up the envelope to the full 20 degrees for sampling and 30 degrees for contact science activities (contact science is just using the DRT, MAHLI, and APXS on surface targets, like what Spirit and Opportunity did with their robotic arms).