A SpaceX  Falcon 9 rocket launched a cargo to the International Space Station today at 3:25pm ET, and then turned around and flew back to a soft landing in the Atlantic Ocean. The cargo, flown on a Dragon capsule, will reach the station on Sunday morning.

It’s the first time a booster rocket has made hypersonic reentry into the atmosphere intact and then splashed down gently without benefit of parachutes. Today’s flight was a major milestone in SpaceX’s quest for reusable orbital rockets....