The launch of SpaceX's Crew Dragon was called off Wednesday about 16 minutes before liftoff at NASA's Kennedy Space Center in Florida.
Flight controllers had reported that all equipment and technical requirements had been met to go forward with the launch. But several weather-related requirements affecting the flight path could not be met. One concern was the possibility of lightning.
NASA has set the next launch attempt for Saturday afternoon.
U.S. astronauts Robert Behnken and Douglas Hurley are aiming to become the first astronauts to launch from American soil since the last space shuttle flight in 2011.
A successful SpaceX launch will make history as the first time a private company has launched astronauts into orbit.
A Falcon 9 rocket is to carry Crew Dragon. Space X designed and built both spacecraft.
The mission will take Behnken and Hurley to the International Space Station, where they are expected to stay at least a month.
I'm Bryan Lynn.