PASADENA, Calif.—The biggest and best-equipped robotic rover ever launched to another planet is hurtling toward a rendezvous with Mars, in a $2.5 billion mission that tests the engineering limits of interplanetary exploration and takes the search for alien life to a new level.

If all goes well, the National Aeronautics and Space Administration's Mars Science Laboratory mission expects to land a one-ton mobile rover, called Curiosity, on the Red Planet in early August to begin...