This is a good article. Some of the failure modes are assumed; based upon fault-tree and most likely scenario. The metric/English unit conversion error was real, but even with the bad navigation, the probe still should have been in a 'safe' orbital corridor.

The other story is how close Pathfinder, Spirit and Opportunity came to biffing it, especially Spirit. All three probes landed at near the burst limit of the air bags (~25m/s), well above the nominal impact velocity of ~15m/s.

Mars has been hard, and Phoenix gives us another chance to see just how hard Mars can be. Nothing routine about these Mars landing missions!

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jwj

The Reluctant Cosmologist