THE US space agency is to send its Mars rover Opportunity on a two-year trek to try to reach a crater called Endeavour. The robot will have to move about 11km to get to its new target - a distance that would double what it has already achieved on the planet. Endeavour is much bigger than anything investigated to date, and will allow a broader range of rocks to be studied. Opportunity arrived on Mars in January 2004 on a mission scheduled initially to last just three months. The performance of the rover - like that of its twin, Spirit - has greatly exceeded what anyone had dared hope. The US space agency (Nasa) concedes, however, that the Endeavour assignment will be an extremely tough one. "We may not get there, but it is scientifically the right direction to go anyway," said Steve Squyres of Cornell University, principal investigator for the science instruments on Opportunity and Spirit. "This crater is staggeringly large compared to anything we've seen before." Opportunity has just emerged from the 800m-wide Victoria Crater. Endeavour by comparison is 22km across. The mission team estimates Opportunity may be able to travel about 100m per day. But even at that pace, the journey could take two years. The rover will stop to study rocks on the way, and in winter months it cannot move because there is not enough sunlight to provide sufficient power for driving. Detailed satellite imagery from the Mars Reconnaissance Orbiter will help pick out the best route ahead; and new software recently uploaded to Opportunity will enable the rover to make its own decisions about how best to negotiate large rocks in its path. At least it does not yet have to cope with the locomotion problems of Spirit. A jammed wheel on the twin means it must now drive backwards everywhere it goes. Since landing on the Meridiani plains, Opportunity has gathered important data about Martian geology, including evidence that parts of it were once soaked by water.      - BBC