Bob Behnken and Doug Hurley were set for a rare ride on Wednesday, but bad weather postponed their trip into space until Saturday.
Their journey to the International Space Station (ISS) was set to begin at 9.33pm UK time in the spacecraft 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 Centre in Florida.
But as the weather conditions became worse, the US space agency postponed the mission for safety reasons.
Natalia Jorquera outlines the billionaire space race above
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.
Nasa, which has strict rules about the conditions for manned crew missions, said of one these rules was being violated just minutes before the launch.
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.
When the launch does go ahead it will be the first time a private company has attempted to send astronauts into space. It comes after a successful practice launch in January.
Here’s everything you need to know about the Demo-2 mission.
What's the purpose of it?
Nasa hope Demo-2 will prove SpaceX’s ability to ferry astronauts to the space station and back safely.
The demonstration mission is the final major step required by SpaceX’s astronaut carrier, the Crew Dragon, to get certified by Nasa’s Commercial Crew Programme for more long-term manned missions to space.
Who are Bob Behnken and Doug Hurley?
Mr Behnken, 48, and Mr Hurley, 53, are experienced Nasa astronauts who have been involved in testing of the Crew Dragon capsule.
Mr Hurley, who was a fighter pilot in the US Marine Corps, was on the final flight of the space shuttle Atlantis in 2011 before it was discontinued.
His son has sketched out how he thinks the launch will go for his dad.
Mr Behnken was a flight test engineer with the US Air Force before joining Nasa, and has spent just over 29 days in space, which includes 37 hours of spacewalking time.
He will serve as the mission’s joint operations commander and take responsibility for the rendezvous, docking and undocking of the Dragon capsule.
Mr Hurley will be in charge of the launch, landing and recovery of the vehicle in his role as the Crew Dragon spacecraft commander.
The two men will be wearing spacesuits designed by SpaceX with help from Hollywood costume designer Jose Fernandez.
How will the astronauts get to the space station?
The Falcon 9 rocket will take off from launchpad 39A at the Kennedy Space Centre in Florida, carrying the Crew Dragon spacecraft with Mr Behnken and Mr Hurley strapped inside.
Shortly after lift-off, the rocket will separate into what is called a first stage and a second stage.
The graphic below details the early stages of ascent and separation of the Falcon 9.
The first stage will return to a SpaceX landing ship which will be stationed in the Atlantic Ocean off the coast of Florida, while the second part of the rocket continue the journey with the Crew Dragon.
Once in orbit, the Crew Dragon will then separate from the second stage and travel at around 17,000mph before being in a position to rendezvous, and dock, with the space station 24 hours later.
What will they do when they get there?
Mr Behnken and Mr Hurley will test the Crew Dragon’s environmental control system, the displays and controls, and the manoeuvring thrusters.
They will also monitor the autonomous docking system during the approach to the space station.
The astronauts will join the three other space station residents – Nasa’s Chris Cassidy and Russia’s Anatoli Ivanishin and Ivan Vagner – to become members of the Expedition 63 crew.
They will perform further tests on the Crew Dragon along with other tasks related to the space station.
But the pair’s main mission is to conclude the validation process that is required by Nasa to ensure the spacecraft designed to carry astronauts can operate safely.
How and when will they return?
The Demo-2 mission is expected to last anything between one and four months. But Nasa said the duration of this mission would be determined by when the next commercial crew will be able to travel to the space station.
The spacecraft will be capable of staying in orbit for at least 210 days.
When it is time to return, the Crew Dragon will autonomously undock with Mr Behnken and Mr Hurley on board and depart the space station.
This graphic breaks the return down into five key stages:
Shortly after firing up its engines to re-enter the Earth’s atmosphere, the spacecraft will deploy four parachutes to slow its descent and splash down in the Atlantic Ocean, just off the coast of Florida.
SpaceX’s recovery ship, called Go Navigator, will be waiting nearby to retrieve the duo and ferry them to Cape Canaveral.
Why did Nasa stop sending astronauts into space?
In 2010, the US started to wind down its space shuttle programme which had carried astronauts into orbit for three decades with the aim of focusing on building technology for Moon and Mars missions.
Nasa asked private companies such as Space X and Boeing to design the technologies which would allow passengers to travel to space, while opting to pay Russia to send American astronauts to the space station as an interim arrangement.
If Demo-2 is successful, Elon Musk's SpaceX will be allowed to go ahead with more manned missions to the space stations as part of their 2.6 billion US dollar (£2.1 billion) contract with Nasa.
Boeing also has a similar deal with the space agency, worth 4.2 billion US dollars (£3.4 billion), to send astronauts to the space station in its CST-100 Starliner crew capsule, although its vehicle is not expected to be ready until next year.
Libby Jackson, human exploration programme manager at the UK Space Agency, said the work done by SpaceX and Nasa is not only a huge stepping stone for the US but also a “major milestone for the global space sector”.
She told the PA news agency: “We have been dependent on just a single way of getting to and from this amazing scientific laboratory we have up there.
“To reinstate what we call dissimilar redundancy – a different way of getting to and from the space station – would be very significant for the future of the space station.”
She added that the mission opens up a new era where space agencies are becoming more open to commercialisation and space tourism, saying it is “indeed where I see some of this heading”.
More on Nasa and space travel: