Twin Mars rovers in instrument mix-up New Scientist

NASA’s Mars rovers Opportunity and Spirit are identical twins – so alike that they even fooled NASA. Researchers have discovered that they sent the robots to Mars with an instrument meant for Opportunity inside Spirit and vice versa. While the bungle does not undermine the main scientific conclusions drawn from the data collected by the rovers, it is an embarrassing slip-up for a space agency that once lost a Mars spacecraft because engineers mixed up metric and imperial units.