I’ve been focusing a lot on the probabilistic assessment that Curiosity has a 98.3% chance of landing successfully (if its hardware works). About 1% of that risk is in the parachute, which is why scientists working the Mars Reconnaissance Orbiter are so keen to catch Curiosity during its descent. But what about the remaining 0.7% terrain-hazard risk?
This risk is the combination of boulders and slopes that could threaten to tip the rover, and craters and mesas with walls too steep for Curiosity to escape. According to Devin Kipp, an engineer on the entry, descent and landing (EDL) team at the Jet Propulsion Laboratory in Pasadena, California, slopes greater than 20% give the rover trouble. “Above that and it gets shaky,” he says.
Allen Chen, the EDL operations lead, was kind enough to share the above image, which shows the areas (in red) that contribute to the 0.7% risk associated with terrain hazards. You can see that the ellipse was purposefully positioned between the steep walls of Gale crater in the upper left corner, and, in the lower right, the steep slopes of Mount Sharp (oops, I mean Aeolis Mons).
But look carefully within the ellipse and, halfway between the centre and the western edge, you can see an angry red pimple: an unnamed 250-metre-wide crater with walls that would probably trap the rover. It’s a hole-in-one that Curiosity certainly wants to avoid. After the jump is a map that shows how that crater’s walls can slope as much as 25 degrees. Chen says that it’s possible that the rover could escape through a slightly less steep section of the crater’s southern wall, but it’s a predicament he doesn’t want to deal with.
So why didn’t the team move the landing ellipse to the north a little, where the landing terrain is all blue? That certainly was considered, Chen says. But there are trade-offs. The team would have eliminated a tiny amount of dangerous terrain, but then the rover would have much further from the base of Aeolis Mons — where the most important mission science awaits.