Anxious scientists are awaiting the planned liftoff Saturday of NASA’s second of two Mars Exploration Rovers — this one named Opportunity. Liftoff is targeted for 11:56:16 p.m. EDT (0356.16 Sunday GMT) and the weather forecast is optimistic. Officials predict a 60 percent chance of acceptable conditions. Once outward bound, Opportunity will join Spirit, which is already en route to the Red Planet after its launch June 10. That spacecraft is on track to come to full stop within Gusev crater, perhaps the site of an ancient martian lake. Opportunity is targeted for touchdown at Meridiani Planum, where the chemical evidence for past water may exist and the site could have been a cozy habitat for life.