A pioneering attempt to cross the Pacific Ocean using a solar-powered plane was hindered by poor weather Monday. Swiss pilot Andre Borschberg, who is flying the Solar Impulse, was forced to make an unscheduled stop in Japan on a trip from China to Hawaii. The flight is the seventh leg in the effort to fly around the world using the energy of the sun. The Pacific crossing, which was expected to take 130 hours, is considered the most challenging part of the journey. The unscheduled stop raises concerns about whether the delay could affect later phases of the flight.