Three Generations of Rovers are pictured in the Mars Test Yard at NASA's Jet Propulsion Laboratory, Pasadena, California in this undated handout photograph. Front and left is the flight spare for the first Mars rover, Sojourner, which landed on Mars in 1997 as part of the Mars Pathfinder Project. The Mars Exploration Rover Project test rover (L) that is a working sibling to Spirit and Opportunity, which landed on Mars in 2004 and shown (R) is a Mars Science Laboratory test rover the size of that project's Mars rover, Curiosity, which is on course for landing on Mars on August 5, 2012. REUTERS/NASA/JPL-Caltech/Handout (UNITED STATES - Tags: SCIENCE TECHNOLOGY) FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. IT IS DISTRIBUTED, EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS