File photo: AP

A final decision on a launch attempt for SpaceX's milestone mission to the International Space Station on Saturday afternoon will take place after assessing the weather that morning, NASA chief Jim Bridenstine said Friday.

Fears of a lightning strike postponed the initial takeoff attempt on Wednesday of what would have been the first crewed rocket launch from US soil in almost a decade, and the first time a commercial company has achieved the feat.

"No decision on weather right now for Saturday's test flight of @SpaceX's #CrewDragon spacecraft. Will reassess in the morning," tweeted Bridenstine.

Earlier in the day, NASA said the chances of a Saturday launch at 3:22 pm Eastern Time (1922 GMT) were 50 percent. The weather forecast currently predicts a thunderstorm. 

The next window, which is determined by the relative positions of the launch site to the space station, is Sunday at 3:00 pm Eastern Time (1900 GMT), and fair weather is predicted.

NASA astronauts Robert Behnken, 49, and Douglas Hurley, 53, former military test pilots who joined the space agency in 2000, are to blast off from historic Launch Pad 39A on a two-stage SpaceX Falcon 9 rocket.

The same launch pad was used by Neil Armstrong and his Apollo 11 crewmates on their historic journey to the Moon, as NASA seeks to revive excitement around human space exploration ahead of a planned return to Earth's natural satellite and then Mars.

The mission comes despite shutdowns caused by the coronavirus pandemic, with the crew in quarantine for more than two weeks. 

NASA has urged crowds to stay away from Cocoa Beach, the traditional viewing spot -- but that did not deter many space fans on Wednesday.

US President Donald Trump, who flew in for the previous launch attempt, is expected to attend again.