NASA is all set to launch the world’s biggest extraterrestrial explorer. A six-wheeled, one-armed Mars rover is due to blast off Saturday morning from Cape Canaveral. The unmanned Atlas V rocket will put the spacecraft on an 8½-month trek to the red planet. The rover, nicknamed Curiosity, is the size of a car. It’s a mobile laboratory holding 10 science instruments that will sample Martian soil and rocks, and analyze them right there on the surface. There’s a drill as well as a stone-zapping laser machine. Curiosity will spend two years looking for evidence that Mars may once have been — or still is — suitable for microbial life. The mission costs $2.5 billion. Forecasters expect decent launching weather.