Weighing as much as a small car, a rover named Curiosity rolls quietly around Mars. Scientific instruments pack its body and cluster at the end of a mechanical arm. To the many NASA workers involved in Curiosity's mission on Mars, the rover is not simple a robot, but an astronaut bravely exploring an alien place. As it roams and explores, Curiosity could help answer the exciting question: Did the water on the planet's surface ever harbor life?