SpaceX began the countdown to send two veteran NASA astronauts to the International Space Station on Saturday although uncertain weather conditions threatened a second postponement of the historic first crewed mission by a commercial rocket.
SpaceX's two-stage Falcon 9 rocket was scheduled to liftoff at 3:22 pm Eastern Time (1922 GMT) from Florida's Kennedy Space Center with astronauts Robert Behnken and Douglas Hurley aboard the Crew Dragon capsule.
"We are moving forward with launch today," NASA chief Jim Bridenstine said in a tweet. "Weather challenges remain with a 50 percent chance of cancellation."
"Proceeding with countdown today," said SpaceX founder Elon Musk, the entrepreneur whose California-based company is seeking to become the first private firm to send astronauts into orbit.
Weather forced the last-minute postponement on Wednesday of what would have been the first launch of American astronauts from US soil since the space shuttle program ended in 2011.
The next window, which is determined by the relative position of the Florida launch site to the space station, is Sunday at 3:00 pm (1900 GMT), and fair weather is predicted.
The mission, dubbed "Demo-2," is the final test flight before NASA certifies the SpaceX spacecraft for regular crewed missions. Behnken, 49, and Hurley, 53, former military test pilots who joined the US space agency in 2000, are to blast off from Launch Pad 39A.
The same launch pad was used by Neil Armstrong on Apollo 11's landmark 1969 journey to the Moon, and NASA is seeking to revive excitement around human space exploration ahead of a planned return to Earth's satellite and then Mars.
Behnken and Hurley were going through the same preparations Saturday that they went through on Wednesday. They donned their futuristic SpaceX-designed spacesuits four hours before launch and exchanged final words with NASA chief Bridenstine, who was wearing a mask because of the coronavirus pandemic.