Mars Rovers in Autumn: A Life and Death Drama on the Red Planet
Although the NASA robots are mechanized twins at birth, each has a distinct personality, Wallace observed.
Undergoing testing here on Earth prior to Mars sendoff, Wallace felt that Spirit exhibited a tendency to be less well-behaved, the more adventurous of the two vehicles. “Opportunity tended to kind of toe the line…a little more staying inside the lines,” he related.
A classic case of anthropomorphization. Spirit must be the younger twin.