A year from now, NASA hopes to be making tracks on Mars once again, this time with two robotic six-wheeled rovers. The agency announced the mission’s objectives this week at the American Geophysical Union’s meeting in San Francisco. The Mars Exploration Rovers (MERs) will be launched separately in May and June 2003. Swaddled in air bags, the roving twins will bump down on different sides of the red planet in January 2004, if all goes according to plan.