NASA has announced that it will purchase three additional flights for its astronauts to the International Space Station on SpaceX's Crew Dragon vehicle.

"After a thorough review of the near-term certified capabilities and responses from American industry, NASA’s assessment is that the SpaceX crew transportation system is the only one certified to meet NASA’s safety requirements to transport crew to the space station, and to maintain the agency’s obligation to its international partners in the needed timeframe," the agency said.

SpaceX first demonstrated the safe flight of humans on Crew Dragon in mid-2020 and has since launched three operational crew missions for NASA, with three more scheduled for now through the spring of 2023.

"NASA commends Boeing for its ongoing investigation of the oxidizer isolation valve issue that was discovered ahead of the planned uncrewed Orbital Flight Test-2 mission to the International Space Station in August, prioritizing safety over schedule while working to solve this challenge," said Phil McAlister, director of commercial spaceflight at NASA, in a statement.

Should this test go well, Boeing could then begin flying operational missions, carrying NASA and international astronauts to the space station for six-month stays.

In its original procurements seven years ago, NASA agreed to purchase six of these "operational" missions each from SpaceX and Boeing.

As NASA wants to fly crew missions every six months, and it has no guarantee Boeing would be ready by 2023, it took the step of extending SpaceX's contract now.