Will it be twins for NASA? The space agency’s Mars czar is “cautiously optimistic” that two interplanetary robotic rovers will be ready in time for sendoff next year to the red planet. Spacecraft engineers and software specialists engaged in preparing the Mars Exploration Rovers (MER) are racing to meet a series of major milestones over the next several months. Overall testing of the robots has proven more costly than budgets allowed, forcing a recent internal overhaul of the program. Later this year and in January, two independent groups are to scrutinize the overall MER project. They are being convened to give a thumbs-up or down on whether the rovers are ready for an assault on Mars and can carry out their assigned duties.