Although neither rover has yet to match the distance record set by Lunokhod 2, the smaller Martian rovers encounter significantly more problems arising from the dusty nature of Mars. Firstly, the progressive obscuration of the solar panels by airborne dust leads to a fall-off in the available power, mitigated somewhat by the beneficial but random action of transient aeolian phenomena, which appear to clear some of the material from the rovers. Secondly, the Lunokhod wheels were not exposed to blown dust grains, whereas the wheel assemblies on each MER are subject to both saltating and suspended particles, which place greater loads on the motor and gear shaft seals. As of late 2005, only the Spirit rover has shown signs of anomalous wheel friction, with both craft working after exceeding
their design benchmark of 90 days by almost a factor of ten. Appropriate driving rules allow that rover to manoeuvre and drive albeit with reduced efficiency. At this same stage the Opportunity rover has a jammed front wheel actuator, and has difficulties in unstowing its arm but after more than one Martian year all payloads are operating well.
Both craft have been exemplary models of the judicious mixture of caution and design innovation needed to produce successful planetary craft. In terms of risk, the project has been regarded as being challenging with successful missions of comparable complexity being developed in historically much longer periods (Dornheim, 2003). It remains to be seen whether follow-on missions will display a similar level of robustness. Indeed, the fact that the continued survival of the MER hardware is unexpected, albeit very welcome, reveals much about the uncertainties surrounding risk assessment in planetary missions and the danger of attempting to characterize unique procedures and systems.
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