The geological and environmental history of Curiosity’s location on Mars is written in the layers of Mount Sharp. As the rover climbs the mountain, it travels from older layers at the base to higher, younger layers above. The patterns of change in the rock composition tell the story.
Over the past few months Curiosity has been drilling at fairly regular intervals to systematically survey how the minerals and composition of the rocks are changing through time. At each site the drilling process produces rock powder, which is collected by the drill. This powder is transferred to instruments inside the rover for analysis.
Normal drilling activities have made use of both rotation and percussion, but percussion has caused intermittent short circuits since Feb. 28, 2015. To test the ability of the drill to collect a sample using rotation only, engineers decided that the hole to be drilled on Dec. 1 would use rotation only, without percussion. It was believed that the material at this site was soft enough that this experiment would go well.
Unfortunately, the drilling never started that day because of a drill fault, which appears to be unrelated to the previous short circuits during percussion. It also appears to be unrelated to the rotation-only test. Instead, it seems to be related to the drill feed process.
Whenever Curiosity prepares to drill a sample hole, the arm with the drill is lowered into position until two posts on the drill are in contact with the rock. Then the drill feed mechanism extends the drill to touch the rock target with the bit. The percussive and rotating mechanisms then start boring into the target to collect the powder sample.
The fault occurred when the feed mechanism did not move the bit to the rock. After completing several diagnostic tests, the experts believe they have found a pattern that matches what engineers would expect to see if a piece of foreign debris was embedded somewhere inside the drill.
It could be a piece of Martian soil or a pebble that somehow got into the mechanism, or it could be something carried from Earth. If the contamination came from Earth, it could have been there before launch, or it could have been generated after launch. Parts inside the drill might have rubbed together over the past four years, creating shavings or fragments that have lodged inside the feed motor.
If operating the drill on Mars caused the problem, engineers might be able to change the way they use the instrument. They could also learn from these problems and improve the design of future drills, such as the one being developed for the NASA 2020 rover.
Right now engineers are focused on how to recover the use of the drill. It is too early to know whether they can fully correct the problem or whether this issue will prevent future drilling. It may turn out that the stalled motor remains an intermittent problem, making it a nuisance for ground controllers but not fatal for the future of the drill.
If the drill cannot be used to collect samples in the future, it would be a significant loss. However, Curiosity still has many other instruments that will continue to collect extremely valuable data on Mars.
As we’ve said before, it’s not easy — even for a machine — to live and work on Mars.