The rocket was a Falcon 9, built by SpaceX, Elon Musk's commercial spaceflight company. On its own, the retropropulsion landing is a major technological accomplishment. But it means even more as a step toward reliably getting humans off of Earth—maybe even permanently. "In order for us to really open up access to space," Musk said in a press conference shortly after the landing, "we need to achieve full and rapid reusability."

That's because space is expensive. A single Falcon 9 costs about $60 million. According to Musk, each Falcon 9 could theoretically be reused for 10 to 20 missions. Filling a Falcon 9 with rocket fuel only costs $200,000 to $300,000, so even counting refurbishments between missions, that means a hundredfold drop in marginal cost per launch.