LOS ANGELES — A solar-powered spacecraft is spinning toward Jupiter for the closest encounter with the biggest planet in our solar system. NASA’s Juno spacecraft fired its main rocket engine late Monday to slow itself down from a speed of 150,000 mph (250,000 kph) and slip into orbit around Jupiter. With Juno on autopilot, the delicately choreographed move comes without any help from ground controllers. The spacecraft is traveling through a hostile radiation environment and rings of debris and dust, “making for very serious hazards,” Juno chief scientist Scott Bolton said during a morning briefing.