Nasa and SpaceX will try again to make history as they seek to launch astronauts into space from the US for the first time in nine years, just days after their first attempt was cancelled.

Preparations are under way for Robert Behnken and Douglas Hurley to travel to the International Space Station (ISS) on a rocket and capsule system built by billionaire entrepreneur Elon Musk’s firm.

The duo were due to lift off on Wednesday, but the mission was aborted less than 17 minutes before launch time over concerns that the event could trigger lightning.

Nasa administrator Jim Bridenstine tweeted to say Saturday’s launch activities were moving forward, but added: “Weather challenges remain with a 50% chance of cancellation.”

Elon Musk, who is the chief executive of SpaceX, said the teams were proceeding with countdown, despite the weather risk.

If all goes as planned, the mission, named Demo-2, will see SpaceX become the first private company to send astronauts into space.

But if the launch is delayed, the next available window will be on Sunday May 31 at 8pm UK time.

Since ending its Space Shuttle programme in 2011, Nasa has depended on Russia’s space agency Roscosmos to transport its astronauts to the space station.

In 2014, Nasa awarded SpaceX and Boeing contracts to provide crewed launch services to the space station as part of its Commercial Crew Programme.

According to Nasa, the aim of the Demo-2 mission is to show SpaceX’s ability to ferry astronauts to the space station and back safely.

It 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.

SpaceX’s Falcon 9 rocket will take off from the Launch Complex 39A at the Kennedy Space Centre in Florida, carrying the Crew Dragon spacecraft where Mr Behnken, 49, and Mr Hurley, 53, will be strapped in.

Mr Behnken will serve as the mission’s joint operations commander and take responsibility for the rendezvous, docking and undocking of the Dragon capsule, while Mr Hurley will be in charge of the launch, landing and recovery of the vehicle in his role as the Crew Dragon spacecraft commander.

Around 10 minutes after lift-off, the rocket will separate into a first stage and a second stage.

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 continues the journey with the Crew Dragon.

Once in orbit, the Crew Dragon will separate from the second stage and travel at around 17,000mph.

The craft is expected to rendezvous and dock, with the space station on Sunday at 3.30pm UK time.

Mr Behnken and Mr Hurley 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.

Mr Vagner tweeted earlier to say he was waiting for the duo at the space station.

The mission is expected to last anything between one and four months, with a number of tests being performed on the Crew Dragon.