Mars Pathfinder

A success story with a scare in the middle. Days after landing on Mars on 4 July 1997, Pathfinder's computer began resetting itself: four resets, on 5, 10, 11 and 14 July. The cause was priority inversion in its VxWorks software, where a low-priority task held a shared resource that a high-priority task needed, while medium-priority work kept the low-priority task from finishing. Engineers reproduced the fault on a duplicate system on Earth and, on 21 July, uploaded a change enabling priority inheritance. The glitch had appeared in preflight testing but was given low priority. Pathfinder is the standard example for teaching concurrency, and for the value of being able to diagnose and patch a system you cannot touch.