In a few days, a robot vehicle no bigger than a golf cart will complete its analysis of a small patch of red Martian soil. Then it will turn south to continue a journey that will become the longest overland trek ever made on another world.

Spirit and Opportunity touched down, encased in huge inflated airbags, on 3 and 24 January 2004 respectively, and have since sent back more than a quarter of a million photographs, survived dust storms, descended into craters and, in one case, climbed a mountain. Opportunity has operated continuously without a hitch, while Spirit suffered one major glitch two years ago when a wheel jammed. However, the little rover has still managed to move around fairly well on its remaining five wheels.