The first launch of the SpaceX NASA Dragon-2 rocket was dramatically cancelled - just 16 minutes before it was due to take off.
Fuel was launched and the astronauts were making final preparations when the decision was made over the weather in Florida.
The countdown clock stopped with 16:53 left.
The launch has now been put back three days until May 30 when they will try again.
It was due to be the first manned space launch from American soil since the Space Shuttle was retired in 2011. The Dragon capsule was set to make the 19-hour trip to the International Space Station but that journey will now have to wait.
The NASA weather report had a 40% chance of breaking the flight's strict safety limits - in this case over the amount of lightning in the atmosphere - so despite the spaceship itself being in perfect shape, the safety of the crew came first.
The crew had to stay onboard while fuel was drained from the spaceship before they could leave and return to base.
British astronaut Tim Peake said: "What a shame - SpaceX launch is scrubbed, but the rules are there in the interest of safety. A great rehearsal and reset for the next launch attempt in 3 days."
