The historic SpaceX flight has been postponed due to bad weather.
Robert Behnken and Douglas Hurley were set to travel to the International Space Station (ISS) on a rocket and capsule system built by billionaire entrepreneur Elon Musk's firm.
An estimated 1.7 million people from around the world tuned in to the launch from The Kennedy Space Center in Florida.
But as the weather conditions became worse, US space agency NASA postponed the mission for safety reasons.
The US Air Force's 45th Weather Squadron, which monitors the weather for air and space operations, had forecast between a 40% and 60% chance of favourable conditions at the launch site in Florida.
With just 16 minutes left before Falcon 9 was set to launch towards the International Space Station, officials said the words "launched scrubbed" - stopping the spaceship from setting off.
The ground crew needed to hit three weather criteria and would have met them 10 more minutes after the 4.33pm launch time, allowing the rocket to takeoff.
The earliest the SpaceX Falcon 9 rocket and the Crew Dragon spacecraft could depart is this weekend, with potential launch windows available on Saturday and Sunday.
Earlier, SpaceX boss Elon Musk said he accepted absolute responsibility if today's historic launch of his Falcon 9 rocket ends in tragedy.
Mr Musk told CBS This Morning: "I'm the chief engineer of this thing so I'd just like to say that if it goes right, it's credit to the SpaceX-NASA team. If it goes wrong, it's my fault."
Asked whether there was one thing about this afternoon's launch that kept him up at night, he added: "There's thousands of things that can go wrong and only one thing that can go right."
The mission, dubbed Launch America, will be the first time in nearly a decade NASA astronauts have lifted off US soil aboard an American made rocket.
If all goes to plan, SpaceX will become the first private company to put astronauts into orbit, something achieved by just three countries - Russia, the US and China.
