¡¡PASADENA, the United States, Jan. 24 (Xinhuanet) -- As NASA scientists struggled to revive their ailing Spirit Mars rover, they also began to prepare for the landing of its twin, Opportunity, which will touch down on the Mars surface about midnight on Saturday.

    The Opportunity will be slowed first by a parachute, then retrorockets. At a height of about 30 feet, the rover, encased in a cocoon of air bags, will bounce and roll for perhaps half a mile before coming to a stop.

    The Opportunity's target is a region known as Meridiani Planum, which contains a deposit of iron oxide the size of Oklahoma. The iron oxide, known as hematite, interests scientists, because on Earth, it usually forms in the presence of water.

    "Even though Meridiani is an easier landing site in some respects, less winds, less slopes, and the rocks aren't as bad as I'm going to be nervous," said Rob Manning, Mars Exploration RoverEntry, Descent and Landing Lead at the NASA.

    NASA on Friday declared the crippled Spirit Mars rover to be in"critical" condition and said the vehicle would likely remain idlefor several days or even weeks.

    Sending two rovers was seen as a way to increase the chance of success, as only one in three international efforts to land on Mars has succeeded. Some of the other spacecraft blew up, crashed or disappeared. Enditem