NASA's Crew Dragon astronauts enter International Space Station

NASA astronauts Doug Hurley and Bob Behnken provide a tour of their Crew Dragon space taxi

Ever since the space shuttle was retired in 2011, NASA has relied on Russian spaceships launched from Kazakhstan to take United States astronauts to and from the space station.

Liftoff was rescheduled for Saturday afternoon.

A reporter asked Hurley and Behnken about what happened to the American flag they hung at the ISS when the duo flew there aboard space shuttle Atlantis in 2011, the last time a flight was launched from USA soil.

In the last hour or two, however, SpaceX did call up to astronaut Bob Behnken to determine the cause of a spacesuit pressure reading slightly below nominal, although the ground controller made sure to clarify that the suit would have still done its job in the event of Crew Dragon cabin depressurization.

US President Donald Trump and Vice President Mike Pence attended the liftoff, which was also watched by almost 4,000 people at the Kennedy Space Center. Inside the craft, astronauts Bob Behnken and Doug Hurley will lift off en route to the International Space Station. The space agency then pays for its astronauts to ride aboard.

The preparations took place in the shadow of the coronavirus outbreak that has killed an estimated 100,000 Americans.

Saiyadh added, "We hardly get five minutes to speak to astronauts".

"When we were watching the vehicle do its stair step maneuver up to our altitude ... we saw it very far away - about 19, 20 kilometers", Cassidy said.

No issues with the spacecraft have been reported - described by a SpaceX commentator as "pretty boring for the astronauts, but that's what we wanted".

"What today is about is reigniting the dream of space and getting people fired up about the future", he said in a NASA interview before the flight was scrubbed.

"Let's light this candle", Hurley said just before ignition, borrowing the historic words used by Alan Shepard on America's first human spaceflight, in 1961. He said he told them: "We've done everything we can to make sure your dads come back OK".

Saturday's launch by SpaceX, the private rocket company of billionaire entrepreneur Elon Musk, represented another milestone for the reusable rockets.

Attendance inside Kennedy Space Center was strictly limited because of the coronavirus, and the crowd amounted to only a few thousand. Accompanied by her 14-year-old daughter and 12-year-old son, she brought face masks and a small tent to protect against the elements.

SpaceX is launching Behnken and Hurley to the International Space Station, where they are expected to stay for 110 days. "It's exciting and hopeful".

"We're doing it differently than we've ever done it before", Bridenstine said.