Two NASA astronauts arrived at a Florida launch pad flashing thumbs-up signs as they prepared to be carried into orbit on Wednesday by billionaire entrepreneur Elon Musk's private rocket company SpaceX in the first spaceflight of U.S. space agency astronauts from American soil in nine years.
There was off-and-on rain and the National Weather Service issued a tornado warning for the area around Kennedy Space Center. A SpaceX official cited a 60% chance that thick clouds over eastern Florida could force a launch postponement. The next launch window would be on Saturday afternoon.
A SpaceX Falcon 9 rocket was due to lift off from the Kennedy Space Center at 4:33pm EDT (2033 GMT), launching astronauts Doug Hurley and Bob Behnken on a 19-hour ride aboard the company's newly designed Crew Dragon capsule to the International Space Station.
"We are go for launch!" NASA Administrator Jim Bridenstine wrote on Twitter earlier in the day.