LOS ANGELES (AP) — A NASA spacecraft has begun firing its thrusters to refine its flight path to Mars.The maneuver Wednesday is expected to last three hours and put the one-ton rover nicknamed Curiosity on course for a landing on the red planet.Curiosity is scheduled to touch down in August in a crater near the Martian equator after an eight-month trip. It carries a suite of instruments to explore whether the dry and barren surface ever had conditions favorable for primitive life.There will be several more chances for the spacecraft to tweak its trajectory before landing.The $2.5 billion Mars Science Laboratory mission is managed by the NASA Jet Propulsion Laboratory in California.