CAPE CANAVERAL, Fla.—SpaceX this weekend will attempt the first U.S. splashdown of returning astronauts in darkness since the Apollo 8 moonshot in 1968. Elon Musk’s company is targeting the predawn hours of Sunday to bring back three NASA astronauts and one from Japan, after dangerously high wind scuttled a pair of earlier attempts. The astronauts—only the second crew to fly SpaceX—will depart the International Space Station on Saturday night aboard the SpaceX Dragon capsule that carried them up last November. They’ll aim for a splashdown 6 1/2 hours later, around 3 a.m. in the Gulf of Mexico off the coast of Panama City, Florida. SpaceX brought back a station cargo capsule with a splashdown in darkness in January. That adds to NASA’s confidence for a nighttime homecoming, said Rob Navias, a spokesman at Johnson Space Center in Houston. “SpaceX has done numerous dress rehearsals and spent a lot of time …