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A few months after it landed, Spirit's left front tire began showing a problem by using much more energy than the others. Here, JPL engineers Eric Aguilar, left, and Joe Melko work in a sandbox to find a way around the problem. The wheel eventually died in 2006 but NASA scientists were able to keep it rolling on five wheels by driving it backwards and devising easier ways to turn.
On a positive note, the dead wheel dug up Martian soil which enabled Spirit's most impressive discovery - a patch of nearly pure silica. This was most likely caused by hot springs or steam vents.
Spirit contained a rock abrasion tool and was able to drill into 15 different rock samples and analyze them.